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  • A Delrin Question

    A simple question. Does Delrin change size with a temperature change? If so, ideas of the shrink available from putting the part in the freezer?

    Thanks in advance

    Pete

  • #2
    It definitely does change size with a temperature change like everything else. There are different grades and manufacturers, I would not be surprised if temperature coefficient is different as well. If you want to be accurate, make a test. Machine a test part from your particular material, I would do a simple diameter on a lathe. Let it stabilize to ambient temperature, then measure size and temperature.
    Next put it in a freezer together with a thermometer or thermocouple. Let it soak for a couple of hours, remove the part and quickly check the size and temperature. Make sure your micrometer is at the same temperature as during first check. Here is your answer, 100% accurate and reliable. Do not forget to wear gloves...

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    • #3
      Originally posted by mikey553 View Post
      It definitely does change size with a temperature change like everything else. There are different grades and manufacturers, I would not be surprised if temperature coefficient is different as well. If you want to be accurate, make a test. Machine a test part from your particular material, I would do a simple diameter on a lathe. Let it stabilize to ambient temperature, then measure size and temperature.
      Next put it in a freezer together with a thermometer or thermocouple. Let it soak for a couple of hours, remove the part and quickly check the size and temperature. Make sure your micrometer is at the same temperature as during first check. Here is your answer, 100% accurate and reliable. Do not forget to wear gloves...
      Excellent answer. If you have a shop notebook it might pay to save that number. Maybe reduce the number to inches per hundred degrees per inch of thickness. That should get you in the ballpark in the future.
      *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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      • #4
        6.8 e-5 in/in/F

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        • #5
          Delrin is a brand name for a specific type of acetal plastic. Often times, what people call Delrin is some other manufacturer's version and the coefficient of linear expansion for various acetals can vary as much as 30% between manufacturers. If it is true Delrin, you can get the properties here:

          http://www.sdplastics.com/delrin/delrin[1].pdf

          It still moves more with temperature changes than aluminum and much more than something like granite or cast iron, but it's not as bad as many other types of plastic. Delrin is pretty good with regard to dimension stability and strength, at least compared to cheaper plastics like ABS.

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          • #6
            Acetal is an interesting material and it doesn't behave much like most plastics we grew up with.

            Since the OP seems to be asking for the sake of some sort of press fit, it is worth the reminder that the acetal will probably harden significantly when cold. If distorting the material is part of pressing it in, that may be significant. At some point, maybe it even microfractures, and then subsequently fails.

            Conversely, if you could internally relieve the material a little bit, and then heat it, maybe that would allow the fit.

            Acetal doesn't soften much when heated. It starts to soften just a little around 170-180F. And if you go just a bit higher, it decomposes into toxic gas and can do so rather dramatically. I've never tried chilling it, so I don't know how it behaves. At what point does it stiffen, get brittle, etc?

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            • #7
              Ductile to brittle transition temperature -1 to -2 C.

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              • #8
                If aiming for a 'press fit' turn a small 7.5° taper on some or all of the length of the parts, small enough that they can slip over each other when the outer is heated and clamp when they are at the same temperature. That way there isn't any stress in the final assembly. That's what I did on the end caps of my surface grinder replacement handles and it's worked perfectly.
                Location- Rugby, Warwickshire. UK

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                • #9
                  I have made a few parts for vintage cameras out of 63mm acetal black solid bar on the lathe.
                  https://app.box.com/s/5xc8mpmbejud8n56g15tycbwz6avu3uf

                  Sometimes I found that the diameter and bore of hollow parts would creep
                  a little while turning and boring to size.
                  I suppose the best way would be to bring it close to final and leave overnight
                  before finishing and parting off the parent bar.

                  Some of the parts are up to 5 years old, and the press fits are still OK.
                  The conical lens hood on the old Graftar lens on right of photo is a press fit,
                  and I wondered if it would fall off. But I just tried to pull it off
                  and it did not move under normal pull so left it in place .

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