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Articles from 2002 In November


Technology Notebook: Improving granulator efficiency

A three blade, open rotor offers good airflow for cooling, while the herringbone or twin shear design maximizes cutting efficiency over the wide cutting chamber. The knives are gapped outside of the granulator for accurate and easy replacement.
Editor?s note: Bob Harrison is product sales manager for Wittmann Inc. (Torrington, CT), a supplier of auxiliary equipment.

The last 10 years have brought significant improvements in granulator design, increasing safety, quality of regrind, ease of use, and energy efficiency.

One key to effective plastics processing is uniform quality of regrind. The bulk density of many virgin plastics is in the 30 to 35 lb/cu ft range. Since the bulk density is greatly influenced by the size and shape rather than the type of material, the bulk density of regrind may be considerably less, in the 15 to 25 lb/cu ft range. This may be due to the thickness of the scrap being ground, but also can be the result of fines, dust, and filaments known as ?angel hair? that are generated in the regrind process. When the mixture of virgin and regrind is introduced into the system, process requirements such as drying, blending, and melting can change significantly.

Uniformity of Regrind
The standard method of measuring regrind uniformity is the sieve test. Regrind is passed through a series of screens with progressively smaller hole sizes. Particles that do not pass through the top and largest screen (typically .2500 to .3125 inch) are considered ?longs? and may present material handling problems because of their large size. Particles that are captured by the .1875- and .0937-inch screens are considered good regrind, and those that pass through the .0937-inch screen are considered fines.

Many variables affect regrind quality, including the physical and mechanical properties of the material. Hard, brittle materials shatter upon impact with the knives. Soft, flexible, energy-absorbing materials must be cut by the knives. Influential operating conditions include feed temperature, feedrate, and method of evacuation. Granulator characteristics that have an impact include rotor design, drive design, cutting angle, knife tip design, knife material, screen location and design, and screen hole size.

As the processor may have little control over the choice of material for a particular job, the main focus must be on the operating conditions and capabilities of the granulator. However, there are certain elements, such as knife gap, that are more or less critical depending on the type of material. Soft, flexible materials that must be cut require a very tight clearance between the rotating and stationary knives. A general rule of thumb is that brittle, energy-impacting materials such as polystyrene or those filled with glass, mineral, or talc tend to fracture upon impact with the blades and require less horsepower to process. Since the material is fracturing upon impact with the blades instead of being cut, the granulate tends to lack uniformity and have a high fines percentage. Regrind quality generally improves when the parts can be processed while still warm and somewhat pliable.

On the other hand, soft, energy-absorbing materials such as flexible PVC, urethane, and styrene butadiene copolymer (like K-Resin) must be cut by the knife and generally require more horsepower. For these materials it is more critical that the knives be sharp, the knife tip angle steep, and the gap between the rotating and stationary knives as small as possible. In general, small clearances between the rotating and the stationary knives favor more energy-efficient knife cutting in contrast to the more energy-consuming, tearing-type cut that occurs when the gap is larger. An analogy would be to think about how a pair of household scissors work?or don?t work, for that matter?when the bolt holding the two blades together loosens and the blades are not held as tightly together.

An extended rotor shaft allows for the addition of an extra flywheel to store energy. This gives more inertia when cutting thick cross-sectional parts. Reducing energy demands on the motor minimizes amperage spikes and increases motor life.
The Right Rotor
There is no single rotor design that is best for all applications. However, the three-blade, open-rotor design is the most versatile. An open-rotor design allows for good airflow through the cutting chamber, and hence good cooling, which can be critical when processing easily degradable materials such as styrene butadiene copolymer. On the other hand, the staggered rotor can be more energy efficient when processing thick cross-section parts. By design, it takes smaller bites. Less horsepower is required to cut the same thick cross-sectioned part and motor amperage spikes are reduced. A drawback is that a solid rotor restricts airflow through the cutting chamber. The solid rotor is not optimum for bulky, thin-walled parts that are becoming more common.

Several granulator manufacturers have designed their rotors with adjustable rotating knives instead of fixed knives. Three major features of adjustable rotating knives contribute to improved granulator efficiency and regrind quality:
  • The cutting circle remains constant even after the knives have been sharpened. With a constant cutting circle, the distance between the knife tips and the screen does not change, thus ensuring maximum cutting efficiency.

  • Rotating knives are adjusted individually, which allows for the smallest possible gap between each knife. With fixed knives, the gap must be set for the largest knife?the one with the largest cutting circle?as all of the other knives have the same or larger gap depending on their height.

  • Adjustable rotating knives offer extended knife life since the knives can be sharpened individually, requiring only the minimum amount of metal be removed from each knife. With fixed rotating knives, the knives must be sharpened as a set and all of the knives dressed down to the knife with the most wear. Herringbone or twin-shear rotors are used to minimize the knife gap over wide cutting chambers.
Various knife tip angles should be available for processing different types of materials. High angle knives (with a high free angle) are more efficient for processing soft, flexible materials. By decreasing the free angle while maintaining a constant cutting angle, knife tips can be strengthened to reduce chipping for processing hard, brittle materials. It is very important for high-quality regrind and energy efficiency to have a relief angle on the stationary knives as well.

Rotor Speed vs. Cutting Efficiency
Drive design features directly impact quality of regrind and energy efficiency. Excessive rotor speeds actually increase the time it takes to granulate most materials. Excessive speeds also lead to a less uniform particle size and a higher percentage of fines. High knife speeds do not allow correctly sized particles to easily pass through the classifying screen holes. Instead, the particles are swept around and around while being cut further into undesirable fines and dust. That same unnecessary cutting also leads to excessive noise, increased energy consumption, and excessive wear on the knives and the cutting chamber.

Rotor speed reduction using lower rpm and higher torque motors can actually increase knife tip force. A 20-hp, six-pole (1200-rpm) motor has the same torque as a 30-hp, four-pole (1800-rpm) motor. To help cut through tough materials using a low rotor speed on larger units, the rotor shaft can be extended to include an additional flywheel mass opposite the solid flywheel drive pulley. The additional flywheel uses stored mechanical energy to drive the blades through thick cross-sectioned parts. It does not require the motor to work harder. Minimizing motor amperage spikes also increases motor life.

If acquiring a granulator or replacing a motor on an existing unit, consider a high-efficiency motor. An example from a motor supplier catalog demonstrates how an additional investment of $178 up front on a higher-efficiency motor can save several thousand dollars over the average lifetime of a granulator motor, which is often seven to 10 years.

The single most important step a processor can take to maximize granulator efficiency is to be sure that the granulator knives are sharp. This is a commonly neglected task.

Some granulator manufacturers offer a knife preadjustment fixture that allows for the knives to be gapped outside of the machine where it is safer and easier to perform. The fixture ensures an accurate and consistent knife gap setting, allowing personnel to verify that the knives have been properly sharpened to maintain the correct knife tip angles. The fixture also allows for a spare set of knives to be gapped in advance, allowing for faster knife change and less downtime. By making it quicker and easier to change the knives, the system makes it more likely that knives are changed more often for maximum cutting efficiency.

Safety and Sound Requirements
A good point of reference, though not mandatory in North America, is the new European safety standard EN 12012-1, which requires granulators to have two sensors (for redundancy) that monitor the rotor motion and signal the electronic interlock to open and allow access only when the rotor has come to a complete stop. This is a far better safety than a time-delay limit switch and provides easier access to the cutting chamber. In addition, a mechanical rotor brake has been designed that is automatically applied when the cutting chamber is opened to prevent the rotor from spinning easily.

Increasingly strict guidelines for allowable noise levels and the increasing popularity of quiet all-electric injection molding machines have created the need for better sound control of granulators. This has been met with soundproofed hoppers, sound enclosures for the base of the machines, and reduced sound levels via higher-efficiency cutting designs. Reducing the time it takes to granulate a given quantity of material also reduces average decibel levels.

