This is the first in a three-part series on the principles of scientific molding, also known 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, "More injection 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 back and watch him go. Bozzelli's normally laconic and laid-back manner evaporates. He leans forward, eyes blazing behind his square-rimmed glasses, and he tells you without hesitation 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 the U.S. molding industry. He's afraid molders are giving away the store. He's doing everything he can to keep them competitive, efficient, and smart. During a typical Bozzelli lecture you're likely to hear these one-liners: "We've got to keep internationally competitive," and "Processing has a history of trying to compensate for whatever else went wrong," or "It's not who is right, it's what is right for the industry."
This Vietnam veteran who spent time on the ground fighting in the jungle might use this analogy to prove his point: many American soldiers, he says, were killed in that war because, when given the choice, they took the worn, exposed jungle path rather than the more difficult, but safer, trek through the underbrush. Similarly, "the injection molding industry is following the path of least resistance," he says. And getting killed.
John Bozzelli is the principal of Injection Molding Solutions, a one-man company that has 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 an infrared camera, Bozzelli and GP technical service representatives spend a lot of time "looking over the machine's shoulder," helping molders find, evaluate, and fix inefficiencies in their processing-spreading the gospel of scientific molding. Along the way he and GP have become some of the molding industry's foremost experts on the technique, otherwise known as systematic molding or decoupled molding. With it, Bozzelli contends, molders can reduce cycle times, increase machine efficiency, and ultimately make more money. And it's not hard.
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 molten plastic into a shape. The focus of molding is the performance of the melt in the mold, not the setpoints on the machine.
Scientific molding is a machine-independent process that provides certain basic parameters to optimize the molding process. Chief among them are:
- Plastic temperature;
- Plastic pressure;
- Velocity of injection;
These data, and others such as shot volume and transfer position, help Bozzelli build what he calls a Universal Setup Card. It's comprised of parameters that can be achieved on any capable, well-maintained press, whether it's two years old or 20. It doesn't matter if the machine is closed or open loop, as long as you push the same amount of material at the same rate at the same position. It allows you to move a mold and material from press to press with setup data that is universal to all machines. Because of this, Bozzelli and GP like 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 is doing in the mold, and the ability to optimize and repeat this pressure shot after shot is one of the hallmarks of a quality process. Unfortunately, most molders don't use cavity pressure 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 the mold 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 and the size of the nozzle on the press. Otherwise known as the intensification ratio, it tells you the maximum pressure your machine can produce. For instance, if the maximum hydraulic pressure is 2000 psi, and the intensification ratio is 10:1, the machine can generate 20,000 psi of melt (or plastic) pressure in the nozzle.
The least reliable-and possibly least relevant-pressure on the press is hydraulic pressure. It is purely a measure of how much force the machine can generate against the ram. Although hydraulic pressure and fill time can be used as a measure of viscosity, to Bozzelli this pressure is the least indicative of material performance in the mold. To illustrate this point, Bozzelli likes to point to graphs that show hydraulic pressure for a part fluctuating wildly from shot to shot, while cavity pressure remains constant. "In scientific molding," he says, "I don't care what the hydraulic pressure is on first stage as long as the machine gives me a consistent velocity each time. And that means quality parts."
The second concept to embrace following the scientific molding method is what he and GP call Delta P, or Delta Pressure. This one is quite simple, but again, most molders do not practice it. The hydraulic system generates pressure to push the hydraulic screw cylinder that pushes the melt; the valve between the pump and screw cylinder allows the molder to adjust the volume of oil or velocity depending upon how much is needed and/or desired. The Delta P principle says that the hydraulic pressure on the pump side of the valve should always be about 200 to 400 psi greater than the pressure on the hydraulic screw cylinder side of the valve; this gives the machine the reserve power it needs to adjust to changes in viscosity during first stage-which are inevitable.
Without that abundant pressure, you are pressure-limited. Your press is maxed out with each shot, with nothing in reserve to overcome increases in viscosity. Viscosity changes as flow rate changes; if flow rate varies, viscosity varies; if viscosity varies, parts will 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 trying to 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 filling stage. The first case in point is a 300-ton rebuilt HPM operated by a molder in Greenville, SC. With a 7.7:1 intensification ratio, the press has a maximum hydraulic pressure of 1750 psi and a corresponding maximum melt pressure of about 13,500 psi.
|Figure 1. This gun shot primer tray measures approximately 4 by 4 inches with 100 evenly spaced holes, each about .25 inch deep. The problem: an actual cycle time of 21 seconds, but quoted at 19 seconds.|
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 shot primer 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 the molder has are a cycle of 21 seconds that was quoted at 19 seconds and sinks near the center of the tray-a problem exacerbated by the suction cup on the end-of-arm tooling on the robot.
Working with Bozzelli at this molder is John McDonnell, a technical service representative for General Polymers, based in Charlotte. To start, Bozzelli and McDonnell hook up an RJG DartPak and its software to read the machine's hydraulic pressure, melt pressure, 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 mold is about 1.05 seconds. This fill time dictates the injection velocity-about 3.5 inches/second in this case-the first of the universal parameters the scientific method requires. Next, Bozzelli and McDonnell turn their attention to the hydraulic pressure curves generated by each cycle and read by the DartPak.
|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 machine ramps up to push melt into the mold. The curve on the gun shot primer tray showed two peaks during fill, something the molder had not noticed previously (Figure 2). The first peak, it's discovered, is caused by a delay as the hot runner valves open, which allows pressure to build before the melt enters the mold. This is solved by adding a .2-second delay between the inject signal and inject begin; this gives the valve gates the time needed to open before the melt arrives. Although the curve looks better, the solution is not an elegant one. "You will never see anyone calculate the cost of that .2 second over 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 Bozzelli himself. In his summary report to the molder he states, "If the job ran for 4500 hours/year and the machine hour recharge rate was $40/hour, this .2 second delay will cost the 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 the Delta 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 like docking 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 to the dock," he says. To do this, Bozzelli and McDonnell set the ideal velocity (based on the rheology curve) and keep it constant, while moving the screw position back incrementally until the part shorts-99 percent filled. This prevents the melt from slamming 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, at which time the part is 99 percent full. This provides the cutoff position for mold and material. 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. Another visit took Bozzelli and GP technical service representative Alan Larsen to a custom molder in Waterford, WI, just southwest of Milwaukee. Of the 38 presses there, more than half were at least 20 years old, and transferred based on time, not position or cavity pressure. Optimizing such a machine is difficult, but still possible. However, transferring on time or hydraulic pressure, Bozzelli points out, does not let the machine accommodate 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 10:1 intensification ratio. Material is a 45 percent glass-filled nylon, precolored black. The part has six threaded inserts that are manually inserted and removed. There are no microprocessor controls on the machine in use. To start, Bozzelli turns off pack and hold and begins fill optimization. Because position control is impossible, the fill time is incrementally reduced until the part is 99 percent filled. When he started, the fill time was 4 seconds; Bozzelli cut that to 2 seconds, and incrementally increased pressure to take 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 mold open 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.
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.
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.