For work I’ve been researching casting epoxies to make some small potted electrical connectors, and this sparked my imagination. Is anyone using casting epoxies to make molds for composites parts? Clearly this would only be a viable solution if the part were small otherwise the epoxy necessary would be outrageous in terms of cost. For small parts requiring a gallon or less of casting epoxy is this a good solution? I would think that you could get by w/o the need for any sort of surface coat. Simply make a plug and box the sides, then pour in the casting epoxy and cure. Is this something that is commonly done? Anything I’m overlooking?
You’re not overlooking anything, we do it all the time, for tricky parts that have some under-cuts we use urethane rubber and cast the parts in the rubber with an epoxy casting resin. Things like clips for parts are done this way. Works like a champ…We still wax the part up to help release it, the one thing is repairs are near impossible on a casted mould. You have to get it right the first time.
You can repair it but the surface quality is never the same and it is time consuming, cheaper to just recast it.
I do epoxy casting with laminating resin and playground sand all the time. You can make one gallon of resin into 3 gallons of cast-able material. I still use a surface coat in this type of application.
Another option is the plastic faced plaster (PFP). In this application you use an epoxy surface coat that is hydrophibic. You brush on the epoxy surface coat, let it gel, then add a second coat of epoxy and immediately pour on the casting plaster. These are much cheaper to make than an epoxy casted molds.
If you need the mold to set-up quick you can use hard urethane casting systems. Many of these systems will setup in as little as two hours and have a hardness of around 82. Some of them can be demolded in 15 minutes but they typically don’t work well for larger masses because they develop too much heat during the cure. They are usually filled making them impossible to polish.
In many thermoplastic applications this is done as well. Sika and Ciba are 2 companies making special epoxies for that.
And indeed there is the PFP system. The plaster is mixed with acrylic resin, and of a composition that does not shrink. We actually make the plaster and the acrylic resin for that (www.acrylicone.com).