Application of Gas-assisted Injection Molding for Large and Medium-sized Plastic Parts in China
1. Forming Principle and Process Classification of Gas-assisted Injection Molding
Large and medium-sized plastic parts usually have problems such as thick wall, surface sink marks, high internal stress, long molding cycle and easy warpage. Gas-assisted injection molding injects high-pressure dry nitrogen into the plastic part after filling a certain amount of melt. The gas advances along the thick channel with the least resistance to form a hollow air channel. The uniform gas pressure compensates the shrinkage of plastic during cooling until the material is fully cured. It is mainly divided into short shot method, full shot method and edge sealing method. The short shot method is widely used for large and medium-sized parts. A part of melt is injected first, and then gas is introduced to push the melt to complete filling, with obvious weight reduction effect. The full shot method is suitable for products with high appearance requirements, and the gas is mainly used for pressure holding. The gas only flows along the preset channel and will not penetrate into the thin-wall area.
2. Core Advantages of Gas-assisted Process for Large and Medium-sized Plastic Parts
Gas-assisted molding can eliminate sink marks and depressions on thick rubber positions such as ribs, columns and assembly bosses, reduce internal stress and deformation of products. The hollow channel replaces solid thick plastic, reducing raw material consumption. The uniform gas holding pressure reduces the required injection and holding pressure, lowers the clamping force requirement, and allows the use of smaller tonnage injection machines, reducing equipment investment and energy consumption. The cooling time of thick wall areas is greatly shortened, and the overall molding cycle is optimized. Plastic parts formed by gas-assisted molding have lower residual stress and better warpage resistance, suitable for long strip and large frame parts that are easy to twist.
3. Key Points of Product Structure Design
The air channel of large and medium-sized plastic parts shall be continuous and smooth, arranged on the original thick ribs, support columns and frame positions, with straight lines or gentle transitions, avoiding sharp bends and narrow bottlenecks to prevent gas bifurcation and penetration into thin walls. The cross-section of the air channel is mostly circular or oval, and the plastic layer thickness on the outer wall of the channel shall be reasonable to ensure structural strength. An overflow groove or sealing structure is set at the end of the channel to control the stop position of gas. When multiple channels coexist, the flow resistance of gas shall be balanced to avoid priority gas entering a single channel. The air inlet shall be arranged away from the appearance surface.
4. Supporting Requirements for Mold Design
Compared with ordinary injection molds, gas-assisted molds are equipped with air inlet components, channel sealing structures and gas exhaust structures. The air inlet pin is the core part, which needs wear-resistant and reliable sealing to prevent plastic backflow and blockage. The parting surface and exhaust groove shall adapt to the filling sequence. The cooling water channel shall be evenly arranged around the air channel, because the air channel position has high heat accumulation risk. Although the locking pressure is reduced, the template and inserts of large molds need sufficient rigidity to prevent mold expansion and flash. Mold flow analysis shall enable the gas-assisted module to simulate melt advancement and gas front, and predict risks such as gas lagging and penetration.
5. Process Parameter Adjustment and Control
The debugging core of gas-assisted molding is to control melt filling amount, gas switching delay time, gas pressure and holding time. Too much short shot leads to poor weight reduction effect, while too little short shot causes gas blow-through. Too short delay time will lead to messy gas flow, while too long delay time will cause the melt surface to cool and crust, and the gas cannot form a stable channel. Nitrogen pressure is controlled by multi-stage curves, low pressure for initial advancement and high pressure for later holding. Nitrogen shall be dry and high-purity to avoid bubbles and silver streaks inside the product.
6. Typical Application Scenarios and Limitations
Gas-assisted injection molding is widely used in large and medium-sized automotive interior parts, large home appliance frames, treadmill handrails and large bathroom panels. It is not suitable for products with mirror appearance requirements, complex structures that cannot arrange continuous air channels, small-batch frequent revision products, transparent materials and high-viscosity special materials.
7. Common Defects and Maintenance in Mass Production
Common defects include gas blow-through, uneven channel wall thickness, gas lagging and residual sink marks. The air inlet pin is easy to wear and fail to seal after long-term impact, and needs regular inspection and replacement. The air pipeline and filter shall be maintained regularly. Raw material batch fluctuation and poor drying will change melt viscosity and affect the gas front. During troubleshooting, the short shot amount and switching delay shall be checked first, instead of modifying the mold channel structure directly. Gas-assisted injection molding provides a different molding idea for large and medium-sized plastic parts. The key of stable implementation is pre-planning of air channel, sealing structure design of mold and accurate timing and pressure control. It can balance quality, production capacity and production cost for suitable products.
