Guides / Materials
Delrin and Acetal Material Properties: A Reference for Engineers
September 25, 2026
Every acetal data sheet lists roughly the same fifteen numbers, and most buyers skim past them because the numbers alone do not say what to do with a part. This guide walks through the properties that actually decide whether Delrin is right for a design, what each one means physically, and where homopolymer and copolymer pull apart.
Treat every figure here as a starting point for comparison, not a specification. The grade, the filler, the supplier and the test method all move the number. A real project uses the data sheet for the exact grade a shop is quoting.
Mechanical properties
Tensile strength and modulus
Unfilled acetal homopolymer runs roughly 60 to 70 MPa in tensile strength at yield. Copolymer sits a little below that, commonly 55 to 65 MPa. Tensile modulus, which describes stiffness rather than ultimate strength, lands around 2,800 to 3,200 MPa for homopolymer and slightly lower for copolymer.
What this means in a part: acetal is stiff for a thermoplastic, stiffer than nylon and most other unfilled engineering plastics in this price range. A thin acetal wall deflects less under the same load than the same wall in nylon or polypropylene. That stiffness is a major reason it gets specified for gears and precision fits, where a part that flexes under load changes the tooth engagement or the running clearance.
Flexural strength and modulus
Flexural numbers track close to tensile numbers in acetal because it does not have a strong fiber orientation effect the way filled or reinforced plastics do. Expect flexural strength in a similar range to tensile strength, and flexural modulus a touch higher than tensile modulus. This matters for a part that is loaded as a beam or a cantilever rather than pulled in a straight line, such as a snap arm or a thin bracket.
Impact strength
This is where acetal’s reputation gets complicated. Notched Izod impact strength for unfilled acetal is moderate, noticeably better than acrylic or polystyrene but well behind nylon, especially conditioned nylon that has absorbed some moisture. The word notched matters. Acetal is far more sensitive to a sharp internal corner than most plastics its buyers expect. Unnotched impact strength looks respectable on the data sheet; a real part with a 0.2 mm internal radius where the drawing called for a sharp corner behaves much closer to the notched number.
The practical rule that follows from this: acetal handles moderate impact well when the geometry avoids stress concentrations, and fails abruptly when it does not. This is a design property as much as a material property.
Hardness
Acetal typically measures in the Rockwell M80 to M94 range, or Shore D 80 to 86. That is hard enough to hold a machined edge and to resist marking in normal handling, softer than most filled or glass reinforced grades, and considerably softer than the steel counterfaces it commonly runs against. The hardness gap between acetal and steel is part of why it performs well as a bearing material: the softer plastic wears, the steel shaft does not.
Coefficient of friction
Against dry steel, unfilled acetal typically shows a coefficient of friction somewhere around 0.35, with PTFE filled grades cutting that meaningfully lower, often into the 0.1 to 0.2 range. This is the property behind acetal’s use in gears, cams and slides: a low, stable friction coefficient that does not require an applied lubricant to perform acceptably.
Thermal properties
Melting point and heat deflection
Acetal homopolymer melts around 175 degrees C. Copolymer melts a little lower, near 165 degrees C. Heat deflection temperature, the point at which a loaded beam of the material deflects a specified amount under heat, sits well below the melting point for both, commonly in the 100 to 160 degrees C range depending on the applied load in the test.
The gap between heat deflection temperature and melting point is the reason the continuous service temperature guideline, around 90 to 100 degrees C, sits so much lower than the melting point. A part does not need to reach its melting point to lose the stiffness a design depends on.
Coefficient of linear thermal expansion
This is the number that catches the most buyers off guard. Acetal expands at roughly 90 to 120 micrometres per metre per kelvin depending on the grade, commonly cited as close to ten times the expansion rate of steel. A 100 mm acetal part warming from 20 to 60 degrees C grows by something in the range of 0.4 mm from thermal expansion alone.
Every fit, every backlash allowance and every inspection procedure on an acetal part needs to account for this. A bore that measures correctly at room temperature in an air conditioned shop can be meaningfully tighter once the assembly reaches operating temperature. Design guides that treat acetal fits the same way as aluminium fits produce parts that bind in service.
Thermal conductivity
Acetal, like most unfilled plastics, is a poor conductor of heat, typically in the range of 0.2 to 0.3 W/mK, roughly two orders of magnitude below aluminium. Heat generated at a sliding interface or by friction does not leave an acetal part quickly, which is one reason acetal on acetal sliding contact runs hotter than acetal on steel: neither surface carries the heat away.
