Nylon Sheet Warping

[Mattyvdv]

Hello again,

I work for a plastics engineering company south of sydney. We mainly work with polyurethanes and HDPE. However, we get the odd nylon (6pla or LFX) jobs.

The supplied sheet always comes oversize in thickness. If we order a 20mm sheet it usually comes in at 21- 23mm. We have a multicam CNC router and everytime we try to surface plane the sheet it always warps and bows all over the place. I can get one side done then it warps as soon as i release the vacuum holding it down. When i flip to do the other side the vacuum is not strong enough to pull the bow out and i cannot surface plane the other side.

We have tried taking small cuts (0.25 - 0.5mm). Also, it doesnt matter if the sheet is 3000x 1200 or 600x600 it still warps and bows. We use the mist sprayer and there is no signs of heat build up. The surface finish we get is fine. Its just not even. Once flipped i get steps because of the warping.

We have also tried clamping around the sheet, machining around the clamps then doing a clamp clean-up cut. The sheet warps in between the clamps haha.

Any tips or techniques to rectify this situation would be greatly appreciated. Even if someone not in the know wants to throw a idea at me im open to suggestions.

[H.A.S.]

Sounds like it is not the heat or clamping pressure. Is there any way you can fly-cut the pieces? I've used that on smaller pieces of nylon. There are so many variations of nylon, it may be worth a try.

[wildwestpat]

I have had problems with Nylon in the past think it was nylon 66 sheet. Most if not all nylons are affected by water and need time to stabilise to the working humidity. Also the sheet material has inherent stresses which skimming may release unequally.

Nylons are also very sensitive to heat and this limits the feed rate and also requires very sharp cutting edges with the correct angle for nylon. All this I am sure you know so it is the water sensitivity and the need to give time for the sheet to stabilise prior to machining.

Regards - Pat

[djr76]

If the plastic you are using can come being made in a compression mold instead of and extruded shape, they are less susceptible to warping.

[Mattyvdv]

Thanks for the input. nfortunately our supplier casts the sheets. Fly cutting does work extremely well but unfortunately with a minimum spindle speed of 9000rpm that is not a option. I have contacted the supplier and they say to take one cut either side rather then small cuts flipping after each cut. This mothod worked perfectly for the first sheet i did. Then the next warped and i was back to square one. I guess i have to figure out the "sweet spot" so to speak and go form there.

[Shane123]

i think i would cut the parts out, leave that extra mm or 2 on the bottom as onion skin, then flip the sheet when done and run a planer over it, plane the excess off.

[DMF_TomB]

1) nylon expands when warm and needs time to cool.

2) nylon often is under internal stress. i have had 6" round stock crack like a wood log every time i drilled a hole through the center.

3) nylon expands easily 0.010" per inch when wet with water and also will shrink this amount when it dries. i have had wheels bored for a bearing with 0.001" press fit and wrote bore size on part and pressed in the bearing. i put part in a drawer and 6 months later bearing fell out. OD can get smaller and ID can get bigger as it dries.

4) standard basic machinist procedure is to take rough cuts as evenly as possible on both sides. Also do not clamp a warped piece straight down to table. you need to use shims to support the warped part without distorting it when clamping it. i use a magnetic base and dial indicator to shim part so it does not move more than 0.001" when clamping down to table. after you take roughing cuts you then take finishing cuts to remove warpage but releasing clamps and shimming and reclamping. it is not unusual to take many finishing cuts. but with nylon you should take time to stabilize temperature of part and i would avoid getting it wet. use air cooling.

5) also when i want a more stable plastic for making part i usually pick Acetal / Delrin. There are other more stable plastics but although some are more tolerant of water warpage almost all of them have a much higher expansion rate with temperature than metal some 10 times a higher expansion rate.

[Mattyvdv]

Very interesting Tom B. Also, 100% agree with you about doing it out of acetal. Since we first got this particular job ive been pushing to do it out of acetal but for whatever reasons they want to keep it nylon.