HDPE high-density polyethylene is a crystalline polyolefin plastic with good melt fluidity, large shrinkage rate, general rigidity and excellent low temperature resistance. However, it is prone to surface shrinkage depression, warpage deformation, dimensional instability, mold sticking, flash and difficult cooling cycle control during molding. Due to its fast crystallization rate, obvious thermal shrinkage and high sensitivity to mold temperature and process, injection molding processing must be standardized and controlled from raw material drying, machine parameters, mold design, molding process and post-treatment to produce qualified products stably.
1. Raw Material Drying and Incoming Material ControlHDPE itself has extremely low hygroscopicity with negligible moisture content under conventional storage environment, and generally does not need long-time drying. Simple air drying is only required when raw materials are damp, mixed with regrind materials or stored in rainy days. Forced high-temperature and long-time drying will easily cause material oxidation and yellowing, molecular degradation, and adversely affect toughness and appearance instead. New materials can be directly put into production. The proportion of regrind materials is recommended to be controlled within 30%. Regrind materials need to be screened for impurities and crushed evenly to avoid appearance defects such as black spots, silver streaks and impurity spots. Meanwhile, keep consistent melt flow rate to prevent dimensional fluctuation caused by batch differences.
2. Key Points of Injection Temperature Setting
HDPE has a clear melting point range. The barrel temperature should be set reasonably in sections. The feeding section temperature should not be too high to prevent premature melting, slipping and bridging. The middle and rear sections are controlled stably according to material grade with the overall barrel temperature range of 180～220℃, no need for excessive heating. Excessively high temperature will cause melt decomposition, product brittleness and surface yellowing; too low temperature leads to poor plasticization, obvious weld lines and reduced strength. The nozzle temperature is slightly lower than the front section of the barrel to prevent material flow and drooling in the runner, avoiding cold material spots and poor bonding caused by cold material entering the cavity.
3. Mold Temperature Control and Cooling RequirementsHDPE has strong crystallization characteristics, and mold temperature directly affects crystallinity, shrinkage rate and finished product rigidity. The conventional production mold temperature is maintained at 30～50℃. Higher mold temperature makes crystallization more sufficient, improved product hardness but increased shrinkage and aggravated shrinkage and warpage; lower mold temperature leads to incomplete crystallization, softer products, larger size and higher internal stress. The mold cooling water channel should be arranged evenly to ensure synchronous overall cooling of products, strengthen cooling focusing on thick-wall parts, and reduce distortion, deformation and surface shrinkage pits caused by inconsistent cooling speed. The cooling time is reasonably matched according to wall thickness. The larger the wall thickness, the longer the required shaping cooling time. It is not allowed to blindly shorten the cycle for forced demolding.
4. Control of Injection Speed and PressureHDPE melt has good fluidity and is forbidden to be molded with high speed and high pressure. Excessively fast flow velocity is easy to produce jet lines, flow marks, air entrapment and scorching, and also easy to generate flash and burrs. Medium and low speed stable feeding is adopted for filling, uniform filling for thin-wall parts, and slow-fast-slow segmented injection speed for thick-wall parts: slow speed in the early stage for exhaust, stable filling in the middle stage, and slow down in the final stage for glue sealing to prevent air wrapped in the cavity from forming voids. The holding pressure should not be too large, just enough for shrinkage compensation. Excessively high holding pressure will increase internal stress and later shrinkage deformation; the holding time matches the gate freezing speed, and pressure relief should be carried out in time after the gate is closed to avoid dimensional out-of-tolerance and demolding difficulty caused by pressure holding. The screw back pressure is set at a low level to ensure uniform plasticization and stable mixing. Excessively high back pressure causes shear overheating and material degradation and discoloration.
5. Mold Design and Exhaust PrecautionsHDPE has a large shrinkage rate, so sufficient shrinkage allowance must be reserved in the early stage of mold design, otherwise the overall size will be too small for assembly after mass production. The gating system preferentially adopts large runner and moderate gate to ensure smooth feeding and timely shrinkage compensation. The gate is arranged at thick-wall or symmetrical positions to reduce uneven shrinkage and warpage. Exhaust grooves should be reasonably arranged at parting surface, dead corners and runner ends. HDPE fills the mold fast and easily wraps gas. Poor exhaust will cause scorching, bubbles and low weld line strength. The mold demolding draft angle should be appropriately enlarged with smooth surface polishing to prevent HDPE products from sticking to the mold, pulling and whitening during ejection due to high toughness. Ejector pins are arranged evenly to avoid local ejection deformation.
6. Common Molding Defects and Avoidance
HDPE is most prone to shrinkage, warpage, flash, weld lines and demolding pulling during production. Shrinkage is mostly caused by thick-wall material accumulation, insufficient holding pressure and insufficient cooling, which can be improved by appropriately extending holding time, reducing mold temperature and optimizing local cooling. Warpage results from uneven shrinkage and large temperature difference between front and rear molds, which can be corrected by balanced water channels, symmetrical feeding and slowing down cooling rhythm. Flash is mostly caused by excessive injection speed, insufficient mold locking force and poor fit of mold parting surface, which can be solved by reducing speed appropriately, increasing mold locking force and trimming mold parting surface. Avoid large-area flat plate structure without reinforcing ribs as much as possible, and use ribs to balance shrinkage and reduce later natural deformation.
7. Post-molding Treatment and StorageHDPE injection parts will still have slow secondary crystallization and shrinkage after demolding, and the size will change slightly within 24 hours. Precision assembly parts need to be placed at room temperature for static setting before dimensional inspection. Finished products should be placed flat during storage, avoiding heavy pressure, sun exposure and sudden high and low temperature changes to prevent accelerated crystallization causing deformation and bending. For secondary processing and assembly, forced bending should be avoided, and appropriate matching should be made by using the material's own toughness to prevent stress cracking under low temperature environment.
8. SummaryThe core key points of HDPE injection molding are avoiding excessive drying due to low moisture absorption, moderate barrel temperature, low and stable mold temperature, medium-low speed and low-pressure molding, mold design with reasonable shrinkage rate and sufficient exhaust. Relying on the characteristics of strong crystallization shrinkage and good fluidity of materials, multi-dimensional cooperation from raw materials, temperature, mold temperature, process and mold can not only avoid common defects such as shrinkage, warpage, flash and mold sticking, but also stabilize product size, improve appearance quality and structural strength, meeting the mass production requirements of various HDPE injection products such as civil use, chemical industry, packaging and daily necessities.
