Fixture Materials
Delrin and UHMW Fixture Details and Contact Pads
The soft half of a fixture: the faces that touch an anodized or painted part and are not allowed to leave a mark.
Delrin acetal and UHMW are the two plastics that do most of the non-marring work in fixtures. Acetal is stiffer and holds size well, so it suits located details, nests and soft locating pins. UHMW is softer, more slippery and more impact tolerant, so it suits wear strips, guides and pads a part slides across. Both are usually inserts on a metal base. Send the print naming which faces are cosmetic; a person quotes in 24-48 hours.
Updated September 2026
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Why a fixture has soft parts at all
Most fixture work happens after the part is finished. It has been anodized, powder coated, painted, polished or plated, and any scratch the fixture puts on it is a reject at the last possible moment. A steel locator will mark an anodized face; an aluminum rest will mark a painted one. So the contact faces go soft, while the base, the frame and the bolt pattern stay in metal.
The usual build is a metal base carrying plastic inserts, or a metal frame with a plastic top plate. That keeps the fixture stiff, keeps the dowels and threads in material that holds them, and puts the soft material only where the part touches, where it can also be replaced when it wears without scrapping the fixture. Mark on the print which faces touch a cosmetic surface and the split follows.
Delrin acetal: the one that holds a size
Acetal, the polymer Delrin is a trade name for, is covered in Wikipedia’s Polyoxymethylene entry (en.wikipedia.org/wiki/Polyoxymethylene). What matters for fixtures is that it is stiff for a plastic, machines cleanly to a good finish, has low moisture absorption so it does not grow in a humid plant, and is dimensionally stable enough to hold a machined tolerance. Distributor data such as Piedmont Plastics’ materials pages (piedmontplastics.com/products/g10-fr-4) is where the published numbers for the engineering plastics bench live.
That makes it the default where a soft detail still has to locate something: a nest pocket cut to the part shape, a soft locating pin, a rest pad whose height is part of the datum scheme, a guide that has to stay where it was set. It is the plastic that behaves most like a metal detail and is quoted the same way. The Delrin and acetal page covers the grade as a machined part in its own right.
UHMW: the one that slides and takes a hit
Ultra-high-molecular-weight polyethylene (en.wikipedia.org/wiki/Ultra-high-molecular-weight_polyethylene) trades stiffness for two other properties: a very low coefficient of friction and high impact and abrasion resistance. It is softer than acetal and it creeps under sustained load, so it is the wrong choice for a detail whose position has to stay exact.
It is the right choice where a part slides across a surface, drops onto it, or is dragged along it: wear strips, chute and guide faces, drop pads, liners on a chute or a conveyor nest. The same softness that disqualifies it as a locator is what makes it gentle on a finished part. A common fixture uses both, with acetal locating the part and UHMW where it lands or rubs.
What each one will not do
Neither plastic belongs where the fixture gets hot. A powder coat cure oven commonly runs in the 350 to 400°F region, and that is not the place for either material; masking and racking fixtures that go through a cure use metal on the hot faces, which masking fixtures covers. Neither belongs next to a weld arc either. Where a fixture detail has to insulate as well as stay stiff, G10 and phenolic is the material that does it.
Neither holds a thread the way metal does. A plastic detail that gets bolted and unbolted often should be through-bolted into the metal base, or carry a threaded insert called out on the print, rather than being tapped and stripped. And UHMW in particular should not carry a location that has to stay put under a clamp, because it will move over time. State the load and the duty cycle on the print and the quote will say which is which.
How the details are drawn and made
Plastic details are milled like any other part and quoted the same way: pockets, profiles, drilled and counterbored mounting holes, and pins turned to size. Tolerances are looser than in metal, because both materials move with temperature more than aluminum or steel does, and both push back from the cutter differently. Call out the tolerance the detail actually needs rather than defaulting to a machined-metal band, and the quote will say if it is not achievable in that material.
The mounting matters as much as the shape. A plastic insert usually sits in a machined pocket in the base so the pocket takes the side load rather than the screws, with clearance holes for cap screws into the metal below. Draw it that way and the detail can be swapped when it wears. CNC machining tolerances covers the general bands.
What to send, what is inspected, and when it ships
Send the fixture print and STEP model naming the material on each soft detail, which faces touch a cosmetic surface, the tolerance each detail needs, the mounting scheme, and the load and duty cycle if a detail is loaded. Any file type is accepted, STEP plus PDF is ideal, and the quote lists what is missing. If you are not sure whether a face should be acetal or UHMW, say whether it locates or whether the part slides on it.
Every detail is inspected against its print before it ships: profiles and pocket dimensions with a height gage and gage pins, thickness with a micrometer, hole position by CMM where the print calls it out. One detail, a replacement set of wear pads, or a whole fixture are all normal orders with no minimum. Standard lead time is 4-6 weeks from approval, the quote comes back in 24-48 hours, and rush at 1-2 weeks is quoted when a date is fixed, and extended is 6-8 weeks when timing is flexible.
Questions
Before you send a job.
01 Delrin or UHMW for a contact pad?
Delrin if the pad also locates the part or its height is part of the datum scheme, because acetal is stiffer and holds a machined size. UHMW if the part slides across it, drops onto it or drags along it, because it is more slippery and more impact tolerant. Many fixtures use both, with acetal locating and UHMW where the part lands.
02 Will a plastic detail hold a tolerance?
Delrin will hold a useful one: it is dimensionally stable with low moisture absorption, so a nest pocket or a soft pin stays where it was cut. UHMW creeps under sustained load and should not carry a location that has to stay exact. State the tolerance each detail needs on the print rather than defaulting to a machined-metal band.
03 Can the whole fixture be plastic?
It can, for light work, but the common build is better: a metal base carrying plastic inserts or a plastic top plate. That keeps the stiffness, the dowels and the threads in material that holds them, and puts the soft material only where the part touches, where it can be replaced when it wears.
04 How should a plastic insert be mounted?
In a machined pocket in the base, so the pocket takes the side load instead of the screws, with clearance holes for cap screws into the metal below. Tapping the plastic itself is what strips after a few changeovers. Where a plastic detail must be threaded, call out an insert on the print.
05 Can these go through an anodize or powder line?
No. A powder cure oven commonly runs in the 350 to 400°F region and neither material belongs on a hot face; neither belongs near a weld arc either. Masking and racking tools that go through a process use metal on the faces that see heat and chemistry.
06 Can I order replacement pads for a fixture I already have?
Yes, and there is no minimum. Send the print for the detail, or a new sample pad with a dimensioned sketch and photos with a ruler in frame, and the quote lists what is still missing before a build-to-print number can be given. A set of replacements is quoted as one order.
Related
Where to go next.
Custom machined fixtures
The hub: every fixture family, quoted from your print.
View →Delrin and acetal parts
The grade as a machined part in its own right.
View →Assembly fixtures and nests
Where most non-marring contact faces end up.
View →Get a quote
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