Molding & rotocasting questions

[ProfKSergeev]

I thought I'd post this here as it's more general than studio. I'd be indebted to the skilled members of this forum for any words of wisdom.

I have experience with two part mold-making and (solid) resin casting, but I've been itching to learn something new, and to that end I'm building a hand-cranked rotocaster using these instructions.

What I'm unsure of is:

1) Which kind of silicone to use for the mold, bearing in mind I do not have a vacuum chamber.

2) And which kind of resin. What setting time and viscosity do I need? I've used Alumilite before, but I've read it produces much more heat than other brands and it cures very quickly.

I look forward to any advice anyone might have, and I'll be sure to update this thread with my progress.

 

[brandomack]

You could try this

Brush-On® Series Urethane Rubber Product Information | Smooth-On

I asked smooth-on the same question, as I also dont have a vacuum chamber.

of course I've learned that in the big picture it's way easier and cheaper to just get the right equipment to begin with.

 

[modelnutz]

I've had good results using Smooth-On's SC-300.
However, Smooth-on sells a resin designed just for roto-casting....give them a call and talk with their teckies...they're great guys to talk to.

One trick that I've learned from experience is to let the mold cavity "breath" while the casting is happening.
(This will prevent that annoying situation where you have a great casting but the dang thing sucks in making concave flats.)
This is achieved by making a pour hole the same size as a short length of silicone tubing ( RC airplane gas line works great ).
After poring in the "shot" of resin, slide the tube in to the hole so that it extends beyond the inner surface of the cavity....1 to 2 inches inside the mold and 1 or 2 inches outside the mold should work well.
Since the tube extends into the cavity beyond where the resin is flowing...you shouldn't loose any resin ( play it safe though...put down a sheet of plastic under the roto machine to catch spills )
Since the tube is silicone, it should pull out of the mold pretty painlessly, allowing you to pour in more resin for follow-up shots.
Do this for each pour and you should get good parts without any altered shape.

Also, don't try to roto the part in one shot....too much resin will cause it to "kick" too fast...resulting in a big lump somewhere in your casting...and thin wall in other spots.
It's best to try for 2 or 3 pours/spins per part.
Just time it so the resin isn't fully cured so the follow up coats of resin will bond to the previous coating.

Lots of trial and error is typically required before you have the "magical" amount of resin to get a good skin coat.....be sure to keep records of spin speeds, resin amount used...and timing of follow up shots.

Hope this info goes a little way to help you get good castings....if you run into trouble and want to "farm it out"...send me a PM...I'd be glad to provide a quote for casting services.

Good luck to you sir !

 

[modelnutz]

One last thing....be careful if you go with a Urethane rubber for the mold....you will need to use a mold release every pull.
Casting urethane in Urethane molds is pretty dicey.
You can easily glue a part in to your mold...thereby ruining the mold.

Check with Smooth-On...they have "trowelable" silicones just for your type of application.

 

[ProfKSergeev]

modelnutz, those are some great tips. Thank you very much!

 

[modelnutz]

Just doin' my part for the community :cool

 

[vaderman1701]

I am pretty sure you can use mold max 30 from Smooth-on to make your mold. They also have a brush on version called Mold Max Stroke.

You may want to PM REL. He built a home made rotocast machine and rotocast some parts that turned out well.

He could probably post some pictures of his.

Scott

 

[ProfKSergeev]

I've ordered some Mold Max 30 and Task 15 resin from Smooth-On, and I'll be sure to keep you all apprised of my rotocasting efforts.

 

[TMFX]

ModelNutz knows his stuff! I personally really like MoldMax 30

Here's another tip for working with silicone when you don't have a degassing chamber:

pour the silicone from a very high height, put the mould on the floor and stand on a stool or countertop. Pour onto the parting wall or avoid the actual master's surface while pouring and allow the level to rise up and over all the detail. The whole idea though, is to get the stream of silicone to draw out to a fine diameter, because it cannot contain air bubbles and they will tear out, leaving the batch "degassed" as it fills the mould. This is even used with a degassed batch of silicone for extra insurance.

Finally, before casting inside a silicone mould, dust with talcum or baby powder on a chip brush, and blow out any excess. This leaves a film of powder on the inner surface which will draw resin into details by capillary action. This works well for vents, or other areas with vanes or small geometry that can trap bubbles. Do not dust a master with talc before pouring silicone, seems like a good idea but actually deters silicone from creeping into detail. It works wonders for low viscosity materials though.

 

[Zack Morris]

There's some good tutorials on youtube:
shoot a question to 'smooth on inc', he'll get back to you, he answered some of my
questions

this is his page:

YouTube - ‪SmoothOnInc's Channel‬‏

 

[ProfKSergeev]

I recently molded the piece and was pleased with the results, though I have decided it won't be necessary to rotocast it.

 

[danbakeronline]

Techie here.

You guys have it pretty well covered. To fill in some gaps, our rotocasting resin (should you need it) is Smoothcast 65D. Smoothcast 45D will also work, and is a bit lower shore hardness. And yes, casting urethane into urethane is gonna be problematic. Use Universal Mold Release for those applications. We'd still recommend a silicone (like you're using) for all this.