PET (Polyethylene Terephthalate) is a high-performance crystalline engineering plastic, featuring high mechanical strength, good fatigue resistance, high transparency, and excellent chemical corrosion resistance. It is widely used in electronic appliances, auto parts, packaging containers, and precision components. However, PET is extremely sensitive to moisture, has a narrow molding temperature range, is prone to hydrolysis and degradation, and has large shrinkage fluctuations. Improper operation easily causes defects such as silver streaks, brittleness, warpage, bubbles, and poor surface gloss, affecting product quality and increasing costs. Strict control in all links is essential for stable production of qualified PET products.
Raw Material Pretreatment: Strictly Control Moisture to Prevent DegradationRaw material pretreatment is critical to avoid hydrolysis and degradation. PET’s molecular structure contains ester bonds, which are highly sensitive to moisture. High-temperature heating of moisture-containing PET breaks ester bonds, causing molecular chain scission and defects like silver streaks and brittleness. Standard PET must be dried with a dehumidifying dryer (dew point ≥ -40°C) at 160–180°C for 4–6 hours, ensuring moisture content ≤ 0.02%. Glass fiber reinforced PET requires 180–190°C drying for 5–6 hours. Ordinary hot air dryers are prohibited. Dried materials are stored in a 120–140°C insulated hopper to avoid reabsorption. Unused opened materials must be sealed and re-dried before reuse. Recycled materials should not exceed 30% of total raw materials, with fixed crushing size and cleanliness to prevent discoloration.
Injection Temperature Control: Precise Management in a Narrow RangePET has a melting point of 250–260°C and decomposition temperature above 280°C, leaving a narrow 20–30°C molding range. Barrel temperature is set in sections: rear zone (230–240°C) for preheating, middle zone (250–260°C) for full plasticization, front zone (260–270°C) for stable melt temperature. Glass fiber reinforced PET’s temperature is increased by 10–20°C but not exceeding 280°C. Melt residence time in the barrel must be ≤ 30 minutes; long shutdowns require cooling to 200°C or emptying the barrel. Nozzle temperature matches the front zone, with fluctuation ≤ ±5°C to avoid blockage or decomposition.
| PET Material Type | Drying Temperature | Drying Time | Dew Point Requirement | Moisture Content Limit |
|---|
| Standard PET | 160–180°C | 4–6 hours | ≤ -40°C | ≤ 0.02% |
| Glass Fiber Reinforced PET | 180–190°C | 5–6 hours | ≤ -40°C | ≤ 0.02% |
Mold Temperature: Ensure Crystallization and Dimensional StabilityMold temperature affects PET’s crystallinity, surface quality, and dimensional stability. Low mold temperature causes incomplete crystallization, poor gloss, and brittleness; high temperature extends cooling time and causes demolding difficulties. Ordinary PET requires 80–120°C mold temperature, while glass fiber reinforced PET needs 100–140°C. Thick-walled products need higher mold temperature to avoid shrinkage; thin-walled products need lower temperature to shorten cooling time. Cooling channels must be evenly arranged, with cavity-core temperature difference ≤ 5°C, and scale cleaned regularly to ensure stable cooling.
Injection and Holding Pressure Process: Reduce Internal StressStable injection and holding pressure ensure color uniformity. Standard PET’s injection pressure is 80–120 MPa (100–140 MPa for reinforced grades). Medium injection speed (30–50 mm/s for standard PET, 40–60 mm/s for reinforced grades) avoids shear overheating or insufficient filling. Holding pressure is 40–60% of injection pressure, with 5–10 seconds holding time to prevent shrinkage. Excessive pressure increases internal stress and warpage; parameters must remain stable after confirmation.
Post-Processing and Equipment MaintenanceHigh-precision PET products require annealing at 120–140°C for 2–4 hours to eliminate internal stress. Glass fiber reinforced PET’s annealing time is controlled to avoid fiber exposure; transparent products need a clean annealing environment. Shutdowns require cleaning the barrel with PP/PE to prevent carbonization. Regularly inspect screws and check rings (SKD61 for reinforced PET production), clean cooling channels, and maintain dryers and feeders for stable operation.
ConclusionSuccessful PET injection molding relies on strict control and stable operation. Controlling moisture, precise temperature/pressure management, good mold/equipment maintenance, and targeted post-processing reduce defects, save costs, and improve efficiency. Strict implementation of these precautions ensures consistent production of high-quality PET products.
