Hey stopmotimators, last June I made a dozen or so puppets. They turned out great. Now, a month and a half later, I'm having repeated collapsing issues using the same setup.

This is my setup:

  • Originals made out of Van Aken plastalina (sulfur free) and wood.
  • Plaster molds (densite plaster from dickblick.com)
  • Vaseline used in plaster casting process as release agent
  • Painted a latex skin inside the two part mold
  • Used a wire armature held together with plumbers epoxy for the puppets
  • Poured in Smooth On FlexFoam-It III

The only variable is the time - it is hotter and more humid now in August, though since I've started casting again this month I've experimented switching from AC to none AC, from a room with a dehumidifier to a none climate controlled room and had the same issue no matter what.

Today, I demolded my creations after only letting them sit in the mold for 45 mins or so (I've left them in for as long as over night before). They looked pretty good right off the bat, but they had a little bit of collapsing around the legs. I returned later to find that they had collapsed further since demolding... they all look like wrinkly old men and not plump little puppets! So this time, at least, the collapse happened not right away but over a period of time.

Also of note - the Smooth On foam that's left in my plastic mix cup does foam up and does not collapse. So it seems the issue is with my mold or setup. 

I called Smooth On, they were stumped. Any suggestions?? Very infuriating, especially considering my success in June.

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Hi Michael,

A few years ago I had exactly the same problem with my foam.I can't remember exactly what brand I was using but we couldn't figure it out.Eventually,we found out that the supplier hadn't been mixing the chemicals properly before they were decanted into smaller containers for sale and shipping,resulting in an inconsistant mixture before we even started using the product!

My advice would be to test using another brand of foam and see if the results are the same,at least then you could eliminate one possibility!

Good luck!

Hi,  I am trying to read through the lines about what you are talking about . First off you don't want to use vaseline as a release for foam latex or latex rubber . If you are just using latex rubber . You want the mold to be dry and porous so it dries fast and just doesn't sit there. The vaseline is plugging up your stone surface not letting moisture absorb and evaporate. You want to use a release like Stearic acid better known as Foam Latex release sold by Monster Makers or Burman Foam Latex give the mold a couple of coats letting each dry , dust off the excess with a chip brush. before applying the next coat . 3 or 4 coats should be good enough . A new mold is still wet and moist, so you need to bake out the mold to get rid of excess moisture before running your first puppet. For your puppets you may not need to have a latex rubber skin onthe outside of your foam latex. If you run your foam latex fairly dense ( low volume ) so it is firmer and denser you don't need it. the foam Latex will give you enough of a skin . So I would say no latex skin , just run your foam latex in the mold . Your foam collapse problems are probably due to your foam not gelling well, a good trick to use is to get foam latex pigments available from the same suppliers and add 10 drops to the gelling agent , mix into the gelling agent so when you add your gell at the appropriate time you can see when it has mixed in well and is a solid color. You may not be refining the foam latex long enough to get rid of excess ammonia, remember that your gell to ammonia ratio has to be just right for you foam latex to gell. As a matter of fact it is always good to do a first test batch to make sure your foam latex will set up, before you commit your mold and armature, there is a lot of prep to do before you run your puppet, why waste it !  Hopefully Monster makers or Burman foam have given you good instructions for running foam latex, remember too much ammonia in your mix and it won't set up, too little ammonia in your mix when you add your gelling agent it will set up faster than you can blink. You want the right amount of ammonia so it will give you enough working time , use a stopwatch to time your mix so you know how much working time you have as well. Test batch first always before you commit your mold . Last but not least keep your foam latex base and components away from cold rooms or freezing temperatures , your stuff will go bad , your foam latex base will freeze it has water in it. Always shake your components thoroughly before using, it is good to date your stuff so you know how old it is and whether you need new stuff . I can go on and on about foam latex but it sounds like your foam collapsing is due to this, tis Smooth-On Flexifoam sounds like a polyurethane foam 2 component product if it collapses the components are bad due to moisture. Unfortunately that happens with these products, they sell a product called BURP which purges any moisture and air from your containers for storage purposes. Once you open these containers it is only a matter of time ! till they go bad..... hope this helps .

