Gas injection molding?

Gas injection molding?

All of us in the rubber industry have known for a long time that people who process plastics do things differently. For many years most of us have manufactured our products with little interest in plastic processes. However, with the advent of a variety of new thermoplastic elastomers, many rubber companies are looking closer at plastic processing and manufacturing techniques. One of the newer of these is gas injection molding.

What is gas injection molding?

Molding of plastic parts is very different than molding rubber. While there are a number of articles that go into these differences, the basic difference stems from the fact that in plastic molding the part is being cooled and changed from a molten state to a solid one. The resin used is "frozen."

As this cooling takes place, many of the physical characteristics of the resin change. This includes flow rates and volume. Because of the change in volume associated with the cooling, typical injection processes for plastics include a "packing" cycle where pressure is maintained on the injected resin from the injection screw. If this pressure is released too soon, dishing of the surface and other distortions occur because of the volume reduction. Even with this packing cycle, thick section parts can still have problems.

Gas injection molding involves first, injecting a short shot of resin into the mold. A gas, typically nitrogen, is then injected into the melt, pushing it out the rest of the way into the mold and forming a hollow channel in the part.

Unlike structural foam, where gas is mixed with the resin and then injected, this procedure involves injection of the gas into the center of the flow. The plastic remains solid around the gas channel. As the volume reduces on cooling, surface changes occur on the inside of the channel, keeping the outside free from dishing, etc. Once the part has cooled sufficiently, the gas is vented out of the part and the mold opened.

How does it actually work?

Actual processes vary depending on the equipment used and the part being made. Most work to date has been done in Europe. Recent litigation over patents has prevented much development of the systems in the U.S. until recently.

The basic process, regardless of equipment or part, is as described above. Specific differences involve how the gas is controlled (controlled volume vs. controlled pressure), the point at which the gas is injected into the mold, how the gas is vented from the part and how the gas flow channels are designed.

Typically, the mold is partially filled with resin from the injection screw. The gas is injected to pressurize the part. Several systems control the gas injection by volume; i.e. a precalculated volume of gas is injected to pressurize the part. In others, gas pressure is controlled during injection. In the pressure controlled system sold by Battenfield, pressure can be controlled in three different stages during pressurization. Gas passes through the part via various flow channels and ribs. These are placed on the back side of the part so as to not affect the appearance side. In addition to providing a path for gas flow, these ribs provide additional structural strength to the part.

The part remains pressurized until the part is no longer molten. This corresponds to the normal packing cycle used in the molding procedure. Once the part is no longer molten, the gas is vented either break through the injection orifices or by providing vent hold in the part.

The gas channels are formed in the part by the gas itself. Since the areas designed for the flow channels are thicker than the surrounding part, the center of the resin in that area remains hotter and more fluid than the outside. As the gas is injected, it pushes through the center of these sections, following the intended flow pattern. As it moves, resin is pushed out against the old surface and a pattern of ribs and channels is formed. As the resin cools, shrinkage takes place on the inside of the channel, eliminating "sink" marks from occurring on the part surface. Also, use of gas rather than packing from the injection screw is reported to reduce problems with flashing of excess resin and molded in stresses. Use of the gas injection process is said to reduce the amount of clamping pressure needed to form parts.

In the Battenfield process, the primary process control parameters have been identified as the following:

* Timing of the start of the gas injection.

* Providing sufficient gas pressure at the start of the injection.

* The gas pressure profile during injection.

* The timing of the release of gas pressure from the part.

If gas is injected into the resin too soon, the outside of the part will not have had adequate time to cool. This will result in too little difference in the flow characteristics in the channel area to form the hollow channels. If this occurs, the gas will blow out through the resin without forming the channel for resin distribution.

It the gas is injected too late so that the part has cooled too much, the gas will be unable to push through the resin and, again, the channel will not be formed.

During the initial phase of gas injection, it is necessary to use greater pressure than after the flow has begun. As a result, according to Battenfield, it is necessary to have some type of pressure profile during the injection phase.

Once the part is formed, the pressure must be vented from the part prior to demolding. If not, the part can be distorted during the demolding process. If the venting occurs too soon, the benefits of gas injection will not be achieved. If vented too late, long cycles and inefficient use of equipment will result.

What are the advantages to the process?

There are a number of problems inherent with molding of plastic parts which the gas injection process helps overcome. As mentioned earlier, this process helps avoid "sinks" on the surface of the part caused by shrinkage of the resin as it cools. This is particularly a problem where ribs and thicker sections are involved. Even using the packing cycle with normal injection molding, ribbed sections and bosses in the part can result in sinks in the surface above them. Using the gas injection process, this problem can be minimized.

