PP (polypropylene) has homopolymerized and copolymerized types. Homopolymerized PP, with 60% - 70% crystallinity due to high molecular chain regularity, offers good rigidity and hardness, fit for auto high - strength parts and precision instrument shells. Copolymerized PP, with 40% - 50% crystallinity from disrupted chains, has excellent toughness and impact resistance, suitable for kids' toys. PP is chemically stable, resistant to acid, alkali, salt, and has a low density (0.9 - 0.91 g/cm³), meeting lightweight design needs.
When ambient humidity > 60% and raw material water content > 0.05%, injection molding problems occur, like uneven melt viscosity, surface defects, filling issues, and reduced product strength. Use hot air circulation drying at 75 - 85 °C for 2.5 - 3.5 h, turning materials every 30 min. For large - scale production, a dehumidification dryer with dew point < - 20 °C is recommended. After drying, check water content with a Carl Fischer moisture tester; it should be < 0.03% for use.
The barrel has back (160 - 180 °C for preheating), middle (185 - 205 °C for plasticizing), front (210 - 235 °C for fluidity), and nozzle (195 - 225 °C to prevent drooling) sections. For high MFR PP, lower each section's temp by 5 - 10 °C. For 1 - 2 mm thin - wall products, raise front and nozzle temp by 5 - 15 °C. For thick - wall parts (> 4 mm), lower front section temp by 3 - 8 °C. For complex molds, increase temp by 3 - 6 °C.
Conventional injection pressure is 80 - 120 MPa. For thin - wall, small - cavity or complex molds, it can reach 120 - 150 MPa. For high - filling - volume products, start with 130 - 160 MPa and reduce to 60 - 80 MPa when 80% - 90% filled. Holding pressure is 40% - 60% of injection pressure. Thin - wall parts hold for 8 - 15 s, thick - wall products for 20 - 30 s. High - crystallinity PP needs slightly higher holding pressure.
Fast injection speed causes trapped gas and internal stress; slow speed leads to insufficient filling. Recommended speeds are 60 - 100 mm/s for thin - wall products and 30 - 60 mm/s for thick - wall products. For multi - gate molds, balance and slow speed when gates open synchronously. Point - gate molds start with high speed and slow down later. Side - gate molds adjust speed according to melt flow.
Normal screw speed is 60 - 90 r/min. For high - viscosity PP, increase to 70 - 100 r/min; for low - viscosity PP, reduce to 40 - 60 r/min. Adjust speed by 5 - 10 r/min based on barrel temp and filling amount. In full - load production, reduce speed by 3 - 6 r/min.
Mold temperature is usually 30 - 50 °C. Lower temps cause rough surfaces and shrink marks; higher temps extend production cycle and make products brittle with size fluctuations. For special fine products, raise temp to 50 - 70 °C. Use an 8 - 12 mm diameter cooling water channel in the mold, spaced 30 - 50 mm apart, with external temp control (±2 °C accuracy). Preheat the mold to set temp (±3 °C deviation) at the start.
Side gates are versatile for regular - shaped products. Point gates are for thin - wall parts with high - appearance requirements but need secondary gate removal. Latent gates are concealed and suitable for auto - production lines. Side - gate width: 1.5 - 2.5 mm for 1 - 2 mm wall thickness, 2.5 - 3.5 mm for 2 - 3 mm wall thickness, 0.6 - 1.2 mm depth, 1 - 3 mm length. Point - gate diameter is 0.8 - 1.5 mm. Place the gate at thick - wall areas, away from appearance - sensitive and stress - concentration points.
The main channel is conical, 0.5 - 1 mm larger at the small end than the nozzle (usually 3 - 5 mm), 50 - 80 mm long, with an inner - wall finish of Ra0.8 - Ra1.6 μm. Shunts are often trapezoidal, 4 - 8 mm in diameter or equivalent, symmetrically arranged. Set 4 - 6 mm diameter and 3 - 5 mm deep cold - material holes at the end of main and shunt channels.
Clean the mold every 1000 - 1500 injections. First, use compressed air, then a special cleaner. Ultrasonic cleaning for stubborn stains. Apply anti - rust oil after cleaning. Regularly check cooling channels, moving parts wear, and mold closing accuracy. Nitride the mold surface regularly.
Silicone oil demolding agent, diluted to 3% - 5% with 0.1 - 0.2 g/cm² spray volume, works well but may leave oil spots. Wax - based demolding agent, heated to 60 - 70 °C and coated 0.05 - 0.1 mm thick, is safe. Use ejector rods (3 - 6 mm diameter, 30 - 50 mm spacing, 0.5 - 1.5 kN output) for high - strength products and push plates (flatness deviation < 0.05 mm, ejection speed 5 - 10 mm/s) for thin - wall, large - area products.
Manual trimming uses a 15 - 30 ° blade - angled knife. Mass production uses a punching press with 1 - 3 kN punching force and 0.05 - 0.1 mm mold clearance. Laser cutting (50 - 100 W power, 20 - 50 mm/s speed) is for fine products. For drilling in PP, use a high - speed steel drill at 1000 - 1500 r/min with 0.05 - 0.1 mm/r feed rate. Polish with 800 - 1200 mesh sandpaper or use chemical polishing. For hardening, increase hardness by 20% - 30% through methods like ion implantation.
Inspect appearance with a 20 - 50x magnifying glass. Control size tolerance within ±0.05 - ±0.2 mm with measuring tools. Test tensile and impact strength per standard. Measure thermal deformation temp at 2 - 5 °C/min heating rate. Wrap fragile products in 3 - 5 mm thick bubble bags (80% - 90% coverage), seal high - precision products in anti - static bags, and use cartons with > 2 kN box - wall strength and pearl - cotton partitions for bulk packaging.
