Plaster mold making for complex pieces

[HaleyChoice]

Hi all, 

Looking for advice on making plaster molds for slip-casting a complex piece with multiple undercuts.

Any advice or good resources for the following would be so helpful and appreciated:

plaster mixing

slip recipes

making two and three piece molds

 

[DangerMDV]

Hey there! 
 

Im also relatively new to slip casting but I like to tell my self I’m a competent maker. With plaster and slip casting Ive had some misses and more recently many more successes. 

Places to start;

Andrew Martin’s book “Essential Guide to mold making and slip casting” is the one. Honestly, just start here. My local library had it, I read it and ive since bought it. Its worth every penny. It has plaster ratios and slip ratios. I buy my slip premade as its cheap and easy. If i were to scale my operation id consider mixing slip but cost is not an issue for me. 
 

as to the 2-3 part mold, Martin covers it in his book but its a great visual problem to solve in in your brain/sketchbook. 
 

The trickiest thing i found when getting started was the plaster to water ratio. I just couldn't get the math right. I started a topic in the forum. (Im trying to attach the link...i dunno). I ended up building a calculator in google docs. 

The quick and easy way to do it is 7parts water 10 parts plaster by weight. 
 

I hope this helps. Happy making! 

 

 

[Jeff Longtin]

The primary advice I would give you is be prepared to make mistakes. They happen. I've been doing this a long time and I still make mistakes.  (Don't anticipate all the undercuts, don't find the right parting line, etc.) It's just part of the process.

When I first started making molds, for profit, I marketed myself as "Complex Molds Made Easy." I made some very complex molds.  (10 part was tops.) Most of those folks did not create businesses around the product. Many because complex molds are complex to use and complex to maintain. If possible, try to simply the form, such that you only need to make it a two - four part mold. This will be easier on you and easier to make the product easily and consistently. 

Thought to consider: only you know how complex the original form is. If you alter it, in some way, to make it easier to mold, the customer will not know. They'll only see the finished product, you present them, and appreciate what you have done. (They won't know you filled in 10 undercuts, instead of 5, like you preferred.) 

I always directed customers to Dept56 products for example. A wonderful example of a form that is highly simplified, to mold easily, and yet, when hand painted, given great complexity. Very few D56 collectors appreciate how simple the forms actually are.

Good luck!

[Hyn Patty]

Here's a great tip for figuring out your mold pieces and if they'll work.  This works best however if you have an original that isn't delicate - such as a rubber or resin/3D printed master that you are making your plaster molds from.   Harder to do if your original is soft clay and easily deformed.

Using mold release on your original, start by using plastilina or other oil clay and 'clay up' each section shape of what you think your plaster mold should be.  As you do each shape, toss the whole thing into the freezer to make the oil clay harden up.  Then see if the section you built will pull off without hanging up on your undercuts.  If it does, remove it and reshape as necessary.  Once it's just right, move onto the next section to oil clay up the next 'mold' section you want to test.  Each piece needs to be easily removeable without deforming the oil clay or sticking to your original master.

Without photos this can be difficult to visualize.  But once you do each section of what will eventually be your plaster master, you can box it all up and start removing one piece of the oil clay at a time.  Then pour your plaster into that void.    When it sets, remove the next piece of oil clay and apply mold release, then pour your next section of plaster.   I use this when I am making complex molds for my horse sculptures.  

Alternatively you can use this to pour silicone or other rubbers for a master mold to make your production plaster molds from.  Just be sure that your oil clay doesn't have sulfur in it as that will react poorly with some kinds of rubber.