CONTACT INFORMATION
Wittmann Inc., Torrington, CT
Bob Harrison
(508) 422-9524; www.wittmann-ct.com

Tooling Corner: Die design for extrusion

Figure 1. These examples show the application of die design guidelines.
Editor?s note: Chris Rauwendaal is a designer, consultant, and seminar instructor who has written extensively on process engineering and extrusion topics.

Die design for extrusion can be rather complicated, since the size and shape of the extruded product varies from that of the die flow channel. Multiple mechanisms affect the size and shape changes in the extruded product, and these can be controlled by die design.

Basic Considerations
The objective of an extrusion die is to distribute the polymer melt in the flow channel such that the material exits from the die with a uniform velocity. The actual distribution is determined by the flow properties of the polymer, the flow channel geometry, the flow rate through the die, and the temperatures of the die and the polymer melt. If the flow channel geometry is optimized for one polymer under one set of conditions, a simple change in flow rate or in temperature can make the geometry less than optimum.

Except for circular dies, it is essentially impossible to create a single flow channel geometry that can be used for a wide range of polymers and operating conditions. For this reason adjustment capabilities are often incorporated into the die. These allow adjustment of the flow while the extruder is running. The flow distribution can be changed mechanically, thermally, or both ways. Mechanical adjustment devices include choker bars, restrictor bars, and valves.

Thermal adjustment involves changing the die temperature locally to adjust the flow locally. Mechanical adjustment capabilities complicate the design of the die but enhance its flexibility and controllability.

Some general rules are useful in die design:
  • No dead spots in the flow channel.

  • Steady increase in velocity along the flow channel.

  • Easy assembly and disassembly.

  • Land length about 10x land clearance.

  • No abrupt changes in flow channel geometry.

  • Small approach angles.
In die design, problems often occur because the product designer has little or no appreciation for the impact of product design details on the ease or difficulty of extrusion. In many cases, small design changes can drastically improve or degrade the extrudability of the product. Some basic guidelines in profile design minimize extrusion problems:
  • Use generous internal and external radiuses on all corners; the smallest possible radius is about .5 mm.

  • Maintain uniform wall thickness (important!).

  • Make walls no thicker than 4 mm.

  • Make interior walls thinner than exterior walls (for cooling).

  • Minimize the use of hollow sections.
Figure 1 illustrates applications of these guidelines to several different profiles.

Figure 2. This is an example of a partition in the flow channel of the die.
Flow balancing. Mechanical adjustment of the die flow channel can be done in two basic ways. The length of the channel (land length) can be adjusted to make sure the average flow velocity is uniform. The other method is to adjust the height of the channel.

Balancing by land length. The land is the final portion of the die flow channel just before the exit of the die where the channel cross section is constant. Flow balancing by adjusting the land length has to be done such that the average flow velocity in one section of the die is the same as in another section. This is generally necessary when the product wall thickness (channel height) is not uniform. For channels with a rectangular shape the land length ratio should be equal to the height ratio raised to the power 1+n, where n is the power law index of the polymer. For most polymers the power law index varies between .3 and .5.

This means that when the height of one channel is 5 mm and 3 mm for another channel, and the power law index is .5, the land length ratio should be (5/3)1.5 = 2.15. The 5-mm-high channel should have a land length that is 2.15 times longer than the 3-mm-high channel. One of the problems with balancing by land length is that cross flow can occur. This can be avoided by incorporating a partition between the thick and thin portions of the channel. This is illustrated in Figure 2.

Figure 3. These two designs illustrate unbalanced and balanced channel height design.
Balancing by channel height. Balancing by land length does not always yield satisfactory results. Another method is to balance by channel height, as shown in Figure 3. This figure shows a U-shaped profile with circular sections. The circular sections are thicker than the walls. Without balancing, the flow through the circular section is substantially greater than the slit section.

If the flow channel is designed as in Figure 3a, flow in the thin sections would be much slower than the thick sections; resulting in considerable distortion of the extruded product. Figure 3b shows the same basic shape but with circular pins mounted in the circular sections of the die such that the wall thickness is uniform throughout. The flow velocities from die 3b will be more uniform than those from die 3a, and, as a result, little distortion would occur in the product extruded from die 3b.

Size and shape changes. The shape and the size of the extruded product are different from that of the die flow channel. The extrudate can expand as it exits the die; this is often called ?die swell?, even though the term ?extrudate swell? is more appropriate. Extrudate swell occurs because of elastic recovery of the plastic; however, swelling can also be affected by air entrapment and foaming.

The extrudate decreases in size as a result of draw down and cooling. Draw down occurs because the velocity at the take-up is higher than at the die exit. Draw down is necessary to have a certain level of tension in the line to keep the extrudate from sagging. In non-circular products draw down changes the shape of the product because the plastic melt in corners flows more slowly than in other regions of the die. As a result, material disappears from the corners by draw down. This is why a square flow channel produces a bulged product (see Figure 4). The shape change is also affected by the elastic recovery of the plastic.

Figure 4. These illustrations show how a corrected die can produce a desired extrudate shape.
Cooling reduces the size of the product because the plastic density increases as the temperature decreases; this is particularly true for semi-crystalline plastics such as LDPE, PP, and HDPE. Shape changes can occur as a result of nonuniform cooling. When the extrudate enters a water bath the outside layers cool rapidly and solidify. As a result, a rigid, solid skin forms that grows in thickness as the extrudate continues to cool along the water bath. On the other hand, the inside material is still at high temperature, and as this material cools it can form shrink voids if the outside layer is too rigid to deform. It is also possible that the outside layer is pulled to the center if the thermal contraction force is high enough to deform the outside layer. An example of a shrink void is shown in Figure 5.

Figure 5. Rapid cooling can cause a shrink void in a part, as shown in the illustrations above.
Size and shape changes can also occur in the extruded product by relaxation. This is the reduction of internal stresses due to changes in molecular configuration over time. When the extrudate is stretched extensional stresses are introduced. The stresses can relax over time. The relaxation of internal stresses can lead to a reduction in length and an increase in cross sectional area. If the internal stresses are not uniform, the relaxation can lead to warping of the extruded product.

From these considerations it is clear that the process that produces size and shape changes in an extruded product is rather complicated with several mechanisms at work at the same time. As a result, it can be quite difficult to predict what die geometry produces the desired product shape and dimensions. This is why die design is one of the most challenging aspects of extrusion engineering.

CONTACT INFORMATION
Rauwendaal Extrusion Engineering
Los Altos Hills, CA
Chris Rauwendaal
(650) 948-6266; [email protected]

Are full hydraulic injection molding machines yesterday?s news? Part 2

Nth degree modularity is built into Arburg?s new Allrounder Alldrive (Allrounder A). The main functions of a press?opening and closing the mold, injection, and metering?are servodriven. But all other auxiliary axes movements, like nozzle touch or mold actions, can be either electrically or hydraulically driven, depending on the application.
In the September/October issue of PM&A we presented the opinions of injection molders and machine manufacturers who believe that it’s too early to write off full hydraulic presses. In this, the second part of the discussion, we’ll hear from those who say time is running out for full hydraulic machines, and also from those who believe there’s room in the market for machines with all types of drive systems.

Those who say full hydraulics are here to stay have strong arguments. Take Gordon Darling of the Northeast machinery rep firm Superior Industrial, for instance. “There will always be hydraulic ram-type machines, and hydraulic toggle-type machines. Metalworking machines (machine tools) went 100 percent all servoelectric in the ‘70s and ‘80s. But machining steel is a far more precise process than injection molding.”

Electric machines are expensive, says Darling, especially in larger sizes. “And, just like God made little green apples, there will always be customers who want to pay a low price for their equipment.

“There will always be about 50 percent of the molding market that really has nothing to gain by buying all-electric, such as a hanger maker in Alabama, or somewhere, who is paying $.03/kWh for his power and the tolerance on his product is ±.250 inch,” says Darling.

