low cost casting slurry

Looking for a casting slurry that can be poured into an open mold that can take up to 200 degrees f. for short periods with a shore D of 55 or better of that costs under $10.00 a gallon.

Here are some suggestions I have heard and would like others:

First apply top or face coat to master positive,

then pour a mixture of lightweight drywall compound w/ fiberglass or hemp and latex

epoxy facecoat then pour polyester resin w/ microspheres and alumina trihydrate

plain old urethane (9lb density), pour and walk away

hydrophobic epoxy face coat and pour ultracal 30

sandwich construct and vacuum bag high heat epoxy facecoat, then tooling gel, then lay up fiberglass and top with tooling dough.

Many of my production mold halves require 5 gallons of displacement and the epoxy systems are too expensive, the sandwich construction is too labor intensive and plaster runs the risk of turning into dust. These are production molds for making 24' diameter urethane gaskets. In the past, manufacturers have used urethane and poured into the master with form ring and wait 8 hours. This seems the simplest of approaches but the urethanes are $20 a gallon and more and we have to make several hundred production molds.

I sure would appreciate anyone who can suggest a slurry that can be poured 30 inches in diameter and 2 inches thick that can take low heat, a production environment and once all the goodies are mixed together, creates a viscous liquid under 15lbs per gallon for under $10.00 a gallon.

Many thanks.

Cheap resin likes to shrink and maybe crack - all the sand stops that. The ceramic bubbles are cheap, but not as cheap as sand - there are all sorts of fillers - crushed walnut shells is another lightweight one. The catalyst is just the normal MEK, but you can goose it a little hotter than normal because of the sand. Maybe the molds would be easier to handle with metal casters, find those galvanized steel 'channels' at a plaster/drywall supplier - they make inexpensive tracks for the casters - strong and inexpensive. Ideally, and eventually, you could have your whole setup like a giant train set, just make sure all your benches are the same height and level. Another is overhead gantries, you move the molds with a block and tackle sliding thingie. You definitely don't want the backing resin to shrink, it will pull and distort the gel coat. The smell will go away, or you can wash them in warm, soapy water. To drastically strengthen the molds, when you are at the plaster/drywall store, get some expanded galvanised mesh, and cut one piece to fit flat in the middle of the mold, put the other half of the backing mush in- now it is steel reinforced.

Remember that no mold is perfect, anticipate the fact that you will have to patch the surface chips etc. A 'bondo' made with epoxy is far superior to one made with polyester. 3m has a marine bondo made with vinylester that I have never used but sounds good.

You have just received over $500 worth of advice in less than 500 words. At least name a mold after me.

Industrially yours Gary ( factory consultant in the 80's)