resin casting and molds

[Rick L]

Obviously there is no advantage to vacuum casting with fast curing plastic.

[Chariots of Fire]

With all of that, I will stick to simple two part molds and keep life simple!!!😎

[oldcarfan]

On 2/14/2025 at 3:27 PM, robdebie said:

Since then I've made a web page with a lot of information on my method of vacuum resin casting: https://robdebie.home.xs4all.nl/models/casting.htm

Here are a few pictures from the page.

Rob

You've just answered a question I've had for years! I worked for a company for a while that made resin cast ship model parts. We had a vacuum chamber for de-airing mold rubber and a pressure tank for pouring resin parts. Glad to know there is a way to use the vacuum pot for pouring parts too,

[Big John]

I have a setup for both vacuum and pressure pot.  The silicon aren't that big a problem however my resin working time is so short that it starts kicking off within a minuet after I start mixing not leaving me time to safely secure the pressure pot lid.  Some times it kicks while pouring into the mold.  Slower setting resins seem to be more fragile.

Edited Sunday at 10:10 PM by Big John
New Pic

[NOBLNG]

4 hours ago, Big John said:

I have a setup for both vacuum and pressure pot.  The silicon aren't that big a problem however my resin working time is so short that it starts kicking off within a minuet after I start mixing not leaving me time to safely secure the pressure pot lid.  Some times it kicks while pouring into the mold.  Slower setting resins seem to be more fragile.

Have you tried Smooth-On? They have a couple of resins with a longer pot life. I don’t find them overly brittle.

 

Edited Monday at 01:39 AM by NOBLNG

[Chariots of Fire]

My casting is for really simple parts so vacuum or pressure casting has no place in my process.  The 300 series casting resin works just fine and allows enough time to get it into position before it sets.  The only issue is to be sure that bubbles are removed from small areas at the beginning of the pour.  Since I'm not doing large quantities of anything, the simpler the better!!😉

[robdebie]

I use the 7-minute Smooth-Cast 305 (with some black dye), and I'm happy with its material properties after curing. I cast some fragile parts, like the jet nozzles right from center and the bomb fins in the center, and never had failures. On the other side, since the cure is slow, the resin doesn't heat up, and a hot cure of a fast resin might give better material properties. The drawback of a hot cure is that you'll have more shrinkage.

Rob

[Rick L]

A few tips on reducing bubbles

Stir slowly and don’t use popsicle sticks. They are too porous and will introduce bubbles. Use a mini plastic spatula or plastic sprue. If it has a long working time, warm the two parts in a water bath this will make the plastic less viscous. The lower the viscosity the easier bubbles rise. If using the “squish” method, don’t pour in the middle of the mold and slap the cap on. Pour it down the side wall of the mold until you have the desired amount. An ultrasonic cleaner will also help bring bubbles up on less viscous plastics. Place the cup with plastic into the cleaner tray and surround the cup with water. You can buy an inexpensive cleaner at Harbor Freight. When you wind up with micro bubbles at the top of the plastic, wave a bunsen torch or breath heavily over it. The carbon dioxide will break them up. I’ve never tried this myself but am told that a spritz of alcohol on the surface of the plastic will also remove those micro pests.

[Big John]

Yes, I like Smooth-on, silicons and the black Onix is great for tires, however I bought some 321 urethane back in covid days stirred the botttles, let stand for a couple of day to outgass, then mixed slowly and methodically to avoid bubbles, and as it cured it more or less turned to foam with small bubbles throughout the part. Smooth on had no solution except that I should use a pressure pot when casting with this product.

[robdebie]

Did you leave the lids off for the outgassing? If so, the resin absorbed moisture, and foamed because of that moisture during the cure.

I try to limit my resin's exposure to air / moisture as much as possible, but with every opening you let in fresh air with new moisture. When I reach the bottom of the bottles, they have had maximum exposure, and the mixed resin will foam, if not put in a vacuum.

Fresh casting resin should not have any of these problems though.

Rob

Edited Tuesday at 08:06 PM by robdebie