Yes, But
Still, we received almost twice as many responses sounding the death knell for full hydraulics as we did for those saying full hydraulics still have a life. Twinshot consultant Joseph J. McRoskey of JMJ Management LLC (Carlsbad, CA) says full hydraulics are history, although Joel Thomson, a colleague of his at Twinshot Technologies & Community Products (Rifton, NY), has a different opinion (see September/October 2002 PM&A). Still, McRoskey doesn’t expect them to become obsolete in his lifetime.

“They represent a lower capital cost, they’re less complex and easier to maintain, easier to repair, and most of us processors have much more experience with them. Some of us enjoy the flexibility of simple accumulator systems that permit fast injection when we need it.

“I have run a lot of machines,” McRoskey says, “and I prefer the electrics. But forget about wholesale replacement. It won’t happen.”

Even though his company has recently installed all-electrics in a new molding cleanroom, Michael McGown of Hy-Ten Plastics (Milford, NH) thinks hydraulic injection molding machines will remain on the scene for some time to come. Why? One word: price.

“Machine cost will be the primary driver of many of the smaller injection molders’ decision making,” McGown says. “I’ve toured no less than two dozen injection molding facilities in China and have yet to view an all-electric machine.”

Nevertheless, Hy-Ten’s new presses are electrics. Why? One word: costs. “As for myself, every machine purchase requires a complete evaluation. Power cost here in southern New Hampshire is quite expensive compared to some of our neighboring states. There is no doubt that the electric machines have the edge here at Hy-Ten Plastics.”

Although it has introduced a small-tonnage all-electric optical disk molding machine, the e-Jet, Netstal expects to add all-electrics in the same tonnage range as its SynErgy series of hydraulic toggles (60 to 240 tons) for Q1 2004.
Time’s Up
The coming obsolescence of full hydraulics is obvious to at least one man—Yoshiharu Inaba, member of the board of directors and executive VP of the world’s largest supplier of all-electrics, Fanuc Ltd. (Oshino-mura, Yamanashi Prefecture, Japan). Inaba says that from subcompacts to midsize presses, all-electrics are multiplying and already have become the predominant machine less than 300 tons in Japan. He also says that larger-tonnage hydraulic rams are on their way out because of developments in larger servomotors and multiservomotor drive systems.

Reasons why include the narrowing price gap, energy savings, and repeatability. Full hydraulics fail to pass muster in the modern plastics technology world for four reasons, according to Inaba:
  • Machine control is susceptible to hydraulic fluid temperature, which varies over long-run operation, making it difficult to maintain process stability.

  • Servovalves and proportional control valves deteriorate over time, making it difficult to maintain the initial performance of full hydraulic machines over the long haul.

  • Hydraulic drives normally require continuous operation with minimum startups and shut downs, making it difficult to improve energy savings. Inaba also reminds us that high-performance hydraulics with high injection-speed acceleration cannot use energy-saving variable-volume pumps.

  • Full hydraulics can be messy and noisy, making it difficult to improve the workplace environment.
The solutions provided by all-electrics begin and end with their servomotor drives. Injection, extrusion, clamping, and ejection are all driven by a-c servomotors on Fanuc presses. Inaba says this results in a system that is highly immune to ambient temperature changes or other external disturbances, thereby enabling long-term molding precision and stability. The Fanuc Roboshot’s pressure control is ±.1 MPa, he says, on par with a machine tool.

A-c servomotors operate only when needed, resulting in two-thirds more energy savings when compared with full hydraulics. "In the case of Fanuc Roboshot, using a regenerative function, the energy in deceleration converts to electrical energy. Therefore the efficiency of energy consumption is higher than other all-electric injection molding machines," says Inaba.

Mold protection also is improved. Upon detecting an abnormal load during clamping, Fanuc presses bring clamping to an abrupt halt, thereby protecting the mold from breakage. They cycle faster, too. The Roboshot’s four motors can be operated simultaneously, and they’re getting faster. Inaba says the Roboshot has a blistering injection speed acceleration time of 0 to 300 mm/second in only 27 ms.

Sumitomo has no plans to discontinue its smaller-tonnage (18- to 80-ton) hydraulic clamp machines. These models may eventually be replaced by all electric models, but only if dictated by industry demand. The company?s hydromechanical SHC and SHA machines (100 to 650 tons) will continue to be produced and aggressively marketed.
Big Electrics
Jerry Johnson of JSW Plastics Machinery Inc. says he believes hydraulic machine makers are shaking in their boots. He says JSW plans to obsolete its own line of small hydraulics (55 to 110 tons) in the very near future, replacing them with all-electrics. Like Inaba, he takes issue with those who believe that the larger-tonnage market will continue to be the stomping ground of full hydraulics.

“During the 1950s through to the 1990s, hydraulic transmission power was the best possible method for driving heavy steel platens and injection screws under a lot of pressure. And, even when the all-electrics were introduced, the first models were small. For example, in 1994, JSW introduced the U.S. market to our all-electric machines, but we only had three sizes: 40, 60, and 120 tons,” says Johnson.

Even though JSW could document the accuracy and energy savings of its electrics, Johnson says they were not taken as a serious threat against hydraulics simply because they were small. However, he says this perception is rapidly changing. The company now offers all-electrics from 500 to 1450 tons, and is looking to increase that range. JSW recently added the 1450-ton all-electric to its line, and is testing a 1660-ton machine.

Speaking of size, UBE Machinery (Ann Arbor, MI) manufactures both hydraulic toggles and all-electric machines—big ones. As a matter of fact, UBE has built and sold the largest all-electrics in the world. A 1500-tonner was on show at NPE 2000. More recently UBE announced the sale of an all-electric with 7 by 4.5 ft between the bars and a clamping force of 2000 tons.

Understandably, UBE’s Jason Forgash believes that full-hydraulic machines are not the latest or best technology available in the market today.

“The advantages of all-electric—including accuracy, repeatability, energy consumption, and environmental issues—outweigh even the newest and best hydraulic machines on the market,” he says.

"I strongly feel that if the electric machine price was similar to hydraulic machines, the sales of hydraulic machines would be a very small percentage of the market. I think that if you asked almost any molder in the country their choice hydraulic or electric (assuming the price was the same), the response would be 99 percent electric."

Forgash says full hydraulics will only hold out as long as this price differential persists. And for those who say that there are applications that only a full-hydraulic machine can cover, he has this to say: "Modifications can be made to the electric machines to meet the requirements of most applications."

Opinions are Changing
The pro-all-electric party is winning in the public opinion polls. Some of our respondents say full hydraulics are goners, even though they’ve yet to buy an electric press. Brad Hilton of automotive lens molder Emrick Plastics (Windsor, ON) is up front and honest in saying that his perceptions of the superiority of all-electrics are just that—perceptions—based solely on what he has heard and read about them.

“We currently run 18 hydraulic machines, and have internally decided to seriously look at an all-electric when we purchase our next machine. Since I have never run an all-electric machine, I must rely on my perceptions,” he says.

Hilton has seven reasons why electric technology is tomorrow’s news. Electrics, he says, are quieter and cleaner (better for ISO 14000 and waste stream management), they consume less energy, have better process consistency and control, have smaller footprints, they’re easier to service, and they’re much faster.

“The only advantage of hydraulic presses that I can see is in mold setup,” says Leo I. Montagna (Sterling, MA) of custom molder Lee Plastics, one of the oldest custom molders in the U.S. “If utility costs are important to you, then electrics are the only way to go. If precise control is your concern, the hydraulic press can’t compete. If you want me to believe the hydraulic press is as good as an electric or hybrid, then prove it!”

