PC Engineering Plastic Mold Development Technical Specifications

Polycarbonate (PC) features high transparency, impact resistance, heat resistance and stable dimensions, widely used in new energy housings, optical lenses, electronic and medical plastic parts. However, PC melt has high viscosity, long cooling cycles and severe internal stress tendency, with strict requirements for mold temperature and exhaust. General mold structures fail to meet its forming demands. This specification defines standardized development rules for PC molds covering steel selection, gating, cooling, exhaust, ejection, polishing and trial molding.

I. Mold Steel Selection

Transparent optical PC parts adopt S136 or STAVAX stainless steel (HRC48–52), with excellent corrosion resistance and mirror polishing performance to avoid surface fogging and yellowing. Glass fiber filled non-transparent PC uses NAK80 or 718H pre-hardened steel, with cavity nitriding to boost wear resistance. Mold frames adopt thickened S50C plates (20% thicker than standard) to resist high injection pressure and prevent flash. All cavity steel requires full tempering to eliminate internal stress and avoid long-term dimensional drift.

II. Gating & Parting Design

PC has poor fluidity, so runners adopt U-shaped sections 1.2–1.5 times wider than ABS molds with mirror-polished inner walls to reduce shear and melt decomposition. Fan or film gates are preferred for transparent thin shells to minimize weld lines and stress whitening; thick optical parts use hot runner gates to lower molecular chain fracture. Tiny pinpoint gates are forbidden for thick PC workpieces to avoid severe internal stress cracking. All parting surfaces reserve 0.01–0.02mm sealing strips, matched with overflow grooves to stop flash under high pressure.

III. Cooling & Temperature Control

PC requires mold temperature 80–120℃. Independent cooling circuits are arranged for cavities and cores, with water channels within 12mm of molding surfaces, spaced under 20mm. Thick-wall areas add spiral baffles to balance temperature. Water pipes minimum Φ8, prioritizing Φ10/Φ12 for sufficient circulation. Hot runner systems install heat insulation sleeves and independent cooling channels to prevent PC thermal degradation. Deep slender cores use internal circulating hot water to avoid inner surface whitening.

IV. Exhaust Structure

PC decomposition produces trapped gas, requiring strict exhaust standards. Parting exhaust grooves are 0.015–0.02mm deep and 5–10mm wide, arranged every 15mm. Weld lines, rib ends and deep cavities are equipped with independent exhaust inserts with 0.015mm clearance through to atmosphere. All ejector pins retain 0.01mm unilateral exhaust gap; transparent molds add air collection troughs to prevent waste gas backflow and surface black spots.

V. Ejection & Demolding

Transparent outer surfaces need ≥3° draft angle, inner ribs ≥2°, and ≥4° for glass fiber modified PC to avoid scratch whitening. Composite ejection with large pins, sleeves and push plates prevents concentrated ejection stress and cracking. Deep undercuts use mirror-polished angle lifters with 0.008–0.012mm fitting clearance; forced demolding is prohibited to eliminate hidden internal stress. All moving ejection components achieve full mirror finish without tool marks.

VI. Machining & Mirror Polishing

Cavities reserve 0.03–0.05mm polishing allowance for low-speed CNC ball milling to reduce tool lines. Mirror EDM uses fine low-current finishing, with discharge white layers fully ground before polishing. Optical parts follow 400#→800#→1500# sandpaper polishing plus diamond paste, reaching Ra ≤0.008μm. All cavities are alcohol cleaned and dust-free wiped before assembly to avoid surface pitting.

VII. Trial Molding Acceptance

Stress whitening/cracking indicates insufficient gate size or draft angle; bubbles/scorching mean inadequate exhaust layout; warpage comes from unbalanced cooling. Qualified molds must produce 20 consecutive defect-free shots without bubbles, weld blackening or stress marks, with stable light transmittance and dimensional tolerance. Complete PC injection parameter manuals are delivered together with finished molds.

Conclusion

PC mold development centers on anti-corrosion mirror steel, low-shear gating, balanced high-temperature cooling, dense shallow exhaust, large draft smooth demolding and precision polishing, which differs fundamentally from ABS/PP mold design. This full set of standards eliminates PC typical defects including bubbles, internal stress cracking and poor transparency, stabilizing mass production quality and extending mold service life for optical, flame-retardant and glass-filled PC products.