Delrin FAQ — Common Questions on POM-H Acetal Plastic

Answers to the most common engineering and procurement questions about Delrin acetal homopolymer (POM-H): what it is, what differentiates it from acetal copolymer, which grade to order, machinability, and compliance.

What is Delrin?

Delrin is DuPont's registered trade name for acetal homopolymer (POM-H, polyoxymethylene homopolymer). It is a semicrystalline engineering thermoplastic with an all-oxymethylene repeating unit (-[O-CH₂]-), first commercialized by DuPont in 1960. Delrin is characterized by high crystallinity (75–85%), which gives it stiffness, fatigue resistance, and dimensional stability superior to most commodity plastics. It is available as rod, sheet, and tube for machined parts, and as pellets for injection molding.

For the complete material overview, see the Delrin material hub.

Is Delrin the same as acetal?

Partially. "Acetal" is the generic family name for polyoxymethylene (POM) plastics. There are two variants: acetal homopolymer (POM-H, which includes Delrin) and acetal copolymer (POM-C, which includes Celcon and Hostaform). These are distinct materials with different properties. Delrin = POM-H only. When someone says "acetal" without further qualification, it could mean either — context matters.

Key differences: Delrin has higher tensile strength and fatigue endurance; acetal copolymer has better hot-water resistance and no centerline porosity in large-diameter rod. See the Delrin vs. acetal copolymer comparison for full details.

What are the different Delrin grades and how do I choose?

The main Delrin grades for stock shapes are:

Delrin 150 — Medium-high molecular weight, optimized for extrusion; best surface finish in stock shapes; the dominant grade in machined parts
Delrin 100 — Highest molecular weight; maximum impact toughness and elongation for shock-sensitive applications
Delrin 500 — General-purpose, lowest MW in the standard series; most used in injection molding; available in stock shapes
Delrin 507 — Internally lubricated; lower friction for sliding/wear applications
Delrin 570 — 20% glass fiber reinforced; higher strength and stiffness, more creep-resistant
Delrin 100AF — PTFE/aramid fiber filled; dramatically lower wear rate and CoF for bearing applications
Delrin 150SA / 500SA — FDA-compliant natural (white) grades for food-contact use

For precision machined parts from stock, default to Delrin 150. For maximum toughness, use Delrin 100. For FDA food contact, specify 150SA or 500SA. See the full Delrin grades guide.

What is centerline porosity in Delrin rod, and does it matter?

Centerline porosity is a porous void zone that forms at the center of large-diameter Delrin (POM-H) rod during extrusion. As the rod solidifies from the outside in, the outer layers freeze first; the inner core solidifies last and undergoes volumetric shrinkage, which can create a porous or voided zone along the rod axis.

This is a known characteristic of all acetal homopolymer (POM-H) and is acceptable under ASTM D6100 — it is not a quality defect. The practical impact: parts machined from large-diameter Delrin rod that use the center of the rod (e.g., parts with through-bores concentric to the rod axis) may intersect the porous zone, which can affect:

Gas-tightness of machined surfaces
Structural integrity in the core region
Visual appearance of machined internal surfaces

For applications where centerline porosity is unacceptable, substitute acetal copolymer rod, which does not develop this characteristic.

Can Delrin be used with hot water?

Not for sustained service. Delrin (POM-H) is susceptible to hydrolytic degradation in continuous contact with hot water above approximately 140°F (60°C). The mechanism is chain-end unzipping — the fully regular POM-H backbone can depolymerize progressively from the chain ends under hot-water exposure, leading to:

Surface cracking and embrittlement
Dimensional instability over months to years
Potential release of formaldehyde degradation products

For hot-water-contact applications, use acetal copolymer instead — the comonomer units in POM-C block the unzipping pathway and provide substantially better hydrolysis resistance. Heat-stabilized Celcon and Hostaform grades are rated to 160–180°F continuous in hot water.

Is Delrin FDA food-grade?

Standard Delrin 150 and Delrin 500 are not automatically FDA-compliant — they may contain additives not listed under 21 CFR 177.2480. The FDA-compliant grades are Delrin 150SA and Delrin 500SA, which use only CFR-listed adjuvants. These grades are available in natural (white) color only.

For food-contact applications involving hot water above 140°F, Delrin SA grades are not appropriate — use FDA-compliant acetal copolymer. See the Delrin FDA food-grade page and the acetal copolymer FDA food-grade page.

How does Delrin machine compared to other plastics?

Delrin is among the easiest engineering plastics to machine. It:

Cuts cleanly without smearing or melting under the tool
Produces short, controllable chips with standard carbide tooling
Holds dimensional tolerances of ±0.001–0.002" on a good CNC lathe or mill
Produces Ra 16–32 µin surface finish with finishing passes
Does not require pre-annealing in most applications
Can be machined dry for most operations

The primary caution: do not laser-cut Delrin — POM generates formaldehyde gas under laser exposure. Use waterjet, bandsaw, or router for sheet blanking.

For the complete parameter table, see the Delrin machining guide.

What temperatures can Delrin handle?

Continuous use: 185°F (85°C) — the heat deflection temperature at 264 psi is 255°F (124°C), but continuous service above 185°F causes creep and long-term property loss
Short-term peak: 220°F (104°C) for brief excursions without structural load
Melting point: 347°F (175°C) — above this, Delrin liquefies
Cold service: Delrin can be used to approximately -40°F (-40°C); impact resistance decreases at low temperatures but the material does not become brittle like some engineering plastics

For applications above 185°F continuous, evaluate PEEK, Ultem/PEI, or nylon 66 (rated to ~250°F).

Can I substitute acetal copolymer for Delrin on a print that calls for Delrin?

Only if the application engineer approves the substitution after reviewing the property differences. The key risks in blindly substituting POM-C for POM-H:

Fatigue endurance: POM-C is approximately 10–20% lower; for gear tooth loading at high cycle counts, this matters
Tensile and stiffness: Marginally lower in POM-C (usually within noise for most structural applications)
Specification compliance: A print that calls out "Delrin 150" by grade name is likely written to include traceability to that grade's property level; substituting without approval may constitute a non-conformance

In practice, many engineering teams accept the substitution for general machined parts where fatigue is not the governing criterion. Always document the substitution in the engineering change control system.

What is the difference between Delrin 150 and Delrin 500?

Both are unfilled acetal homopolymer (POM-H), differing primarily in molecular weight:

Delrin 150 is medium-high molecular weight, optimized for extrusion. It is the preferred stock-shape grade: better surface finish in extruded rod and sheet, slightly higher elongation, better melt stability in extrusion.
Delrin 500 is lower molecular weight, the standard injection molding grade. Slightly lower viscosity for faster mold fill. Properties are marginally lower than Delrin 150 (tensile: 10,000 vs. 10,500 psi).

For machined parts from rod or sheet, prefer Delrin 150. For injection-molded parts, Delrin 500 is the standard.

Where can I buy Delrin rod, sheet, and tube?

Delrin rod, sheet, and tube are stocked in standard sizes by industrial plastic distributors including FedMat. Standard sizes ship from stock; custom sizes (non-standard diameters, custom lengths, cut-to-size sheet) are available. See the Delrin rod page and Delrin sheet page for stocked sizes and ordering.

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