[tstanley01]

I am trying to make a small vacuum plenum for my CNC that is it is about 18" by 30". I made one out of 3/4" Polypropylene and it has a horrible warp in it. What type of plastic would remain flat when grooves are machined in one side and not the other. I have very little experience machining plastic, outside of cast acrylic and even then it is almost exclusively 6mm. Also, I need to mention that it needs to be completely air tight. I am using a small vacuum pump that pulls down to almost 28" but has very low cfm. I am limping along with PP plenum that I have, but it is not ideal and am ready to upgrade.

[mactec54]

I am trying to make a small vacuum plenum for my CNC that is it is about 18" by 30". I made one out of 3/4" Polypropylene and it has a horrible warp in it. What type of plastic would remain flat when grooves are machined in one side and not the other. I have very little experience machining plastic, outside of cast acrylic and even then it is almost exclusively 6mm. Also, I need to mention that it needs to be completely air tight. I am using a small vacuum pump that pulls down to almost 28" but has very low cfm. I am limping along with PP plenum that I have, but it is not ideal and am ready to upgrade.

Because there is stress in all plastic sheet, the only way that can help is by stress reliving the sheet and this has to be done by heat treating the material this to can be tricky to get just right, it can be done before machining and after machining sometimes, before and after are needed for the same piece

Some plastic materials are worse than others, there are some that are stable, but it depends on what you process needs

[tstanley01]

Because there is stress in all plastic sheet, the only way that can help is by stress reliving the sheet and this has to be done by heat treating the material this to can be tricky to get just right, it can be done before machining and after machining sometimes, before and after are needed for the same piece

Some plastic materials are worse than others, there are some that are stable, but it depends on what you process needs

Well what material would you suggest? This is turning out to be a lot more difficult that I expected to get a little flat plenum. Maybe I could attach it to some sort of backing material like melamine to keep it straight?

[peteeng]

Hi Tstan - The higher density plastics like acetal or PET are less affected with warping. This is mainly due to moisture content and PP is stretched when formed so it has lots of internal stress. If you can get it into an oven at 60-80C for a couple of hours it may straighten. Or since you have a good vac pump put it into a vac bag overnight at full vacuum. That will flatten it and draw out the moisture (but the moisture will return but if the new shape is supported the moisture may not matter) . If you can vac it and heat it all the better.... Peter

the other option is to do it in MDF then coat with epoxy (2 coats) very stable cheap and easy to repair when you cut into it. I'd go that way.... made lots of vacuum clamps. plenums and things that way.

[mactec54]

Well what material would you suggest? This is turning out to be a lot more difficult that I expected to get a little flat plenum. Maybe I could attach it to some sort of backing material like melamine to keep it straight?

You have to say what the material is in contact with, chemicals / water Etc. Delron / Acetal can be used in most cases, they all will bend when machined though

Phenolic sheet would be a good choice if you can use it, and it will not normally bend when machined

[tstanley01]

You have to say what the material is in contact with, chemicals / water Etc. Delron / Acetal can be used in most cases, they all will bend when machined though

Phenolic sheet would be a good choice if you can use it, and it will not normally bend when machined

Just air. It is setup on a cnc router.

[mactec54]

Just air. It is setup on a cnc router.

Then the Phenolic sheet would most likely stay flat the best, don't get the glass filled as this won't machine as well

[joeavaerage]

Hi,
I've had pretty good results with acrylic. It does not absorb moisture and remains flat after machining. It is very inclined to gum up tools
when its cutting, being very heat sensitive, so I use flood cooling, thereafter it cuts fine.

Acrylic is one of the cheapest plastics so you can afford to get a thick piece thereby avoiding distortion when cutting.

Acetal is my go to material for making parts, it cuts well, again with coolant, and remains dimensionally stable with minimal water adsorption.
PEEK is also good but both Acetal and PEEK are both quite pricey, great for small parts, for larger parts cost becomes an issue.

Craig

[mactec54]

Hi,
I've had pretty good results with acrylic. It does not absorb moisture and remains flat after machining. It is very inclined to gum up tools
when its cutting, being very heat sensitive, so I use flood cooling, thereafter it cuts fine.

Acrylic is one of the cheapest plastics so you can afford to get a thick piece thereby avoiding distortion when cutting.

Acetal is my go to material for making parts, it cuts well, again with coolant, and remains dimensionally stable with minimal water adsorption.
PEEK is also good but both Acetal and PEEK are both quite pricey, great for small parts, for larger parts cost becomes an issue.

Craig

Check the size of the sheet that he is using Acetal (Delrin) will not stay flat in a sheet that size and thickness he is using

If he is able to use the 3/4" 19mm thick it may stay flatter than the PP sheet but if it is 6mm thick it will be like a banana a sheet that size