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  • Silicone molds, and urethane casting

    Discussion in 'Prototyping & low-volume processes' started by stewlenox, Jul 31, 2009.

    1. meloncollie

      meloncollie Member

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      Stewart,

      Thanks again for your advice and taking to time to have a look.

      I've cut down a positive to shape and size today and ordered the materials and so I should be able to give it ago in about 72hrs time... I'll let you know how successful I am!

      Cheers,
      Meloncollie
       
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    3. meloncollie

      meloncollie Member

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      My moulding and casting stuff all came in the post this morning and my positive is just about ready.

      However, the supplier has sent me Brown pigment by mistake when I ordered Black.
      I have little time to get this done and was wondering if another way of colouring the polyurethane when casting would work. I.e. food dye, or something like that? Or can I only use specially formulated pigments?
      I'm not sure I have time to send the pigments back and wait for a replacement order to arrive.

      Thanks,
      Meloncollie
       
    4. stewlenox

      stewlenox Active Member

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      I've used a variety of pigments. Try mixing a small sample of your material with some black artists oil paint....if you have a good art supply store nearby, you might even be able to get some dry pigment. Dry pigment will work if you mix it into the thinner half of the 2 part urethane very thoroughly first, and let it soak for a bit. Basically almost any really strong pigment will work if you use the minimum amount possible. Test it first though.
       
    5. meloncollie

      meloncollie Member

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      Hi!

      Just to let you know, I managed to get some dye and I managed to cast today. Will post pics once the camera is charged.The polyurethane thickened even faster that the claimed work time on the bottle, so the process got a little tricky and messy with that, the long narrow shape, the cores and me possible making my pour holes a little too small.

      But I managed to get a half decent cast considering it's my first ever attempt. Usable for my project prototype anyway. It bends and stays where it's bent and is a nice uniform profile right the way through.
      It would be good to have a better quality finish one though for the degree show though and I have enough urethane for another go.

      So another couple questions;
      The finish is quite glossy and almost kinda tacky to the touch. It's attracting a lot of dirt and dust. Would making a matt surface finish help? If so, is there an easy way to alter the silicone mould to a matt finish to replicate this in the casting?

      Also, I've sliced off the sprues with a sharp scalpel, but there are still some marks. Do you have any suggestion of how to finish these better / minimise them?

      Thanks,
      Meloncollie
       
    6. stewlenox

      stewlenox Active Member

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      Hi,
      You might be able to reduce the gloss by dusting the mold with talc, or cornstarch before pouring....maybe this will help for the sticky bit too. Another trick is to preheat the mold. I'd do both. You can dust the parts when they come out too. Sometimes a post de-mold heat cure helps, but I can't know if this is the case with your material.
      The cut off sprues and vents will look like ejection pin marks that would normally be on most parts anyway. There's not much you can do about them now that the mold is made. If you decide to make another, try to minimize the point at which they connect to the part, and inject the casting material. I've even cut flaps that self close after injection. You don't need to buy an injection gun for this....the tubes will work all by themselves if you just push the plunger with the butt of a screwdriver handle. Here's the kind of cartridge I'm talking about:
      http://www.sh-nsj.cn/en/product_show.as ... 0Cartridge
      It's the first one. It has three parts...A nozzle, the cartridge barrel, and the plunger seal. You pour your urethane into it with the nozzle already poised in the fill hole, then insert the plunger seal, and push it in with the screwdriver handle. I have the guns, but rarely use them unless the urethane is thickened.
      One more trick...urethanes cure better in high humidity, so if you can heat them in the oven with a pan of water in it, it might improve the surface cure.
       
    7. Uidearp

      Uidearp New Member

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      Hi Everyone,

      Vacuum Casting is relatively labour intensive and typically requires between 2 and 5 days to produce the first casted product. The technology is best suited to low volume productions.we need make a master model by SLA or CNC, then apply whatever surface finishing to the master model. We then pour liquid silicone rubber around the master model in casting box. Once cured, we cut out the silicone rubber mould with which we cast up to 20 PU (polyurethanes) parts per silicone mould in vacuum machine.
       
    8. johntolhurst

      johntolhurst New Member

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      I'm building a bicycle with suspension for Race Across America. Shocks are usually compression, but I need one operating in tension. So I picture a stack of donut shaped elastomers, sitting on a cup welded to the mainframe, and with a rod passing through a cap, then through the stack of donuts, through the cup and onto the suspension link.

      I am posting here for suggestions on the shape of the elastomer, and how to formulate it.

      I need 6cm of wheel travel, which means 3cm of elastomer compression. Bike and rider weigh under 100 Kg, loaded 50/50 front and rear.

      My stack of donuts can be 35mm outside diameter, space for the rod means an ID of 6 or 7mm. The donut stack can be 75mm tall, or it could be even 100mm longer than that, but 75mm is a better look.

      I trust someone will let me know if there is a better area of the forum for this question.
       
    9. Rison Rapid Prototyping

      Rison Rapid Prototyping Member

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      Silicone mold is a commom way for low volume production, the same principle as steel, we can even process overmolded part with it, so it's very suitable for new production publishing due to lower cost and quick delivery, except silicone mold, 3D printing and RIM mold are also good choce, better quality but higher cost.
       

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