Trying to understand undercuts in relation to liquid latex casting

Kelly Mahler

Just hoping to clear up a bit of confusion on my part. I understand that undercuts can be a huge problem when making things like foam latex, but I can't seem to find solid information on whether undercuts matter if you are just casting using regular liquid latex. Some sources I've seen still mention the undercuts in the sculpt, whereas others, simply talk about getting the clay out from the mold. I sculpted in WED and plan to seal with Krylon, and then make my mold in Hydrocal.

So, I'm trying to understand- are undercuts something to be concerned about if you are not doing foam latex? I have my sculpt finished, and I want to know if there is anything I should worry about before I cast (considering my sculpt does have some undercut areas). 

Hoping someone can clear this up a bit for me. Thank you!

Kai Rottmann

Hey Kelly :-)

There are no problems with undercuts.

We have to follow simple rules in terms of mold making:

solid casts- soft molds= undercuts are okay
soft casts- solid molds= undercuts are okay
solid casts with solid molds= undercuts are bad

Here is a simple sketch with my hightech pro animating software (Windows paint xD)

This is a cut through a sculpture with a done two- part mold:



And this is a picture which shows, why it is so important, that the mold is done right. The sculpture will break and maybe its fully lost.
The sculpture is only captured in the mold.
But if the mold is weak in the undercut areas, it will get damaged, too. Then is the whole sculpture lost. 

Try to imagine what happens, if the mold is out of plaster (solid) and the casting material you will use is plaster (solid), too?
The cast and the mold will get locked and maybe cant be opened without damaging the cast and mold.

If you have a plaster (solid) mold and you will cast latex (soft) in there= no problem.
The latex will pull and squish and whatelse out of the mold without doing damage.

I hope you understand what i mean and i also hope, that this answers your question, hihi.

Have fun with your projects and never stop creating ;-)

Greets

Kai

Kelly Mahler

Thanks! Yes, that helps. So I won't worry about the latex, then, in this case. Just need to make sure I don't lock my two halves of my mold.

Kai Rottmann

Okay :-)

Hmm this can happen, if you doing the keys of the two parts wrong. When youre doing an undercut there, they will lock together xD
You cant doing anything wrong, if youre doing the mold- walls in a 90° angle and the keys in form of a half hemisphere.
If you would do these half hemispheres more than a half, it will lock.

I remember that Timothy Martin explains in his course where to go with the outline of the mold and which release agents are important, so that the mold halves cant lock ;-)

I wish you all the best for your project :-)

Greets

Kai 

Chris Ellerby

A+ for illustrations!

/Chris