How to break up investment after casting flasks

Hi everyone. I’m just a humble hobbyists who never was professionally trained and learn things the hard way and from the internet that I’m very thankful for. The occasional book helps too.

Anyways, while I normally cast small wax items, I’m starting to print castable resin models with an SLA printer. I will be using Plasticast from R and R but I think it may be a good idea to use boric acid water to make the investment even stronger.

(my understanding is that unlike wax, when castable resin models are burned out, they expand enough to damage the surface of the investment before vaporizing and this expansion can destroy the surface of the investment which causes porosity on the model)

But the investment will be very difficult to break apart if I use boric acid water.
What are the best ways to break up hard investment after I cast them? I only use 2.5 by 3 inch flask or something in the range. I’m worried about bending/breaking the cast pieces if I have boric acid made investment.

How do professionals do it? I have a cheap ultrasonic it would fit in, overnight in the ultra sonic? That probably won’t work. I have a rock tumbler, wonder if sticking it in the rock tumbler with stainless steel shot overnight? That’s sounds better to me. I have a 3200 PSI pressure washer but I’d probably lose the pieces or bend them.

Hi Rick,
In general if you’re using a castable resin (depending on the particular characteristics of the castable resin) Plasticast should be more than strong enough for a good cast with no breakdown. No need to use boric acid. If you’re doing small casts then an ultra sound will work for getting the excess investment off, you’ll just be cleaning it after each use. We use a home made pressure washing system. Costs about $600. to make (instead of $5000. to purchase)and uses 1600psi presuure washer at 1.2 gal per minute. Enclosure lasts around 3 years before it has to be replaced. Glad to share how we made it, just let me know and I’ll post all the info necessary to make your own.
Hope this helps,


Hi Randy, thank you so much with sharing this information. I think what I should do is first try some castings with just using plain water and Plasticast investment, then if my surface quality is fine, I will stick with that. But if i do find out I want to use boric acid water and I’ll need help breaking up investment, I’ll reach out to you for how you built your pressure system, that’s awesome thanks!

Appreciate you offering help to a broke school teacher like me, LOL.

Plastics often leave a poor surface with burn-out. Boric acid has never been necessary for any investment in my experience. Modern investments are quite durable especially if designed for plastics. As far as getting the piece out of the flask, a plunge into water and shake out frees the piece. A few minutes in the ultrasonic removes excess investment.