Opinions Have Changed
Others, with hands-on experience in all-electrics, have already begun to write the full hydraulics obituary. Molder Nypro Inc. (Clinton, MA) is among them. For Nypro, full hydraulics are history. “The answer, for us, is yes, at least for the smaller machines (200 to 400 tons) with which we are most familiar,” says Nypro’s Al Cotton, summarizing the answers he got from his colleagues at Nypro’s headquarters. Cost savings, especially power savings in the 50 to 60 percent range, is the main reason.

Cotton says there are a number of qualitative—as opposed to quantitative—reasons as well, which are important to the Nypro technicians who oversee these machines. “For instance,” says Cotton, “oil leaks are the bane of every injection molding technician, particularly in cleanrooms, and they are virtually eliminated with electrics.”

Cotton says Nypro has at least one electric in every one of its many plants around the world, “and we see them mushrooming as a percent of our equipment in the near future.”

Netstal Machinery has earned its reputation over the years fielding high-performance, high-speed, hydraulically driven toggle presses for thin-wall packaging and demanding technical parts. Reto Morger of its Swiss parent company, Netstal Machinen AG (Naefels, Switzerland), says such applications will keep hydraulic and hybrid machines in fashion.

Nevertheless, Morger says Netstal knows that the market is going for both concepts—hydraulic and all-electric. “Therefore,” he says, “we will provide at least from the next K show, a fully electric injection molding machine for standard applications.” At K 2001 Netstal displayed an all-electric machine for molding optical disks.

“Yes, hydraulic machines are yesterday’s news,” says Gordon Sanford of insert molding specialist PMT Inc. (Seymour, CT). “Their only redeeming factor is the lower initial price.”

Sanford, like many others, says that once the prices drop on the servodrives, it will be possible for OEMs to build electric machines for less than a hydraulic. “We have seen steady price reductions in the servo components used in our in-house automation over the last 10 years, and expect the same will happen to the molding machines.”

Sanford says all-electric presses get the job done a whole lot easier than full hydraulics. “The all-electric conversions of energy to motion have a simpler, shorter path to control the pressure and speeds that are the heart of the injection molding process.”

Van Dorn Demag?s Cadence series of full hydraulic presses (25 to 125 tons) reportedly are repeatable, energy-efficient, high-performance machines with generous tiebar spacing, but without an all-electric?s price tag.
Are All-electrics Yesterday’s News?
As they did in the first installment of this series, many of our respondents were hesitant about saying the day of the full hydraulic machine has passed, while at the same time strongly voicing their belief that electrics were the presses of tomorrow. One such respondent, a spokesperson for a major, lean-thinking automotive molder, who requests anonymity, says his company got burned when first trying to flip the all-electric switch.

"We have nearly 500 molding machines globally and 95 percent of them are hydraulic," this source says. "Initial capital costs are an important factor. Generally, during the introduction of the electric machines there was a 50 percent premium for electric technology. Also during the introduction, the infrastructure to support machines in the field was not so good."

His field engineers back then were much better at diagnosing hydraulic presses. "We introduced electrics to the molding floor on two occasions. Within two years we returned the machines for not being capable in the simplest form. Three years later we again attempted an electric introduction. This time the results were very good until there was a breakdown. It's a great machine until it breaks."

The repair diagnosis process and parts re-supply caused frustration and discontent while the company waited for the repair. He says his company did see one major benefit.

"The electric nature of the electrical machine was excellent for the integration of robots and pickers," he says. "We could start the descent of the picker arm directly and before full mold open for simple cycle improvements without modifying to the actual cycle parameters."

Introductory training for the electrics was found to be very important. He says all-electrics are far less forgiving of processing errors and mistakes. These errors cause downtime. However, even after the relatively bad experiences, this source says electric machines will become the machine of the future at his company.

Here’s why. "The noise level of the electrics is much less. We have a very high density of machines to floor space and the decibel level is very important. No oil and no oil cooling is another inherent advantage. Moving, cooling, filtering, and testing oil are all non-value add activities, and the electrics will have the decisive advantage when their reliability is better and their initial purchase cost becomes more in line with the hydraulics."

All-electrics still need work in other areas, this source says. Some very serious improvements to the back end of the machine have to be made, for one thing. All-electric proponents celebrate their fast injection speed. That much, he says, is true. "However, the electric knockout speed is the biggest deterrent to the electric machine here. We need high velocity KO speed to eject our parts. The electrics just aren't capable of the kind of KO speed we need at this point in time."

Core-pull sequencing in all-electrics also is found to be lacking. "The electrics do a good job at straightforward A-B plate molding, if you can live with a relatively slow eject stroke. But they must rely on a hydraulic add-on for more sophisticated core-pull molds. To use an electric machine to turn a hydraulic pump to power the core-pull sequence seems inadequate."

He says that complex molding—side-action with core pull sequencing—requires that kind of technology to be on-board. All-electrics will not become the machines of choice for complex molding without an integral core-pull sequence capability at his company. "For us, the electric/hydraulic hybrid may actually be the machine of the future."

A spokesperson for Boy Machines asks, ?Does the full hydraulic machine make a good part, a consistently good part? Isn?t that all that it?s about?? He feels his company?s 60-ton Boy 55A does just that for less money.
More on Hybrids
Laurie Weeks of Cascade Engineering (Grand Rapids, MI) says full hydraulics are definitely becoming yesterday’s news. However, she feels that all-electrics may eventually share the same fate.

“With the introduction of the all-electric machine we saw some very repeatable results on the injection units of these machines,” Weeks says. “These results have allowed for some greater complexities in tooling to take place over the years.”

But Cascade has also learned that the all-electric clamp lacks the power necessary to hold the mold closed during filling and packing. This, says Weeks, is leading the industry toward hybrid machines.

“It is my belief that the best injection molding machine built will have a full hydraulic clamping system and an all-electric injection unit. We will really be able to mold some complex products then,” she says. “Another example of the hybrid machines is the combination of the molding processes that is happening now—injection molding combined with compounding, for example.”

Weeks then offers an even more compelling vision: “If an OEM is going to play in the future markets of injection molding machines, it is going to have to package its equipment as modules, meaning that you, as the buyer, will choose the style of clamp unit that you want with the injection unit. So all the machines of today will still be used, just in a different fashion.”

Room for Everyone
Some of our respondents believe there’s room for full hydraulics, hybrids, and all-electrics. One is the noted author and industry expert Jack Avery, manager of operational assets at GE Plastics. “My thoughts on this are that electrics will have the best fit in the small machine market where the added costs of all-electrics are not as significant as in the larger machines. Plus, the more environmental operational aspects of electrics are important in many markets, including medical and electronic.”

Avery believes that the midsize machine market will be the segment penetrated by hybrids. “Utilizing electric components for the material delivery system is the most economical means. Hydraulics for the clamp system reduce cost vs. all-electrics, as well as the commercial availability issues with large electric motors.”

Finally, he sees hydraulics maintaining their toe-hold in the market for big iron, primarily due to the high costs of the large servomotors required for the clamping system of large all-electrics.

Van Dorn Demag (Strongsville, OH) covers all the bases—from full hydraulic ram-type presses, to toggles, to hydromechanicals, to all-electrics. And, if you care to put the Demag Ergotech USA El-Exis machine under the VDD umbrella, it even has machines with hybrid hydrostatic transmission systems. So VDD’s marketing manager, Larry Doyle, has a well-balanced view. Overall, he says, there is room for full hydraulics, all-electrics, and any other type of molding machine.

For starters, he says that full hydraulics at 300 tons and less are losing market share and that most machines in this tonnage range will be electrified in the next five to seven years.

“Prices on a-c servomotors and drives, as well as ballscrews, are stabilizing,” Doyle says. “As a result, all-electric machines in this size do not carry as high of a premium in price as they once did. This all-electric technology has become more affordable and ultimately a better buy.”

With all of this said, VDD’s Cadence series of hydraulic machines (28 to 125 tons) reportedly remains a good buy. The machines have a dual-pump hydraulic system and servovalved injection. “In addition, the generous distance between tiebar specifications allows molders to run larger molds in smaller-tonnage machines,“ says Doyle.

