PS Plastic Injection Molding Guidelines

PS (Polystyrene), including GPPS and HIPS, is a non-crystalline, brittle and highly flowable general plastic with good transparency, ease of coloring and low cost. It is widely used in packaging, stationery, household appliances, disposable products and optical components. However, PS is sensitive to internal stress, prone to cracking, warpage, flow marks and brittleness. Proper processing control is critical to ensure quality and yield.

Raw Material Drying and Handling

Although PS has low moisture absorption, trace moisture and dust can cause silver streaks, bubbles and haze, especially in transparent GPPS. Materials must be dried at 70–80°C for 2–3 hours before processing. Excessively high drying temperature causes yellowing and degradation. Closed hoppers prevent re-absorption. Recycled material ratio should be controlled within 10%–20%, and lower for transparent products. Prolonged residence time in the barrel must be avoided.

Barrel Temperature Control

PS has wide processing temperature range but is sensitive to overheating. Barrel temperatures are set in a gradient: rear 160–180°C, middle 180–200°C, front 200–220°C, nozzle 190–210°C. GPPS requires slightly higher temperatures, while HIPS can be 5–10°C lower. Temperature exceeding 240°C causes thermal degradation, odor, brittleness and discoloration. Nozzle leakage and stringing must be prevented by proper temperature control.

Injection Speed and Pressure

High flowability means excessive speed causes jetting, gas traps, burn marks, flash and high internal stress. Multi-stage medium-low speed is recommended: slow start for stable feeding, medium speed for main filling, slow end for improved venting. Injection pressure should be moderate, only sufficient to fill the cavity. Excessively high pressure dramatically increases internal stress leading to cracking. Holding pressure is set at 30%–50% of injection pressure, with transfer at 95%–98% filling.

Mold Temperature and Cooling

Mold temperature significantly affects surface finish, transparency, internal stress and warpage. For GPPS transparent parts, 40–60°C improves gloss and reduces stress. For HIPS, 30–50°C is sufficient. Uniform cooling is essential to avoid warpage. Cooling time is determined by wall thickness to ensure stable ejection without deformation. Insufficient cooling causes shrinkage and stress; excessive cooling reduces efficiency.

Mold Design Requirements

Gates should be appropriately sized to avoid excessive shear. Preferred gate types include side gate, pin-point gate and submarine gate. Cavity surfaces must be well polished, especially for transparent parts. Adequate venting is critical: vent grooves of 0.015–0.025 mm depth at flow ends and weld line areas prevent burns and bubbles. Balanced ejection with sufficient ejector pins or plates prevents white marks and cracking. Sharp corners must be rounded to minimize stress concentration.

Internal Stress Relief and Anti-Cracking Measures

High internal stress is the main cause of cracking in PS products. Measures include lower injection and holding pressure, moderate speed, increased mold temperature, sufficient cooling and avoiding over-packing. Annealing at 60–70°C for 2–4 hours effectively relieves residual stress and significantly reduces cracking in critical parts.

Troubleshooting Common Defects

Silver streaks and bubbles indicate moisture or overheating: improve drying and lower temperature. Cracking and white marks mean high stress: reduce pressure and speed, increase mold temperature. Visible weld lines require better venting and moderate speed increase. Flash indicates high temperature, speed or insufficient clamping force. Warpage requires balanced cooling and optimized gate location.

Summary

PS injection molding focuses on temperature control, speed management, pressure adjustment and internal stress reduction. By standardizing drying, temperature, multi-stage injection, mold cooling, venting and ejection, and applying stress relief treatment when necessary, manufacturers can achieve stable appearance, dimensional accuracy and low cracking rate, ensuring efficient and high-quality mass production.