MDPE (Medium Density Polyethylene) offers balanced rigidity, toughness and chemical resistance, with properties between LDPE and HDPE. Due to its high crystallinity and significant shrinkage, injection molding requires precise control over temperature, pressure, cooling and mold design to avoid warpage, shrinkage marks, poor appearance and dimensional instability. The following key points provide practical guidance for stable production.
1. Raw Material PreparationMDPE has extremely low moisture absorption, normally below 0.01%, so drying is usually unnecessary. However, if the material has been exposed to moisture or stored improperly, it should be dried at 60–80°C for 1–2 hours to prevent silver streaks and bubbles. Avoid mixing MDPE with other polyolefins such as LDPE, HDPE or PP, as mismatched melting and shrinkage properties can cause delamination, poor surface quality and unstable dimensions. Thoroughly clean the barrel before production to eliminate residual materials.
2. Barrel and Nozzle TemperatureThe melting point of MDPE is around 125–135°C, with an ideal processing range of 170–210°C. Temperature should be set in a gradient from rear to front: rear section 160–175°C, middle section 175–195°C, front section 190–210°C. The nozzle temperature should be 5–10°C lower to prevent drooling and cold slugs.
Temperatures below 170°C lead to insufficient plasticization, poor filling and low strength. Temperatures above 220°C may cause thermal degradation, yellowing, brittleness and excessive flash. Stable temperature control is essential for consistent melt quality.
3. Mold Temperature and CoolingRecommended mold temperature is 40–60°C. For thin-walled parts, 40–50°C speeds up cooling and shortens the cycle. For thick-walled or structural parts, 50–60°C improves crystallization, reduces internal stress and enhances dimensional stability. An evenly arranged cooling system minimizes temperature differences across the cavity.
Low mold temperature causes high internal stress, warpage and cold marks. Excessively high mold temperature extends cooling time, causes sticking and increases shrinkage. Uniform cooling directly improves part stability and appearance.
4. Injection Pressure, Speed and Holding PressureMDPE flows well, so injection pressure can be moderate: 50–80 MPa for standard parts, 70–100 MPa for thin or long-flow parts, and 40–60 MPa for thick simple parts. Injection speed should be medium to low to avoid jetting, turbulence and trapped gas.
Holding pressure is critical for MDPE’s high shrinkage, usually 50–70% of injection pressure, with a holding time of 5–20 seconds until the gate solidifies. Adequate holding reduces shrinkage marks and dents, while insufficient holding leads to obvious sinking and unstable dimensions.
5. Back Pressure, Screw Speed and Mold DesignBack pressure of 5–15 MPa ensures uniform plasticization and proper degassing. Screw speed is set at 60–100 rpm to avoid excessive shear heat.
MDPE shrinkage is 1.8–2.5%, so mold dimensions must include sufficient compensation. A draft angle of 1°–2° prevents sticking and stress marks. Effective venting and balanced ejection are required to avoid burns, voids and surface damage. Gates should be designed to reduce sudden flow changes and internal stress.
6. Cooling, Post-processing and Defect ControlCooling time accounts for 60–70% of the cycle: 10–15s for 1–2mm wall thickness, 15–25s for 2–4mm, and longer for thicker sections. Insufficient cooling causes warpage and sticking. High-precision parts may be annealed at 70–85°C for 1–4 hours to relieve stress.
Shrinkage marks are improved by increasing holding pressure and cooling time. Warpage results from uneven cooling and can be fixed by balancing cooling channels and optimizing flow parameters. Bubbles and streaks usually relate to moisture or overheating. Sticking issues are solved by longer cooling, increased draft and polished cavities.
7. Startup, Shutdown and MaintenanceClean the barrel with PE material before starting. For short stops, lower the temperature to around 150°C. For long downtime, purge the barrel with LDPE to prevent degradation. Apply rust protection to the mold and keep cooling channels clean and dry.
In short, stable MDPE injection molding relies on reasonable temperature settings, proper pressure matching, sufficient cooling and scientific mold design. Strict parameter control and routine maintenance effectively reduce defects and support efficient, consistent production.
