HDPE Plastic Injection Molding: Complete Processing Guidelines

HDPE (High-Density Polyethylene) is one of the most widely used general thermoplastics, valued for its high toughness, corrosion resistance, low-temperature performance, and good flowability. It is commonly used in packaging containers, industrial parts, pipes, daily products, and household appliance components. Compared with LDPE, HDPE has higher crystallinity, density, and rigidity, so its injection molding requires stricter control over temperature, pressure, mold design, and processing parameters. The key principles are accurate temperature control, reasonable pressure, sufficient shrinkage compensation, thermal degradation prevention, and internal stress reduction. This article summarizes complete and practical guidelines for stable HDPE injection production.

Raw Material Preparation and Selection

HDPE is non-hygroscopic, so under normal conditions, pre-drying is unnecessary. However, if the material has been stored for a long time, exposed to moisture, or used for high-precision products, low-temperature drying at 60–80°C for 1–2 hours is recommended. Overheating must be avoided to prevent yellowing or changes in melt flow rate.

Material purity must be strictly controlled. Mixing with other plastics such as PP, ABS, or PVC must be prohibited. PVC, in particular, decomposes at high temperatures and releases corrosive gas that damages the barrel and screw and causes black spots, brittleness, and odor in finished parts.

Different HDPE grades have distinct melt flow rates (MFR). Low MFR grades (2–8 g/10 min) provide high rigidity and strength, suitable for large thick-walled parts. Medium to high MFR grades (8–20 g/10 min) offer better flow and are ideal for thin-walled or complex components. Correct grade selection directly reduces defects such as short shots, sink marks, and warpage.

Equipment and Temperature Control

HDPE melts at approximately 130°C and is typically processed between 180°C and 240°C. The temperature must not exceed 270°C, as thermal degradation accelerates significantly above this level, resulting in yellowing, silver streaks, and odor.

Barrel temperatures should be set in a gradient: rear zone 170–190°C, middle zone 190–220°C, front zone 200–230°C. The nozzle temperature should be 5–10°C lower than the front zone. An open-type large-bore nozzle is recommended to prevent material stagnation and degradation.

The screw should be a gradual-type with an L/D ratio of 20:1 to 25:1 and a compression ratio of 3:1 to 4:1 to ensure stable plasticization without excessive shear heat.

Mold Design and Temperature Control

As a crystalline plastic, HDPE has a high shrinkage rate of 1.0%–2.5%, usually averaged at 1.5%–2.0%. Mold cavities must be sized with proper shrinkage compensation. Thick-walled sections require higher shrinkage compensation.

Wall thickness should be uniform with rounded corners to avoid stress concentration and sink marks. Draft angle should be at least 1°–2°, and 2°–3° for deep cavities. Mold temperature is controlled at 40–60°C for general parts, 50–70°C for thin-walled or high-quality surface parts, and 40–50°C for thick-walled parts to balance cooling efficiency and dimensional stability.

Adequate venting (0.01–0.02 mm) is essential at flow ends and deep sections to prevent burns, bubbles, and short shots. Gates should be designed for balanced flow; side gates, fan gates, or submarine gates are commonly used to ensure smooth filling.

Injection Molding Parameters

HDPE processing follows the principles of medium-low pressure, medium-low speed, sufficient holding pressure, and adequate cooling.

Injection pressure ranges from 50 to 100 MPa, and up to 120–140 MPa for thin or long-flow parts. Velocity should be controlled stepwise: slow at the start to prevent jetting, moderate in the middle, and slow at the end to reduce impact.

Holding pressure should be 50%–70% of injection pressure and maintained until the gate freezes to compensate for volumetric shrinkage. Backpressure of 5–20 MPa improves homogenization and degassing. Screw speed is kept at 30–60 rpm to avoid over-shearing.

Cooling time depends on part thickness: approximately 10 seconds per 1 mm of wall thickness. Parts must be fully cooled before ejection to avoid deformation.

Defect Control and Post-Processing

Common defects include warpage, sink marks, bubbles, silver streaks, flash, and sticking. Most can be solved by adjusting temperature, pressure, cooling, venting, or mold structure.

Before shutdown, the barrel must be purged with PE or PP to prevent material degradation. Stress-sensitive parts can be annealed at 60–80°C for 1–4 hours to improve resistance to environmental stress cracking.

Strict raw material control, stable equipment, standardized parameters, proper mold design, and trained operators together ensure low reject rates and consistent quality in HDPE injection molding.