A) Part Strength and Rigidity
Structural integrity and part stiffness are major advantages for conversion to Gas Assist. Flat, relatively thin wall parts can be made rigid with the addition of a gas channel or channels where allowable in the part, with accommodation for proper filling of the cavity.

The only revision to a part would be the channel or channels, with resin gated to the channel and the subsequent gas injection phase displacing that resin for final tool fill. The part would be “short shot”, with the flow/gas channel filling with resin as well as up to 99% of the remainder of the cavity. The nitrogen is injected, evacuating the flow/gas channels and completing fill of the remainder of the cavity.

Note that the cavity, in this example, never is completely filled with resin. Resin in the channel is displaced to fill the last-to-fill sections of the tool under reduced pressures. The channels(s) will result in a tubular cross section, which will significantly increase rigidity of the part, while low pressures facilitate a stress free part and reduce cycle time.

B) Long Flow Lengths
Long flow lengths are readily accomplished utilizing gas injection. A common example is chrome plated front facia molding approximately 70 inches long, 3 inches wide, with a .100-inch nominal wall. The example part is molded in plating grade ABS with one gate into a post in the center of the part, of a diameter of approximately .150 inches.

In this example, the cavity would appear impossible to fill, and if accomplished, would result in a very stressed, warped molded part with several weld lines. However, at the front edge of the molding there is a flow/gas channel, which is approximately .220 inches in diameter. At the time of injection of the short shot, the melt fills approximately 96% of the cavity, including and from the flow/gas channel.

At the point of gas injection, the gas displaces the resin in the channel, completing the filling of the thin wall section of the molding. Effectively, the flow length in the nominal wall, under any pressure at all is 3 inches, not 36 inches, as the final filling of the thin wall section is from the flow channel, not from an adjacent thin wall area of the cavity. Consequently, the part can be molded under minimal pressures; stress, warp and weld lines are eliminated, and close dimensional tolerances are held.

C) Heavy/Problem Ribs and Bosses
The natural tendency of the pressurized gas is for it to flow to the path of least resistance in the tool. This area will be where the resin is the thickest, and consequently, the most fluid, as higher temperatures are retained in the heavier sections. Correspondingly, gas channels are directed to locations under bosses and intersecting ribs, allowing those locations to eventually become the gas channels.

Because nitrogen vacates the resin in the thicker areas, sink marks are eliminated, as there is no longer a thick section of resin to cause excessive shrink. Furthermore, uniform nitrogen pressure is held on all internal channel sections, thereby packing resin in those areas against the wall of the cavity. This further facilitates shorter cooling cycles. Ribbed sections in a molded part will not cause sink marks to read through to the Class A surface.

D) Structural Foam Products
Structural Foam Products frequently have the highest per-part cost reductions available. This is because with usually minor tooling modifications on existing products, resin usage and cycle times can be significantly reduced. This is accomplished by strategically locating gas channels to increase part strength on the reduced wall-thickness product.

Cycle times can be reduced from as low as 180 seconds and as high as 6 minutes, down to 60-90 seconds, depending on part size and resin selection. Frequently, existing tools, without modification of wall thickness, can utilize Gas Assist by eliminating all gates except one central sprue, and by adding properly designed gas channels to orient the resin and gas to the desired locations.

A uniform-wall part of any nominal wall thickness and without channels would not be acceptable for the Gas Assist process, but a part with the differentials resulting from flow/gas channels and intersecting ribs will usually be a candidate for this type of conversion.