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December 17, 1998
21 Min Read
.svg?width=850&auto=webp&quality=95&format=jpg&disable=upscale "Scientific Molding, Part 1: Filling")
This is the first in a three-part series on the principles of scientific molding, alsoknown as systematic molding or decoupled molding. Part two will focus on pack and hold;part three investigates the benefits that infrared imaging can bring to your operation.
When John Bozzelli gets going, he's hard to stop. Say something to him like, "Moreinjection molding machines were sold into Asia than the U.S. last year," or,"What's the most common problem you find when you visit a molder?" Then sit backand watch him go. Bozzelli's normally laconic and laid-back manner evaporates. He leansforward, eyes blazing behind his square-rimmed glasses, and he tells you withouthesitation what's wrong, with rising intensity and volume. It is, as he likes to say,"a significant emotional experience."
Bozzelli is concerned in a most serious way about the current state of affairs in theU.S. molding industry. He's afraid molders are giving away the store. He's doingeverything he can to keep them competitive, efficient, and smart. During a typicalBozzelli lecture you're likely to hear these one-liners: "We've got to keepinternationally competitive," and "Processing has a history of trying tocompensate for whatever else went wrong," or "It's not who is right, it's whatis right for the industry."
This Vietnam veteran who spent time on the ground fighting in the jungle might use thisanalogy to prove his point: many American soldiers, he says, were killed in that warbecause, when given the choice, they took the worn, exposed jungle path rather than themore difficult, but safer, trek through the underbrush. Similarly, "the injectionmolding industry is following the path of least resistance," he says. And gettingkilled.
John Bozzelli is the principal of Injection Molding Solutions, a one-man company thathas him on the road almost every day of the week, often with General Polymers (GP),visiting molders across the country. With a laptop computer, an RJG DartPak, and aninfrared camera, Bozzelli and GP technical service representatives spend a lot of time"looking over the machine's shoulder," helping molders find, evaluate, and fixinefficiencies in their processing-spreading the gospel of scientific molding. Along theway he and GP have become some of the molding industry's foremost experts on thetechnique, otherwise known as systematic molding or decoupled molding. With it, Bozzellicontends, molders can reduce cycle times, increase machine efficiency, and ultimately makemore money. And it's not hard.
Under Pressure
To understand and apply scientific molding, you must first comprehend, accept,and embrace one basic concept: The injection molding machine is a tool to mold moltenplastic into a shape. The focus of molding is the performance of the melt in the mold, notthe setpoints on the machine.
Scientific molding is a machine-independent process that provides certain basicparameters to optimize the molding process. Chief among them are:
Plastic temperature;
Plastic pressure;
Velocity of injection;
Cooling.
These data, and others such as shot volume and transfer position, help Bozzelli buildwhat he calls a Universal Setup Card. It's comprised of parameters that can be achieved onany capable, well-maintained press, whether it's two years old or 20. It doesn't matter ifthe machine is closed or open loop, as long as you push the same amount of material at thesame rate at the same position. It allows you to move a mold and material from press topress with setup data that is universal to all machines. Because of this, Bozzelli and GPlike customers to understand the distinction between hydraulic pressure, melt pressure,and cavity pressure.
The preferred pressure is cavity pressure. It's the best indicator of what the melt isdoing in the mold, and the ability to optimize and repeat this pressure shot after shot isone of the hallmarks of a quality process. Unfortunately, most molders don't use cavitypressure because it requires a pressure transducer in the mold. "I guarantee,"says Bozzelli, "that the cost of inserting a slot for a pressure transducer in themold more than pays for itself during the life of the job."
In lieu of measuring cavity pressure, the next best pressure is the nozzle pressure.Nozzle pressure is a function of the hydraulic pressure relative to the size of ram andthe size of the nozzle on the press. Otherwise known as the intensification ratio, ittells you the maximum pressure your machine can produce. For instance, if the maximumhydraulic pressure is 2000 psi, and the intensification ratio is 10:1, the machine cangenerate 20,000 psi of melt (or plastic) pressure in the nozzle.
The least reliable-and possibly least relevant-pressure on the press is hydraulicpressure. It is purely a measure of how much force the machine can generate against theram. Although hydraulic pressure and fill time can be used as a measure of viscosity, toBozzelli this pressure is the least indicative of material performance in the mold. Toillustrate this point, Bozzelli likes to point to graphs that show hydraulic pressure fora part fluctuating wildly from shot to shot, while cavity pressure remains constant."In scientific molding," he says, "I don't care what the hydraulic pressureis on first stage as long as the machine gives me a consistent velocity each time. Andthat means quality parts."
