Gas Assisted Injection Moulding

The process known as Gas Assisted Moulding is a proven and beneficial moulding method of improving a variety of injection moulding conditions. Initially, the process was used to core out thick sections of injection-moulded articles and has been the subject of many papers presented at earlier conferences.

In the last several years, the Gas Assisted Moulding process has also been used to improve the surface quality of molded parts that have ribs, bosses or pillars that are required geometry in injection moulded parts.

Gas channels can be added along the length of these ribs, and by injecting gas in the centre of the channels will pressurize the immediate area and eliminate sink marks.

Properly located gas channels can also reduce the tendency for warpage in the moulded part that is otherwise caused by the induced stress during the packing phase of the standard injection moulding cycle.

The benefits of these techniques have made significant improvements in many moulded products. As an example, nearly all TV cabinets produced today use this Gas Assisted Moulded method.

There are still, however, moulding challenges. Shadow marks can be caused during the injection of gas. If the plastic flow hesitates or stops, marks or flow lines will be evident on the surface and, therefore, unacceptable on the Class A surface finish.

“Fingering” is another undesirable condition caused when the gas is not retained in the thick section of the part and spreads to the nominal wall section. Although state-of-the-art gas control devices may eliminate some of these occurrences, there will continue to be part geometries that still produce this problem.

When gas is used to pressurize multiple channels the gas pressure applied is obviously greater at or near the gas channel. Naturally, the pressure dissipates the further the distance is from the gas channel. The results are surface imperfections such as indentations or weld lines.

Conventional gas molding also requires an entry and exit hole that is retained as a blemish on the part.

It is our belief these problems can significantly be improved or eliminated with the use of a new process developed - External Gas Moulding.

The work on this process began in the 80’s and was invented in the U.S.A. by Dr. Robert Carroll and was patented in 1988. The “Injection Compression Process”, as it was called, was the first patented use of gas as a packing or compression technique in injection moulding. The process, however, was not brought to commercial status.

At the same time, Asahi Chemical of Japan was independently working on a similar process and also achieved promising results. Recently Asahi purchased the entire ICP worldwide patent portfolio. In 1999 Incoe obtained licensed rights to use and sell this patented technology in North America.

A quantity of plastic is injected into the mould. This is accomplished using the normal filling phase of the injected moulding machine. The moulding machine applies no pack pressure or additional material.

Gas is then injected at low pressure (typically less than 1000 psi) through the core side of the mold. Gas is not injected into the plastic material, but onto the plastic creating a layer or “blanket” of gas across the entire sealed surface area or selected sealed areas of the moulded article.

The gas is introduced via a porous metal insert, gas pin, or poppet.

Gas pressure is held constant until the plastic has cooled and is self-supporting. Gas pressure is then either fully vented by the same manner as introduced, or may be reduced with some remaining gas pressure retained for use in encouraging ejection.

The gas must be sealed at the parting line and any ejector pin areas to prevent escape. The part geometry near the parting line must be modified slightly to create the seal and is critical to the process.

The Automotive industry frequently uses a process known as "backmolding" where a laminate containing a design or texture is molded into the part on the class A surface side. This process requires the injection-molding machine to perform a special molding sequence referred to as "coining". This form of compression molding is used to apply a uniform pressure on the laminate. Normal injection molding would cause imperfections to the laminate during the high-pressure packing phase. To incorporate this function in the molding machine is costly, and in some cases also requires the use of additional cylinders or “stampers” in the mold. These can be eliminated using the EGM process, which is easily adapted to any injection-molding machine.

The advantages of the EGM process are as follows:

• The entire surface of the molded part is evenly pressured. No pressure drop is realized between gas channels. The process is used to pressurize the article, in place of the pack phase normally performed by the injection-molding machine, and compensate for the volumetric shrinkage during cooling.
• The process allows pack pressure remote from the material injection point. This can be beneficial for applications such as in-mold decorating.
• Gas control device can be simplified.
• Ejector pin marks are either minimized or eliminated since the part is either assisted in ejection by the gas or fully ejected by the gas, in place of ejector pins.
• Ribs and bosses are fully packed and sink free on the Class A surface finish without the use of gas channels.
• Thinner wall sections can be obtained since the pack pressure is equally distributed and at significantly lower pressures. In conventional molding, thin sections require high fill pressures often resulting in a warped part.
• Hesitation marks, shadow marks and permeation all are eliminated since the molded article is full at the time of pressurization.
• The molded article is free of molded in stress normally produced during conventional injection molding, since the process produces a result similar to compression molding.

The elimination of sink marks are attained by the gas pressure moving the material uniformly against the cavity surface as shown in this comparison.

We believe this process will benefit many markets now using the gas assist process, and additionally provides the potential to expand into other markets and products that are not now using the gas assisted processes.