Cold Casting: What am I doing wrong?

[Ein]

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:





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?

 

[zorg]

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

 

[Luke0312]

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.

 

[Ein]

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.

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.

 

[Daniel Nelms]

@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.

 

[Kevin Gossett]

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.

 

[cavx]

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.

 

[Ein]

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.

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?

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.

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.

 

[Endless Wonder]

What your process when mixing the resin and powder?

 

[Ein]

What your process when mixing the resin and powder?

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.

 

[coofunkcurly]

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.

 

[Endless Wonder]

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.

 

[Ein]

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.

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!

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.

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

 

[Endless Wonder]

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!

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

 

[Ein]

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

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.

 

[Kevin Gossett]

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

 

[Ein]

Well damn. 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.

 

[Kevin Gossett]

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

 

[cavx]

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.

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

 

[Kevin Gossett]

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

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