Delta P
The second concept to embrace following the scientific molding method is what he andGP call Delta P, or Delta Pressure. This one is quite simple, but again, most molders donot practice it. The hydraulic system generates pressure to push the hydraulic screwcylinder that pushes the melt; the valve between the pump and screw cylinder allows themolder to adjust the volume of oil or velocity depending upon how much is needed and/ordesired. The Delta P principle says that the hydraulic pressure on the pump side of thevalve should always be about 200 to 400 psi greater than the pressure on the hydraulicscrew cylinder side of the valve; this gives the machine the reserve power it needs toadjust to changes in viscosity during first stage-which are inevitable.
Without that abundant pressure, you are pressure-limited. Your press is maxed out witheach shot, with nothing in reserve to overcome increases in viscosity. Viscosity changesas flow rate changes; if flow rate varies, viscosity varies; if viscosity varies, partswill vary. And the effect is additive: the slower the fill rate, the higher the viscosity;the higher the viscosity, the slower the fill rate. Says Bozzelli, "It's like tryingto drive up hill with a four-cylinder car when you really need a V8."
Cases in Point
Scientific molding involves the optimization of temperature control, filling, packing,cooling, and part removal. This installment, however, focuses primarily on the fillingstage. The first case in point is a 300-ton rebuilt HPM operated by a molder inGreenville, SC. With a 7.7:1 intensification ratio, the press has a maximum hydraulicpressure of 1750 psi and a corresponding maximum melt pressure of about 13,500 psi.
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The mold is an eight-cavity hot runner tool using pneumatically actuated valve gates;at the time, the mold had no pressure transducers in the cavity. The part is a gun shotprimer tray, about 4 by 4 inches with 100 evenly spaced holes, each about .25 inch deep(Figure 1). The material is a precolored black polypropylene. The primary problems themolder has are a cycle of 21 seconds that was quoted at 19 seconds and sinks near thecenter of the tray-a problem exacerbated by the suction cup on the end-of-arm tooling onthe robot.
Working with Bozzelli at this molder is John McDonnell, a technical servicerepresentative for General Polymers, based in Charlotte. To start, Bozzelli and McDonnellhook up an RJG DartPak and its software to read the machine's hydraulic pressure, meltpressure, fill time, and cycle time. With it, they build an on-machine melt rheology curve(see sidebar, pp. 122-123) and determine that the best fill time for the material and moldis about 1.05 seconds. This fill time dictates the injection velocity-about 3.5inches/second in this case-the first of the universal parameters the scientific methodrequires. Next, Bozzelli and McDonnell turn their attention to the hydraulic pressurecurves generated by each cycle and read by the DartPak.
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Figure 2. Chart shows the double peak in hydraulic pressure caused by hot runner valve gates. |
A typical hydraulic pressure curve shows one peak during the fill stage as the machineramps up to push melt into the mold. The curve on the gun shot primer tray showed twopeaks during fill, something the molder had not noticed previously (Figure 2). The firstpeak, it's discovered, is caused by a delay as the hot runner valves open, which allowspressure to build before the melt enters the mold. This is solved by adding a .2-seconddelay between the inject signal and inject begin; this gives the valve gates the timeneeded to open before the melt arrives. Although the curve looks better, the solution isnot an elegant one. "You will never see anyone calculate the cost of that .2 secondover the life of the mold," says Bozzelli, "But it will cost you."
Actually, someone did calculate the cost of that .2-second delay, and it was Bozzellihimself. In his summary report to the molder he states, "If the job ran for 4500hours/year and the machine hour recharge rate was $40/hour, this .2 second delay will costthe molder $1690/year. If the tool ran for five years, it's more than $8000 lost."
Next, Bozzelli and McDonnell start optimizing the fill stage of the cycle. Like theDelta P philosophy, the fill philosophy uses a boat metaphor. Most molders, Bozzelli says,are in the habit of slamming the shot against the cavity. Metaphorically, it's likedocking a boat without slowing down, slamming the boat into the pier at full speed."We're trying to find the point at which we can cut the power and coast the boat tothe dock," he says. To do this, Bozzelli and McDonnell set the ideal velocity (basedon the rheology curve) and keep it constant, while moving the screw position backincrementally until the part shorts-99 percent filled. This prevents the melt fromslamming the cavity and leaves 1 percent of the part to be filled during pack and hold.Since the mold did not have a pressure transducer, transfer is done based on position, atwhich time the part is 99 percent full. This provides the cutoff position for mold andmaterial. The molder then has two important universal parameters: velocity and position.
