HELP - Molding and casting problem

[Mego Magyar]

I've started doing some molding and casting and have run into an issue and am wondering if I'm doing something wrong. I'm using OOMOO 30 and Smooth Cast 300.

When I make a mold of a soft ( rotocast? ) item, on what should be a smooth surface I get little bumps in the mold. These aren't air bubbles that mad a pocket but the opposite, extra mold material on a smooth surface. When I make a cast there are tiny potholes on what should be a smooth surface. I at a loss on how the mold compound can appear in a place that was smooth.

[megowgsh]

Please post before and after pictures of what you are describing

[David Lee]

Sounds like moisture to me... ya gotta mold in a dry place, outside won't do... Moisture is your enemy!

Dave

[piecemaker]

I'm probably completely wrong but this is the only thing i can think of that Might cause that problem.

Instead of a visible air pocket which would cause a bump on your casting.(top pic)

You may have a hidden air pocket just below the surface of the silicone.(middle pic)

When you pull the head out,the hidden air pocket expands the silicone wall out and when you cast something you end up with a pothole in your cast head.(bottom pic)

[img]http://piecemaker.smugmug.com/photos/i-cND6xxH/0/M/i-cND6xxH-M.jpg[/img]

[Mego Magyar]

Here is a photo of how the cast comes out. If I look inside the mold there are bumps in the mold where this happens. The original is totally smooth. I'm doing this doing this inside and these are fresh molds and castings. I've had a few of these on casts in the past but this was the worst.

[madmarva]

Rocketeer?

[mikeoz]

I've had a similar problem when I've used too much mold release, or, have not let the mold release dry a bit before molding something.

[Mego Magyar]

I've had a similar problem when I've used too much mold release, or, have not let the mold release dry a bit before molding something.

That could be it, if I remember correctly I did use a good dose of the release but I thought I had let things dry enough.

[dr_cyclops]

I use gloss-coat (and allow at least two days to dry) on projects to be molded.
A thin layer of baby powder in the mold before casting, has been a great help, (with the products I'm using).

[tylerh]

With smooth on, you actually shouldnt need mold release - Silicone doesnt really stick to anything other than silicone -

I would wash the original you are molding with soap, water, and maybe rubbing alcohol, and make sure its REALLY dry, then try again -

I assume you are doing a 1 piece mold then cutting the head out of the silicone block??

Also, do you use a pressure pot or a vac chamber??

Tyler

[piecemaker]

Are you molding/casting these under pressure?I knew i had a similiar problem but not to the extent you are showing with some early molds of mine which were not pressure cast.I found one and cross sectioned it at the "bump" and there is an air pocket behind it.Maybe not the problem you are having but another thing that can cause the same problem.

[img]http://piecemaker.smugmug.com/photos/i-NdgrMdf/0/L/i-NdgrMdf-L.jpg[/img]

[ctc]

Hmmmm....

Weird. If the little pits on the parts are 'cos of portrusions in the mold, you might not be letting the mold cure long enough, and you're getting pulls when you remove the part. The pits on the part look like what you get with an old mold that's strting to crumble, but a fresh one shouldn't do that.

I've never used mold release, but I put a thin coating of baby powder in the mold before casting. I don't treat most originals before making the mold, but I use GI 1000, and it sticks to very few things.

Don C.

[Captain Awesome]

I don't use mold release either. I have used a little baby powder now and again and that seems to work well. All I usually do is wash the part real well anake sure it's dry. The cast usually comes out pretty well. Having said that, iI have had the same problem as you're having. I usually do a lot of prep work on my heads anyway, so if I ever do have that problem, I drill the spot out a little and Milliput it.

[megowgsh]

Using the correct equipment is always the right answer. You need to be using a pressure pot to make all of your molds. I NEVER have these problems unless the item I am molding is already flawed.

[Mego Magyar]

To answer questions:

1 piece mold that I'm cutting open.

No vacuum or pressure pot - a paper cup on my kitchen table.

The bumps in the mold are firm and not half loose pulled away pieces. I haven't cut them yet to see if they are air bubbles, will be doing that later.

EDIT: I cut open the mold and it is air pockets. Don't know why it's happening this much now when it wasn't before.