Special Molding Process Requirements of Injection Molds for Transparent Plastic Parts
Transparent plastic parts realize functional performance via light transmission; internal bubbles, silver lines, fusion lines, sink marks and surface scratches damage light transmittance directly. Specialized standards on steel selection, cavity polishing, gate layout, vent design and ejection system focus on high surface finish, low internal stress and zero molding defect for transparent product mold development.
1. Mold Steel Selection and Surface Polishing Requirements
Cavity and core of transparent product molds adopt mirror pre-hardened stainless steel including S136 and STAVAX with impurity removal smelting to eliminate internal sand hole and sulfide inclusion and avoid rust-induced pit after long-term production. Polishing proceeds in three phases: rough sanding, fine grinding and mirror finish with diamond paste, finally achieving cavity surface roughness below Ra0.02μm for flawless transparent outer surface after molding. Split mold structure is minimized as splicing gap accumulates residual melt and decomposes into black speck contaminating transparent surface under heat; integrated integral cavity structure is prioritized for small transparent components.
2. Optimization Design of Gating System
Gate shape and location influence melt shear heating remarkably; excessive shear triggers local overheating, material degradation, yellowing and cloudy pattern on transparent parts. Pin-point gate, fan gate and tab gate are preferred to reduce shear rate and avoid thermal cracking from partial temperature rise. Gates are arranged at thick-wall position far away from visible light transmission surface to prevent residual gate mark spoiling appearance. Main runner and sub-runner adopt smooth transitional enlargement design with inner wall mirror polishing to cut flow resistance and eliminate carbonized black speck from detained degraded raw material. Hot runner mold is widely used in mass production to remove cold slug and avoid impurity spot mixed from cold material inside cavity.
3. Specialized Design of Venting Structure
Trapped gas forms white fog and internal void inside transparent parts, thus vent standard of transparent mold is stricter than conventional plastic mold. Parting line vent depth is controlled between 0.01mm and 0.02mm with width 5~12mm arranged continuously along melt flow direction; breathable steel insert or ejector pin vent is installed at rib, blind corner and fusion line position to exhaust trapped air via pin clearance. Extra overflow well is set at cavity terminal and melt convergence area to discharge residual gas with minor cold slug and eliminate internal bubble fundamentally. Mold clamping tightness is precisely controlled to avoid excessive clearance-induced flash or over-closing blocking vent passage.
4. Contour Cooling Channel Design
Uneven cooling brings severe internal stress leading to post-molding whitening, crack and light distortion of transparent parts. Cooling channels are laid close to cavity surface with uniform distance between channel and mold wall to restrict integral mold temperature difference within ±3℃. Spiral irregular cooling channel is applied for thick-wall transparent components to accelerate local heat dissipation, shorten cooling cycle and balance shrinkage. Mold temperature is set based on raw material property: 85~110℃ for PC and 60~80℃ for PMMA; stable mold temperature reduces light transmittance defect caused by uneven crystallization and stress whitening after molding.
5. Process Specification of Ejection and Guiding Structure
Clearance of ejector pin and sleeve is limited within 0.008~0.015mm: undersized clearance causes pin sticking and surface scratch while oversized gap generates flash damaging transparent finish. Ejector position avoids visible light transmission area with multi-point uniform ejection layout on large transparent shell to disperse ejection force and prevent local indentation whitening. High-precision wear-resistant guide pin and guide bush prevent abrasive debris dropping into cavity and forming foreign black spot during opening and closing; cavity and runner are thoroughly dedusted before mold assembly and production startup.
6. Regular Maintenance Specification of Transparent Mold
Maintenance of transparent mold differs from common mold: cavity is cleaned with soft cloth before shutdown and startup with rigid scraper forbidden to scratch mirror cavity. Remaining melt is shot out intermittently during short downtime to prevent carbonized degraded material dropping into mold. Anti-rust oil is coated evenly inside cavity before long-term storage with sealed protection against rust from air oxidation. Vent groove is disassembled and cleaned periodically to clear accumulated carbon blocking exhaust and triggering batch bubble defect in subsequent production.
Overall mold process control centers on elimination of impurity, bubble, flow mark and internal stress via high-standard steel, mirror polishing, reasonable gate & vent layout and balanced cooling to ensure uniform light transmission and clean appearance of transparent molded parts meeting optical-grade production standard.
