Author Topic: Another Polyfoam question Crazy Clayer
posted 12-27-2000 07:39 PM
Does the mold you pour it into need a place for the polyfoam to overflowif you pour too much in?

thanks a lot,
peter Murphy.....I need to know soon im finishing the mold today
http://members.spree.com/sci-fi/crazyclayer/images/bitehand2.jpg

crazy lou
posted 12-27-2000 08:20 PM
all my polyfoam molds have an escape hole so it doesnt overpressurize the mold...better to have to trim then to have to start again

later
Tom

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Jeff F
posted 12-28-2000 01:58 AM
One or more escape holes (depending on the size of the molds) is necessary. Even if you calculated the amount of foam needed to fill the mold exactly (including expansion) the air in the mold needs somewhere to escape.

Drill one or more escape holes on the positive side of an appliance mold - the surface that will be glued against the skin. Place escape holes in the most unobtrusive spot(s) for props. Clamp the mold shut. Cut away the excess foam that is extruded after curing is complete.

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Jeff F - Moderator
Magic and FX
Amazing the Masses

morpherguy
posted 12-28-2000 08:19 AM
Well, it really depends on what you want to accomplish with the polyfoam. If you just want to use it to fill a void then there is no real reason to have a closed mold. If all you want to do is backfill a latex or urethane shell then go ahead and drill holes so the exess foam can escape.

If you are trying to produce a polyurethane foam casting that has a good skin then you will want to have back pressure in the mold. In this case, the only vent holes you will want are the ones that are needed to make sure that trapped air does not keep the foam from expanding into all areas of the mold. Very small holes that are not much bigger than the thickness of a paperclip. The foam will expand and fill the mold. When the mold is full, the foam will keep expanding and will create pressure that will help cause the foam to form a nice skin in the mold that will capture all of the detail. Of course, it depends on the foam that you use. Some water blown foam does a better job at forming a skin than others. None of them is as good as the old FC blown foam systems but, unfortunately, they are no longer available... There is a company in California called Foamex that has many varieties of urethane foams. Find them and call them. They will probably be willing to send you some free samples. Their foam systems come in just about any color you want and they will custom mix colors for you..

Jeff F
posted 12-28-2000 03:19 PM
I've never had a problem with a complete skin forming using cold foam in a mold with escape vents. I have had foam turn out denser than I wanted with insufficient venting.

I play it safe and mix enough to have plenty of run off, it ends up being cheaper than doing repeated castings trying for a complete skin. Experiment with the brand and density of foam until you get the results you want.

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Jeff F - Moderator
Magic and FX
Amazing the Masses

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