Casting and Molding Troubles- Bubbles, Undercuts, and Pressure

[nick daring]

I'm trying to mold and cast an 8 inch tall statue and have been running into some problems and would love the forum's experience and knowledge on how best to approach it correctly.

The sculpture in question-




Here's my journey so far-

Brush on mold with Rebound 25 with Plasti Paste mother mold. The silicone mold is simply slit up the back. Yes, I used the brush-on starter kit. My first mold!

View attachment 255212

Not bad! I did get a little too excited and started slitting the rubber before making the mother mold. Dumb, I know, but it lined back up okay.

So I made my first casting out of Smoothcast 300-


As exciting as it was to get a big chunk of resin in the shape of my sculpture there were problems. Mainly bubbles.

There were a couple crevices in the silicone mold that caught a couple bubbles during the molding process but a pointy file can pop those out without too much trouble. Not bad but not perfect.

The main issue was bubbles in the resin casting itself. This statue has a bunch of pockets and crevices for bubbles to get stuck. The worst area was his (it's?) elbows and a couple spines on the back. Big undercuts.



To fix this, I started to fill the mold halfway and tilt the mold in an attempt to "burp" the trapped bubbles in the elbows and then fill the rest. It sorta worked a but It was hard to tell if I had got them out or not during the pour. Inconsistent results.

I was also getting bubbles in his face and other places that weren't too badly undercut. Just bubbles that wanted to stick to the surface.



The best trick I've found so far was using talcum powder as a release agent. It did a pretty good job of keeping most of the surface bubbles off. Still not perfect though. Again Inconsistent results.

I decided to remake the mold. This time I'd make it a 2 Part mold, try to not get bubbles in the mold itself and I'd put vents in for the elbows.



Again Rebound 25 was used and a fiberglass mothermold was built with an MDF frame. I thought the built-in stand was a pretty clever idea.

The elbow vents were made with two wires that ran from the elbows to the top of the mold. I figured I could just snap them off and pull them out with needlenose pliers after each casting. Sadly I think the wire I used was too thin and no helpful venting occured.

Also for some reason I ended up with more bubbles in my silicone mold than the first time. I may have gotten sloppy and impatient from this being the second time doing the same subject. Not sure.

I was also a bit dissapointed with my seam line. The flash was a little more pronounced than with my single slit version before. Nothing terrible but surprising since I figured the slit method was inferior.

I then struggled with a resin that was inappropriate for the job for a while (Smooth-On Onyx FAST is troublesome in larger pours). I realized after a bunch of wasted resin that the fast curing resin was just getting too hot and deforming and blistering the statue as it hardened. Really weird.

I also tried to slush cast an initial layer to get a clean surface free of bubbles and then do a larger pour to fill the void. I had mixed results and it made the whole process more complicated and take longer than I'd like.

I was now desperate to get consistent good castings so I made a pressure pot.



I cut down the base of my original brush-on mold to fit and made a quick casting only to realize the vital importance of a bubble free mold when doing pressure casting.



Every subsurface bubble in the rubber expanded and made a lump in the resin. In many places the pressure actually popped the trapped bubble and pulled the resin inside creating hundreds of little resin pebbles forever stuck in the mold. It would be super fascinating if it wasn't so heart breaking.

I will say that the pressure did shrink the giant bubbles in the elbows to a much managable size though. Still not enough though.

I'm now at a point now where I'm thinking about making a third mold.

A big block mold that is made under pressure with a single slit up the back and 2 elbow vents that lead to the main body/block.



Hopefully I can just pour my resin, set it in the pressure pot. Pressurize for 30 minutes, remove and have a good casting.

Before dumping a 100 bucks of silicone into this new big fat mold I thought I'd consult the board with my dilemma.

What molding and casting methods should I be using for this project? I've approached it from so many angles that I'm not sure how to go forward confidently.

Thoughts and opinions are desperately desired.

 

[robn1]

Pressure will definitely make a better mold, every mold I've made under pressure has been perfect. It may also work with Rebound, I've never use it so I'm not sure. But if you can brush it on and then pressurize it should work.

If you go with the poured mold, keep it as small as possible to save on rubber, them make a mold jacket like before.

 

[nick daring]

I've also thought about doing a matrix mold so that it can be pressurized. Something similar to this Hellboy bust mold- Hellboy bust and matrix mold making tutorial

But honestly I'm to the point where I don't mind wasting some silicone to get a good solid simple mold. Time is getting more valuable than silicone now.

I had no clue you could pressurize a brush-on mold. Would you do a layer then drop it into the tank then wait for tackiness and pull it back out, etc? That seems like you'd asking for something to go wrong while moving your model back and forth to pressurize each layer. I would also think the thickener in the brush on layers would keep the bubbles from pressurizing as well.

I think I might just go ahead and pour the big slab tomorrow and chop up the old molds to fill in the gaps on the side to help out on silicone usage. I should have just enough left over to fill the octogon mold walls I have built. I'll mix up about half the silicone I need, pour into the void, and start dumping in chunks of old silicone to raise the level as much as I can. Then mix up enough to top it off.

