Milling plastics

[agniusm]

Perhaps there is someone with experience machining plastics. I have some 10mm sheet of POM milled and it all deformed. I read that there is annealed POM, but would it make a difference, will it stay flat after machining? Any other rigid plastics i could use? How would reinforced glass fiber board would behave?

 

[Dauntless]

Polyformaldehyde sure sounds like something you wouldn't want to mill, but I don't really know the material. A good tough material that you're not supposed to use around acids. Yes they mill it but I'm guessing it's in a controlled environment, not around the house. You're going to do your cutting with a coolant, if you're doing this in a home workshop, what little I know of it is not encouraging. I suppose if you had a laser in a hazmat environment. (Now, I might be too cautious, it might not be as dangerous as I'm assuming. Some of it is food safe, surprisingly.)

You didn't mention what you're trying to make with it. If it's deformed, a thermoplastic will go soft and sag right back onto a flat surface. Your sheets were extruded, so you won't get so perfect as you had with a sag molding. It's forming temperature (soft but not melted) will be over 250 degrees Fahrenheit. You could flatten it, form and shape it, etc. I think there's versions of it getting down to 200 F.

 

[agniusm]

I need to make 18650 tray to hold 84 cells (14S6P). I did first sample out of 10mm POM and it deformed. It would straiten out in the application but i need to keep uniform pressure on all of the cells. A lot of material is taken out and it bows on x and y axis. I need to find solution so it does not bow or find a rigid material that does not have or has little residual stress.

 

[Dauntless]

Misunderstood, I thought you meant warpage. If it's just distorting under pressure creating the box shape will take some of that out of it. Even with some ribbing there's bound to be a little flex, rather than uniform. Even if you had the center set on the fender, there'd be settling encircling that spot.

It's all a question of how perfect the uniform pressure has to be. If you cut strips that are 90 degrees of the angle they were cut from the sheet you could literally line the entire bottom, (Big job) but that won't be so perfectly flat. You then put neoprene under the batteries. But it won't be perfect.

 

[DanGT86]

You may just have to mill it in a few extra operations. Its annoying and time consuming but will usually work. Find a way to hold it while you are milling that does lets it sit naturally as in you are not putting it into a machine vise bent then hammering it down straight. You don't want to be the one causing the stress or it will just bend back when you remove it from your workholding device. Make all of your rough cuts for the pockets and other features leaving a stock for finishing. Re-face it back to flat again and then finish your features. This way your final cuts are made in a stress relieved part.

I had the same issue with some Delrin plastic I was machining. I pocketed out one side and it bowed really bad when I removed it from the machine. I'm sure there are plastics that don't do this but if you like what you have you may just need to work around it by adding more cutting operations.

Good luck.

 

[agniusm]

Thanks. But how did you got rid of residual stress? I have read it must be annealed. It takes a fare amount of time and an oven. I could in theory make a controlled kiln of some sorts but that is another operation and i would like to keep it simple. I found a material which i could use and it cost roughly the same, only it does not look that nice. Its called Tufnol or Novotext maybe similar to Bakelite.

 

[DanGT86]

I actually never really got the residual stress out of it. It was just a low tolerance holder for a some cell logs so I didn't really care that it warped. Could you contact a material supplier and find out if they have a more stable or less stressed material?

 

[Dauntless]

Baekelite, good lord, so hard to believe that's still out there. That is NOT a green material. But I assume that won't flex on you. Your materials aren't just similar, they ARE Baekelite.

What is the shape of this tray? Is it just flat? That could help understanding.

Could you possibly make a box of your Polyformaldehyde then put some Plexiglass or Baekelite across the bottom to take the flex out?

http://www.bakelitemuseum.net/

 

[Deafcat]

POM? as in delrin? it's usually extremely easy to machine and can be cut to very precise sizes (though, not as precise as Ertalyte).

as for the application, if you can provide some drawing/images and tell me a bit more about the performance requirements, I can suggest you an ideal material for reasonable price (none of the crazy high-tech expensive EPPs).

I've worked extensively with pretty much every machinable synthetic out there and have designed many products from a wide range of them, I'd be glad to point you in the right direction regarding material selection, part design, and machining/processing. if I don't reply promptly here feel free to PM me.

 

[adam333]

When machining plastic, you usually want to remove material from both side of the sheet equally to avoid the piece to warp afterward.

It also help a lot of the vise is at least as large as the material you put in it.

finally, do not tight the vise too hard.

 

[Chalo]

POM? as in delrin? it's usually extremely easy to machine and can be cut to very precise sizes (though, not as precise as Ertalyte).

That's what I was thinking when I saw the OP. I've machined a lot of Delrin, but it's never misbehaved for me.

 

[agniusm]

Thanks everyone. This is what i need:

Here is the deformation visible:

I want it as flat as string.
Material i have used POM - Polyoxymethylene, 10mm
Derlin i think is the brand not the material

 

[Deafcat]

yea, that deformation would occur even with 6061... so much material removal from one side and no stiffness provided by the back side.

I would add a triangular webbing structure to the rear face if possible, to add stiffness across the part and keep it flat, this would add almost negligible weight and only increase the thickness a small amount.

like so

 

[agniusm]

To what extent it would help? Would it be done before milling main section or after?

 

[Chalo]

The curvature of your part suggests that it's the result of residual stresses, and might be counteracted by starting with a thicker piece of stock and cutting material from both sides (even if one side is machined smooth).

If your process results in more than a little heat and smell, try slowing down the tool RPM and taking a heavier chip.

 

[agniusm]

Thats what I thought about residual stress. There are annealed sheets but I cant get them here. I dont do the parts myself so I would need to instruct the shop how to do it. Last ones they made were single pass, perhaps that could be a problem?

 

[Chalo]

I would start by asking them to use the next thicker size of plate, and cut away a roughly equal thickness from both surfaces before making your part. Residual stress is concentrated in the skin of the material, and usually is not a problem once you remove both opposite surface layers.

 

[agniusm]

I would need 20mm of material. Its about 6mm removed. On 10mm sheet I could live with removing 2mm of thickness.

 

[agniusm]

I just wonder if that web structure would help? It also kinda looks cool. Did mine:

2mm enough?

 

[Deafcat]

at least 1mm depth of the webbing would be enough, with say a 2mm floor thickness. thus, the thickness of the ribs in the webbing are 3mm.

I'd machine the webbed face first, then flip it over and do the interior. if you can, slightly increase the thickness of the outer walls, along the long sides of the part.

about that web pattern of yours: if those holes in the middle long rib are thru-holes, be sure to add some extra rib material around them (otherwise they are no longer contributing much stiffness since they are "broken" by the hole)

 

[agniusm]

Yeah, i figured that part after i posted, also changed the diagonal ribs to start from the corners:

 

[Chalo]

If the problem is residual stresses, the ribs will do more harm than good if they include the stressed skin.

 

[agniusm]

..if its in the skin as you mentioned before i suppose. I wander id G10 or FR4 would behave differently?

 

[Chalo]

..if its in the skin as you mentioned before i suppose. I wander id G10 or FR4 would behave differently?

Yes, that would be more dimensionally stable. Stronger and stiffer, too.

 

[Deafcat]

G10 is quite a bit heavier than delrin, and it will pose it's own unique machining challenges as it's very abrasive on tools. once the carbide starts to dull, the finish and cut sizes will immediately show noticeable degradation. I like using it, but I'm sure once you get this delrin part sorted out you'll be happy with the results

the modified profile you posted looks good. I'd try that (cut that face first), then flip the part over, tap it down flat and let er rip. remember to beef up those long sidewalls, I think that will give a great boost to lengthwise flatness.