Cold Casting: What am I doing wrong?

Discussion in 'Replica Props' started by Ein, Jul 2, 2015.

  1. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    My method so far has been as follows:

    With the mold clean and open, dust it down with metallic powder; in this case I'm using -200 mesh aluminum powder from Smooth-On. Get a good coating, tap the excess back into the container, brush the unnecessary powder off the non-model parts of the mold with a soft brush.

    Mix some resin - the last batch I did I used Onyx (Slow) from Smooth-On, but previously I have tried using clear resins with comparable results. Do a 1:1:1 mixture of Part A to Part B to Aluminum Powder, mix thoroughly. I inject the resin into my model with a large-gauge syringe because it's a smaller mold and the syringe allows me to put a bit of pressure behind the resin to ensure it fills the mold thoroughly. Once I'm certain the mold is full, I throw it in a pressure pot at 40-60 PSI to crush any air bubbles out of solution.

    Pop the model out, rub it down with some mineral spirits, clean up some of the excess garbage with a knife. All good so far! Model looks pretty great, but it's a dull grey at this point.

    This is where it falls apart for me. I can try using steel wool, or high-grit sandpaper, or polishing compounds on cloth... end result is always the same. I end up with a 'mottled' metallic finish, rather than something that looks more consistently metal. Here's an example:

    [​IMG]

    [​IMG]

    Ignore the slight casting defects on that for the moment. What is causing this? What am I doing wrong? How do I get a more consistent finish?
     
  2. zorg

    zorg Master Member RPF PREMIUM MEMBER

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    First thing I would try is do everything you have done but into a basic small open top mold to try a test pour but miss out the syringe part.

    -z
     
    Last edited: Jul 2, 2015
  3. Luke0312

    Luke0312 Sr Member RPF PREMIUM MEMBER

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    Which way are you orienting your mold while the resin is curing?

    I would recommend laying the mold with the finished side down (or if all sides have details, the most critical side down). I believe when cold casting, some of the metallic powder settles to the lowest point, leaving other spots with more resin than powder.
     
  4. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    My mold's not designed that way because I didn't really think about it, but I have to assume that's not always the case? I've seen people do things like cold cast helmets with a uniform finish, and there isn't exactly a 'down' when it comes to that.

    I will definitely try it, though. Maybe I can plug the open end after putting the resin in well enough to keep it from leaking.
     
  5. Daniel Nelms

    Daniel Nelms Sr Member RPF PREMIUM MEMBER

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    @Ein I never dust my molds personally. Hit it with some 220 grit sand paper, don't go crazy on it or anything just enough to get the dull surface off. Then polish that up with some 0000 steel wool.
    Always works for me.

    Also if you use clear resins add some black pigment in to it also, that really helps to bring out the metal sheen and be a lot less grey/ashy. I've never tried cold-casting with onxy which i believe cures black but you might add a few drops in there anyway for good luck.

    Finally, you want to use a resin with a fast cat. The longer it takes to setup the more metal particles are going to sink to the bottom. I've been using MPK70 lately which cures white by itself, I hit each batch with about 16-20 drops of so-strong and that does the trick.
     
  6. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    I cold cast with Onyx Fast with no issues whatsoever. The deep black color helps give a more realistic look, and the fact that Onyx is polishable helps even more. The metal particles will settle, so you want to have the detail face downward when casting.
     
  7. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    A couple of things -

    1. How smooth was the original part? Silicone will pick up all the details including dust and fingerprints on the part. You can only get out what you put in.
    2. Why are you rubbing down the parts with a spirit after de-molding? Some of these are abrasives and will remove the shine from the surface. If I ever have to wash a part, I used warm soapy water only.
    EDIT: 3. Powdering the mold will also take away the shine. I use Talc to purposely give my clear parts a 'frosted' look. They will come out water clear if I skip this part.
     
    Last edited: Jul 3, 2015
  8. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    I'm working on remaking my molds right now to have the detail face downward, but what do you do in situations where you're casting an object that needs a good finish on both sides?

    Also, I've been pressure-casting my casts in order to eliminate bubbles - getting a pitted surface on a cold-cast seems irreparable, because there's no way you can fill in the holes with anything that won't also have completely ruined the metallic surface. How do you use Onyx Fast without air bubble issues?

    1. Very smooth; these aren't fingerprints or dust on the casting. I had the original master part sanded up to 1500 grit.

    2. The Smooth-On site says to do as much, so I figured I'd do it. Check here, step 17.
     
  9. Endless Wonder

    Endless Wonder Sr Member

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    What your process when mixing the resin and powder?
     