Getting a good mold will solve most of my headaches at this point I think.

If this one goes belly up for some reason I should do a matrix mold just to say I've used every method available on this thing.

 

[Leigh]

I know you've been following my progress & you know my limited experience but I thought I'd offer my 2p worth

With some of my moulds I had some trouble with bubbles forming in undercuts. I used a 1mm drill to make holes in the rubber & I've had success. I also found that a very small hole just doesn't work, obviously you don't want massive holes though. I didn't drill into the jacket but I did have to use petroleum jelly on the inside surface of the jacket so that any seepage wouldn't stick.

Another trick I've used is slitting the mould in the problem area, I've cut a slit right through the rubber when I've done this but not the jacket. With the slitting option you must make sure you have a good even covering of talc on the mould & right into the slits. Its important that you get it into the slits! the talc draws the liquid plastic into the slit & will form a very thin flash line but the void you were getting should have gone. at the end of the day IMO its much easier to remove a little extra flashing than fill a hole

Another option if you going to make a new mould, make a sprue that runs from the tip of the elbow to either side of the pour spout, make it at least 3mm (1/8") thick getting wider as it goes to the top. If you make your mould a 2 part mould (front & back) you could cut the sprue into each half of the mould.

HTH

 

[robstyle]

the best advice I can give is dont over think things. The evaluator isnt needed but since you have it, try evacuating the raw material A and B separately BEFORE mixing. Then from that point work at a slower more controlled pace with the material from adding A to B, to mixing to pouring. Dont invite any air into the material even during the rotation of the mold to cover its undercuts and cavities. Im not sure how well the material will do in multiple pours but one thing to try is a 1/4 or so batch to faux rotocast with, then simply pour the second batch. This could however lead to leeching. Again I dont use the materials you are working with.

The talc powder will attract and absorb the material but its no solution.

In the end however there is only so much you can expect from the material. Everyone here is a semi blind fan of Smooth On as its widely used, but a fan I am not. Ive had many issues with their products in the past from contamination to shelf life while working for others that support the brand. Each time the solution was to switch materials after every attempt to correct a problem failed. The issue at hand here could easily be contamination. have you cleaned the mold proper prior to casting? Are you tinting the material and if so with what and how much? Is mold release being used and if so, why?


Ive made and poured many molds then run parts out of them with more undercuts than your dealing with here. But I use materials I know will perform the first time every time. Its the justification of spending more on materials where its needed.

 

[mfxreplicas]

The talcum helps but also thinning the resin with a little acetone (start at around 10%) and doing a slush/hand roto cast first with this thinned mix. It makes it harder for the bubbles to stay in the mix. Then slush a couple of layers behind before filling solid. Also always use the absolute minimum amount of resin to get an even coat rather than having lots of excess. Even do a hand roto and let the excess drain and then de-mould this thin layer and weigh it so you know the absolute minimum it takes to coat the surface. I'm not overly familiar with Smooth cast and its' properties but the resin I use for something like this is easi flo 120 as it has a slower cure curve because its designed for roto casting rather than having a snap cure which some resins have. Its also very fluid to begin with but even so still needs that bit of acetone.

 

[bookface]

There's a couple of things I can suggest before you go dumping more cash into a third mould. First, you say you've tried slush casting a beauty coat into the mould. Have you tried brushing a beauty coat in? Take a disposable brush and try getting in there with that, stipple it into all the details and crevices on the mould. I've used this for things with really tiny detail on them, way more detailed than what you have there, and it works like a charm.

Second, have you tried using a filler? It's surprising, but straight resin can actually hang onto bubbles more than a resin that's been mixed with a powder. If you're using smooth on stuff, try the Urefil 3 filler, it's also called fillite from other sources. It's like very fine grey sand, and I've found that to help when trying to eliminate bubbles in a casting too.

I haven't used smooth on stuff in years now, but it could also be worth trying something with a longer cure time. As I remember it, the 300 is very abrupt in curing.

 

[andy19422]

Why not try slush casting it? The second mold look easy to bolt a lid too, then when it's set you can back fill with resin to make it heavy. You could even use plaster for this to make it cheaper

 

[nick daring]

I just wanted to update you guys with how things went.

I went ahead and created a third brush on mold using the techniques that worked best from my other two molds.

My first two silicone layers were very carefully dabbed in trying to avoid any trapped bubble in the deep tight crevices. The results were pretty clean. Only a few that that create resin bubbles that pop out very easily.

I cut a slit in the back instead of doing a two part mold and the results are really nice. The seam is almost completely invisible after casting.

And concerning casting I went with people's suggestions here and slushed two thin layers of resin into mold before filling the cavity and the results are very good! The undercuts pose no problem anymore although it is a bit of work to get a casting poured up.

For the mother mold I recreated a slightly smaller version of the previous version. A bit less cumbersome easier to handle when rotating by hand.

Thanks for all the advice and help everybody. I'll put up some pics soon.