Manufacturing Process and Precision Control of POM Injection Molds

1. Core Mold Requirements Based on POM Properties

POM (polyoxymethylene), divided into homopolymer (POM-H) and copolymer (POM-K), is a high-density crystalline engineering plastic with excellent wear resistance and mechanical strength. However, it has poor thermal stability and is sensitive to copper—copper catalyzes its degradation, so copper materials must be completely avoided in mold parts contacting the melt. Its melt viscosity drops sharply with temperature, and it has high post-molding shrinkage, requiring molds to have precise temperature control, low retention risk, and adaptive venting design.

2. Key Manufacturing Processes of POM Injection Molds

2.1 Mold Material and Pretreatment

Cavities and cores prioritize pre-hardened alloy steel (e.g., 718H) with hardness HRC 32-38, balancing wear resistance and processing stability. Due to POM’s sensitivity to metal ions, all molding surfaces undergo passivation to prevent oxide layer detachment and material degradation. Guide pillars/bushings use SUS440C stainless steel instead of copper alloys to eliminate degradation risks.

2.2 Mold Structure Design Specifications

Gating System: Main runner diameter is 3-6mm. Gate size matches product wall thickness—round gate diameter ≥ 50% of wall thickness; rectangular gate width ≥ 2× wall thickness, depth = 60% of wall thickness. Gate length ≤ 0.5mm to reduce melt retention.

Temperature Control System: Zoned heating/cooling maintains mold temperature at 80-90℃ (temperature difference ≤ ±2℃). Cooling channels are spaced 15-20mm, ≥10mm from cavity surface for uniform crystallization.

Venting System: Vent thickness is 0.01-0.02mm (POM-H) and 0.04mm (POM-K), both 3mm wide, densely arranged at last-fill areas to avoid gas trapping.

2.3 Precision Machining and Surface Treatment

Cavity surfaces are processed by high-speed milling and EDM, with roughness Ra ≤ 0.4μm. Coordinate measuring machines ensure positioning accuracy ±0.002mm. For paint-ready products, plasma pretreatment improves surface energy for coating adhesion.

3. Key Precision Control Technologies

3.1 Molding Parameter Optimization

Temperature Control: Melt temperature is 215℃ (POM-H) and 205℃ (POM-K), with barrel temperature gradient 10-15℃. Melt retention time is strictly controlled; barrels are cooled or cleaned with PE during shutdown.

Injection & Pressure Parameters: Medium-to-high injection speed is used, with back pressure ≤ 200bar. Holding pressure is 60-70% of injection pressure, with time adjusted by product thickness.

3.2 Dimensional Accuracy & Deformation Control

0.8-1.5% shrinkage compensation is reserved in mold design. Sequential valve gating balances filling. Draft angle is 40′-1°30′ to avoid demolding damage. High-precision parts undergo annealing to reduce residual stress and prevent cracking.

3.3 Inspection & Maintenance System

Laser scanners inspect cavities (repeatability ±0.001mm). Guide pillar clearance and cavity surface are rechecked every 5,000 cycles; wear exceeding limits triggers repair. Molds are coated with anti-rust agent during storage.

4. Application Verification

In automotive gear molds, 718H steel and zoned temperature control achieve 800,000-cycle life. Products meet IT7 tolerance with Ra 0.2μm. Optimized venting significantly reduces gas trapping defects, verifying process reliability.