• Welcome to engineeringclicks.com
  • Silicone molds, and urethane casting

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

    1. stewlenox

      stewlenox Active Member

      Joined:
      Jul 2009
      Posts:
      34
      Likes Received:
      0
      Hi all,
      If any of you need to know the "tricks" of casting in color using silicone molds and urethanes I can probably help you out, and maybe save you some money too. I have extensive experience with it.
       
    2.  
    3. GarethW

      GarethW Chief Clicker Staff Member

      Joined:
      Jul 2009
      Posts:
      1,424
      Likes Received:
      5
      Hey stewlenox,

      Thanks - I'm sure your experience is going to prove extremely useful in these forums. That's exactly what this place is all about!

      If you already have any design guides for this kind of thing please feel free to add them to this thread, or create a relevant thread.

      By the way, is silicone mold/urethane casting the same as "vacuum casting"? What are the main applications (& quantities) for this kind of technique?
       
    4. wangyangzhong

      wangyangzhong Member

      Joined:
      Oct 2009
      Posts:
      19
      Likes Received:
      0
      usually we use such silicon molds for samples or prototypes,
      the molds life is very short, around 10-50 shots(depends on part and mold structure).
      but cost and lead time are probably 1%-5% of making a steel mold.
      additionally, it's ok to make some special features like under-cut.
      while impossible in steel molds.

      Stephen
       
    5. stewlenox

      stewlenox Active Member

      Joined:
      Jul 2009
      Posts:
      34
      Likes Received:
      0
      Hi Gareth,
      There are several types of "vacuum casting" these days. The jewelry industry now uses vacuum casting in the lost wax process. It uses a slightly porous mold which is vacuumed from the bottom as metal is poured into the top. This is replacing centrifugal casting.
      The Vacuum casting you are probably asking about is indeed used with silicone and urethane. It works very well, but equally good results can be achieved without the expensive equipment. I vacuum the components to de-gas them and to minimize bubble formation. This can be done in a smaller vacuum chamber (less than 5 gallons). I like to pressurize the molds to shrink any other trapped air in the mold. I also cut reservoirs into the sprue and vents of the mold so that the material can back-feed into the mold.
      When I make the silicone mold itself, I like to put it on a vibrating table for about two hours while it is curing. Stephen is also correct about the mold life. It depends on how aggressive the casting material is.
      I've cast low-temp metals like pewter into a silicone mold with decent results too. It's very versatile from a prototyping standpoint.
       
    6. Ycxmould

      Ycxmould New Member

      Joined:
      Jun 2010
      Posts:
      1
      Likes Received:
      0
      VC (Vacuum Casting) is widely used process in rapid prototyping industry, typically used for making 20 to 40 pieces of functional prototypes out of broad range of polyurethanes and other polymers available.
       
    7. meloncollie

      meloncollie Member

      Joined:
      May 2011
      Posts:
      6
      Likes Received:
      0
      Hi,
      I'm an industrial design student and need a little help making a prototype. I want to mould an elastomer part with metal cores.

      Hopefully the picture below will illustrate what I mean. It is a intended to be a flexible arm for an adjustable product that bends and stays in it's bent position. Working much like a french curve would.

      I need to mould at least two, it's approx 25mm wide by 12mm tall and 500mm long. It has 3 5x5mm cores, two of which I need to fit some inserts into, the middle empty, to channel a power cord. The inserts are square section aluminium armature wire, and some stainless steel splints to stop the aluminium kinking.

      [​IMG]

      A few questions;
      What would you recommend I make the mould from?
      What would you recommend I make the moulded part from?
      Should I mould it with the metal cores embedded, or insert them after?

      Thanks,
      Meloncollie
       
    8. stewlenox

      stewlenox Active Member

      Joined:
      Jul 2009
      Posts:
      34
      Likes Received:
      0
      Hi meloncollie,
      To answer your questions: First, use silicone for the mold. I can tell by your spelling (mould vs. mold) that you are in the UK, so unfortunately I can't recommend a supplier (I'm in the US)....but Dow, Shin-Etsu, and GE all make comparable products. Personally I like the platinum cure, translucent silicone better than the tin cure, opaque material from a mold-cutting standpoint, and also because it's more durable.

      Second, urethane can be obtained in just about any durometer. I'm guessing somewhere around 40-50 shore A would be about right for you. Here in the States, I would look at Smooth-On company, but In the UK maybe Ciba. I'm sure you will find a supplier on-line.

      Third, cast the metal in place, but use a spray release on it first so it can slide a little so that the urethane does not prevent it from holding position. Because you are using two metal cores, you are going to need a little room at the ends for the metal to move into. The urethane will bond to itself perfectly if you need to cast in multiple pours. If I were doing this, I would cast the metal in place, then pull it out a little, then cut it shorter than the channels. When pushed back in, this would leave some air gaps at the ends. Then, I would probably figure out a way to seal the ends with the same urethane, leaving the air space intact. You could incorporate this by making a variable length mold, and standing it on end for each seal.
      As for the hole for the wire, I'd be tempted to cast a piece of flexible tubing in place, and either leave it in, or for more room pull it out, leaving the channel.
      By the way, here are two commercially available flexible curves...FYI. :

      http://www.dickblick.com/products/alvin ... le-curves/

      http://www.dickblick.com/products/staed ... le-curves/

      I hope this helps
      -Stewart
       
    9. meloncollie

      meloncollie Member

      Joined:
      May 2011
      Posts:
      6
      Likes Received:
      0
      Stewart,

      Thanks for your reply. Very helpful indeed.
      I have some flexible curves I've taken apart, and noted the air gaps at the ends.

      I will look at UK suppliers for the materials you have recommended, and may post links to double check, if I'm not sure before I go ahead and purchase.

      Thanks again for your help, owe you one!
       
    10. meloncollie

      meloncollie Member

      Joined:
      May 2011
      Posts:
      6
      Likes Received:
      0
    11. stewlenox

      stewlenox Active Member

      Joined:
      Jul 2009
      Posts:
      34
      Likes Received:
      0
      Hi Meloncollie,
      The silicone is a little soft, so if you use it, make a mold frame that can be re-used with each casting. I'd choose a little stiffer silicone if you can find it, but if not, what you linked to will work.
      The urethane looks like a good choice. Make sure that you have some way to pressurize the mold while the part is curing, or you will most likely get bubbles from the reaction. I use an industrial paint tank that I converted. You need about 50 psi. to squeeze the bubbles down to nothing. You could probably make a chamber out of heavy PVC pipe, with a screw-on cap on one end. Be careful when you test it!
      Try to find a silicone that takes 12 hours or more to cure so that bubbles can escape the mold. I'm assuming that you do not have access to a vacuum chamber to de-gas the just-mixed silicone. It's OK...but also make sure that the silicone is not thickened....you need to be able to pour it easily.
      Mix the silicone thoroughly, but slowly, to reduce bubbles.
      One more thing....Flexible urethane has a short shelf life unless you can eliminate its contact with air. It cures faster in high humidity as well. Almost all RTV silicones and urethanes cure faster with heat and humidity, however excessive heat may change the properties of some urethanes.
      Good luck!
       

    Share This Page