Pressure casting mold?

[Joe S.]

I have some of that pink silicon rubber from smooth on that I am trying to use to make a mold. Ed (parklandturner) made me some cylinders that have a rooster engraved on the end, and I'm trying to make a mold so I can cast more of them to make finials. The problem is I keep getting air bubbles in the engraving so it looks like the rooster was peppered with a shotgun! I think casting under pressure would solve the problem, but I have no setup to do that. Are there any one time workarounds I could use here? Thanks for any advice in advance.

 

[commercialbuilder]

I have used the pink molding material from a different supplier and I found that if I cast the item with it facing up it worked better for me. I also used a vibrating sander under the mold to help get the bubbles to rise away from the item being cast. I always poured the pink material into one corner of the mold very slowly to let the air escape. Good Luck
Mike

 

[Joe S.]

Harry (longbeard) suggested that I heat the rubber before pouring it, so I'm giving that a try now.

 

[Ed McDonnell]

Hi Joe - I would use vacuum, not pressure, to solve the bubble problem with silicone. I'm guessing if you had a vacuum pump you would already have tried that. Here's something else you can try.

Mix up a small amount of silicone, being as careful as you can to not introduce air. Yeah, I know...good luck with that...stir, don't beat.

Pour the mixed silicone out into a thin sheet (if you have a nylon cutting board or something similar this will make this part less messy). Use a pin / toothpick / whatever to pop the bubbles or move them out of the way so you have a bubble free area. You could then try pouring your bubble free resin into your mold, but the stuff will probably be too stiff to pour easily at this point.

Instead you could try to transfer this bubble free silicone to your mold using a popcicle stick that you sanded to a thin edge (like a spatula). Just fill the rooster flush. If any bubbles show, use the pin / toothpick. When the silicone has cured, pop it out leaving you with a little silicone rooster (those legs may be a little tough to get out).

If you have a whole rooster, then set up a vertical casting mold and glue the rooster to the bottom of the vertical mold. Now you can pour resin. And make your finial blanks.

Alternatively, you might find the silicone putty smooth on sells (or the castin craft stuff at local stores) to be a much better medium for your purpose. You can stuff the putty into the rooster cavity, insuring a complete fill, and then build your mold cavity around it.

Good luck.

Ed

 

[Jim Burr]

Harry (longbeard) suggested that I heat the rubber before pouring it, so I'm giving that a try now.

I'd guess an issue might be that your using rubber???!! How and where did you steal that?

 

[Joe S.]

The pink silicon rubber from smooth-on. I think it's called moldmax 10 or something. I think smooth-on has some liquid mold making materials that might work...