Doyle feels that all-electrics above 300 tons are cost prohibitive at this time. The larger servomotors and drives are non-standard products and tend to increase machine costs. And as ball-screw diameters increase, he says, this leads towards yet another expensive, non-standard product, or to new, unproven technologies.

Larger all-electrics using multiple motors on one axis require multiple drives, another cost driver. "In addition," says Doyle, "synchronization of these motors on one axis involves special sequencing in the program and could involve additional hardware requirements. Ultimately, the costs associated with these machines put them at a price most molders will not pay."

From his vantage point with a company with an all-encompassing line, Doyle makes one observation others often fail to make when debating the merits of the different drives—namely, the impact on floorspace. VDD’s two platen hydromechanical machines use 25 percent less space than a three platen machine. Most all-electric machines are toggles, so, as their tonnage increases so does the footprint of the machine.

"Only rarely do we see hydraulic toggle machines above 700 tons. These machines take up entirely too much space in a manufacturing environment. This will be something for molders to ponder when they evaluate all-electric machines in the larger sizes," Doyle says.

When it comes to applications, Doyle believes some are better suited to all-electrics, particularly those requiring what he says are their superior positioning accuracy, environmental friendliness, and energy efficiency. Still, he says machine control technology is an important issue to consider.

"Servomotor encoders do an excellent job in counting pulses of light to provide very accurate positioning. But it is the machine control technology that will make the all-electric machine respond better to variables in the molding process." Doyle tells us VDD’s next generation Pathfinder control on its IntElect Series all-electrics allows the machine to automatically compensate for temperature and material viscosity process variables.

For some applications, though, all-electrics need not apply. Thin-wall applications with high length to thickness ratios require high-performance injection specs that are out of the reach of all-electric technologies, according to Doyle. Accumulator-assisted injection on hydraulic machines meets these requirements at an affordable price and with proven technology.

Many molders and machinery suppliers believe the global marketplace is big enough for all-electrics, hybrids, and full hydraulics, like this Goldstar 610H. It still may be too early to write the latter off.
Apples to Apples?
David Geiger, industrial systems engineering manager at Moog Inc., heads a group that develops solutions for electric machines, hydraulics, and sealed electric/hydraulic actuators.

Geiger believes that there is a big misunderstanding in the marketplace regarding electric machines. Most of the misunderstanding is coming from sales organizations using data that he says compare old hydraulic technology to modern electric servo technology. Others compare the costs of sequential hydraulic machines to the parallel movement possible with electric machines. “The end user must look for the best technology for his application,” says Geiger.

“All technologies can reach the precision required in the most demanding application, if you use the best products available. It is difficult to summarize which technology is the best without going into the specific application. I recommend that the customers investigate in detail their application before choosing a drive technology.”

Keeping your application in mind, Geiger believes there are five main areas of concern:
  • Do you require the most cost-effective sequential movement machine? If so, hydraulic is preferred.

  • Do you require a parallel movement machine? If so, go for an electric or hybrid.

  • Do you require a large amount of force and speed? If so, go hydraulic.

  • Do you have environmental impact concerns? If so, go electric.

  • Do you have environmental impact concerns with parts that require large amounts of force or speed? If so, sealed electric/hydraulic actuators are preferred.
Hydraulics Today
“Milacron responds to customer requirements based on their application needs and regional preferences,” says Dale Werle of Ferromatik Milacron North America (Batavia, OH). “Today, these requirements are filled by all types of machinery designs, including all-electric, hybrid, two-platen, and traditional hydraulic.”

With such diversity, Werle admits that it’s hard to make generalizations about the life-span of full hydraulics, but he feels that all-electrics are taking over at 200 tons and less. "There is strong preference for our advanced all-electric IMMs in this range, and our mid-sized Powerline Plus series electrics have excelled in the high-speed packaging markets," he says.

Produced in North America and Germany, Werle says Milacron’s Powerline machines are making inroads into the medical and packaging fields where high-speed full hydraulics have dominated. They are now available up to 1125 tons.

"A key to the growth of larger electrics will be the electrification of mold motions, which is well under way at D-M-E. That said, we are still successfully selling the all-hydraulic Babyplast micromolding machine and a very small 'true hydraulic' machine, the Edge, to satisfy a segment of the market that clings to this technology."

Regarding its other hydraulics, Werle says Milacron’s Magna series presses have been successfully transplanted to India, where they are now the market leader. And he adds, "The pure hydraulic machines we build in Germany are still very much in demand due to the mature, large customer base that continues to be delighted with this type of technology."

Electrics Tomorrow

The company’s U.S.-built hydraulic two-platen Maxima Series machines, from 300 to 6600 tons, have enhanced Milacron’s hydraulic line-up’s speed, footprint, and oil conservation. Maxima machines are available in conventional or hybrid configurations, in Europe and the U.S.

But Werle has this to say about hybrids: "We have made such hybrid machines for 25 years. While the market has been stirred up about hybrids with 'lipstick on the pig' salesmanship, we are certain it is not a technology of the future, but a competitive reaction to our strong all-electric patents and considerable investments in technology and supply chain development."

All told, Werle definitely believes all-electrics will be tomorrow’s news. “Without revealing specific numbers, I can say that all-electric IMMs make up a higher percentage of Milacron’s overall sales each year—much higher than the SPI numbers reported for the broad market, “ he says.

With each new year, Werle says, all-electric technology will become more accepted and inexpensive, and bite off another machine-size range or application that had been the exclusive domain of hydraulics. “The pace of this transition will vary from year to year, but it is as inevitable for molding as it was for robots, where many persisted in thi

Direct extrusion with twin-screw extruders

In direct extrusion, raw ingredients such as polymers, fillers, and additives are mixed, reacted, and devolatilized in a high-speed, twin-screw extruder and directly made into a final product, bypassing pelletizing. Although high-speed, twin-screw extruders have been used for many years to produce sheet, film, profile, and fibers, only in the past five years has there been a concerted effort to use them to perform direct extrusion.

Direct extrusion using twin-screw technology was initially mandated, in desperation, by the need to produce formulations that were adversely affected by the second heat and shear history inherent in the single-screw extrusion step. Once the technical viability of direct extrusion was demonstrated, it became apparent to the marketplace that substantial cost savings were also possible using this technology.

Additional benefits include the ability to adjust formulations inline to accelerate development efforts, and to maintain a proprietary in-house manufacturing process. Materials that benefit from direct extrusion include filled olefins, TPE/TPO/TPVs, polyesters, PVB, wood-fiber composites, adhesives, foamed polymers, nylons, and degradable polymers.

Corotating twin-screw extruders allow screw rotation up to 1200 rpm, and dominate the high-speed, twin-screw market for applications that require intensive mass transfer. Counter-rotating intermeshing and nonintermeshing twin-screw extruders are also being used for direct extrusion applications. Typical processes performed in the twin-screw extruder, regardless of the mode of operation, include reactive processing, devolatilization, alloying, and compounding particulates into plastics.

High-speed, twin-screw extruders are starve-fed devices in which the output rate is determined by the feeder(s), and screw rpm is used to optimize compounding efficiencies. Feeders maintain consistency of the formulation, introduce ingredients in the proper order along the length of the process section, and regulate the extent of mixing. For direct extrusion, the feeding and materials handling system to the twin-screw extruder is critical to maintain front-end pressure stability.

Loss-in-weight feeders are typically specified for direct extrusion where the auger speed is modulated up or down to maintain a consistent mass flow to the extruder, based on the rate of weight changes in the hopper situated on a load cell. Twin-screw systems may use up to eight or more feed streams.

Volumetrically controlled feeders are generally not acceptable, even for premixes, due to the inherent fluctuations in feedrate that result in pressure fluctuations at the die inlet. If pellets were the end product this would be a nonissue, since ±20 percent dimensional stability for a pellet is often acceptable.

