Acetal Filled Grades: PTFE, Glass, and Carbon Compared
When unfilled POM isn't enough for your bearing or wear application, these specialty grades extend the envelope. Here's exactly when and how to use each one.
Start With Unfilled POM. Upgrade Only When PV Limits or Wear Rates Demand It.
Unfilled acetal handles the majority of gear, bushing, and cam applications without modification. Specialty filled grades cost more, can be harder to source, and in the case of glass-filled POM, introduce mating-surface wear concerns. This guide helps you identify the specific conditions where a filled grade delivers a measurable performance advantage — not just a theoretical one.
Unfilled Acetal: The Baseline
Before you upgrade to a filled grade, you need to know what unfilled POM already delivers — it handles most gear, bushing, and cam applications without modification. Know the baseline before evaluating upgrades. Unfilled POM is already a high-performance bearing material for most applications.
When Unfilled POM Is Sufficient
For gears, bushings, and cams running at PV below 0.08 MPa·m/s with dry contact and ambient temperature, unfilled POM-H or POM-C is the correct specification. Adding a filled grade adds cost and sourcing complexity for no measurable benefit in these conditions.
PTFE-Filled Acetal (AF Grade)
If your bearing or bushing application exceeds unfilled POM's dry PV limit of ~0.10 MPa·m/s, PTFE-filled acetal is the first filled grade to evaluate. The self-lubricating bearing grade. PTFE fiber content (typically 18–20%) reduces dry friction dramatically and extends wear life in high-duty-cycle applications.
PTFE-Filled Acetal Properties (Delrin AF / equivalent)
When to Specify PTFE-Filled Acetal
- Application PV exceeds 0.08 MPa·m/s continuous
- Part runs at medium duty cycle (>30 minutes/hour)
- Mating metal surface is soft (aluminum, brass) and unfilled POM wear rate is too high
- Application requires the lowest possible dry friction coefficient
- Bearing surface temperature risk is moderate — PTFE filler helps dissipate heat
- Part is subject to high structural load (PTFE filler reduces tensile strength ~20%)
- Application requires food-contact compliance — verify PTFE grade FDA status first
- Cost is critical — AF grade is 2–3× unfilled POM price
Glass-Filled Acetal (25–30% GF)
If your part needs higher stiffness or better creep resistance at elevated temperature, glass-filled acetal doubles the modulus — but you must design around mating-surface abrasion. The structural upgrade grade. Glass fiber reinforcement doubles stiffness and significantly improves elevated-temperature performance — at the cost of increased friction and mating-surface abrasion.
GF25 Acetal Properties (vs. unfilled baseline)
Critical Warning: Mating Surface Abrasion
Glass fiber in POM is abrasive to softer mating materials. Do not run glass-filled acetal against:
- Aluminum alloys (6061, 7075)
- Brass or bronze
- Soft steel (< HRC 30)
- Other plastics as mating gear or bushing
Safe mating materials: hardened steel (HRC 45+), hard chrome, stainless steel 17-4 PH.
Carbon-Filled Acetal (CF Grade)
If your application requires ESD dissipation or the lowest possible thermal expansion from a plastic part, carbon-filled acetal is the premium option to evaluate. The precision structural and ESD (Electrostatic Discharge) grade. Chopped carbon fiber provides maximum stiffness, lowest CTE, and electrical conductivity — for ESD-sensitive environments and high-precision structural plastic parts.
CF20 Acetal Properties (vs. unfilled)
When CF Acetal Earns Its Place
- ESD-sensitive environments (semiconductor handling, electronics assembly jigs)
- High-precision structural brackets where minimal thermal expansion is required
- Bearing applications combining load and moderate sliding velocity where self-lubricating carbon fiber improves PV limit
- Wafer handling fixtures, PCB handling trays, and tooling where static charge dissipation is critical
CNC Machining in All Acetal Grades
MakerStage machines unfilled acetal (POM-H, POM-C), PTFE-filled (AF grade), and glass-filled acetal for CNC precision parts. The quote review helps confirm which grade best matches wear, stiffness, and sourcing needs before production.
Review Acetal Grade RequirementsFull Grade Comparison Table
Use this table to compare all four grades side by side when making your material selection — it covers every property that affects your design and sourcing decisions. All grades at 23°C (73°F), dry conditions unless noted. Filled grade properties vary by specific filler loading — values shown for typical commercial grades.
| Property | Unfilled POM | PTFE-Filled AF | Glass-Filled 25% | Carbon-Filled 20% |
|---|---|---|---|---|
| Tensile Strength | 69 MPa (10.0 ksi) | ~55 MPa (~8.0 ksi) | ~110 MPa (~16.0 ksi) | ~80 MPa (~11.6 ksi) |
| Tensile Modulus | 3,100 MPa | ~2,500 MPa | ~6,200 MPa | ~7,500 MPa |
| Dry CoF vs steel | 0.20–0.35 | 0.05–0.15 | 0.35–0.50 | 0.15–0.25 |
| PV limit (cont.) | ~0.10 MPa·m/s | ~0.20 MPa·m/s | ~0.06 MPa·m/s | ~0.15 MPa·m/s |
| Wear rate (Taber) | 6–12 mg/1k | 2–5 mg/1k | 3–6 mg/1k | 3–7 mg/1k |
| Elevated-temp stiffness | Good | Good | Excellent | Very Good |
| ESD dissipative | No (insulator) | No (insulator) | No | Yes (10³–10⁶ Ω/sq) |
| Machinability | Excellent | Excellent | Good | Good (abrasive) |
| Mating surface abrasion | Low | Very Low | High — avoid soft metals | Moderate |
| Cost vs unfilled | Baseline | 2–3× unfilled | 1.5–2× unfilled | 3–5× unfilled |
| FDA natural grade | Yes | Check grade | Generally No | No |
Further Reading
- Acetal Bushings and Bearings: Low-Friction Design Guide — PV limits, clearance fits, and shaft surface requirements.
- Acetal Plastic Gears: Design Rules and Material Guide — when to use filled grades in gear applications.
- What Is Acetal (POM/Delrin)? Complete Engineer's Guide — unfilled acetal hub guide.
Frequently Asked Questions
Quote the Right Acetal Grade
CNC machining in unfilled, PTFE-filled, and glass-filled acetal grades, with quote-stage review of wear, stiffness, and environment fit before production.
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