I second all that.
Maybe you meant the vaseline release was only to stop the second half of the plaster mould from sticking to the first half, and was not applied to the part of the mould where the latex goes? I prefer to use clay and water for that - easy since I use water based clay for the clay wall when making the mould, so I always have it right there. I don't like oil or petroleum based products anywhere near latex.
For release in the actual inside of the mould, I use the stearic acid based release that comes with the (Burman or GM) foam latex kit, or I use a solution made by shaving a bar of soap into hot water. I don't usually put a skin of latex on first, just the foam. When I have tried that, I've sometimes got a partial foam collapse inside, with parts shrinking and distorting. Possibly it's because the latex skin seales it so,the moisture is trapped inside - plaster with just the soap or supplied release can still breathe a bit, which is better.
Gelling is a combination of mixing time and amount of gelling afent used. A new fresh batch has more ammonia in it and takes a bit more mixing time or gel (but I still use less than the instructions say to). I keep notes so I can see how much I used on a similar mould in the past, and how long I did each phase of the mixing, plus how it turned out. The difference between gelling before you can get it out of the bowl, and not gelling at all, can be a fine one.

Very helpful advice, except FlexFoam-iT is a polyurethane foam, not latex foam. Just mix part A & B, pour in your mould and leave to cure.

Are you using the same stuff from June (same container) because I don't think it keeps very well once it has been opened.

Edit: Just noticed, this is pretty much what Richie mentions at the end of his story (I didn't read it to the end 1st time becuase he was talking about foam latex). :)

The collapsing problem with urethane foam is very common and usually the casting can be saved. With large castings that do this, punch it with your fist. I know that sounds stupid but you can puff the foam back up by beating on it. For a small casting like a puppet, you should be able to simply squeeze and unsqueeze the parts that are collapsed. Try this and you may find that the foam puffs back up again. What I believe is happening is that there is some sort of vacuum effect happening inside the cell structure. By squeezing it you are working air into the cells. I could be wrong about what the squeezing does, but it does work. Try it.

I don't believe your mold has anything to do with the problem since the flexfoam is not touching the plaster, it has to be either the flexfoam is a bad batch (got contaminated) or the latex you're using to skin the mold got contaminated and is reacting badly with the flexfoam.

John Douglass said:

With large castings that do this, punch it with your fist. I know that sounds stupid but you can puff the foam back up by beating on it.


I know this may not be the case but are you mixing the right ratio? Being rich in one part or another can give you different qualities. I just tried this tonight (not mixing 1:1) and got the same effect. But what is different is that it is almost more like there was a vacuum effect going on. It is looks more like all the air was sucked out of the piece from the inside instead vs. not having enough to hold its shape.

The squeezing and punching technique works best when the foam is very fresh and new and the latex has not taken on the memory of the collapsed position. The longer it sits collapsed, the harder it is to get it to return to its proper form. Glad to hear you had some success.

I used to work with the top foam guy in the UK, Dave Chagouri, and he could often be seen to put a board over his larger foam casting and walking/jumping on top of it to push out the excess gases as a preventative measure to shrinking and collapse. :)

Dear Michael,

Just read your post. I am new to the world of Smooth-On FlexFoam and planning to use it for the same procedure of making stop motion puppet as yours. I am a little confused about which FlexFoam it (III, V or X) to go for. Have you tried the other FlexFoam (V or X) types for your puppet? And are your puppets the in the 8-10 inches height?

Hope your problem got solved. How did the foam respond to the armature movement? Did the latex (as outside skin) and FlexFoam III (as the inside volume along with armture) combination work well to the subtle animation movements of the character? 

Wish you the Best,


I believe the 2 part urethane foams have a shelf life, like most materials. I think that is what the smooth- on stuff is? But generally i find mine becomes unusable because the brown stuff has crystalised. If the 2 parts are still liquid it generally works. Don't know.

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