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A missing link is found in closed loop control

June 1, 1997

7 Min Read
A missing link is found in closed loop control

Milko Guergov, president and CEO of M&C Advanced Processes Inc. (Wyandotte, MI), says that what he wanted to do was to bring mathematical principles and analyses to the injection molding process. In so doing, he's developed a new kind of closed loop process control system, one that has many molding machine makers, resin suppliers, molders, and moldmakers extremely excited. Why? It's because they believe Guergov may very well have finally, in one stroke, removed much of the "black art" from the science of molding.

Basically, Guergov's mathematical analysis shows that a critical variable exists in the molding process that, up until now, has largely been overlooked - there's air-pressure resistance in a mold cavity when a part is shot. The internal melt pressure of the shot must be counterbalanced to compensate for the commonly recognized shrinkage forces and the air resistance in the cavity during fill and solidification. In this way, proper control over flow, stress, sink, shrinkage, and part density can be achieved.

The method and process control for injection molding in a prepressurized cavity, according to Guergov.

Guergov's solution is to prepressurize the system with shop air, typically at 200 psi, from the injection nozzle to the mold cavities. He uses air resistance itself to build and maintain a minimum static internal melt pressure. The cavities are sealed with neoprene rubber. Air pressure is equalized from the nozzle to the cavity. Pressure transducers in the injection nozzle and at the last place to fill in the mold cavity measure all air and melt pressure displacement transitions during the fill, pack, and hold phases of the cycle in real time.

It is these real-time measurements that generate the feedback for PID calculation. It is these signals that, through a PLC, are sent to the molding machine controller and hydraulics to adjust the ram, so the internal melt pressure is maintained at desired levels that are greater than shrinkage forces and air resistance, but are small enough to prevent stress and shear. Molding is accomplished from the material's point of view, rather than from the machine's point of view.

The loop is closed from the melt to the machine. This method eliminates the need to compensate for variations either in mold temperature, resin temperature, or viscosity. And it eliminates the need for a velocity-to-pressure switchover. Molders can control the actual condition of the melt in the cavity.

Most molded parts can be produced by a single process variable, namely, the internal melt pressure profile, which - again - is based on the minimum static pressure within the melt to counterbalance shrinkage forces and air resistance. This static pressure also gives the melt more mobility, so it can feed the faster solidifying areas of the part, eliminating sink while controlling shrinkage and density.

Algorithms in Guergov's software calculate the internal melt pressure profile based on signals from the nozzle and cavity transducers, taking into account the flow rate, melt temperature, modulus, shrinkage factors, part area, and part stress levels during solidification - mostly basic specs provided by materials suppliers. The mathematics, polymer sciences, and control theories behind this system may be daunting to some, but the significant results can easily be grasped just by looking at some of the parts Guergov has shot.

GCP process control allows molders to monitor and control the internal melt pressure of the shot. Therefore, molders can control the formation, movement, and growth of the gas bubble, even in long channels. Channel walls are uniform, and there's no unintentional skin penetration. And, localized gas bubbles can be formed. These prototype handle parts were shot using dried shop air to form the channels, rather than nitrogen.

Improved Process Control

Guergov's patented process control system is trademarked "GCP." That's GCP as in gas counterpressure. In this case, though, the "gas" is air, and the "counterpressure" is used to control the molding process. And, unlike the old gas-counterpressure systems used in structural foam molding, structural properties of parts are not lost. They are improved. The GCP process in this case is for improving the conventional molding process, and is not for structural foam. By the way, the "M&C" in the company name stands for "molding and casting." For almost 23 years, Guergov has been building molds, molding machines, and diecasting equipment. He's also been involved in setting up turnkey manufacturing plants, from start to finish.

A complete GCP system, including the machine-mountable pneumatics, strain-gauge transducers, and controller, sells for about $100,000 for retrofit to existing machines. Licenses are included in the base price. Machine modification and setup can be accomplished in two to four days. It's adaptable to all molding machines and molds. Guergov estimates the cost will be two to four times less if it's sold as a standard option on new machines. Mold modification costs from about $3000 to $5000, depending on mold size. When it comes to new machines, he does not plan on going in exclusively with any one OEM. All the major players are talking to him.

The caster-mounted controller is very easy to use. Next to the main display screen are two digital transducer readouts - one for the nozzle, one for the cavity. All the operator has to do is check every now and then to make sure these two numbers are the same.

If they are, the benefits are tremendous, and not just when it comes to shrink and sink control. (Guergov intentionally short-shot parts - they have no sink marks.) Cycle times can be reduced up to 20 percent due to faster cooling with thermoplastics, or faster heating with thermosets. Remember, the cavities are pressurized with air. Molds can be run at lower temperatures. The higher internal melt pressure forces the plastic against the cavity wall, so surface appearance improves.

Warpage is reduced or eliminated because there's balanced and equalized pressure and force as the plastic cools during fill and pack. This internal pressure balancing act also helps reduce or eliminate surface stresses as surface layers of the melt cool during fill. It increases the laminar nature of flow, and improves both part structural integrity and mechanical strength.

There's more. Part weight can be reduced up to 25 percent if blowing agents are used, because densities are equalized. There are no materials limitations. Reinforcements don't show up on the surfaces of filled or reinforced parts. Clamp tonnage can be reduced. GCP even improves the environment out in the shop, since air and decomposition gases displaced in the pressurized system can be vented outside. And imagine the freedom it provides in designing new parts, since designers don't have to engineer around normal molding problems with things like ribs and bosses.

Mold modifications for GCP processing can cost from $3000 to $5000, depending on mold size. Cavities can be sealed with neoprene rubber for pressurization. This production mold is used to make flush pedals.

GCP also can improve the performance of unconventional molding systems. Take gas assist, for instance. By controlling internal melt pressure with GCP, molders can control the formation, movement, and growth of the gas bubble, even in long gas channels, so the walls of the part have uniform thickness with no skin penetration. Bubbles can be grown simultaneously with cavity filling, either through a channel or in a localized area, so there are no witness lines. GCP gas assist allows molders to use dried shop air instead of nitrogen, and "gas" injection can be through a standard ejector sleeve and pin, rather than through some fancy nozzle.

Then there's multimaterial molding. Guergov has learned that GCP can allow molding of a part with two different-but-compatible materials on a standard, single-barreled molding machine. Material combinations, like 75 percent regrind:25 percent PP or 50 percent PP:50 percent PC/ABS, can be mixed together in a single, standard machine hopper. GCP process control builds the internal melt pressure of the mixed melt so that the higher modulus material solidifies faster. Like a sponge, shrinkage squeezes out the lower modulus, still-liquid material. In turn, the higher modulus material, now the core material, is centered in the cavity by the equalized melt pressure of the still liquid, lower modulus material, now the skin. There's no core shifting. Parts with some combinations of certain PP-PC/ABS blends that Guergov shot at M&C are paintable.

GCP shows promise in improving process control in metal injection molding, which involves high-density loadings of micron-sized metal particles. And optical disk production will benefit from faster cycling of more stress-free substrates. Thin-wall, complex parts in medical and consumer electronics markets, automotive parts, even toys - GCP can help molders produce better parts, cheaper. "You can't control that which you can't measure," Guergov says. By using mathematical models, theoretical details, and common sense to find and measure a critical missing variable, he may just have brought this "black art" of molding a bit more under control.

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