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Thread: Plastics advice needed.

  1. #1
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    Plastics advice needed.

    I need to buy some plastic for a project, and need advice on the best for my needs.

    These are side guards for an equipment seat.
    It needs to be.........

    12 inches by 8 inches.
    1/4 inch thick.
    Black in color.
    STIFF and as inflexible as possible.
    Won't crack after drilling holes and bolting mounting tabs on it.

    The tabs will bolt to the bottom of the plate and up at least 4 inches.
    The tabs fit in a slot with the plates sticking straight up.

    This will be subject to side loading against the plates.

    What plastic?? Delrin??

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    Forum Administrator Schuetzenman's Avatar

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    Delrin maybe. https://www.mcmaster.com/#standard-p...heets/=1bxzqxt Check Mc Master Carr I've put one link in.

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    Senior Member NAPOTS's Avatar

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    UHMW Poly?

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    I thinks it's down to Delrin and HDPE.

    Both sound pretty good for the purpose.
    I contacted an online seller and asked which.

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    Administrator Krupski's Avatar

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    Quote Originally Posted by Schuetzenman View Post
    Delrin maybe. https://www.mcmaster.com/#standard-p...heets/=1bxzqxt Check Mc Master Carr I've put one link in.
    Quote Originally Posted by NAPOTS View Post
    UHMW Poly?
    I suggest Lexan (polycarbonate).

    Delrin absorbs moisture like nylon does and gets brittle in cold temperatures.

    Polyethylene exhibits cold flow (things bolted to it go loose as the plastic "flows" out under the pressure. HDPE is rather soft, but strangely also brittle. If bent beyond it's plastic deformation limit, it shatters.

    These plastics are also degraded by UV radiation from sunlight.

    Lexan costs quite a bit, but it's the most rugged plastic out there (a 1/2 inch thick pane of Lexan will stop a 5.56 bullet!).
    Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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    Forum Administrator Schuetzenman's Avatar

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    Lexan aka Polycarbonate also absorbs moister and what is worse is gasoline and other solvents will kill it in 30 seconds. Very poor choice IMO says the guy with 35 years as a Molding Process Engineering career.

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    Administrator Krupski's Avatar

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    Quote Originally Posted by Schuetzenman View Post
    Lexan aka Polycarbonate also absorbs moister and what is worse is gasoline and other solvents will kill it in 30 seconds. Very poor choice IMO says the guy with 35 years as a Molding Process Engineering career.


    Lexan will absorb some moisture, but that has no detrimental effect on the properties.

    Yes, agreed many solvents will weaken or destroy Lexan, but that's true of most other polymers. HDPE, LDPE and Acetal Homopolymer (Delrin) are a lot more resistant to solvents.

    A good (better?) choice of plastic may be ABS since it's fairly soft (not brittle) and easily solvent bonds using THF (plus is a lot less $$$ than Lexan).

    I spent quite a few years at a medical device manufacturer and have a fair amount of experience with injection molding, the properties of many common polymers and the characteristics of thermoforming vs thermosetting plastics.

    Your 35 years of experience, of course, trumps mine by quite a bit, so I'll defer to your knowledge and advice.
    Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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    Advice from a plastics company recommended Delrin, but that HDPE would work.
    They said Delrin would cost more so that might make HDPE the best choice.

    I think I'll go with Delrin but I'm still not sure about how cracking resistant it is.
    If pressure is directed around the holes where 1/4 inch bolts go, I'm wondering if Delrin will crack, or if HDPE would be better.

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    Senior Member NAPOTS's Avatar

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    my experience with delrin is limited to using it in applications to resist wear where we have moving components but want to avoid direct contact without getting into fancier bearings and such (like occasional contact) to protect cables from chafing etc. It seems to work great from that standpoint and is pretty chemically stable. It is tough stuff but very expensive.

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    Forum Administrator Schuetzenman's Avatar

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    Quote Originally Posted by dfariswheel View Post
    Advice from a plastics company recommended Delrin, but that HDPE would work.
    They said Delrin would cost more so that might make HDPE the best choice.

    I think I'll go with Delrin but I'm still not sure about how cracking resistant it is.
    If pressure is directed around the holes where 1/4 inch bolts go, I'm wondering if Delrin will crack, or if HDPE would be better.
    Well there's Homopolymer and Copolymer POM resins. Delrin is a trade name for what is usually a Homopolymer version of POM plastic. Copolymer is more felxible. Both of them have high surface lubricity, meaning slippery in nature. They often get used to mold gears because of this low friction and the toughness of it. I've molded leaf springs out of it because in thin sections it is very flexible and can do many cycles without cracking or breaking. If plastic wont cut it, there's always aluminum.

    Oh and if you catch it on fire it gives off Formaldehyde gas, don't breathe the fumes, you won't like it.

  11. #11
    Senior Member NAPOTS's Avatar

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    Quote Originally Posted by Schuetzenman View Post
    Well there's Homopolymer and Copolymer POM resins. Delrin is a trade name for what is usually a Homopolymer version of POM plastic. Copolymer is more felxible. Both of them have high surface lubricity, meaning slippery in nature. They often get used to mold gears because of this low friction and the toughness of it. I've molded leaf springs out of it because in thin sections it is very flexible and can do many cycles without cracking or breaking. If plastic wont cut it, there's always aluminum.

    Oh and if you catch it on fire it gives off Formaldehyde gas, don't breathe the fumes, you won't like it.
    I forgot that it was a homo polymer, the gayest of the plastics

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