  10. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    I have clear plastic cups - I get an equal volume of Part A, Part B, and Aluminum Powder in three cups, then dump Part A into the aluminum powder and stir it into a very thin paste, then dump Part B into that and stir vigorously for about 30 seconds until the mixture looks pretty uniform. Then I'll usually either pour it straight out of that cup or use a feeding syringe with the end cut a bit shorter to push the resin into the mold.
     
  11. coofunkcurly

    coofunkcurly Sr Member RPF PREMIUM MEMBER

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    Are you shaking your Onyx fairly regularly? I do find the components can settle quickly and not cure correctly when mixed. I like to shake both components well about an hour before I'm gonna use them.
     
  12. Endless Wonder

    Endless Wonder Sr Member

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    Bizarre. It looks like it's just not uniformly mixed in the photos, but it sounds like you're mixing it just fine.

    I would do as zorg suggested and try it without the syringe and see what happens. It's possible it's forcing more of the powder into certain areas than others.
     
  13. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    I'll try it on my existing mold right now, though I'm concerned the resin's working time might not give me enough of a window to fill the mold and get it in a pressure pot. We'll see! :D

    I'm shaking the hell out of it right before use, so maybe that's part of it? Who knows.
     
  14. Endless Wonder

    Endless Wonder Sr Member

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    You might try it without the pressure pot as well. Or try a lower PSI.
     
  15. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    Happy to give it a try, but I'm almost positive I'll have air bubble pitting on the surface of my cast if I don't pressurize it, and I don't have a vacuum degasser or anything.
     
  16. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    I never have issues with bubbles using Onyx fast without a pressure pot. Never used one.

    [​IMG]
     
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  17. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    Well *. Those look lovely.

    I just had a go at using the Onyx I have (slow) and doing the whole non-pressure-pot, non-syringe-feed thing. For reference, Onyx Slow has a pot life of like 5 minutes, so it's still the shortest-life resin I have on hand right now. Maybe I'm adding too much aluminum to the mix (1:1:1) but it seemed too thick to really flow through the mold properly - I didn't see material coming back up out the other hole on my mold. It might also be because my mold is fairly thin and a fairly small piece and the Onyx was just too viscous to get through it. Still playing around with it.
     
  18. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    Also, if you're shaking the Onyx right before pouring, you'll need to let it degas a few minutes to release the trapped air. That may be why you've gotten bubbles before
     
  19. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    What about #3? As I said, the talc destroys any chance of glass or water clear. Maybe don't the molds.
     
  20. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    I always powder my molds with the metal powder when cold casting. Yes, it will take the "gloss" away from your cast, but it doesn't prevent it from being polished
     
  21. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    Yeah, when I say I'm 'powdering the molds', I mean with the same aluminum I'm casting with.
     
  22. cavx

    cavx Master Member RPF PREMIUM MEMBER

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    I understand why one talcs a mold. The parts release easier and it extends the length of the mold. As we both know, silicone is not cheap, so we want to get the most life we can from them.

    Talc is pretty fine, but is the this aluminum powder as fine or more course? If it more course, then it will be roughing the surface of each cast.
     
  23. Risu

    Risu Master Member RPF PREMIUM MEMBER

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    I can't speak to the 200 mesh, but I've got some 400 mesh aluminum powder and it's about as fine as flour. I've actually had some of the same problems with my cold casting, so I've been following this thread. My problem is that even the lightest polishing with steel wool goes right through the aluminum powder and exposes the resin mix underneath, which doesn't look metallic at all.
     
  24. AtrumAntics

    AtrumAntics New Member

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    iv had a similar issue cold casting with bronze powder the powder itself if 400 mesh which is super fine but like Risu when i polish it i seem to polish straight through the powder despite using the 1:1:1 mix ratio i'm also using onyx fast and this sets of to quick to degas and using a syringe to get it into my mold quick as it was to thick to pour in before the flash-off point
     
  25. kwalsh0000

    kwalsh0000 Sr Member RPF PREMIUM MEMBER

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    I have always dusted the mold and then done a light coating of resin with powder (just a light one to back the dusting) before the standard tinted resin.
    The helmet there was just done with 00 wool and is weathered (not even 0000 or metal polish to give it a shine) so dusting a light coats still keeps a shine. Maybe try lighter coats?
    [​IMG]
     
  26. Luke0312

    Luke0312 Sr Member RPF PREMIUM MEMBER

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    You dust the mold to assist with wicking the resin into all areas. I'm not even sure it assists with extending the life of the mold, the best thing to use for that is mold release,.

    Even if the aluminum powder did cause a slightly rougher surface, you're polishing it afterwards, so that wouldn't make a difference.
     