Almost all high-speed, twin-screw extruders use segmented screws that are assembled on high-torque hammered and splined shafts. Barrels are also modular and can be configured from feed, plain, vent, side stuff, and liquid addition sections. Each barrel section is electrically heated, and is internally cored for high-intensity cooling near the process melt.

This transparent view of a parallel twin-screw extruder illustrates the structure of the basic hardware that accomplishes compounding and end product extrusion in one step without subjecting the material to interim pelletizing and multiple heat histories. Virtually infinite variations of the hardware can be specified, depending on the desired product.
The modular nature of twin-screw extruders offers extreme process flexibility. Barrels can be rearranged, the L/D can be increased or decreased, and screws can be modified. For direct extrusion applications, the machine is usually somewhat longer so that the latter part of the process can be dedicated to pumping with a more stable pressure than is mandated for pelletizing (see photo at right).

Twin-screw Extruder Function and Design
Twin-screw compounding extruders perform these basic functions: feeding, melting, mixing, venting, and developing die/localized pressure. The segmented nature of the twin-screw extruder in combination with the controlled pumping and wiping characteristics allows specific screw and barrel geometries to be matched to the required process tasks. This enables the same machine to perform both dispersive and distributive mixing, which is a major benefit for certain products that are fabricated by direct extrusion. One example of this scenario is the mixing of glass microspheres into an extruded part so that the spheres never experience the high-shear stress associated with plastication in the single-screw extruder.

Screw design is the heart of any twin-screw compounding extruder. An infinite number of screw design variations are possible. There are, however, only three basic screw elements: flighted, mixing, and zoning. Flighted elements move material past barrel ports, through mixers, and out of the extruder to the die. Zoning elements isolate two operations within the extruder. Screws can be made shear intensive or passive, based on the elements used in the design. (It is interesting to note that almost all materials that are processed in single-screw machines were compounded on a high-speed, twin-screw extruder.)

Mixing in the screws may be dispersive or distributive. The wider a mixing element, the more dispersive it becomes. Elongational and planar shear effects occur as materials are forced up and over the land, and more energy is imparted into the process. Narrower mixing elements are distributive in nature with high melt division rates and significantly less elongational and planar shear (see diagram below).

The pressure gradient in the twin-screw extruder is determined by the selection of screws. Since the twin-screw extruder is a starve-fed device, flighted elements are placed strategically so that the screw channels are not filled and there is no pressure underneath downstream vent/feed sections. This facilitates downstream feeding of fillers (e.g., calcium, flame retardants, talc, titanium dioxide, and so on). The zero-pressure feature also facilitates single or multistage devolatilization.

The viscosity of melting resin is high, so in the early stages of the extruder the strain rates can produce high stress rates. These may be critical to attain dispersive mixing, but can also cause degradation of shear-sensitive materials. In the latter stages of the extruder the viscosities fall such that high strain rates factored against a decreased viscosity produce comparatively low stress rates that enable heat- and shear-sensitive materials to be mixed with a minimal peak shear.

Above is a ZSE-50 twin-screw extruder with gear pump front end and Vulcan downstream profile system. Gear pumps are typically protected by an upstream screenchanger that filters out contaminants. Alternatively, a single-screw pump front end attachment can be specified in place of a gear pump. At left is the ZSE-40 twin-screw extruder with single-screw pump from Merritt. Both the ZSE-50 and ZSE-40 overall extrusion systems are designed and supplied by Leistritz. Some components come from other suppliers, including those noted.
Process Design
Combining compounding/devolatilizing with direct extrusion in a high-speed, twin-screw extruder presents significant process design challenges. The system requires high mass transfer in combination with consistent pumping. The selection of screw elements must take into account the mixing requirements, and also provide stable pumping to the die or front-end device.

For direct extrusion the machine is longer than a standard compounding extruder. The last sections of the screws are dedicated to building and stabilizing pressure. The vent is normally approximately 10D back from the end of the machine to allow enough process length to build a steady state pressure, as compared to 6D for a standard compounder. Distributive mixers are often used towards the end of the screws for thermal homogenization of the melt stream, which would normally not be required for pelletizing.

To maintain dimensional tolerances, a gear pump front-end attachment may be used to build and stabilize pressure to the die. A gear pump dampens out pressure fluctuations by approximately a factor of 10 (i.e. ±200 psi on the inlet of the pump equals ±20 psi on the outlet of the pump). Gear pumps typically have upstream protection provided by a screenchanger. This prevents damage by filtering off-spec substances away from gear teeth (see photo at right).

Alternatively, a single-screw pump front-end attachment may be specified in place of a gear pump. The length is approximately 10D, essentially the length of the metering section of a single-screw extruder. This device is more stable than a standard single-screw extruder since plastication and compression are not present, which is the main cause for pressure instability in a pellet-fed, single-screw extruder (see photo below).

The twin-screw compounding system for direct extrusion is significantly more complex than a single-screw extrusion line. The feeding system to a twin-screw compounder sets the formulation tolerance and also plays a major role in pressure stability. There may be as many as eight or more feed streams into the high-speed, twin-screw compounding extruder. Typically, a PLC-based master control system is required to manage the system as well as facilitate recipe retrieval and data archiving.

The residence time of 15 seconds to 2 minutes for materials in a twin-screw extruder must be taken into account for the pressure control algorithm. A possible control scenario is for the gear pump rpm to be locked to set a constant volumetric rate for the process melt stream to the die. Feeders are adjusted in very small increments every few residence times to maintain a stable front-end pressure over the long term. Screw rpm is continually adjusted, within limits, to maintain front-end pressure over the short term.

If a screw’s rpms are allowed to adjust without limit, widely varying degrees of imparted shear might adversely affect the quality of the final product. When sequential feed streams are introduced into the twin-screw extruder at various points in the process, the closed loop control obviously becomes more complicated, as various residence times must be managed.

The control software uses an algorithm program to analyze the inputs from key points in the system, make numerical calculations, and apply corrections to the screw rpm and feedrate. The objective of the algorithm is to maintain the gear pump inlet pressure at its setpoint. If this is successfully accomplished, the backpressure in the twin-screw extruder is held constant, which provides for consistent shear and mixing. With a stable melt delivery to the gear pump, the discharge flow and pressure to the die is uniform—the ultimate objective of the entire system.

Mixing can be primarily dispersive, as with the wider element, upper left, or distributive, as with the narrower element, at right.Various elements can be combined in screw design to meet whatever mixing challenge is at hand.
Delivering a Consistent End Product
Providing a usable melt to the die and downstream system is only half the battle. To deliver a high-quality end product, the appropriate die and downstream equipment is required, whether the product is a film, sheet, fiber, or profile. The following examples identify products successfully made by direct extrusion:
  • Battery separator/sheet. A PE/silica formulation is premixed and fed into the extruder feedthroat and oil is injected into a barrel section in the early stages of the process. The materials are mixed and devolatilized in the twin-screw extruder process section, which is directly coupled to a sheet die. After the die, a high-pressure calendar “squeezes” the extrudate and sets the final dimension, eliminating the necessity for a gear pump and closed loop pressure control.

  • Foamed profiles. The polymer is fed into the twin-screw extruder and melted prior to gas injection. The polymer is intimately mixed at high pressures with high-division distributive mixers to minimize viscous heating. The latter part of the process section uses low-energy-input pumping elements so that the barrel sections serve as heat exchange devices to cool the process melt. A screw or gear pump front-end attachment may be used, depending on the particular application. Performing this process in a twin-screw extruder provides an alternative to the tandem single-screw systems traditionally used.

  • Filled film/sheeting. The polymer(s)/rubber/additives are fed into the main feedthroat and melted prior to downstream introduction of fillers (carbon black, talc, calcium carbonate) into the process melt stream via a side stuffer. The materials are mixed/devolatilized in the twin-screw extruder, which is typically mated to a screenchanger and gear pump front-end assembly. Olefins, TPOs, and fluoropolymers all use this system configuration.

