Zero draft angles and moulds

Discussion in 'Materials' started by Dresca, Apr 12, 2008.

  1. Dresca
    Joined: May 2007
    Posts: 29
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    Location: UK

    Dresca Junior Member

    Hi all,

    Just wanted to grab some other peoples experiences with this. I have to regularly make panels with zero draft sides to a depth of about 10cm. They are couple metres squared in some cases. I use compressed air to release them, female mould. Never easy to release due to the zero draft, but at 10cm deep sides they pop, especially with a little Urethane resin in the mould mix to give some flex. But we get friction scraping on the zero draft mould sides when releasing parts. This wears over time usually killing the mould.

    I was reading today engineers recommend 3 degrees draft on moulds for ease of pulling pieces. I can't do this as the panels have to be square on the sides. 2 part mould is not practical as laying radius fillets over joins always took too long with this method.

    Anyone else using or ever had to use zero draft angles in pieces over production runs?

    Many thanks
    Dresca
     
  2. Herman
    Joined: Oct 2004
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    Location: The Netherlands

    Herman Senior Member

    Demoulding a part quite early can help, but zero draft moulds usually are a real ***** to work with. Always problems, and always a lot of wear.

    Given the fact that each tooling material will shrink somewhat, or distort just a bit (even when advertised they do not...), there might be areas that have negative draft.

    Whenever possible, steer away from zero draft designs, otherwise get split moulds, or use what you are using now, and deal with it.

    At least get yourself a maximum slip mould release. Tell the release guys what you are facing, and have them recommend something.

    ----
    I usually get people at work that I need to advice on materials and design. However, usually I get involved whenever problems arise, and it is then that I see zero draft or even negative draft moulds, and when asking, there is no reason for designing it that way. It's just what they thought of as simple construction of the plug. Usually the learning is done the hard way...
     
  3. Dresca
    Joined: May 2007
    Posts: 29
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    Location: UK

    Dresca Junior Member

    Thanks Herman

    Thanks mate

    Appreciate the reply, I've been checking in each day to see if anybody had replied!

    We use the semi-perm polymer mould release agents high-slip, pieces are seriously polished, trouble is the friction is never equal on both sides of the box so pieces tend to shoot up under the compressed air and wedge halfway up! I know what you mean about shrinkage, I'm been trying some of the zero shrink aluminium TH filled tooling resins at the moment to reduce shrink. They seem excellent but some v. slight shrink has been measured, less 0.5% I would say. In real life I just don't have time to leave pieces sitting on forms 2 weeks to allow a good post-cure & I don't have a big oven atm. The Urethane resins made heat distortion worse so I'm backing away from those atm.

    I find shrinkage is a real pain with Polyester, despite bracing etc etc. I get some real wastage from shrinkage!

    Having given it some thought I'm returning to split forms at the moment with this box shape. Funny really given such a simple piece the trouble its caused! Again thanks for the reply & have a good day out there in NL.

    Dresca
     
  4. Herman
    Joined: Oct 2004
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    Location: The Netherlands

    Herman Senior Member

    I have yet to come across a decent tooling system that is not only sold as "zero shrinkage" but also is.

    Polyester based materials show considerable shrinkage, and in tooling systems this is counteracted by adding an expanding polymer, which greatly reduces shrinkage, but it is very hard to balance these two out. Expansion and shrinkage are dependant on curing temperature, which may vary all over the newly produced mould. So different shrink ratios at different places.

    One small warning: Whenever you are tempted to use RTM light or press moulding, don't even think about using that for the product you described here. The product will shrink somewhat, and will get stuck nicely over your top mould. A situation you really want to avoid.
    For the rest nothing bad about the RTM light process. It really works well. Tooling is more expensive, and you need a certain level of accuracy when producing them, but it pays of in the end. The only thing is that it really needs that 3 percent draft angle.
     

  5. Dresca
    Joined: May 2007
    Posts: 29
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    Location: UK

    Dresca Junior Member

    Vacuum moulded acrylic

    Herman,

    Thanks for reply. I can see what you mean about the piece sticking to the top mould, I had looked at RTM for the future, but since then had another idea.

    I saw on cable TV recently a program about a bath making business. They take a big Acrylic sheet, suck it into a huge Vacuum mould, pop the sheet and blow glass over the back. Perfect form every time. I'm not sure about the tolerances here. Have you ever heard of/have any experience of this technique?

    I checked on the tooling costs for this, and they are pretty huge, but it could be a fast system when I happen to have some big cash knocking around!

    best regards
    Dresca
     
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