Almanac: The fundamentals of Decoupled Molding

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May 31, 2005

Editor's note: DECOUPLED MOLDING is a service mark of RJG Inc. For style purposes, it's referred to in this article as Decoupled Molding. There are all makes and models of hot runner systems. But no matter what kind you run, all hot runner systems have one thing in common—the word preventive, as in maintenance. One expert walks you through the dos and don'ts.

Molding isn't just an art anymore. It's about fluid and thermal dynamics and controlling pressure and flow and maintaining shot consistency. How do you get your machine to make a good part each time? Here's some advice on how you can apply scientific principles to the molding process.

As the demands of global competition and complexity of part designs have increased, the injection molding process has developed into a far more sophisticated endeavor than has previously been required. New molding machines have become far more skilled in their ability to do a variety of gymnastics to make good injection molded parts. Often, however, we find that molders on the shop floor have not kept pace with the sophistication of, or know how to apply, the new controls given to them on injection molding machines. In addition, those with older machines are discouraged by the dizzying degree of complexity surrounding these newer controls and wonder how they can ever compete. The technique called Decoupled Molding addresses these issues.

In a sense, Decoupled Molding is a classification system, as opposed to traditional molding. Decoupled Molding is further broken down into various forms (Decoupled I, Decoupled II, and Decoupled III).

Decoupled Molding allows process capability to be achieved beyond that of traditional molding techniques and allows the molder to use the full potential of the new machine's sophistication.

Equally important, Decoupled Molding (when applied to an older machine with some rather simple upgrades) allows molders with old equipment to perform at world-class levels, thus enabling them to compete effectively in the new world environment.

Surely it sounds too good to be true, but let's address just exactly what we're talking about.

Traditional Molding

To understand the differences in molding techniques, it is important to define what we mean by a traditional molding process, and to then differentiate it from Decoupled Molding.

Injection molding evolved from a manual process in the 1940s, when machines were virtually unsophisticated arbor presses in which plastic was squeezed into a mold that was manually clamped by hand. Material was fed into a cylinder, heated by heater bands, and a plunger was used to squeeze the melted plastic into a cold mold. Pressure was maintained until the part solidified, and then the mold was opened and the part was taken away. The main controls were pressure and time: how hard and how long one squeezed plastic into the mold.

Injection molding and the concept of how molding occurred evolved from this basic technique. This is what we now call traditional molding.

As the process became more sophisticated, a pump was added to the machine and the plastic was injected using hydraulic force. Early machines still had only one stage: squeeze. The pressure was set so that the plastic filled the mold, the part was packed to an appropriate level, and it was held there until solidified. If parts were insufficiently filled, we squeezed more. If parts were overfilled, we squeezed less.

As the machine evolved further, two-stage machines became standard. But the technique remained the same: Squeeze the plastic into the mold and pack it during the first stage, and then simply switch to a smaller pump (the low volume) during the second stage (or holding phase), primarily to conserve energy. The holding pressure was left either at the same pressure as the first stage or, in some cases, slightly lower to minimize overpacking at the gate.

This was largely the technique of choice over the next 20 years and is still widely used throughout the world. However, during the 1970s, there was an increase in the understanding of rheology, which made it clear some advantages could be gained by using an alternate molding methodology. Before we move on to Decoupled Molding, let's review two molding-related phenomena:

Non-Newtonian flow. When plastic is exposed to a shearing action during flow, it undergoes a dramatic viscosity change. This is called non-Newtonian behavior and is widely recognized as the standard behavior of polymers during flow. Faster flows (higher shear rates) cause a reduction in viscosity. In traditional molding, the velocity of the “squeeze” was virtually uncontrolled because it was not recognized how important this non-Newtonian phenomena really was. Today, we recognize that this is a major variable of the process and, thus, we wish to control the velocity of injection during the filling process.

