Material Properties and Process Logic

Extremely low moisture absorption: PE is a non-polar material, with a water absorption rate that is almost negligible (<0.01%), usually requiring no pre-drying treatment, which is a significant processing convenience.

Excellent thermal stability and narrow processing window: PE has good thermal stability under inert atmospheres, but prolonged heating in air can cause thermo-oxidative degradation, resulting in chain breakage, yellowing, and performance decline. Its melt temperature range is relatively narrow and requires precise control.

Melt viscosity is highly sensitive to shear: PE is a typical "shear-thinning" material, with apparent viscosity decreasing sharply as shear rate increases. This means injection speed is the most powerful way to control its flowability, far more effective than simply increasing temperature.

Density and performance gradient: From LDPE to HDPE, as density increases, stiffness, heat resistance, and barrier properties improve, but impact toughness (especially low-temperature toughness) and transparency decrease. Different density PEs require differentiated processing strategies.

Detailed Key Points of Injection Molding Process

1. Preprocessing: General drying rules: No drying required. Under conventional storage and production conditions, the material can be used directly. Exceptions: Only when the pellet packaging is seriously damaged and has been exposed for a long time to extremely humid environments, where the surface may absorb moisture, it is advisable to dry in a hot air circulating oven at 60-80℃ for 1-2 hours as a precaution.

2. Molding temperature should be precisely set according to the type of PE. The overall principle is to use the lowest temperature possible while ensuring proper filling. Barrel temperature settings: LDPE: The melt temperature range is relatively wide; 160-220℃ is recommended. Lower temperatures can be used to achieve more stable processing. HDPE: Due to a higher crystalline melting point (about 130℃), higher processing temperatures are needed, recommended 180-240℃. Excessively high temperatures can cause degradation and poor surface gloss. Zoning strategy: It is recommended to use a 'low front, high rear' or 'flat' temperature setting to avoid large front-to-rear temperature differences. For example, for HDPE: rear zone 170-190℃, middle zone 190-220℃, front zone 200-230℃. Critical warning: Do not exceed 300℃. PE rapidly degrades above 270℃. The nozzle temperature should be 5-10℃ lower than the front zone to prevent drooling.

3. Mold temperature: Mold temperature is a key parameter for controlling PE crystallization rate and crystallinity, thereby determining product shrinkage, dimensional stability, and surface gloss. Range: 30-70℃. Specific settings should meet the core product requirements. Low mold temperature (30-50℃) strategy: rapid cooling, lower crystallinity. Advantages: short production cycle, better flexibility of the product, suitable for thin-walled daily-use items with low dimensional accuracy requirements. Disadvantages: higher and uneven shrinkage, poor surface gloss, and prone to warping. High mold temperature (50-70℃) strategy: slow cooling, promoting complete crystallization. This is essential for achieving high dimensional accuracy, high surface gloss, low warping, and stable shrinkage products. High mold temperature allows sufficient time for molecular alignment, resulting in more uniform shrinkage, lower internal stress, and effectively reducing sink marks. For HDPE products, a mold temperature of 50-70℃ is recommended. Cooling system: Must be uniform and efficient to ensure stable mold temperature.

4. Injection pressure and speed Injection speed: medium to high speed injection should be used. This is the core feature of the PE process. Utilizing its strong shear thinning effect, high-speed injection can significantly reduce melt viscosity, achieve fast and smooth mold filling, and obtain excellent fusion line strength and surface quality. Injection pressure: The pressure required is moderate, generally 40-80 MPa. Due to its good flowability, extremely high injection pressures are usually not required. Holding pressure and time: crucial. To compensate for the large crystallization shrinkage of PE, a high holding pressure (about 60%-80% of the injection pressure) and a sufficiently long holding time must be applied. Insufficient packing pressure is the direct cause of serious sink marks, voids and dimensional instability of PE products. The holding pressure should continue until the gate is frozen. 5. Back pressure and screw speed Screw speed: medium to high speed can be used, recommended at 50-100 rpm. The right rotational speed helps to plasticize evenly. Back pressure: A lower back pressure (about 5-15 bar) needs to be used. Excessive back pressure increases shear heat, which can lead to high melt temperature and degradation, and does not help improve the plasticization quality of PE. 6. Mold and auxiliary design exhaust: Good exhaust is required. Although PE itself is not easy to produce decomposition gas, high-speed injection is easy to trap gas. The exhaust groove depth should be 0.02-0.03 mm. Gating system: The runner and gate should be designed short and thick to reduce pressure loss. The gate size should be large enough to facilitate pressure retention and contraction. For large parts, multi-point gates help reduce flow distance and orientation. Cooling system: The design must be reasonable to ensure uniform cooling, which is the cornerstone of controlling warpage deformation. Release slope: Due to the flexibility of PE, the release slope can be appropriately reduced, but it is still necessary to ensure a ≥ of 1° for deep cavity products.

Common Defects and Countermeasures

Sink Marks and Voids: The primary cause is insufficient holding pressure or too short holding time. Holding pressure and holding time should be significantly increased. Another cause is too low mold temperature, which leads to premature surface solidification and hinders material compensation; mold temperature should be appropriately increased. Optimize gate location and size (thicker, closer to thick walls).

Warping and Deformation: Mainly caused by uneven cooling and anisotropic shrinkage during crystallization. The core solution is to raise and evenly distribute the mold temperature. Optimize the cooling channel design to ensure consistent cooling rates across all parts of the product. Adjust gate locations to change flow direction and reduce orientation differences.

Poor Weld Line Strength: Increase melt temperature and mold temperature; significantly increase injection speed so that the melt fronts remain hot when they converge; add vent grooves at weld line locations.

Uneven Surface Gloss or Flow Marks: Increase mold and melt temperatures; optimize the injection speed profile to avoid stagnation during filling; ensure consistent surface finish in the mold cavity.

Degradation (Yellowing, Black Spots): Check if barrel temperature is set too high, especially the nozzle and front zone; check if material residence time in the barrel is too long (e.g., due to slow cycle or oversized equipment); clean the barrel and screw.

HDPE and LDPE Processing Differences

Processing Temperature: HDPE generally requires temperatures about 20-40°C higher than LDPE. Mold Temperature and Shrinkage: HDPE has higher crystallinity and greater shrinkage (especially flow-induced shrinkage), requiring higher mold temperatures to stabilize dimensions. LDPE has higher but more uniform shrinkage. Flowability: At the same temperature, LDPE flows better than HDPE, giving LDPE an advantage in molding thin-walled or complex products. Product Rigidity: HDPE products are rigid and not easily deformed after demolding; LDPE products are soft, requiring care to prevent stretching deformation during ejection.