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Any substitute for Smooth-Cast 300? Answered

We have our own homegrown substitute for Oomoo thanks to the local boy genius Mikey77

But is there any substitute for it's companion product, the very expensive ($100/gallon), two part liquid plastic stuff called "Smooth-Cast 300"?

Does anyone know of a way to mix an inexpensive something up and pour it into a mold and have it harden up into a strong thing?  It doesn't have to be as invincible as Smooth-cast, but .. is plaster the only option?

Update:  What am I casting?  I found a really great ornamental tile at Habitat for Humanity and want to make copies of it

Update:   I'm wondering now, what if I used something from the wall/flooring department, like Mastic?  Its about $34 for two or three gallons.  It fills gaps and hardens to such a degree that it can't be chiselled easily.  I may try this.

Update:  Tried mastic, it works but takes far too long to dry, on the order of three days or longer...  Maybe if I mixed in some corn starch (ala oogoo?)

Thank you


How about polyester resin ?

I haven't any experience with it, but I'll investigate it, thank you

"Loading" the resin with a filler increases the volume you have for a given volume of resin, reduces shrinkage, and improves wearability. Its an important trick to know about. Mix in the filler BEFORE you add the hardener though.


5 years ago

You could potentially use machinable wax if your mold can tolerate moderate heat. You can make your own machinable wax from candle wax and freezer bags (https://www.instructables.com/id/Machinable-Wax/), or buy it.

There are kitchen-science ways of making your own plastics (https://www.instructables.com/id/Homemade-Plastic/) which are super cheap in volume and sound like you'll get a decently hard material at the end of it but I have no idea about curing time, shrinkage, or how easy it is to fill a detailed mold with the soft plastic.

To be honest, while $100 is enough to give you sticker shock it's not actually terribly expensive for a whole gallon of an uncommon chemical which does exactly what you need. A gallon of silicone sealant to make Oogoo with wouldn't be cheap either (though definitely cheaper than a gallon of Sugru!) Perhaps see if you can buy smaller quantities if you only need a little bit, or split the purchase with someone else who wants to do their own casting.

I hadn't seen the machinable wax one before. Very interesting!

Yeah, it may come to that, I may have to spend $100.

Why ? I can buy it in pints (well, 500mL) in the UK ? Shop around. Try sculptor supply stores.

Sorry, I thought you wanted a moulding material, what you want is a casting resin instead. Try a loaded polyester resin - resin mixed with a filler like limestone.

the homemade plastic looks too lumpy and cottage cheesey

The clues in the name : Machinable. It shrinks like a ba%$^$&rd when its cooling from liquid. Not a problem, if you're machining it

I've been watching a lot of videos on using 2 part expandable foam poured into molds - it's pretty amazing stuff. Let me know if that would work for what you are doing.

What are you casting?

Have you considered other mold options and using the Oomoo as your casting material?

I found a lovely ornamental tile at Habitat for Humanity and I want to make copies of it.