Pressure-limited injection. If traditional molding techniques are used, we have a paradox. If we use just enough pressure to pack the mold, we do not have enough pressure to fill fast. In other words, the speed of fill is limited by the amount of packing, creating a pressure-limited condition during fill. Here, the first two stages (filling and packing) are coupled together and cannot be controlled independently. Even worse, if only one pressure setting is used throughout the cycle, all three stages (filling, packing, and holding) are coupled together.

A Classification System

Decoupled I. An improved technique of molding, which can be achieved on certain types of parts, is Decoupled I. This technique was used in the 1970s when cavity pressure control was initiated. With this technique, the mold is filled at a controlled velocity until the mold is volumetrically full. At this point, the machine is transferred to a set holding pressure and melt inertia (kinetic energy and the decompression of the melt) is used to pack the mold. Filling is disconnected from packing, but the inertia of the first-stage fill is the major component of the packing process. This is a process that requires a high degree of machine repeatability and is not for the faint of heart. It is generally only used in a very limited set of specific applications.

Decoupled II. If we are to achieve faster fill rates to take advantage of rheology, we must be able to fill quickly and consistently. The only way to do this is to fully separate the filling phase from the packing phase. If we do not separate the fast fill from the sudden stop at the end (when the cavity is volumetrically full), the melt inertia will cause a rapid buildup of pressure when the plastic hits the end of the cavity, producing flash. This is analogous to driving your car into the back wall of the garage to stop it.

A better approach is to slow down before hitting the end of the cavity, thus a decoupling the fast fill stage from the packing stage. Using Decoupled II, this is accomplished by transferring from fill into second-stage pressure when the mold is 95% to 98% full. This is analogous to driving fast on the way home from work and slowing down before parking in the garage.

Similarly, we can fill as rapidly as we'd like as long as we stop short to dissipate the melt inertia before we pack the mold. This is a fundamental concept of Decoupled II. Packing and holding are still coupled together; however, packing is done during second stage. The speed of packing is not controlled directly but is controlled by the second-stage pressure. Second stage is then set to pack and hold the part appropriately, without slamming into the end of the cavity.

Decoupled III. The latest evolution of the Decoupled Molding technique has been to separate the process into three distinct stages: fill, pack, and hold. The first stage, fill, is achieved at one or more velocities (multiple speeds may be necessary depending on part geometry).

The packing phase is decoupled from the filling phase; however, instead of simply squeezing the plastic in under second-stage pressure, packing is done using a low-speed, controlled velocity stage until a pressure setpoint inside the mold cavity is reached. This low packing rate absorbs most of the melt inertia and allows precise levels of packing to be achieved. This is similar to driving slowly into the garage and stopping exactly when your windshield touches the tennis ball hanging from the ceiling. Hold is then used to prevent backflow of plastic out of the mold until the gate is sealed (putting the car in park and setting the brake, to extend the analogy).

Summary

Decoupled Molding can provide unsurpassed process repeatability by segregating the molding process into the logical stages of fill, pack, and hold. Maintaining consistency of these three stages of molding separately allows the molder to build a simple, repeatable, robust process with minimal complexity.

The Decoupled Molding technique can be used on new machines, and on old machines that have been properly upgraded. Only position transfer capability is required to do the simplest Decoupled II technique. Decoupled III provides the capability for world-class consistency but requires the ability to control low-velocity pack speeds and to transfer from a cavity pressure input or an external contact closure. This three-stage technique provides a process consistency three to seven times better than the Decoupled II process.

Many of you may already be doing Decoupled Molding. However, the classification system in itself can make a better molder. The molder knows what he's doing, why he's doing it, and what the limitations are.

Rod Groleau, chairman of RJG Inc. (Traverse City, MI), has more than 30 years of experience in the plastics industry. Rod got his Bachelor of Engineering degree from GMI (now Kettering) and a Master's degree from Michigan State University. Matt Groleau joined RJG in 1991 and worked in many aspects of the business before taking the position of president. RJG provides production and process control systems, cavity pressure sensing technology, and training. Contact them at (231) 947-3111 or [email protected].

 

 

 

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