Precautions for GPPS Plastic Injection Molding
2026-05-09 14:53:13
Plastic Injection Molding
GPPS, general-purpose polystyrene, is widely used in transparent shell, lighting accessories, daily necessities and packaging parts due to its high transparency, good fluidity, easy forming and low cost. However, it has inherent characteristics of brittleness, easy cracking, internal stress whitening and sensitivity to high temperature and humidity. The whole process including raw material drying, machine parameters, mold structure, forming technology and post-treatment needs to be strictly controlled to avoid forming defects and product failure.
Raw Material Drying and Storage Requirements
GPPS has weak hygroscopicity, but surface adsorbed moisture and piled damp materials will cause silver lines, bubbles and reduced transparency during injection molding. Conventional new materials are dried with hot air at 60-70℃ for 1-2 hours. Recycled materials and severely damp raw materials need drying at 70-75℃ for 2-3 hours. The temperature shall not exceed 80℃, otherwise the material will turn yellow and become brittle after thermal degradation. Raw materials shall be sealed and moisture-proof in storage to avoid direct contact with workshop humidity. Crushed recycled materials need dust filtration and impurity separation. Excessive dust will cause black spots and pits, and induce stress concentration and cracking of products.
Injection Temperature Control Points
GPPS has a wide forming temperature range requiring stable temperature control. The barrel temperature is set at 160-180℃ for rear section, 180-200℃ for middle section and 190-210℃ for front section, with nozzle temperature 185-200℃. Too low temperature will lead to poor fluidity, insufficient filling and obvious texture, increasing injection pressure and leaving large internal stress to make products brittle. Excessively high temperature causes thermal degradation, yellowing and reduced transparency, and the toughness decreases significantly, resulting in edge chipping and cracking during demoulding and assembly. High-gloss transparent products adopt the lower limit of temperature for stable plasticization to avoid overheating degradation.
Mold Temperature and Cooling ControlThe recommended mold temperature is 30-50℃. Too low mold temperature leads to rapid cooling and rapid freezing of molecular chains, leaving large residual internal stress, making products easy to crack under stress, bending and bonding in later stage. Too high mold temperature extends cooling cycle, causing product shrinkage, deformation and mold adhesion. The cavity of transparent GPPS mold needs high-gloss mirror polishing with uniform water channel layout to avoid stress difference caused by local uneven cold and heat. Extend cooling time appropriately at gate and thick wall positions, and avoid mold opening and ejection prematurely to prevent product whitening, deformation and cracks.
Injection Pressure, Speed and Holding Pressure SettingGPPS has good fluidity without high pressure and high speed filling. Medium and low speed plus medium and low pressure are adopted for injection. Excessively fast injection speed is easy to produce jet lines, air lines and welding lines, and aggravate internal stress. Stable low-speed filling improves appearance uniformity and transparency. Holding pressure shall not be too high or last too long. Excessive holding pressure will cause material accumulation and huge internal stress, leading to cracking under ambient temperature change and assembly force. The holding pressure shall only eliminate shrinkage, and segmented gradual pressure reduction is more stable.
Mold Structure and Gate Design RequirementsGPPS products avoid sharp corners, thin ribs and sudden wall thickness change. Transition fillets shall be designed at right-angle positions to eliminate stress concentration. The wall thickness should be as uniform as possible, and excessive thickness difference will cause shrinkage and internal stress cracking. Side gate, horn gate and point gate are preferred, arranged away from stressed assembly surfaces and transparent visible surfaces. The gate section shall not be too small, otherwise shear overheating will occur to cause yellowing and stress marks, and increase internal stress. Reasonably arrange parting surface and exhaust grooves to avoid burning marks and air lines damaging the transparency of transparent parts.
Demoulding and Ejection Technology
GPPS has low toughness and is easy to crack. Unbalanced ejection and sparse ejector pin layout will cause obvious whitening, cracking and edge chipping. Ejector pins shall be arranged evenly to increase ejection stress area. The demoulding draft angle is appropriately enlarged, not less than 1.5° for high-gloss transparent parts to reduce mold clamping force. The mold surface shall be finely polished to prevent product scratching. Forbid forced demoulding and violent part taking, adopt slow and stable ejection, and handle finished products gently to avoid invisible cracks caused by collision.
Recycled Material and Color Matching Specification
GPPS can be mixed with an appropriate proportion of recycled materials, which is recommended to be controlled within 20%. Excessively high proportion will reduce toughness, increase yellowing and impurities, and raise cracking probability. Recycled materials must be screened, iron removed, dedusted and fully dried without mixing other miscellaneous plastics. High-temperature resistant color masterbatch with good dispersion is selected for color matching to avoid color spots and flow lines. Transparent products prioritize pure new materials without or with a small amount of recycled materials to ensure light transmittance and appearance purity.
Post-Treatment and Storage EnvironmentFreshly injected GPPS products have large residual internal stress, and bending, drilling, bonding and assembly operations shall not be carried out immediately. Placing at room temperature for 24 hours is conducive to stress release. Annealing at 60-70℃ for 1-2 hours can effectively eliminate internal stress and reduce later cracking risk. Finished products shall be stored away from strong sunlight, alternating high and low temperature and humid closed environment, and kept away from organic solvents and glue corrosion. Contact with alcohol and paint thinner will quickly cause surface whitening and cracking.
Common Defects and CountermeasuresSilver lines and bubbles are solved by strengthening drying, reducing barrel front temperature, slowing down injection speed and optimizing exhaust. Product brittleness and cracking are improved by lowering injection pressure and holding pressure, increasing mold temperature, enlarging fillet and draft angle, and annealing stress relief. Yellowing and poor light transmittance need properly reduced barrel temperature, decreased recycled material proportion and barrel carbon cleaning. Shrinkage depression is optimized by adjusting wall thickness, reasonable holding pressure and extended cooling time. Ejection whitening and cracking are solved by increasing ejector pins, enlarging draft angle, polishing cavity and slowing down ejection speed.
