Low-density polyethylene (LDPE) is soft, tough, with acceptable transparency and excellent chemical stability. It has a low melting point and outstanding melt fluidity, commonly used for injection molding of daily sundries, packaging accessories, hose connectors, thin-wall small parts and sealing gaskets. However, it has a large shrinkage rate, easy warpage, low heat resistance, and is prone to flash and sink marks during molding. Processing requires standardized control from raw materials, temperature, speed and pressure, molds and post-treatment.
1. Raw Material Drying and Storage ControlLDPE has extremely low hygroscopicity and does not require long-term drying under conventional conditions. Only when damp, mixed with recycled materials or stored in humid environment, drying at 60 to 70 degrees Celsius for 1 to 2 hours can remove surface moisture. Raw materials shall be sealed and stacked to avoid mixing with dust, impurities and other plastic materials. Mixing with foreign materials such as ABS, PP and PVC is strictly prohibited to prevent color mixing, silver streaks, brittleness and poor welding. The proportion of recycled materials is recommended to be controlled within 30%. Repeated recycling will lead to decreased toughness and melt fluidity, requiring appropriate adjustment of process parameters.
2. Barrel and Nozzle Temperature Setting
LDPE has a narrow melting temperature range with excessive temperature easily causing material decomposition, product yellowing, surface stickiness and bubbles; insufficient temperature leads to poor plasticization, pattern defects, incomplete filling and dull surface. The conventional temperature range is set as rear section 160 to 180 degrees Celsius, middle section 170 to 190 degrees Celsius, front section 180 to 200 degrees Celsius. Nozzle temperature is slightly lower than the front section to prevent material drooling. Use pure LDPE for machine purging during material replacement to avoid carbon deposits and black spots caused by high-temperature retention.
3. Injection Pressure, Speed and Holding Pressure ControlLDPE melt has good fluidity without high pressure and high speed molding. Set injection pressure at a low level and adopt medium and low speed for smooth filling to prevent jet lines, flow marks and flash caused by excessive flow speed. Appropriately accelerate injection speed for thin-wall small parts and slow down for thick-wall large parts to ensure stable laminar flow filling. Set low holding pressure and short holding time. Excessively high and long holding pressure will cause internal stress, demolding deformation and ejector pin whitening; insufficient holding pressure leads to sink marks at thick-wall positions, requiring fine adjustment according to product wall thickness. Set low back pressure only to ensure uniform plasticization. Excessive back pressure will increase shear heating, resulting in material degradation and increased flash.
4. Mold Temperature and Cooling Key PointsConventional mold temperature is controlled at 20 to 40 degrees Celsius, and normal temperature water cooling can meet molding requirements. High mold temperature slows down product cooling, increases shrinkage and aggravates warpage deformation; low mold temperature leads to rapid surface solidification, uneven surface gloss and excessive internal stress. Arrange cooling water channels evenly with separate temperature control for thick and thin areas, strengthen cooling at thick ribs and columns to reduce surface depression and bending distortion after molding. Precisely control clearance of parting surfaces and inserts. LDPE has excellent fluidity and slight clearance will easily cause flash. Regularly fit parting surfaces and clean sundries in exhaust grooves.
5. Mold Structure, Exhaust and Gate Design
Point gate, side gate and submarine gate are optional with moderate size to avoid shear heating causing flow marks and edge scorching. Adopt multi-point gating for large-area flat products to balance flow path and reduce warpage. Set reasonable exhaust grooves with small depth adapting to low viscosity characteristics of LDPE to prevent flash caused by excessive exhaust depth. Appropriately increase demolding draft angle and conduct smooth polishing on product surfaces. LDPE is soft and easy to cause ejector pin whitening and pulling damage. Optimize ejector pin layout with uniform ejection force to avoid deformation indentation caused by single-point forced ejection.
6. Common Molding Defects and AvoidanceCommon production problems include flash, sink marks, warpage deformation, flow marks and ejector pin whitening. Flash is mainly caused by excessive temperature, fast injection speed and large mold surface clearance, which can be improved by lowering temperature and speed and fitting molds. Sink marks result from thick material accumulation and insufficient holding pressure, which can be solved by appropriately extending holding time and optimizing product wall thickness structure. Warpage is mainly caused by uneven cooling and unbalanced gating, which can be corrected by adjusting mold temperature, optimizing water channels and changing gate position. Flow marks and jet lines can be relieved by slowing down injection speed and raising mold temperature. Pulling damage and whitening can be solved by increasing draft angle, polishing cavities and uniform ejection.
7. Product Post-treatment and Molding EnvironmentLDPE molding produces little internal stress without annealing treatment generally. Soft products just ejected can be placed for static shaping and natural cooling to avoid permanent deformation caused by stacking and extrusion. Keep production environment clean to prevent dust adhering to product surfaces and affecting appearance. Regularly clean machine hopper and barrel to prevent carbon deposits and burnt materials mixing into products and forming black spot impurities for stable long-term production quality.
