Guides / Materials

Acetal vs PTFE: Seals, Valve Seats and Low-Friction Parts Compared

October 14, 2026

PTFE and acetal are the two plastics most often considered for sliding, sealing and low-friction parts. They look similar on a shelf, both white and waxy, but they are different materials with almost opposite strengths. PTFE is nearly inert and slides on almost anything. Acetal is stiff, strong and holds shape. Choosing the right one depends on which of those properties the part needs most.

Typical properties side by side

Property	Acetal	PTFE
Flexural modulus	2.6 to 3.0 GPa	0.4 to 0.6 GPa
Tensile strength	60 to 70 MPa	20 to 35 MPa
Density	1.41 g/cm3	2.1 to 2.2 g/cm3
Dry friction on steel	0.2 to 0.35	0.05 to 0.2
Continuous use temperature	90 to 100 C	up to about 260 C
Chemical resistance	Good, weak to strong acids and oxidisers	Excellent, nearly inert
Creep under load	Low to moderate	High (cold flow)
Machining	Cuts cleanly, holds tolerance	Soft, springs back, harder to hold tolerance

Values vary by grade and test method, so treat them as a guide and use the datasheet of the actual material. The material properties guide lists typical acetal values in more detail.

Where PTFE wins

Chemical and temperature resistance. PTFE resists almost every acid, base and solvent and survives temperatures that would soften or degrade acetal. For aggressive chemicals, chlorinated water or high-temperature fluids, it is often the only plastic that works. The chemical resistance guide shows where acetal stops.

Lowest friction. For light-load sliding without lubricant, PTFE has a very low coefficient of friction, and it has little stick-slip. It suits dry bearings at low load, slide plates and non-stick faces.

Sealing. PTFE conforms to a mating surface and seals on a soft face. Lip seals, gaskets and envelope seals use that behaviour.

Where acetal wins

Stiffness and strength. Acetal is five times stiffer than PTFE and several times stronger. A gear, a cam, a roller, a bearing sleeve under load or a structural spacer needs those properties. PTFE would deform.

Creep. PTFE flows slowly under constant load, so a PTFE part that is clamped or preloaded loosens over time. Acetal creeps much less, so a fastened joint or a valve stem seat holds.

Machining tolerance. Acetal cuts cleanly and stays at size, so tight bores and fine features are practical. PTFE is soft and elastic, springs back after cutting and expands with temperature, making close tolerances hard. See the tolerances guide.

Cost and availability. Acetal stock is cheaper per kilogram and per part, and it is stocked in more sizes. PTFE costs more and is denser, so a PTFE part of the same volume weighs and costs more.

Wear against hard counterfaces. Pure PTFE wears quickly under load. Acetal wears better in most loaded sliding pairs. See the bushing design guide.

Seals and valve seats

Valve seats sit between two jobs. They have to seal, which suits a soft material, and they have to resist the pressure of the closed valve, which suits a stiff one.

Condition	Better choice
Water, air or mild chemicals, moderate temperature	Acetal seat, stiffer and holds its sealing face
Aggressive chemicals or high temperature	PTFE or related fluoropolymer
High pressure with a soft seat	PTFE with a backing ring or a filled grade
Frequent cycling	Acetal for wear, PTFE-filled acetal if friction is a problem
Drinking water	Check approvals and chlorine resistance for the exact grade

For drinking water, the grade needs approvals, and chlorinated water can attack acetal. The valve components guide goes into what to check. PTFE is generally better against chlorine.

PTFE-filled acetal as a middle path

Some parts need acetal stiffness and a lower friction surface. Acetal blended with PTFE, as fibre or powder, gives lower friction and wear, with some loss of strength and a higher cost. It is used for dry sliding on a shaft, a cam follower or a slide guide. The filled acetal guide lists the common blends.

Another approach is to use acetal for the structure and a PTFE insert or coating where friction matters, though that adds an assembly step and a joining problem. See the bonding guide for why PTFE is difficult to bond.

Quick selection guide

If the part needs…	Start with
Load, stiffness, tight tolerance	Acetal
Chemical inertness or high temperature	PTFE
Very low friction at light load	PTFE
Low friction with some load	PTFE-filled acetal
Lowest cost	Acetal
Soft sealing against a rough face	PTFE

How the shop machines each material

The two materials need different handling in the shop, and that shows up in cost and tolerance.

Aspect	Acetal	PTFE
Cutting	Clean chips, good finish	Soft, gummy, burr forms easily
Tool geometry	Sharp, positive rake	Very sharp, high rake, polished
Holding	Normal clamping, light force	Very light force, it flows under clamps
Tolerance	Tight tolerances are practical	Looser, springs back and moves with temperature
Measurement	Stable at a set temperature	Soft, needs low-force and non-contact methods
Surface finish	Fine and bright	Matt, shows tool marks

PTFE also changes size with temperature in a step near room temperature, because it passes through a crystal transition at around 19 C. A PTFE part measured in a cold room and a warm room can read differently by more than the tolerance. Acetal has no such step. See the tolerances guide and the inspection guide.

Cost follows the material and the difficulty. PTFE stock is more expensive per kilogram, and the machining takes more care. For a part that works in either material, acetal is usually the cheaper and faster route. Ask the shop for a quote on both when the choice is open.

Checklist before you request a quote

• State the load, speed, temperature and fluid the part sees.
• Say whether the part is a seal, a bearing or a structural piece.
• Name the mating material and its finish.
• Note creep or preload requirements.
• Ask the shop which material they would choose, and why, before fixing the grade.

Send the drawing and we match it with up to three shops that machine acetal and reply within 24 business hours.

Frequently asked questions

Keep reading

• Black vs Natural Delrin: What the Colour Changes and What It Does Not
• Acetal vs PET and PBT: Choosing for Precision Mechanical Parts