Carcass casting/ foam problem

[AFWS]

First time carcass casting a medium sized critter  I've got issues with the foam being brittle(It's  4lb foam) one leg broke 2 inch from end of foot(didn't put a wire in it ) the other broke down the seam most of the way and bout 2 inches from foot  My mold is plaster if that helps and I did put mold release if that has something to do with it.There is gaps where the two sides meet(foam comes out the sides of mold) ,if that has anything to do with how strong the part is  I've marked my cups to make sure my two parts are the same and waited about 30 to 40 before I opened the mold. Any ideas 

[clifftax]

How old is the foam ?

[AFWS]

How old is the foam ?

Just got out of the box

[Mike B.]

The moisture in a plaster mold could possibly have a negative effect on your foam. The mold should be sealed prior to using it just the release alone may not be enough.

[Doug Bridges]

Instead of just using pure foam, of which you will never get a good firm foam or a skin on the foam in a carcass cast, use bondo in the legs and to skin the form in the plaster mold. After you skin both sides of the main portion of the mold put them together and work more bondo on the seams from inside the form. Then, once the bondo as set, pour foam into the remaining cavity of the mold to fill the space.

The legs should be 100% bondo with rods or wires to reenforce. Run the rods up into the body cavity also.

Now you will have a surface to break the plaster from. Breaking the plaster is a chore. Use a repeated tapping to fracture and break the plaster instead of trying to break it away all at one time.

After all the plaster is gone, you can use bondo or other methods to do the correctional sculpting of the form. Or, use oil based clay to correct and sculpt in more detail and then recast over the clay to refine your casting.

[SCT]

If your molds are plaster with no real strength you won't be able to pour a real dense form. That's why commercial forms are cast in a fiberglass mold. Having said that though, try wrapping the mold in duct tape after you seal it and use release agent. Put some wire in the legs of the mold as well. Pour a larger quantity of foam in so some pressure can build and leave it in for at least 45 minutes.

[Leaf-art]

Brittle foam has to do with mixing. Too much of one part = brittle, too much of the other part = mushy. Unfortunately I always forget which part does what. If I'm pouring a smaller animal (ie not much foam) I use a large syringe to measure out my parts to get them as equal as possible.

[Jim B]

You didn't say how long you let your plaster cure before using the mold.A plaster mold that you plan to pour foam in should be allowed to dry for about a week,then seal it well.It doesn't take much moisture to mess up a foam pour and you know how much water went into that plaster.It takes time for all that moisture to leave.When plaster is completely dry,you should be able to sand it with sand paper and only make dust,it shouldn't stick to the sandpaper.

[John C]

The mold should be reinfroced with burlap or even fiberglass mat.

The mold should set for about two weeks in front of a fan.

Then several layers i..e. coats of SHELLAC.. I always thin the first couple of coats of shellac 50/50 denatured alchohol.. this will penetrae the plaster and it does add some strngeth tit he surface.

Make sure the shellac has dried several days

Then polish with 2000 grit sand paper, wet sanded..

Wax and pull several partial cast wax between each cast..

This will season the mold..

You can put wires in the form or not, althread works well too. (all if you plan no more changes to the form)

Make sure the foam is mixed 50/50 mix until it warms in a room at lest 70F.

You can warm the plaster mold some too.  Clamp very firmly and it should work perfectly.

[AFWS]

Hmmm were to begin  only allowed mold to dry for two days(beginning of disaster) didn't seal mold  didn't put wires(on purpose) after reading some of the earlier responses I sealed the mold then put release on  tried making both back legs yesterday and it separated at the seam(4lb foam)  got a book that has sculpting,making fiberglass molds, making "FORMS"  and other stuff.It said if you try to pull the form out to early something would happen(can't remember) and if you LEFT IT IN to long it would become BRITTLE and break(mystery solved? )  2 time around I used 2.5 lb foam from WASCO and pulled it earlier and all went good.This morning I made the body with 4 lb, nothing stayed in the mold ,but it's NOT as STRONG as the 2.5 lb legs that I made yesterday.Both the back legs and the body molds was made the same day.So is it possible I got some bad foam? cause even what's leftover in the cup is brittle too  my cups have lines on them ,so I know I'm getting the same amount of both parts 

[Mike B.]

You could possibly have bad foam. Is it very old? It is susceptible to moisture contamination just like the urethane casting resins. So always keep the lids on when not it use. How much are you mixing up a one time? If you are just a little off on a small batch it is a much greater % than on a large batch being just a little off. Are you measuring in two separate containers and then pouring into one? The hardener side is a little thicker and more will stay in the cup than the resin side when mixing together.
Just different thoughts.

[kinetic]

This is really ghetto but it might help.

I once spent about a month on a plaster mold, drying it out and shellacing the hell out of it. Even with a good amount of mold release the cast still stuck to the mold and was ruined. So after large amounts of swearing I decided to cut a thin trash bag into halves and line the molds with them and letting the leftover mold release keep it in place. I used a ton of straps to seal it up, poured more foam than the cast needed, and the pressure of the foam pushed the plastic into all the crevices. The edges of the mold started to come apart, so I did the only (in)sane thing I could think of, I sat on it for half an hour. The mold opened up nicely and I peeled off the plastic from the mold.

This leaves wrinkles in the form and it's not as accurate, but I carve/sculpt the details back in. I've had good luck with saran wrap for smaller things and I've been able to make a mold and cast in the same weekend without all the mold prep. I'm just a hobbyist, and I know this is a really ghetto method, but I've never had a cast breaking getting out of the mold.

But if your problem is a brittle foam, can't help ya there 

[Becky P]

Jan Van Hoesen did a carcass casting seminar here and she didn't let it set for two weeks before foaming. Her release agent was paper towels dipped in a powder clay/water slurry so there was lot's of moisture. The foam came out just fine. She did use some straps around the plaster mold to hold it tight while foaming.

[ShakyArcher]

I've poured tens of thousands of turkey head forms (used to work for a prominent freeze dry outfit) & here's my 2 cents...

1. I agree that it could be bad/old foam. Who knows how long the stuff sits on the shelf at the supply house.
2. Moisture & temperature affect that foam a great deal. Example, a cool room can make it set up funny.
3. A weak mold or clamp job could cause separation at a seam. If it's a good sculpture or one you like, consider making a fiberglass mold.
4. Accurate mixing ratios are crucial, especially in a small cast. I always used a digital postal scale in a plastic bag & measured the foam by weight.
5. De-molding too soon can cause seperation at a seam, too, but that doesn't sound like it's the problem.

[bmdakk (Bryan)]

Jan Van Hoesen did a carcass casting seminar here and she didn't let it set for two weeks before foaming. Her release agent was paper towels dipped in a powder clay/water slurry so there was lot's of moisture. The foam came out just fine. She did use some straps around the plaster mold to hold it tight while foaming.

we learned how to do this at school last week and that is exactlty how we we were taught to do it. casted a fox form and from start to finish was about 3 hours( from mixing plaster - to the actual foam mannequin) i was taught to put in a little salt in the plaster mix to help it "kick". we actually used a dry potters clay mixed with water and layed paper towels on that and then brushed on smoe more of the clay. worked great and didnt have any problems with the form being brittle.