Optimizing POM Injection Molding Properties and Process Insights

From precision gears under car hoods to safety-critical medical device components and food processing equipment that comes in direct contact with consumables, modern manufacturing demands materials with exceptional strength, wear resistance, and corrosion protection. Polyoxymethylene (POM), an engineering plastic, has emerged as the material of choice for these demanding applications. This article provides a technical deep dive into POM injection molding, covering material properties, design guidelines, and processing parameters.

Polyoxymethylene, commonly known as POM or acetal, is a high-performance thermoplastic widely used in applications requiring strength, rigidity, wear resistance, and chemical stability. POM comes in two primary formulations: homopolymers and copolymers.

Homopolymer POM (such as DuPont's Delrin®) features a highly ordered crystalline structure, delivering superior strength but requiring more precise temperature control during processing. Copolymer POM offers wider processing temperature ranges but with slightly reduced mechanical properties and lower crystallinity. While multiple suppliers offer copolymer POM, DuPont remains the sole manufacturer of Delrin® resin, a homopolymer with exceptional performance characteristics. Delrin® grades are classified by strength, stiffness, viscosity, and resistance properties, making them ideal for both injection molding and CNC machining applications.

Understanding POM's material characteristics is fundamental to successful injection molding. The following table compares physical, mechanical, and processing properties of common POM grades, with Delrin® 100 being the only homopolymer listed:

The data reveals POM's excellent tensile and flexural strength, though with relatively high shrinkage rates. Homopolymer Delrin® demonstrates the highest tensile strength due to its highly ordered crystalline structure. Certain POM grades can be modified with additives to enhance mechanical strength, corrosion resistance, or UV stability.

As a high-strength engineering thermoplastic, POM offers numerous beneficial properties for demanding applications:

• Fatigue Resistance: POM excels in applications with repeated loading cycles (e.g., gears). Homopolymers like Delrin® offer superior fatigue strength compared to copolymers.
• Creep Resistance: POM maintains minimal permanent deformation under sustained loads, making it ideal for automotive components and connectors.
• High Strength: With outstanding rigidity and mechanical properties, POM serves well in conveyor belts and safety harness applications.
• Low Friction: POM's excellent lubricity and wear resistance make it perfect for sliding components like bearings and gears.
• Food Safety: FDA-compliant grades enable use in food processing equipment where direct contact occurs.
• Dimensional Stability: Despite molding shrinkage, finished POM parts maintain precise tolerances in harsh environments.
• Chemical Resistance: POM withstands most fuels and solvents, though it degrades with phenols and strong acids/bases.
• Heat Resistance: POM maintains properties at continuous temperatures up to 105°C, with homopolymers offering better short-term heat resistance.

Product designers should follow these manufacturing best practices:

1. Wall Thickness: Maintain 0.030-0.125 inches with variations under 15% of nominal thickness.
2. Tolerances: Account for POM's significant shrinkage, especially in thicker sections.
3. Radii: Minimum 25% of wall thickness (75% ideal) to prevent stress concentrations.
4. Draft Angles: 0.5-1° typically suffices due to POM's lubricity; zero draft possible for gears/bearings.
5. Copper Avoidance: Prevent contact between molten POM and copper to avoid decomposition.
6. Mold Material: Stainless steel recommended; chrome-plated molds may flake over time.
7. Assembly Methods: Design for snap-fit or mechanical fastening as POM bonds poorly.

Four critical factors impact POM processing:

1. Heat Control: Maintain melt temperatures below 210°C to prevent decomposition.
2. Moisture Management: Despite low absorption (0.2-0.5%), pre-dry resin for 3-4 hours.
3. Molding Parameters: Use medium-high injection speeds with 70-120 MPa pressure.
4. Shrinkage Compensation: Account for 2-3.5% shrinkage during cooling and post-ejection.

While POM presents processing challenges, properly molded components deliver unmatched performance in lubrication, mechanical strength, and fatigue resistance. Following these material-specific guidelines ensures successful production of high-quality POM parts for demanding applications across industries.