Castings of engraved parts

[banjo_art]

I have made several Aluminite plastic castings from silicon rubber molds. I'm getting a few tiny pin holes showing on the surface of the plastic casting. Of course when paint is applied, it doesn't look very good. The silicon mold is perfectly smooth so this is some sort of tiny trapped air bubble etc.

I've tried both heating the silicon mold and leaving it at room temp. Also I am following the directions to the letter. This is the tan Aluminite supplied in a 28 OZ package. It sets very quickly, about 90 sec. I do not use any talc since there is no mold release problem.

Are you folks using this same material for castings or something with a longer cure time to allow the entrapped air to escape?

Other than a few pin holes, these castings look great.

Please advise.

Thanks
--Art

 

[Andrew Biggs]

Hi Art

I had the same problem and found that any moisture in the air caused havoc onthe castings.

What helped a lot was a small cheap fan heater blowing warm air over the area I was casting in. It seemed to cure the problem.

Hope this helps

Cheers
Andrew

 

[John B.]

Some people have used a thin layer of paint sprayed into the mold before pouring.
Others have painted in a thin layer of the casting material while blowing dry air gently into the mold to break up any bubbles and pouring on top of this coat.
The fast set stuff is very touchy to any moisture.
I have use a product called "purge" to exclude any air from the part used container before re-sealing it.
But this stuff is a little expensive.
Personally I switched to slow dry epoxy resin. And I paint a little in while blowing with air and then add the rest on the top.
This was a tip I got from Sam Welch and he makes some killer castings.
The epoxy takes a little time to dry but is also a little kinder to your molds and they last longer.
Mike Bissell does a lot of fine casting and maybe he will jump in on this.
John B.

 

[jimzim75]

Hi All,
This will help jewellers who are trying to get rid of the tiny pin holes. Anybody who dose their own jewellery will
have a vacuum chamber to get out air bubbles in the plaster investment used in the molds. The same vacuum works
quite well on Aluminite casting also. The equipment is expensive to buy, but if your handy. You can make a vacuum
chamber, and keep the cost down.

You can use the chamber when doing the original rubber mold also.

 

[KCSteve]

Hmmm....

One of the many, many kitchen gadgets we have laying around is a high-speed marinating kit. The key part is a dish you can pull a light vacuum on (it helps pull/push the marinade into the meat).

A quick Google turns up the 'VacuVin Instant Mariantor' which looks like the thing but Amazon says they're out of stock and they don't know when (if every) they'll get them back in stock. Checking other links brings up things related to the Tila Food Saver food vacuum. I'd bet you could adapt the jar sealing adapter to the lid of a Tupperware type dish of the appropriate size. You won't pull much vacuum but it'll pull some.

 

[John B.]

Hi KCSteve.
Thanks for the memory jogger. I had kinda forgotten about that and you are right on with the food machines.
I have used a vacuum seal food machine in the past that I bought in a thrift store.
Some people must get these for gifts and just don't want them and wind up donating them to charity.
It had one outlet with a plastic hose for sealing the lids on the rigid plastic jars that came with it.
I just pushed the end of the hose on to a little fitting that I put in a see-through plastic salad bowl that I bought at Ikea for $5-$6.00.
Set the bowl down on a soft rubber table pad and it pulled enought vacuum to do a pretty good job.
Just be sure to remember that the stuff swells up when you pull avacuum on it and don't over fill the molds at first.
You can add a little more material to top them up later because you have pulled the air out of the portion that is in contact with the design and the back doesn't matter as much.
Also, someone else mentioned mixing in a plastic bag by gently kneeding a two part mix together, cutting off the corner of the bag to get a slow, smooth pour.
This works very well too, much better than mixing in a jar with a stick and pouring from the jar.
John B.