Electrical properties
Unfilled acetal is a good insulator. Volume resistivity commonly runs above 10^14 ohm-cm, and dielectric strength is typically in the range of 20 to 30 kV/mm depending on sample thickness and test method. This is why acetal shows up in terminal blocks, standoffs, bobbins and other parts that need to hold a precise dimension while keeping current where it belongs.
When a part needs the opposite property, to bleed static rather than block it, dedicated ESD or carbon filled grades exist specifically to bring resistivity down into the dissipative range. Unfilled acetal is not a starting point for that requirement; it needs a different formulation entirely.
Chemical and environmental properties
Moisture absorption
This is arguably acetal’s signature property. Unfilled acetal absorbs roughly 0.2 percent moisture at 24 hours immersion and under 1 percent at full saturation, dramatically less than nylon, which can absorb several percent depending on grade and humidity. That low absorption is why dimensions machined into acetal stay put across a range of humidity conditions in a way nylon dimensions do not.
Chemical resistance summary
Acetal holds up well against fuels, oils, most solvents, and neutral to mildly alkaline solutions. It does not tolerate strong acids, strong oxidizers, or prolonged exposure to hot water and steam, particularly the homopolymer grade. Copolymer trades a small amount of stiffness for meaningfully better hydrolysis resistance, which is why it is the more common choice for parts that see repeated hot water or clean in place cycles.
UV resistance
Unstabilised natural acetal chalks and loses surface integrity under prolonged UV exposure. Black grades, which carry carbon black as a UV stabiliser, hold up considerably better. A part destined for permanent outdoor service should specify a UV stabilised or black grade rather than relying on the base resin.
How homopolymer and copolymer diverge, property by property
| Property | Homopolymer | Copolymer |
|---|---|---|
| Tensile strength | Higher | Slightly lower |
| Stiffness | Higher | Slightly lower |
| Fatigue resistance | Better | Good |
| Melting point | Around 175 degrees C | Around 165 degrees C |
| Hot water and steam resistance | Weaker | Better |
| Thick section uniformity | Can show centreline effects | More consistent |
| Alkaline cleaning resistance | Weaker | Better |
Neither grade is categorically better. The choice depends on which property matters more for a given part, which is the subject of a separate comparison if you have not settled that question yet.
Reading a data sheet without being misled
A few habits keep property numbers from leading a design astray.
Check the test method. ASTM and ISO test methods for the same property do not always produce directly comparable numbers. Two data sheets quoting different standards are not necessarily describing different materials.
Check the temperature and conditioning. A property measured dry as molded is not the same as the same property measured after conditioning at 50 percent relative humidity. Acetal moves less on this axis than nylon, but grade to grade comparisons still need the same conditioning basis.
Treat single numbers as typical, not guaranteed. A data sheet reports a typical value from a batch of tests, not a minimum the resin producer promises on every lot. For a part where a property is truly critical, that belongs in a specification with a stated method of verification, not a data sheet citation.
Ask the shop what grade it actually stocks. Data sheets describe a resin family. The grade on a specific shop’s shelf, with its specific filler content and specific supplier, is what actually gets machined into your part. A conversation with the shop closes the gap between the reference table and the real material.
Putting the numbers to work
None of these properties matter in isolation. A gear needs stiffness and fatigue resistance together. A food equipment part needs chemical resistance and moisture stability together. A bushing needs friction, hardness and thermal expansion together, because the running clearance depends on all three at once.
If you are choosing between Delrin and a different plastic entirely, start from the property that is failing in your current design, not from the full data sheet. If you are choosing between homopolymer and copolymer, the environment, not the mechanical numbers, usually decides it.
Have a part where the material choice is not obvious? Send your RFQ with the service conditions and we will match you with up to three shops that machine acetal regularly and can weigh in before you commit to a grade. Reply within 24 business hours.
Frequently asked questions
What is the tensile strength of Delrin?
Unfilled acetal homopolymer typically runs 60 to 70 MPa in tensile strength at yield, with copolymer a little lower, around 55 to 65 MPa. Filled grades move in different directions depending on the filler, and any number quoted without a specific grade is only a starting point for comparison.
What is the maximum continuous service temperature for acetal?
Around 90 to 100 degrees C is the commonly cited figure for continuous use, with brief excursions somewhat higher. Mechanical properties fall off steadily below the melting point, so treat the continuous figure as the design limit, not the point where the part visibly fails.
Is Delrin an electrical insulator?
Yes. Unfilled acetal has high volume resistivity and good dielectric strength, which is why it shows up in spacers, terminal blocks and other insulating parts. ESD and carbon filled grades exist specifically to change this property when a part needs to bleed off static instead.
Need a quote for this part?
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