1.Control Systems and Accessories
The Gas Assist injection molding process is made possible through finite control, regulation, and injection of high pressure, (up to 10,000 PSI) nitrogen gas. This involves a means to intensify gas pressure, control of the high-pressure gas, and delivery of the gas to the desired areas within the molded product.

Senior Gas Kits receive purified nitrogen at a minimum pressure of 500 PSI (2,000 to 2,500 is ideal and recommended), intensify and store the gas at 10,000 PSI, and with PLC controllers, regulates and delivers the gas with precise control of pressure and time. Junior Gas Kits and Satellite I systems receive the high-pressure gas from a Senior Gas Kit or central high-pressure system, and operate in exactly the same manner.

The Gas Kit uses a unique phased pressure control system, providing up to six phases of time and pressure following the gas injection delay period. (Gas injection delay may be from 0 seconds to any delay desired. Usually, the delay will be less than one second.) The Gas Kit system requires minimal interface with the molding machine; only two signals are mandatory. 1) A signal that the mold clamp is closed, and 2) a signal indicating the injection ram has completed its forward stroke. “Ram forward” is the set-point to begin the gas injection sequence. The gas delay timer begins, and the subsequent time and pressure phases follow.

All control is through a “touch screen” Allen Bradley PLC controller; the process is programmed by the injection-molding technician at the time of molding machine and tooling set-up. System setup is a simple process, requiring only connection of the signal inputs to indicate clamp closed and ram forward position, and inputting of the gas delay and pressure/time phases. After setup, the process is automatic, controlled by the PLC and valves in the Gas Kit.

2.Injection through Sprue
The most common and simplest method of gas injection is through the sprue. With this method, the part is gated directly into the tool cavity (such as with center gating) or via a cold runner or runners, which feed to the flow/gas channels of the tool cavity. The tool may have multiple gates to a single cavity, as long as there is careful attention to balance of the fill of the cavity.

An out-of-balance resin fill will cause a condition of out-of-balance gas distribution. This method is also the most cost effective method, as tool build costs are minimized, and potential problems such as plugging of gas pins are avoided. This method may be utilized on single or multiple cavity tools; on multiple cavity tooling, it is imperative to carefully balance the cavities. Although this is true in any multiple cavity tool, as the Gas Assist process is a “short-shot” process, it is impossible to compensate for unbalanced filling by “packing-out”.

It must be noted that with gas injection through the sprue, a hot runner system may not be utilized, as the nitrogen will either mix with the melt and create foaming: cool the resin causing defects in the product; or contaminate the melt characteristics of the resin in a manner that will be detrimental to the appearance and mechanical attributes of the final product. The success of the process is based on displacement of resin in channels and intentionally designed heavy sections; not mixing with the resin or creating hollow areas in a product.

3.Injection through gas pins
Gas injection through a pin in-article or in-runner is the second most common method of injecting nitrogen into a part. The same features are accomplished, however the high pressure gas is introduced through precise injection and venting through one or more gas pins into the tool cavity. In many cases, this is the preferred method, such as in the case of a hot runner multiple cavity tool being converted to Gas Assist.

It does add some complexity to tool design, and adds cost for acquiring, assembly and maintenance of the pins. This is said not to discourage the practice, however, as with gas injection through pins in the cavity, the process is enhanced far beyond the minimal cost differentials involved. Done properly, this is a sophisticated method that will furnish the desired results.

The decision of whether to inject nitrogen “through the sprue” or via gas pins should be based solely on the design of the part to be molded. As an example, a product may be designed where it is desirable to have a “nominal wall thickness” at the same point where it is advisable to edge-gate the part.

Gas should never be injected into or through a nominal wall of the molded product, but into the flow/gas channel, thereby immediately displacing the resin from the channel into the nominal wall. If gas is injected into the nominal wall, control of the flow-path of the gas is lost, and so is control of the entire process. Distribution of the gas will vary, and product integrity will be compromised, as the remaining resin in the nominal wall will be inadequate for the product design.