  • Adhesive compounding. Rubbers, tackifiers, fillers, and oils are mixed and devolatilized in the twin-screw extruder with a gear pump front end upstream from a film/lamination die, or a rod die for direct glue stick profile extrusion. In some instances, the twin-screw process has proven superior to separate twin-screw compounding and single-screw extrusion operations, as a demixing effect often occurs as the materials coalesce in the single-screw process.

  • Wood-fiber/composite products. The primary functions of the twin-screw extruder in this application are to remove water and distributively mix the natural fibers. Because this type of product typically cannot be passed through a screenchanger, a single-screw pump front-end attachment is used. Products include profiles for decking and sheeting.
Direct extrusion from a high-speed, twin-screw extrusion system can improve quality and reduce costs when compared to a two-step compounding/production operation. The downside of this emerging technology is that the intricacies of the overall system increase as the complexity of the upstream materials handling/feeding equipment are now combined with the nuances associated with sizing and cooling an extruded part. In spite of the associated challenges, there are many successful direct extrusion installations of a high-speed, twin-screw extruder, many of which are highly proprietary and confidential. The product mix and anticipated volumes need to be carefully assessed to determine whether direct extrusion is the preferred manufacturing method, based on the prevailing economic and product performance issues.

CONTACT INFORMATION
Leistritz, Somerville, NJ
Charlie Martin; (908) 685-2333
[email protected]

Words of Wisdom: The need for a general purpose standard screw

Michael F. Durina, president of Md Plastics, has extensive experience in designing general purpose and special purpose screws for plastics processing.
Over the last 50 years there has been a considerable amount written about the plasticating screw designed for injection molding machines.  From introductory plastics textbooks like Processing of Thermoplastic Materials, originally printed in 1959 when the plunger was still being compared to the reciprocating screw, to more recent publications such as Melt Rheology and its Role in Plastics Processing, printed in 1995, the subject matter focuses on a single stage compression screw known as the general purpose (GP) design.

Research into currently used designs shows how the GP design has evolved. The GP design (Figure 1) is a single flighted compression screw with a feed zone, a compression (transition) zone, and a metering zone, representing 50 percent, 25 percent, and 25 percent of the flighted length, respectively. The pitch or lead of the screw is equal to the diameter (square pitch) with the thread of the screw having a flight width equal to 1/10 of the diameter. The taper of the compression zone is cut on an involute pattern, creating a compression ratio that varies from 2 to 3:1, calculated by dividing the flight depth in the feed section by the flight depth in the metering section. The L/D ratio, or the flighted length of the screw divided by the diameter of the screw, is between 16:1 and 23:1.

The feed and metering depths are missing in this description. Without these depths, the compression ratio is meaningless. It is not uncommon to see a 60-mm, 20:1 L/D GP screw from different suppliers with a feed depth range between 5 and 9 mm. The depths affect the feeding or solids conveying of the feed-stock, the volume of plastic present in the screw channel, the throughput rate, shear rate, and overall melt quality.

Regardless of the exact depths, lengths, and flight widths used, the GP screw brings value to the industry. The screw supplier and plastics processor both recognize two resilient characteristics of the GP design that are testament to it?s continued usage: The design can process a variety of resins in a variety of shot sizes and cycle times without destroying the properties of the polymer, and the GP design is the least expensive screw design to manufacture.

Figure 1. The overall configuration of the general purpose screw is long established in the injection molding industry, yet there is no benchmark by which variations on the design can be compared and contrasted by molders.

Functions of a Plasticating Screw
Whether the molding application requires that we process an LCP material with a cycle time of 30 seconds or a rigid PVC with a cycle time of 95 seconds, every plasticating screw has to perform certain functions in order to process the polymeric material effectively and produce a high quality molded part. The functions are:

  • Feed the resin consistently.
  • Melt the resin uniformly.
  • Pump the fluid steadily.
  • Mix the resin homogeneously.

The GP screw, if designed properly, can perform these functions satisfactorily, but within a narrow processing window at best (see Figure 2).

Cycle time, shot size, pellet geometry, back pressure, and screw geometry determine residence time. Looking at the graph in Figure 2, when the cycle time/shot size relationship is outside of the curve, the GP design has a difficult time performing the melting and mixing functions. When the application is on the low side of residence time it is most likely that the design will not have enough melting capacity and/or mixing ability to properly prepare the melt pool. Conversely, when there is too much residence time, the screw is most likely to be too deep in the metering section to ensure good melting and mixing, and a good melting strategy plays a major role in achieving an adequate melt pool.

Developing a standard
Standard, as defined by The New Merriam-Webster Dictionary is: something set up as a rule for measuring or as a model to be followed. A standard as far as the Society of the Plastics Industry (SPI) is concerned is: a formally generated, approved, and published document that has third party (American National Standards Institute) certification. The process for obtaining an SPI standard is rigorous, exacting, and time consuming. Most standards are written with some safety consideration in mind. Another way to adopt a standard is to create a guideline through the SPI as well as with the European Committee of Machinery Manufacturers for the Plastics and Rubber Industries (EUROMAP), an institute that is well regarded throughout the international community.

A guideline is a document that is developed by industry for industry and simply outlines a recommendation on a recognized industry safety practice, a design recommendation, or a procedural recommendation. A guideline does not go through the rigorous approval procedure that an ANSI standard does. Typically, the guideline is developed by manufacturers and users and then balloted by each group. A guideline can in some cases be a first effort prior to a standard. For example, there is an SPI (AN-113) and EUROMAP (19) recommendation or guideline for determining the “plasticating capacity of an injection molding machine” that OEM’s use to rate the screw performance on the injection unit. Another example is the work done by the Components Section of the Machinery Div. of the SPI that has published some guidelines giving direction for the manufacturing of single barrels (BI-105) and single screws (BI-106). Standards are a necessity for every industry.

Difficult Task
In defense of the GP screw, the injection process requires that the plasticating screw be very versatile.  Not only does it have to perform the above-mentioned functions, but it has to reciprocate the equivalent of five diameters in length. Effective melting length is changed and the solids bed is disrupted. Cycle times generally vary from 4 to 120 seconds, which affects the residence time of the polymer. The RPM reaches 1000 mm/sec in circumferential speed to facilitate an extremely short cycle time, and the hydraulic back pressure control can reach 5000 psi of plastic pressure. This affects the pumping ability and the pressure flow of the drag flow equation.

If the application were similar to an extrusion process where most of the variables are known and the screw is in a fixed position, it would be easier to study the process and design a screw to meet the objectives. This explains why all of the math models and melt simulation packages are supplied for the extrusion process. In an injection application there are more variables involved and more importance put on adopting a melting strategy that requires good decisions by the technician.

Figure 2. As indicated by the area under the heavy blue line, the conditions under which a general purpose screw will produce the quality of melt needed to perform precision molding are fairly limited.

The Need for a GP Standard
The GP design has been around for more than 50 years. There is no secret as to the geometry, there are no patent infringements to concern us, and there is no great difference in perceived value or performance, so why can?t we adopt a standard? 

A standard would do wonders for the industry, providing a benchmark against which all other iterations are measured. Let?s discuss what a standard would consist of, how to achieve a standard throughout the industry, and the ultimate benefits to the molder. There is a mechanism in place to develop such a standard or guideline; the work has to be done by the group that stands to benefit the most from the document?the injection molding community.

The GP Design
The development of a standard for a GP screw requires considerable research and testing. Collaboration is needed among machinery builders, molders, screw designers, resin manufacturers, and independent testing laboratories.

A specific design would be available for all to see. It would have a known melt temperature, throughput, and melt quality rating, established by using a scale of between 0 and 10 (as shown in Figure 2) for a specific residence time used on the molding machine over the full range of screw diameters and L/D?s.

