Avoiding Delrin Warp

For the last year or so I’ve been making jigs for one-off electronic test probes that we use internally. I’ve been using Delrin 150 FG (for no particular reason other than its easy to machine and non-conductive) without issue on small jigs, but it seems like anything over two square inches and the Delrin starts to bend if I pocket more than 1mm from a 1/4" sheet.
I’m using a vacuum table, a spindle speed of ~24K and a feed rate of 800mm/sec (tried slower and it didn’t seem to make a difference), I’m also doing Z-Axis Measurements (ie: warping) on my g-code so everything is as close to spec as possible.

Workarounds I’ve come up with are stacking thinner stock (eg: 1/8” sheets, they still bend but are more malleable), creating holes and plugs on two different 1/4” layers to lock the Delrin in place (PITA to get it right). None of these are anywhere near what I’d call good solutions.

So I thought I’d ask the pro’s if they recommend a different material, different process, etc?

Delrin, polyacetal, now often called polyoxymethylene (POM), has a crystalline structure. The molecular bonds continue to form after the initial solidification, leading to post molding shrinkage and residual stresses in extruded sheet. The accepted procedure for determining ultimate size of molded parts is to anneal sample parts at 200*F for an hour, or one hour per inch of thickness, and allow to cool. Annealing the sheet before machining may be helpful. It may also make the sheet warp, but at least it should be stable after annealing.

Dennis

When you say "molded" are you referring to extruding Delrin into a mold? If so, that's not what I'm doing, I'm using a CNC to mill Delrin sheets.

Yeah, but my experience is being a molder... but, how do you think your sheets were made? Basically, extrusion and injection molding are related processes, they both heat the material until it flows (the plastic state) stuff it into, or through a die, then allow it to cool in a new shape. The shrinkage that molders chase to get parts on size just presents itself as residual stress in sheet stock. Remove an unequal amount of mterial, and watch the part change shape. Which is why I think the annealing process that molders use to determine MAX shrinkage will work to stress relieve your sheet before you cut it.

Or, pick a material with less residual stress to begin with, like cast ABS sheet.

Dennis

Modelman said:

Yeah, but my experience is being a molder... but, how do you think your sheets were made? Basically, extrusion and injection molding are related processes, they both heat the material until it flows (the plastic state) stuff it into, or through a die, then allow it to cool in a new shape. The shrinkage that molders chase to get parts on size just presents itself as residual stress in sheet stock. Remove an unequal amount of mterial, and watch the part change shape. Which is why I think the annealing process that molders use to determine MAX shrinkage will work to stress relieve your sheet before you cut it.

Or, pick a material with less residual stress to begin with, like cast ABS sheet.

Dennis

Click to expand...

I'll shoot for ABS sheet, thanks