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  • Molded tube and draft

    Discussion in 'The main mechanical design forum' started by Gilles, Mar 21, 2017.

    1. Gilles

      Gilles New Member

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      Hi everyone,

      I have to design a plastic tube that will be glued on a carbon tube. The assembly will then be fitted into an aluminium hub in which it will rotate (I have attached a drawing).

      I would like to have the plastic tube injection molded rather than machined for cost reduction. But I need to have a draft on the inner part of the tube so it can be molded. But with a draft I feel like I will not have a titght fit between the carbon shaft and the plastic tube, and I am a bit afraid that I will not acheive good alignement between the axis of those two parts.
      What is your feeling, Have you ever seen an injection molded tube without draft ? shaft.jpg
       
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    3. Neil D

      Neil D New Member

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      Hi Gilles,

      I've heard of some parts molded with rapid injection molding not having a draft, but anytime anyone I've known has tried to get around putting a draft in it seemed to send a shiver through the crowd.

      If you need a really tight fit can you keep it injection molded and use an epoxy to fill in the gap? How tight of a fit do you need?
       
    4. Gilles

      Gilles New Member

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      In fact what I really need is to have the center axis of the carbon shaft and the palstic tube aligned. A deviation of .1 mm is acceptable.
      Epoxy will fill the space, but will allow an orientation default between the two axis won't it ?
       
    5. Erich

      Erich Well-Known Member

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      How long is the plastic tube? What is the diameter? You can sometimes get away with some narrow no draft ribs running the length of the bore They can even be designed for a low force press fit, with the rest of the bore providing the proper space for the adhesive. This way you get concentricity and a good glue joint all in one.
       
    6. Dana

      Dana Well-Known Member

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      Can you make the carbon tube with a taper that matches the plastic tube's draft?

      Alternatively, if you have the same draft on the ID and OD of the plastic tube (constant wall thickness), you might be able to press the shaft into the tube, expanding it not at all at the entrance end and more farther in, when pressed all the way in it will have stretched it out to a constant OD to slip into the aluminum hub.
       
    7. Gilles

      Gilles New Member

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      Thanks for all your answers :)
      The Ø are shown in the picture :). The plastic tube ID is 6mm OD is 8mm.
      The length of the plastic tube is 40mm.

      Erich:
      The ribs solution is interesting, I had something like that in mind:
      upload_2017-3-23_8-52-48.png
      Is it what your a talking about?

      Dana:
      Having the carbon tube tappered so it matches the plastic tube draft would work, but I would like to avoid any milling operation on the carbon tube :(

      I like the idea of stretching the plastic tube. Do you have any example where that kind of solution has already been implemented ?
       
    8. Erich

      Erich Well-Known Member

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      Yes, that is what I was talking about. With a diameter of only 6mm I would be using fewer ribs than shown. 6 to 8 max.
       
    9. K.I.S.S.

      K.I.S.S. Well-Known Member

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      Hi,
      You can also consider a stripper plate in the mould. These are handy for thin walled components that cannot support ejector pin pressures over small wall thicknesses with zero draft.
      Or if your budget allows, you can use heated water or gas to control the inner wall thickness with zero draft.
       

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