In typical plastic injection molds, ribs are kept as thin as possible to avoid the problems with sinks. When using the gas process, it is necessary to use a much wider rib. In order to achieve the most beneficial effect from the process, it is necessary to have the gas channel near the surface of the part. This is done by tapering the rib so that it is wider near the finished surface than it is at the base. Because of the differences in flow, as described earlier, this will direct the channel through the thicker section near the surface.

In many plastic parts, external runner systems are required to move the resin to the proper location in the mold. Because of problems with sinks, ribs are not large enough to carry resin to all locations in the part. With the gas process, ribs are much larger. As a result, they can carry greater amounts of resin through the part. Thus, the need for external runners is reduced or even eliminated. This, in turn, can reduce tooling costs and the costs associated with reuse of the runners generated.

Another significant advantage is reduction of stresses in the molded part. Using gas injection, pressure is kept equal throughout the mold. Normal molding procedures result in uneven pressures that increase stress in the part. This increased stress can result in warpage of the part when demolded. The internal runner system formed by the gas process results in evenly applied pressures and significantly reduced stress gradients in the part.

The reduction in internal stress also allows for production of parts with different wall thicknesses. Shrinkage and stress problems make it very difficult to produce most plastic parts where there are significant differences in thickness in the part. In addition, there are flow problems associated with these differences. Gas injection reportedly overcomes these problems. In addition to these benefits, the gas injection process is said to result in faster cycles. This is particularly true when comparing to the molding of structural foam. Structural foam, while similar in some respects to the gas injection process, mixes gas with the resin, resulting in a foam with much lower thermal conductivity.

What are the drawbacks?

While there are a number of advantages to the gas injection process, there are also some problems. First, vent holes are required. This often will result in a hole somewhere on the part that must be filled or repaired. Care needs to be taken to ensure that vent holes are placed in locations that will not cause subsequent finishing problems or result in structural weaknesses. While this process alleviates sink problems associated with shrinkage in thick sections, the gas channel is reported to sometimes result in a difference in the gloss level on the surface above the channel. This is referred to as a surface "blush." Whether or not the blush appears and the extent of it is related to both processing conditions and the exact resin used. The problem can be masked by applying a surface "grain" on the mold or by painting the part.

Unfortunately, the gas injection process is much more difficult with multicavity molds. As noted before, this process requires a very closely controlled shot of resin in the cavity. When the gas is injected into the cavity, the melt flow has already stopped. Since the gas cannot push the resin from one cavity to another, for a multicavity tool, the resin shot in each cavity must be the same. If the cavities don't produce exactly the same part, control of the shot size can be extremely difficult.

Another important aspect of this technique is temperature control in the mold. Temperature control in the gas injection process is much more important than in conventional injection molding. Formation of the gas channel along with its thickness is totally dependent on the rate of cooling of the resin. Differences in the surface temperature of the mold will result in different cooling rates along the channel. This can then result in problems in the gas channel and the part made.

Molds for this technique must be different than conventional molds. Design must take into consideration the formation of the gas channels. These, in turn, must interconnect and be capable of being vented. If the channels are not properly vented, blisters typically occur in the part.

While similar in some respects to blow molding, the gas injection process is not suitable to large, hollow thin-walled parts. Trying to use gas injection for these will generally result in variation in the wall thickness and blow throughs.

What about application to TPE?

Little information is available regarding suitability of different resins and materials to this process. It has been stated that there are differences between resins. Also, it has been stated that several thermoplastic elastomers have been used. Reportedly, the process works best with high viscosity, high molecular weight materials. Also, amorphous materials are supposed to perform better than highly crystalline materials. Thermoset materials are said not to function well in this process. With thermosets, flow properties in thick sections are not conducive to channel formation.

Summary

As thermoplastic elastomers become more general in use in our industry, it is important that we become more versed in plastic processing technology. Plastic processing offers a number of unique benefits - and problems - compared to conventional rubber materials. In order to make the best use of available materials for the products we produce, we must understand the benefits and problems related to both physical characteristics and manufacturing processes.

The gas injection process appears to be one of the newer ones that will become much more prominent in the plastics industry in the future. Further information on procedures and techniques can be obtained from Battenfield of America, Inc., Klockner Ferromatik Desma, Inc., Krauss-Maffei Corp, Plastics Machinery Division and Stork Plastics Machiney B. V., among others.