Gas Assist / Structural Web
Gas Assisted Injection Molding or what is also known as structural web has some particular advantages. Here is how gas assist works:
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.
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.
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.
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.
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.
The gas assist molding process:
Information above provided by Steve Ham Plastics, 537 Hickory Street Highlands, NC 28741 USA
Channel used during gas assisted molding process.
Above left. Interior rib show-through on part which did not use gas assist during molding. Above right. Smooth surface finish on part when gas assisted molding was used. (click image to enlarge)
The advantages of gas assist and structural web:
- Thinner walls and hollow parts for large structural components
- Smoother surface finish that rivals high-pressure injection molding
- Consistent surface color and finish – no swirls
- Low pressure molding advantages
- Aluminum molds are less costly than steel
- Maximum part size with minimum clamp tonnage