Gas Assist / Structural Web Molding
What is Gas Assist and Structural Web?
Gas-assisted molding technology, also known as structural web, compliments structural foam molding. Specialized nozzles, inlets and channels are incorporated into the part and tool design.
During the injection stage, inert pressurized gas, usually nitrogen, is injected into the tool and resin using a specialized nozzle or gas pin. The pin design and gas work together to prevent plastic backflow.
With the resin shot completed, the pressurized gas continues. This keeps the flow front moving by forming a bubble inside the piece stretching the skin and material to the furthest extremes of the tool.
After the tool has been completely filled the gas bubble continues to be pressurized creating an internal cushion to compensate for the resin shrinkage and fully form the outer skins of the part.
Finally, once the part has cooled adequately and the skin has its strength, the gas pressure is vented before the mold is opened the part is ejected.
Hollow sections within a part can be formed using gas-assist.
How does gas-assist work?
Inlets and channels are incorporated into the tool and part design.
During the molding process, pressurized gas usually carbon dioxide, is introduced into the melt via specialized nozzles. This maximizes resin flow to the furthest extremes of the mold, and into difficult to reach sections of the part.Get a Quote!
Benefits of Gas-Assisted Molding
Large molded parts with attractive outer surface cosmetics can be produced with gas-assist.
Swirl is eliminated and secondary painting operations are not needed with gas-assisted molding.
Gas-assisted molding gives designers more latitude and options for introducing various component features such as bosses and ribs.
Gas-assist can help reduce cycle time.
Other Design and Product Benefits
- Taller and thicker bosses, ribs and wall sections can be molded.
- Sink is significantly reduced or even eliminated.
- Surface cosmetics are greatly improved by reducing swirling and flow lines.
- Hollow thicker sections can be molded. This allows for features such as handles to be incorporated in the part.
- Can eliminate finishing work such as coring out or thick sections.
- Creates the look of a traditional injection molded part while still getting the benefits of structural foam molding
Gas Counter Pressure
Gas counter pressure is another molding technique used to improve the quality of single-nozzle structural foam molded parts.
Inert gas, usually nitrogen, is used to charge the cavity end of the mold. The pressure slows the flow velocity and controls the foaming expansion during the low-pressure injection process.
This improves the formation of the outer skin and reduces the swirling effect on the surface of the part. The counter pressure controls the foaming rate, shape, size and density of the leading edge of the fill.
The benefits of gas counter pressure
- Eliminates swirl.
- Eliminates sink marks.
- Reduces part shrinkage.
- Produces a smoother outer surface.
- Can be used to create a textured outer surface.
- Reduces molded-in part stress.
- Allows the use of a smaller tonnage press.
- Reduces cycle time.
How the Process 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.