Core Development Essentials for Gas-Assisted Molds of Deep-Cavity Injection Molding
Deep-cavity plastic parts always feature deep depth, uneven wall thickness and long narrow ribs. Conventional injection molding frequently causes shrinkage dents, internal shrinkage cavities, heat accumulation scorching, demolding scratching and warpage deformation. Gas-assisted injection molding forms hollow channels inside molten plastic via high-pressure nitrogen, replacing traditional screw packing with continuous air pressure packing. It effectively eliminates shrinkage defects on thick deep-cavity sections and reduces the wrapping force of plastic parts against cores to lower scratching risks during demolding. Gas-assisted molds cannot be modified from ordinary deep-cavity molds directly. The layout of gas pins, sealing structure, cooling system and gas channel layout must be designed synchronously. The whole development system centers on gas injection layout, cavity sealing, cooling matching, gas channel design, exhaust optimization and demolding structure upgrade.
1. Layout of Gas Pins and Air Inlet Structure
Gas pins serve as the core component for nitrogen injection. Hidden embedded gas pins are preferred for deep-cavity products, installed on thick-wall bosses and thickened base areas rather than visible appearance surfaces or assembly matching surfaces to avoid gas pin marks affecting appearance and dimensional precision. Deep cylindrical products adopt single-point central air intake to push nitrogen straight to the bottom of cavities and form smooth through gas channels, while multi-rib shell products use distributed multi-point gas pins with independent control of air intake sequence and air pressure for each inlet to prevent disordered airflow and local short shots. A 0.8mm diversion groove is reserved at the tip of each gas pin. Molten plastic wraps the gas pin end during injection, and high-pressure nitrogen is fed after 90% to 95% cavity filling to stop plastic backflow and gas pin blockage. Each gas pin base is equipped with an independent cooling sleeve to prevent plastic adhesion and coking blockage caused by high temperature. All gas pins are designed to be detachable for replacement without disassembling the whole mold cavity during later maintenance. The air intake action is linked with the injection stroke of molding machines, and residual plastic at the cavity opening seals the end to avoid nitrogen breaking through plastic parts and causing perforation defects.
2. Air-Tight Sealing Structure for Deep-Cavity Molds
The nitrogen pressure in gas-assisted molding ranges from 8MPa to 25MPa, requiring much higher sealing performance than standard injection molds. A full-circle sealing rib is added on the parting surface to achieve airtight bonding under mold clamping pressure. Inclined surface inserting sealing is adopted for splicing joints and corners instead of flat fitting to stop high-pressure nitrogen leakage which leads to failed packing and collapsed gas channels. The fitting clearance of ejector pins and ejector sleeves is narrowed below 0.005mm, and fluorine rubber sealing rings are mounted on pin tails for double sealing against pressure leakage. Wear-resistant sealing inserts are embedded on sliding positions of sliders to balance sliding smoothness and air tightness and prevent clearance expansion and air leakage after long-term mass production. An outer locking ring is added on molds deeper than 120mm to prevent the parting surface from being pried open by lateral air pressure, avoiding pressure relief and flash generation in mass production. Steel for sealing areas is heat-treated to HRC 52 or above to resist compression deformation after repeated mold opening and clamping.
3. Custom Gas Channel Design Matched with Deep-Cavity Shapes
Main gas channels are arranged along the depth direction of plastic parts, and circular cross-sections are prioritized for uniform air pressure distribution without dead zones of pressure retention. The diameter of gas channels inside cylindrical thick walls is set to 60% to 70% of wall thickness, and the width of auxiliary gas channels inside ribs equals rib thickness. Main gas channels extend to the end of deep cavities and branch into tiny sub-channels covering all thick ribs and raised bosses prone to shrinkage. A 3mm to 5mm solid plastic edge is reserved at the terminal of each gas channel to prevent nitrogen breaking through the part edge and causing air leakage and perforation. Gas channels are transited with circular arcs at stepped positions with large height differences to avoid stagnant airflow and insufficient local packing that causes concave deformation. Multiple connected gas channels maintain consistent internal cavity pressure and prevent plastic warpage resulting from pressure differences in different zones. Open grooves on deep cores are forbidden for gas channel forming, and closed internal hollow gas channels formed naturally by guided nitrogen are adopted to keep outer surfaces of plastic parts flat.
