Gas Assist Injection Molding: Not just for hollow parts
Less Material. Less Weight. Less Cost. Better Quality.
Gas assist injection molding offers a variety of process and design advantages for Outdoor Power Equipment and Medical Equipment application. It involves the injection of a short shot of resin into the cavity and follows by injection of nitrogen gas under high pressure through the nozzle or mold wall. Nitrogen is the ideal gas for the job because it is an inert, dry and non-toxic gas and melts at a high temperature while preventing oxidation. During this process, after the polymer is injected into the mold, a skin layer next to the mold begins to solidify, but the inner core remains molten. Injected gas displaces the polymer from the core region and forces it into mold extremities. As the polymer cools, the gas pressure continues to force it against the cavity wall, helping prevent volumetric shrinkage of the part.
Gas Assist injection molding produces parts that are smooth and extremely cleanable, while offering increased strength and rigidity, weight reduction, design flexibility, and improved surface finish. Gas assist may also lower production costs by reducing cycle time compared with producing solid sections.
Gas-Assist Plastic Design
Gas-Assist Injection Mold Design
Gas-Assist Mold Manufacturing
Gas-Assist Injection Molding
Gas-Assist Injection Molding Types
Due to its availability, cost and inertness, Nitrogen is typically used for external and internal gas assist molding.
External gas assist molding
In an external gas assisted injection molding application, the gas is used on the outside surface, inlets and channels are incorporated into the part design and injection mold. Specialized nozzles introduce pressurized gas into the molten plastic during the molding process. This helps push the resin to the furthest extremes of the mold and into difficult to reach sections of the part. External gas assist is used to enhance surface definition.
Example of lawn mover part which was produced using gas assist injection molding. It enables us to achieve an attractive exterior finish while using low-pressure molding. The location of the gas channels can be seen on the hidden back side of the part.
Internal gas assist molding
Internal gas assist injection molding is a low pressure, conventional injection molding process that injects pressurized nitrogen gas into the interior of a short shot of melted material. The gas flows through strategically designed and placed gas channels, following the path of least resistance to displace the material in the thick areas of the part by forming hollow sections. This process reduces material usage but does not compromise the appearance and functional performance of the part. Next, the pressurized gas packs and holds the molten resin against the cavity walls until the part solidifies. Constant gas pressure helps keep the part from shrinking as the part begins to cool and prior to being ejected from the mold. Once the part solidifies, the gas is vented from the tool.
Gas-Assist Injection Molding Material
Most thermoplastic materials are capable of being used in gas assist application, including ABS, PC/ABS, Acetal, PC, PPO, PBT, PET, PA, PVC, HIPS, PP, and HDPE. PC+ABS and PC are more rigid and look attractive when painted. They have a great finish and solid feel, but of course are much lighter. PA is frequently used for canoe paddles due to its overall ruggedness, while handles for many applications are made out of PC or PBT.
Gas Assist Design Considerations
- Optimizing part design for gas-assist injection molding should focus on channel layout optimiztion, channel sizing with respect to the part and balancing the plastic fill pattern.
- Never place gas channels where two melt fronts come together forming a weld line.
- Make runners and injection points slightly larger than those for conventional injection molding
- Make sure mold walls are evenly heated and cooled
- Sharp corners create thinwall sections so it may be advantageous to cool the mold separately in these locations
- It’s imperative to exactly balance melt flow into the cavities of multicavity tools
- Cavities should be arranged symmetrically in the mold and each should have its own separate gas needle
Gas Assist Injection Mold Manufacturing
To speed up the mold manufacturing process without sacrificing quality, you need the right technology. We use UG to design and manufacture molds. The 2K Mould manufacturing was done toally according to the 3D mold design. For manufacturing the 2K mold, we use different manufacturing machines such as Mikron CNC, AgieCharmiles EDM, Sodick Slow Wire Cut machine, Grinding Machine and Gun Drill Machine. After machining all the components, we have to measure the key size on the CMM machine. It is very important to avoid flash at the parting surface.
Advantage and Application of Gas-assist Injection Molding
Advantage of Gas-Assist Injection Molding
- Increased parts integration possibilities
- Allow plastic to move evenly and efficiently throughout a given mold.
- Reduce sinks and molded-in partstress, especially in parts that have thick-walls and/or flat surface areas with opposing ribs, post, or other engineered structures.
- Lower clamp tonnage requirements and Lower molding pressures, thus extended the life of the tool as well as producing larger parts more economically and with much improved cosmetics.
- Greater design freedom, allow engineers to develop thin-walled components and thick but hollow portions to thin-walled products.
- Shorter the cycle times and increase plastic injection molding machine output.
- Reduced part weight
Gas-Assist Injection Molding Application
- Lawn mower handles
- Large parts with ribs and bosses
- Interior and exterior automotive handles
- Large covers and panels and outer casings
- Large molded plastic parts that require good cosmetic surfaces
- Medical equipment housings
- Appliance handles
- Steering wheels
- Bus Chairs
- tubes, armrests, handles, and frames
- access covers, panels, shelves, and chassis
Stebro Mold has experience helping customers successfully use gas assist to solve challenges with medical equipment applications, as well as numerous other industries. Every application is different, and it is important to collaborate with a gas assist injection molding specialist to develop a design that is manufacturable — so the mold and the part do not cost a fortune, and the process will run at a reasonable cycle time. The goal is to design a good part that can be made efficiently and have it look good too.