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Gas Assist Injection Molding
Traditionally, product designs for injection molding needed to follow tried and true design guidelines for optimal results:
Nominal wall sections must be maintained.
- Ribs must be 50-75% of the nominal wall.
- The part must be gated into the nominal wall.
- Material must flow from thick to thin.
With gas assist injection molding, the best design may deviate from traditional wisdom using the gas assist process and still produce a product with better structure and aesthetics.
A brief explanation of gas assist molding: Gas assist injection molding uses high pressure nitrogen to displace and push material from specific areas in the mold cavity. The typical process includes the following steps:
Molten plastic is injected into the mold.
- Pressurized nitrogen is then pumped into the mold cavity.
- The gas pushes against the walls, keeping the material from shrinking as it cools.
Short shot gas assist molding injects nitrogen into a mold only partially filled with plastic resin. The gas channel forms pushing the plastic to the edges of the cavity. The gas pressure is maintained in the channel until the plastic is set up. The result is fully formed part with no sink marks visible from the outside and with significant weight reduction. The elimination of sink marks creates a more attractive surface finish for finished parts.
Case in point? Gas assist injection molding is the technique we use at KASO to produce Werner Paddles, some of the world's finest recreational paddles (Read more about KASO's work with Werner Paddles).
Gas assist can be incorporated when the part design has structure, like thick-walled bosses and ribs, for functional requirements but will certainly have cosmetic flaws as a result that the consumer will reject. After plastic resin is injected into the mold and the traditional injection molding process commences, additional packing can be performed by introducing nitrogen gas through special pins in the thick-walled structure. Plastic that would normally sink in those areas instead will be pushed against the cavity from a small pocket of nitrogen from within.
Aesthetics aren't the only reason for using gas assist injection molding. Large parts mean long flow lengths for plastic resin. Longer flow length causes increased pressure drop. More pressure drop means a part will be increasingly susceptible to flash, sink, and surface texture variance. Traditional injection molding design solves this by increasing the runner and number of gates, increasing runner weight exponentially and increasing weld-lines with each additional gate.
With gas assist design the additional runner can be built into the part to allow material to flow through the cavity with less resistance and pressure. That same internal runner system can be filled with nitrogen to push out any sink marks that would typically occur with a thicker wall. Weight is therefore reduced, reducing overall project costs.
Other benefits from gas assisted molding include faster drying times and the ability to produce larger parts with smaller machines, as less tonnage is required for tooling that use the gas assist process.
Gas assist is an effective addition to other traditional injection molding techniques. Along with cost savings and reduced cosmetic flaws, it also opens the door for new product design and fresh innovation.
Typical gas pressures used during molding is 400-3500 psi. 3500 psi sounds extreme for molding, but the traditional injection molding process is pressurizing plastic resin between 4000 psi and 20,000 to 34,000 on the high end. The traditional process puts a lot of pressure on the front end, but the pressure drop after material enters the cavity causes pressure to drop off quickly the further it goes and as the material cools. With gas assist you can direct the pressure to specific areas and maintain it longer over an equal gradient in the gas channel.
Article Contributed by:
KASO Plastics Injection Molding & Thermoplastic