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  1. #1
    Join Date
    Aug 2018
    Posts
    18

    Which plastic?

    Hi everyone.

    I am a cnc noob looking to machine forming blocks for sheet metal. i was intending on doing it with mdf but it gets everywhere and i don't have a dust extractor. i have heard about using plastics like acetal but that stuff is insanely expensive where i am, ie: 2000mm x 600mm x 50mm = $3100.

    can anyone recommend a similar material that is:

    dimensionally stable
    has great compressibility strength at least compared to mdf (no distortion)
    fairly easy to machine
    sold at a reasonable price

    the aim is to make a form/die, place a alloy sheet on top of it and then squash it with my 250T rubber press to form the final part, ie aircraft wing ribs.

    i have heard of polyurethane 90A being able to do this, does anyone here have any experience with it? thanks

  2. #2
    Join Date
    Dec 2013
    Posts
    5717

    Re: Which plastic?

    Polyurethane would work for your ''rubber'' press plate, but would not work well for for the die. Polyurethane is pretty squishy. Maybe aluminum dies would be better, at least it's less expensive than acetal. Acetal is expensive everywhere, I buy a lot of it. The other day I bought a piece 600x600x100mm, $1100. Given that each wing rib is different, you would be better to use MDF and install some dust collection. Even a big shop vac works pretty well and is cheap compared to buying acetal or aluminum.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
    Join Date
    May 2006
    Posts
    803

    Re: Which plastic?

    Quote Originally Posted by Jim Dawson View Post
    Polyurethane would work for your ''rubber'' press plate, but would not work well for for the die. Polyurethane is pretty squishy. Maybe aluminum dies would be better, at least it's less expensive than acetal. Acetal is expensive everywhere, I buy a lot of it. The other day I bought a piece 600x600x100mm, $1100. Given that each wing rib is different, you would be better to use MDF and install some dust collection. Even a big shop vac works pretty well and is cheap compared to buying acetal or aluminum.
    How about Kirksite forms?,
    totally recycleable perfectly stable
    make Styrofoam shape, a casting box and sand, melt, pour, cool, machine .
    Did this on Nacelle skins 1970's
    Did a lot of ribs spars etc.
    Been doing this too long

  4. #4
    Join Date
    Aug 2018
    Posts
    18

    Re: Which plastic?

    i understand about the rubber press plate. i was only thinking about the lower forms. alloy is an option but i think it would take much longer to machine. i think kirksite shrinks? i have a bismuth/tin mix thats better than kirksite, its dimensionally stable, hard, reuseable and its good for making molds but not for making parts from drawings, too complex and time consuming, im making 8000 parts. is pvc good?

  5. #5
    Join Date
    Dec 2013
    Posts
    5717

    Re: Which plastic?

    Quote Originally Posted by lostit View Post
    i understand about the rubber press plate. i was only thinking about the lower forms. alloy is an option but i think it would take much longer to machine. i think kirksite shrinks? i have a bismuth/tin mix thats better than kirksite, its dimensionally stable, hard, reuseable and its good for making molds but not for making parts from drawings, too complex and time consuming, im making 8000 parts. is pvc good?
    8000 parts? How many different forms do you need? It might be less time to have somebody with a high speed CNC mill make the forms out of aluminum. We run aluminum at 200+ IPM all the time, with high MRR.

    I think PVC does not have enough compressive strength. ABS is a bit harder, so maybe that would work.
    Jim Dawson
    Sandy, Oregon, USA

  6. #6
    Join Date
    Aug 2018
    Posts
    18

    Re: Which plastic?

    Quote Originally Posted by Jim Dawson View Post
    8000 parts? How many different forms do you need? It might be less time to have somebody with a high speed CNC mill make the forms out of aluminum. We run aluminum at 200+ IPM all the time, with high MRR.

    I think PVC does not have enough compressive strength. ABS is a bit harder, so maybe that would work.
    8k is a bit of an exaggeration but several thousand, one of each unique part. so thats how many forms i need. does pvc machine well? i can use urethane 90a for really critical stuff but how many ton/psi will pvc hold up to? i am trying to find out. i saw a chart saying pvc rigid can take 53 mpa or 7500 psi? that would be sufficient if true because the press box is big and i don't think the psi would actually get over that.

  7. #7
    Join Date
    Dec 2013
    Posts
    5717

    Re: Which plastic?

    Given the number of parts, I guess it's worth experimenting with some different materials. Buy some PVC and try it, that's what I would do.
    Jim Dawson
    Sandy, Oregon, USA

  8. #8
    Join Date
    Aug 2018
    Posts
    18

    Re: Which plastic?

    im ordering some rigid pvc and will see how it goes, its still way more expensive than mdf (2440x1220x30mm = $700 ish) but my lungs will thank me

  9. #9
    Join Date
    Jun 2011
    Posts
    692

    Re: Which plastic?

    Wouldn't it make more sense to get a proper dust extraction rig? That's a one-time expense.

  10. #10
    Join Date
    Mar 2008
    Posts
    683

    Re: Which plastic?

    I doubt PVC will work.

    There is tooling board available from companies lie Freeman Supply out of Seattle. Or Huntsman. For budget they have wood tooling board that have anti-static additives or they have knot free tooling boards from maple or pine. There are also plastic tooling boards that are quite a bit more expensive but they are very durable.
    Depending on your router you might be able to get away with cutting aluminum. It's not that bad to cut.
    https://www.freemansupply.com/
    https://www.huntsman.com/

    Thousands of individual parts. Wow.

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