|Gas Assisted injection Molding|
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What Is Gas Assisted Injectio Molding
Gas assisted injection molding process is a unique variation type of conventional injection molding process that forces a short shot of molten material to fill mold cavity by using pressurized nitrogen gas to drive the material to pre-destined thick wall-thickness area while forming a hollow section in that area. Gas Assisted Injection Molding is a highly technical form of injection Molding, that uses both plastic and gas (usually nitrogen) to fill an injection mould under pressure. The basic knowledge of Gas Assisted Injection Molding is unsurprisingly like regular injection molding.
Process of gas assisted molding
1. Closed tooling
2. Injection nozzle move forward
3. Resin Injection 鈥?The polymer is injected into the mold as a short shot or partially packed cavity.
4. Primary Gas Penetration 鈥?Gas is introduced into the molten core forming a bubble. The gas bubble displaces some of the molten core, pushing it into the unfilled cavity and completing the mold filling
5. Secondary Gas Penetration 鈥?Secondary gas penetration begins at the end of the filling stage when the polymer has reached the end of the mold. The gas bubble extends as the part cools and the material shrinks. The extra cavity volume created as the material shrinks is taken up by the gas bubble. The pressure in the bubble also provides packing of the part during secondary gas penetration.
6. Injection nozzle withdraw back. (Release gas)
7. Open mould
8. Ejection out parts
Injection parameter configuration
鈥?Material drying condition is as same as conventional method.
鈥?Melting temperature in press nozzle is kind of higher than normal, keep as hot as possible to help gas get through.
鈥?Tooling temperature need enhanced, it needs uniform cool down, which also help gas to blow up hollow melting material
鈥?Injection pressure is as same as conventional parameter.
鈥?Injection speed needs higher speed, because that need help gas to blow up hot material.
GAS Assisted Injection Molding Type
1) External Gas Assist Injection Molding
As soon as the molten resin is introduced into the cavity of mold, the gas is injected in a micro-thin layer between the back surface of the part and the mold core surface, thereby forcing the front surface of the part against the opposite side of the mold cavity. As the molten resin cools with pressurized gas guarantees the front side of the part precisely replicates the surface of the mold.
Conventional injection molded (CIM) parts show evidence of resin shrinkage on the front side areas of the rib/ wall junction. For external gas assist injection molding (EGAIM) gas holding on the reverse side of the part prevents front side shrinkage.
Application of external gas assisted injection molding
This process is best for high profile and large surface area plastic parts, especially those requiring detailed fine textures and superior surface aesthetics. Warpage and sink marks are eliminated because the gas hold on the resin as it solidifies. As with internal gas assist, support ribs and bosses can be molded into the part yielding similar dimensional stability and tight tolerances. Molding pressures are also likewise reduced generating less residual stress on parts and wear on molds.
2) Internal Gas Assist Injection Molding
Gas assist injection molding is a low-pressure process that requires the injection of pressurized nitrogen gas into the interior of part in a mold. The gas flows through strategically placed gas channels on the part to displace the material in the thick areas of the part by forming hollow sections. The pressurized gas pushes the molten resin tight against the cavity walls until the part solidifies, and the constant, evenly transmitted gas pressure keeps the part from shrinking while also reducing surface blemishes, sink marks, and internal stresses. This process is ideal for holding tight dimensions and complex curvatures over long distances.
Benefits of Gas Injection Molding
鈼?Reduction in molded plastic weights, and therefore cost of material reduction.
鈼?Reduction in molding time cycles, and therefore cost of production.
鈼?Reduced in-mold pressures, and therefore less wear on molds.
鈼?Reduced warpage and distortion
鈼?Reinforces ribbed parts, Parts have greater strength and rigidity.
鈼?Increased design options for complex parts, reducing multiple part assemblies, and metal to plastic conversions
鈼?Elimination of sink marks.
鈼?Reduced in-mold pressures by up to 70%, and therefore reduced press lock forces enabling larger moldings on smaller machines.
鈼?Reduced power consumption.
鈼?Reduced molded in stress, and therefore improved dimensional stability with no distortion.
鈼?Hollow structures can be produced
Material selection for gas assisted injection molding
Most thermoplastics materials can be employed in gas assist injection molding, such as ABS (acrylonitrile butadiene styrene), PC (polycarbonate), HIPS (high impact polystyrene), PA (nylon), PBT (polybutylene terephthalate), PET (polyethylene terephthalate), PPE (polyphenylene ether) and PP (polypropylene)
Gas assisted injection molding application
Gas assist injection molding is a process that produces large plastic parts with complex designs and superb cosmetic surface finishes. The process is especially ideal for large handles, equipment covers, doors, bezels, cabinets and skins. Some applications benefiting from gas assist plastic injection molding include:
鈻?Medical equipment housings
鈻?Interior and exterior automotive handles
鈻?Large covers and panels for office equipment
鈻?Lawn mower handles
鈻?Basketball backboard frames
鈻?Long tubular shapes, such as shovel handles
鈻?Large parts with ribs and bosses.
Case StudyInjection Molding