Gas assisted injection molding.
Gas Assisted Injection Molding or what we call structural web has some particular advantages. Here is how gas assist works:
Critical steps and processes to deal with shrinkage during injection molding. Structural foam molding offers advantages in manufacturing of large plastic components and parts
In the initial phase, the resin is injected into the empty mold cavity. The resin is coming in from the left of the article through an edge gate.
Gas injection begins while plastic injection continues thus preventing a hesitation of the flow front during the structural foam molding process
In phase two, gas injection begins as the resin injection continues thus preventing a hesitation of the flow front. The gas is injected through a special gas pin. The gas pin design and gas pressure have to work together to prevent plastic back flow.
Gas injection keeps flow front moving as bubble forms inside part stretching the skin to the end of the mold during manufacturing process
In phase three, the resin shot is completely injected into the mold as gas injection continues. This gas injection keeps the flow front moving as the bubble forms inside the article thus stretching the skin to the end of the mold.
Pressurized gas bubble creates an internal cushion to compensate for resin shrinkage during structural foam molding manufacturing
In phase four, the material has completely filled the article, the skins are fully established, and the gas bubble continues to be pressurized thus creating an internal cushion to compensate for resin shrinkage.
Gas is vented in the final phase of creating the highest quality structural foam molded products
In the final phase, the article has cooled adequately to establish skin strength so the gas pressure can be vented. The gas must be vented prior to mold opening to avoid explosion. The gas pin retracts to accomplish this venting action.

Information provided by Steve Ham Plastics, 537 Hickory Street Highlands, NC 28741 USA