A graph similar to Figure 2 would be generated. With this graph, a molder could determine if the GP design is adequate to mold parts to requirements. Similar graphs would also help the molder decide what rpm, back pressure, and barrel temperature settings should be used. While melt preparation can be more of an art than a science, this work to develop a guideline would be a scientific step forward. Injection molding machinery builders as well as independent screw designers and manufacturers could also use the testing mechanism developed to evaluate proprietary designs.

Figure 3. The Posi-Melt screw developed by the author is one effort to create an injection screw that has a wide range of applications but which does not labor under the limitations of current general purpose designs.

A Standard Benefits the Industry
The current situation in injection molding is not conducive for molding perfect parts for these reasons:

  • The typical molder does not know exactly what screw and valve design is installed on the injection unit, nor do they know what to expect from the designs they receive until they have had time to run a few jobs with the equipment.
  • There is no standard screw design to use as a point of reference.
  • There is no standard testing method for making comparisons on the impact of plasticating screw design on the injection molding process.

Without these tools to work with, the molder can?t formulate a precise scientific molding strategy.

It is my opinion that the molding community should insist that screw design guidelines be developed. The weak arguments that have been given in the past for not adopting a standard because of proprietary concerns or for the protection of intellectual property rights have to be overcome for the benefit of the injection molding industry.

CONTACT INFORMATION
Md Plastics
Columbiana, OH
(330) 482-5100
www.md-plastics.com

Editorial: Predicting the future

As we reach the end of 2002, it seems appropriate to reflect on the year behind us and look forward to the year ahead. The first issue of this magazine for which I take credit or blame was the January/February 2002 issue. It has been a rewarding year. Readers have welcomed our concept and our efforts. All that is good, but it is looking backwards. Now we look toward the future.

While the direction of the economy and its impact on the plastics processing business is a deadly serious matter, it can help to take a lighter view of the business of predicting that future.

We know of at least three ways to predict the future, each of which offer equally poor results. The first is to use a crystal ball. But it?s been a tough year for crystal balls. At least one member of the plastics industry has been heard bemoaning ?My crystal ball is broken.?

Another technique is to use a dartboard. It is reported that a leading financial publication periodically chooses a portfolio of stocks by putting the stock listings up on the wall and choosing investments by throwing darts at it. If nothing else, the result is likely to be the oft-recommended diversified portfolio. Sometimes the dart-chosen portfolio even outperforms a portfolio chosen by esteemed investment advisors.

The third technique, apparently widely used in government circles, was brought to us by a prominent member of the plastics industry who shall remain unnamed. It involves gathering masses of data in whatever form available. After careful analysis of this data, highly placed personnel predict the future direction of the economy. The technique is known as SWAG, which stands for Scientific Wild Ass Guess. This technique has broad application in predicting sales, setting budgets, and so on.

In predicting the future, we like to get more favorable results than any of these three techniques provide. We find that if we stick to a few specific predictions, our batting average is really good.

Try these predictions:

  • The National Plastics Exposition will take place in Chicago in June 2003.
  • NPE 2003 will be held in McCormick Place.
  • Those who plan well will get better results from the show than those who don?t, whether as attendees or exhibitors. (PM&A will help you plan).

OK, it?s true that we?re not exactly going out on a limb. It is also true that we will continue to provide the best in plastics processing equipment coverage in the future. We need no crystal ball, darts, or complex data to predict that.

Happy Holidays, and see you in 2003.


Merle R. Snyder
Editor
Plastics Auxiliaries & Machinery

MAGAZINE CHANGES NAME . . . SLIGHTLY

With this issue, we are officially changing our magazine?s name to Plastics Machinery & Auxiliaries (PM&A). This is not a very dramatic change from the former Plastics Auxiliaries & Machinery, and just one in a series of design changes made to the magazine over the past year under its new ownership by Canon Communications. The editorial mission is the same. Note that we are the only magazine highlighting auxiliaries in our title.

This magazine was originally launched as Plastics Auxiliaries. In due course ?& Machinery? was added, yielding the title Plastics Auxiliaries & Machinery. Canon Communications bought the magazine late in 2001, and retained the title until this issue.

Some readers will remember the important role that the no-longer-published magazine Plastics Machinery & Equipment served in providing information to processors regarding machinery and equipment. The current-day PM&A is in some ways a successor to that previous title.

PM&A continues to be the only North American new product tabloid magazine with the mission to inform you, our readers, about advances in the technology of machinery and auxiliaries needed to compete in today?s plastics processing business environment. And to increase the level of service to our readers, PM&A will publish nine issues in 2003, up from six in 2002.

Date stamps work with other stamp plugs

The Compact Series date stamp plug requires as much as 60 percent less space than traditional stamps, allowing more room for waterlines, slides, and other mold details. The 20 Series plug is a compatible replacement for the Euro style plug, while the Retro Series plug replaces the Euro style and other plugs. The Progressive Components name is etched on the side of each date stamp, providing moldmaking and molding personnel with a quick reference when replacements are necessary. Templates help users find the right date stamp plug for the application. All date stamp plugs have a revised arrow detail that fits standard slotted screwdrivers for easy installation. MicroDaters are also available in their entirety or as replacement rings and plugs.

Progressive Components, Wauconda, IL;
(800) 269-6653; www.procomps.com

Chillers target injection molding

Designed with the injection molding industry in mind, the Model ACP air-cooled stand-alone chillers are equipped with hermetic scroll compressors and stainless steel pumps, frames, and tanks. Stainless steel evaporators are also used, and temperature control is via solid-state electronics. Other features include a hot-gas bypass, a water flow switch, a freeze thermostat, and non-rusting water piping. The key to the chillers is reported to be the design of their stainless steel brazed plate evaporators. If 85F plant water is available for the condenser, water-cooled units up to 10 tons can be used. The chillers use coaxial tube-in-tube condensers. Large units use brazed plate condensers, with optional shell-and-tube design.

1st Choice Portable Chillers, Markham, ON
(877) 513-8310; www.1stchoicechillers.com

Portable chillers, standard and custom

Milacron CA portable chillers are designed for process temperatures from 20 to 65F. Capacities range from 1/4 to 4 tons. Process pumps provide 1/4 to 3/4 hp and deliver .6 to 10 gal/min. Microprocessor controls are used on 2- to 4-ton models. Status indicators include power on, compressor, capacity control, alarm, reservoir level, refrigerant circuit, and low flow. Enclosure panels are constructed of stainless steel. All units are mounted on casters for portability. Fan-style condensers are used on air-cooled models, with tube-in-tube condensers used on water-cooled models. Air-cooled models are recommended where water supplies are inadequate or contaminated, or where process heat recovery for plant heating is desired. Air-cooled units are available up to 30 tons. Water-cooled units are used where tower water, city water, or other plant water sources are available for condensing and heat rejection. Water-cooled units are available up to 40 tons. If one of the standard models does not match the specifications required, a custom unit can be designed.

Cincinnati Milacron Specialty Equipment, Batavia, OH
(513) 536-2584; plastics.milacron.com

Bulk loader control system for three pumps

A microprocessor-based central control system for bulk loaders can operate both weigh and nonweigh loaders from a single location but still allows individual control at each unit. The T-Link control system offers a back-lit, touch-screen control that uses a menu-driven format. This allows operators access to all control parameters while permitting input or editing of new data. Twisted-pair wiring connects the main control and individual units.

T-Link tracks actual materials consumption, inventory for each loader, and controls fill by weight, time, or volume. When paired with T-COM2 software, the T-Link can perform two-way, real-time, remote control of loaders through a PC. This reportedly creates complete and accurate management of all processed materials, with continuous monitoring of each loader.

Standard features include three-pump and single- or dual-material control, priority fill stations, nonfill alarms, communication failure alerts, and password protection.

Mould-Tek Industries Inc.
Scarborough, ON
(416) 285-5400; www.mould-tek.com