When Position is Not an Option
Transfer by position, much less cavity pressure, is not always an option. Anothervisit took Bozzelli and GP technical service representative Alan Larsen to a custom molderin Waterford, WI, just southwest of Milwaukee. Of the 38 presses there, more than halfwere at least 20 years old, and transferred based on time, not position or cavitypressure. Optimizing such a machine is difficult, but still possible. However,transferring on time or hydraulic pressure, Bozzelli points out, does not let the machineaccommodate viscosity variations, nor does it assure repeatability of position.
This part is a pump housing on a 440-ton HPM, early '70s vintage, with a 10:1intensification ratio. Material is a 45 percent glass-filled nylon, precolored black. Thepart has six threaded inserts that are manually inserted and removed. There are nomicroprocessor controls on the machine in use. To start, Bozzelli turns off pack and holdand begins fill optimization. Because position control is impossible, the fill time isincrementally reduced until the part is 99 percent filled. When he started, the fill timewas 4 seconds; Bozzelli cut that to 2 seconds, and incrementally increased pressure totake the part to 99 percent full and ready for pack and hold optimization.
The next installment in the November issue of IMM will discuss optimizing cooling,calculating material volume, load compensation, and simple adjustments to automating moldopen and close.
The advent and philospohy of Scientific Molding |
There are several different names for this method of molding. John Bozzelli and General Polymers-with whom Bozzelli frequently works-have adopted the name scientific molding. You may also have heard the term decoupled molding (coined by RJG's Rodney J. Groleau), so called because it separates control of the filling stage (or injection speed) and the packing stage (plastic pressurization) into two distinct phases of the process. The progenitor of this concept, however, is systematic molding, a phrase constructed and defined some 20 years ago, also by Groleau, founder of RJG Assoc. and RJG Technologies, both based in Traverse City, MI. RJG Technologies president Brad Watkins says the concept grew out of the recognition that molding "was falsely looked upon as a black art and not an exact science. Opinions about how to mold a part varied widely, with very little data to support any certain methodology. In many shops, this paradigm still exists today. The fact is, there's a logical reason for everything that happens in molding, even though a lot of molders don't believe that. Laying out a systematic approach helps people realize that molding is more than an art." The RJG system started by Groleau focuses mainly on the behavior of the material in the mold, not the machine or how it's behaving. To the systematic molding way of thinking, it is less important to know what the press is doing or how it does it, as long as it repeatedly produces quality parts in a consistent cycle as efficiently as possible. This method uses pressure measurements in the cavity so you can define a set of parameters for the mold and material independent of the machine's operating parameters. Today, the scientific/systematic molding torch is carried most prominently by General Polymers, Bozzelli, RJG, Glenn Beall, John Klees, and other molding experts. These advocates strongly recommend the use of cavity pressure sensors. This, they say, is the only way to truly know the condition of the melt, and therefore assure part consistency, in the mold. Growing out of this is RJG's line of process analysis hardware and software: DartNet, DartScanner, DartPak, and DartWin. RJG's equipment is designed to help molders analyze and interpret machine capability from the plastic's perspective by measuring hydraulic pressure, cavity pressure, and other process-related parameters, including fill time. These are Bozzelli's tools, and with them he assesses and often improves the efficiency of a press. The Bozzelli Philosophy Conversely, the Bozzelli/General Polymers mantra whenever they come within eyeshot of a press is "data, data, data!" Bozzelli is loathe to make a change on a machine without knowing exactly what the effect will be. This means knowing the diameter of the nozzle, the intensification ratio, the melt pressure, the injection pressure, the melt temperature, and the cavity pressure, if available. With hard, indisputable numbers at his fingertips, Bozzelli can quantify everything the machine does. It's the best way, he says, to optimize the press. "My settings are based on data only," he says. "If anyone wants to change those settings, they've either found a better way, or they have a significant emotional experience." On the business side, Bozzelli is irked by "the Big Fix," an American business attitude that demands quick solutions, without paying attention to the larger picture and recognizing that the Big Fix leads to inefficiencies that ultimately cost the molder money. "A molder," he contends, "sells time on a machine, not parts. The sooner we recognize that, the more competitive we'll be." Says RJG's Watkins, "There are some very simple and fundamental things molders can do to improve