  27. Ein

    Ein Sr Member RPF PREMIUM MEMBER

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    My goodness, I just had a go with using Onyx Fast and barely even had time to get the mold filled. I have no idea how you guys are doing this. Those of you using Fast, are you working in open-faced molds or something? The things I'm generally trying to cold cast are at least two-part molds.
     
  28. Zombie Killer

    Zombie Killer Sr Member

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    Wouldn't it be better to mx the aluminum into part A or B only? Then you can take your time degassing before mixing. I find most of the bubbles come from the resin manufacturing process and not from mixing unless your going crazy mixing. Onyx has always been a pain for me to use do to its super fast setting time. I'm very interested in this thread too, I have some 200 mesh powder from Smooth on to try on my MIB pulsar but this is all new stuff to me. I think I have some onyx around here somewhere I need to try some test pieces. If I can get close to what Kevin Gossette posted I will be happy. Nice stuff Kevin!
     
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  29. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    Thanks!

    With the Onyx Fast, I have enough time to mix and pour into 4 different molds, 3 of which are just simple open-face and one two-part mold (belt buckle). If you're finding that you don't have enough time for a single pour, chill your resin in the fridge prior to using it. That will give you some extra working time.
     
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  30. Circassian

    Circassian New Member

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    Hi everybody

    I have a big problem about blackening my cold cast bronze objects

    There are a few spotty parts on my cast after laying down the cast in black patina liquid you can see the picture below

    http://www.dosyaupload.com/4Qnx


    If you can help me about this I would be very happy
    Am I doing something wrong ?
     
  31. robstyle

    robstyle Master Member

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    That link is either click bait or being misread by my browser. Either way, an older thread that could still use some info.

    Keep in mind I do not use Smooth On products, and the following is based on my own experience and others ive seen first hand. I cant speak for the lucky ones that have had success with the prior mentioned methods and materials.

    With metal powders the mesh number, in this case 200 and 400, just isnt the best for the task. I may be wrong but I assume the reason 425 mesh metal powders are the most commonly sold by retailers is due to its mesh size alone. The finer the powder (mesh) the more airborne it becomes and the need for expensive respirator systems is needed. My assumption is 425 mesh seems to be less likely to cause "lets sue them" circumstances.
    Ive tried numerous times using various metal powders in the 325 to 425 mesh, cold casting, brushing into the mould, mixing into A or B or both.... Ive had no real success with various materials. The only real success I have seen first hand with 425 mesh powders was when the casting material was clear polyester resin tinted black. Basically surfboard resin. Then steel wooled for the metallic sheen to shine through. The issue with casting resins is the inherent soft surface combined with the cure process. To get the sheen you need to scratch the surface. For the material to cure it pushes the metal away...

    I use MPK90 in black. Its a high impact and fast curing plastic. But its also expensive. In the end I know what works by trial and error. Why try and save a percentage in materials when you lose that in trial and error with lower cost materials? I also relegated 425 mesh metal powders to castings that would need to have a magnet attached. For metal powders I use industrial grade in the 10,000+ mesh. When you get into what the term "mesh" means, thats an incredibly fine number. So fine in fact when my lungs were pumped a few years ago from a fluid build up there looked to be a T1000 lurking in the fluid. Once that metal gets into your lungs, it only comes out in death or happenstance such as having your lungs pumped. For an idea of what that metal powder handles like, its like running a felt tip marker over paper, yet its powder. The outcome is no need for steel wool and perfect castings every time. I have tried to explain the process to a couple people but they had no success. Again I use a tried and true material, they used another brand. I wont get into casting in color as I cant for the life of me type that out. Ive even done in in front of an experienced mould/cast person and they were dumbfounded.

    Some examples of castings:

    Strait from the mould knives with the original, cast in color:
    [​IMG]

    Strait from the mould badges and brass knuckles:
    [​IMG]

    Transformers badges, completed but again, no steel wool, just a black wash.
    [​IMG]
     
  32. Friendly flyer

    Friendly flyer Well-Known Member

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    Wow, that's an amazing job!
     
  33. FoamSmith

    FoamSmith New Member

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    You can fix bubbles in cold cast parts. Use the old CA trick but with your metal and not talc. Keep in mind that the repaired section will be shinier when polished since it is almost entirely metal.

    Coldcast Repair.JPG
     
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  34. razorsharp192

    razorsharp192 Sr Member RPF PREMIUM MEMBER

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    Whenever i've moulded metal parts, I always do 1:1:2 so the aluminium powder is equal to the total amount of resin, never had any issues. Alternatively have you tried aluminite metal powder? they work great.
     

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