Delrin Warping After Machining: Why It Moves
Delrin warping after machining comes from stress in the bar's skin. Why one-sided cuts curl a part, and how balanced roughing and stress relief fix it.
The part had always been aluminum. Someone switched it to Delrin, kept the same program, and the edges lifted off the fixture like a potato chip. That's Delrin warping after machining, and it catches a lot of people.
Why Does Delrin Curl Up When You Hog Out One Side?
Because the stock already holds stress, mostly near its surface. Cut a lot away from one side and the stress left on the other side is no longer balanced, so the part bows.
That's the story from an r/Machinists post. The top replies were practical: leave stock on every feature, flip, and take a light pass on everything. Face both sides before cutting features. One machinist said the material carries "lots of residual tensile stress in the skin". Another, who used to make a 12 x 14" part from Delrin, said having the material stress relieved at a heat treater made the job easy.
What actually stops Delrin warping after machining
Boedeker Plastics' annealing guidelines list three causes of machined-in stress: dull tooling, too much heat, and removing a lot of material "usually from one side of the stock shape". For flatness, it suggests roughing, annealing, then finishing "with a very light cut", and says balanced machining on both sides of the centerline helps too.
Boedeker notes most stock shapes are stress-relieved by the manufacturer after molding, but it still lists heavy one-sided material removal as a cause of machined-in stress. It's also honest about how often you need the oven. Boedeker says very few machined plastic parts need annealing after machining. So try the cheap fixes first:
Rough both faces, leaving stock. Let the part sit unclamped. Then finish each side with matching light passes.
Hold it gently. A vise that squeezes the blank will release when you unclamp, and the part moves anyway.
Keep the tool sharp and the chips thick. Heat is one of the causes on Boedeker's list.
If it still won't stay flat, design around it. Thicker sections, ribs, or a pocket split between two faces all help more than a tighter flatness note. Metal plate does a version of this too, covered in why thin plate bows after milling one side.
Delrin on our machines
Delrin is on our materials page for "bearings, bushings, precision plastic parts". If you're choosing it over printed nylon, read snap-fits in MJF vs Delrin and tapped holes in printed nylon vs Delrin. Put the flatness you need on the drawing so the process can be planned around it. If you think the part needs annealing between roughing and finishing, ask us when you quote.
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