the process, and they're not doing them. A lot of molders make money in spite of themselves." Money made, money lost, and profit potential are typically examined in great detail during a Bozzelli/ General Polymers visit. Many molders, he's found, don't understand that they sell time, not parts. As a result, they don't comprehend the impact that inconsistent and inefficient cycles have on their bottom line. According to Bozzelli's calculations, profits per $100,000 of sales decrease 50 percent for every 12 percent increase in cycle time. Conversely, every decrease in cycle time increases profit. This applies not only to cycle time, but to rejects, loss due to contamination, blocked cavities, and other inefficiencies. Finally, Bozzelli and General Polymers want to see the four "circles" of injection molding come together: material, design, toolmaking, molding. "On most projects," Bozzelli says, "decisions in each of these four areas are made in a vacuum. And in the end, the molder is left holding the bag, trying to make up for all of the poor decisions made along the way." And that, he says, is the most expensive way to manage a molding project. "We must have better communication; we need to have all parties involved up front," he says. "Costs are less against profit if changes occur at the development level." |
On-machine melt rheology | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
By focusing on the finished part and building molding parameters based on volume, velocity, and position, scientific molding lets the machine attenuate the scads of variables that plague the molding process: viscosity, melt temperature, residence time, regrind, moisture, and others.
Chief among these is melt viscosity, which is affected the most as all other variables change. As viscosity increases, the machine must work harder to overcome the resistance; as viscosity decreases, the machine does less work. Bozzelli theorizes that there is, for every material, an ideal viscosity-one that makes the least work for a machine, provides the fastest cycle, and produces a better part. To find this ideal viscosity, one of the first things Bozzelli and General Polymers do when visiting a molder is to build an on-machine rheology curve. One example of this process was performed at the molder in Greenville, SC. To recap, the machine is a rebuilt HPM 300-tonner, built in 1984. It is molding gun shot primer trays in an eight-cavity mold. The trays are about 4 by 4 inches and have 100 holes each, about .25 inch deep (Figure 1). The material is a black polypropylene and the intensification ratio on the press is 7.7:1. To build the curve, Bozzelli and General Polymers follow these steps outlined below, which can be done by any molder:
Shear rate = 1/.71 second = 1.41 sec-1 Next, calculate the relative viscosity for each shot. Relative viscosity is the hydraulic pressure at transfer, times the intensification ratio (Ri), times the fill time. For the first shot: Relative viscosity = 1474 psi x 7.7 (Ri) x .71 second = 8058 psi/second Plotted on a graph, the on-machine rheology curve for the material looks like a sloped "L," with the first shot at the bottom and the last shot at the top, where viscosity is highest (Figure 4). Using this curve, you can see how viscosity for the material changes, and pick a fill time that takes advantage of the material.
While one might be tempted to pick the first fill time (.71 second) because it has the lowest relative viscosity, the Bozzelli/General Polymers rule of thumb is to use a fill time that's on the plateau of the curve, but closer to the elbow. You will note that though the viscosity on the first shot is low, the hydraulic pressure required is a relatively high 1474 psi, just a few hundred short of the 1750 psi max for the press. The potential for pressure limiting the press is greater. In this case, Bozzelli chooses a fill time of 1.05 seconds for the material and mold, still relatively quick, but with enough pressure in reserve to accommodate viscosity changes. This time dictates the injection velocity and allows better fine-tuning of the fill stage of the cycle. As a cautionary note, Bozzelli mentions that there are times when you find that the machine cannot inject fast enough. As a result, you may not get past the corner of the L-shaped curve. Actual Velocity
The calculation is just a matter of distance over time. To prove the point, Bozzelli set the cutoff position at .70 inch. With injection starting at 3.31 inches, the total distance of travel for each stroke is 2.41 inches. Over a period of 11 cycles, the velocity on the machine's closed loop control is incrementally reduced, with the fill time recorded after each shot. The math, then, is simple. The distance (2.41 inches) is divided by the fill time, providing the actual velocity for the shot. What the molder found is that a controller-set velocity of 7.5 inches/second has a real-world velocity of 3.5 inches/second; and on down the line it went, as shown in Figure 5. In such cases, which Bozzelli says are not uncommon, he recommends that the controller be programmed to provide an alarm whenever the set velocity is not reached. |
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