4. Differentiated Layout of Independent Cooling Circuits
Two mutually independent cooling water circuits are deployed for the cavity and core of deep-cavity gas-assisted molds to avoid excessive temperature difference between the two sides which causes uneven cooling and twisted plastic parts. Conformal surrounding water channels are applied on front appearance cavities, 8mm to 10mm away from plastic surfaces, ensuring uniform cooling, consistent gloss and no light & dark marks on appearance surfaces. Slim water transport needles are inserted inside deep cores and extend all the way to the bottom of deep cavities to solve the problem of heat accumulation and insufficient cooling at deep positions. Separate small surrounding water channels are arranged within 3mm around gas channels to control plastic solidification speed properly. Premature solidification traps nitrogen and distorts gas channels, while delayed solidification leads to expanded gas channels and bulged outer surfaces of products. All water circuits are treated with pressure resistance and rust prevention to prevent water seepage from damaging the airtight environment inside cavities and rust blockage of slender core water channels during long-term production.
5. Optimized Exhaust System for Deep-Cavity Working Conditions
Deep cavities feature closed molding space with poor natural exhaust performance. Exhaust inserts are embedded at material flow terminals, fusion positions and dead corners at deep bottoms. The front section of exhaust slots is strictly controlled at 0.008mm to stop flash and air leakage, while the rear section is widened and deepened to discharge trapped air rapidly. Vertical slender exhaust slots are opened on core sidewalls to exhaust air accumulated at deep cavity bottoms from top to bottom and eliminate whitening and scorching defects at the bottom. Micro exhaust holes are scattered inside sealing ribs to remove residual air before mold clamping and avoid indentations formed by trapped air pressure. All exhaust paths avoid the extension route of gas channels to prevent high-pressure nitrogen escaping through exhaust slots, which would collapse gas channels and disable pressure packing.
6. Optimized Demolding Structure for Deep Cavities
Gas-assisted molding reduces the wrapping force of plastic parts against cores by 40% to 60%, yet deep-cavity demolding requires long strokes that still bring risks of mold adhesion and scratching. Core surfaces are finished with fine matte texture to lower vacuum adsorption force, and high-gloss polishing on cores is prohibited to avoid large-area mold adhesion. A combined ejection system of ejection sleeves and pins is used. Ejection sleeves wrap the outer side of plastic part bottoms for uniform stress bearing to prevent skewing, ejection whitening and deformation caused by single-point ejection. Extended guide posts are installed on molds deeper than 150mm to guarantee stable ejection plates without inclination during long-stroke ejection. A low-pressure air blowing structure is reserved on core tops to form an air film between plastic inner walls and cores at the moment of mold opening for smooth scratch-free demolding. All undercut structures are demolded via slider core pulling instead of forced stripping, as rigid stripping would crack thin deep-cavity plastic parts.
Conclusion
The development of deep-cavity gas-assisted molds follows the principle of airtightness first, reasonable gas channel layout, balanced cooling, precise exhaust and stable demolding. Timing control of gas pins and gas channel arrangement eliminate shrinkage cavities and dents on thick deep sections. High-precision sealing structures prevent high-pressure nitrogen leakage and stabilize packing effects. Independent front and rear water circuits eliminate heat accumulation deformation, while specialized exhaust and composite ejection structures resolve scorching caused by trapped air and scratching during demolding. Following the above development rules can greatly improve the yield rate and dimensional stability of deep-cavity products, reduce mold revision frequency and realize stable mass production.
