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Three Part Mould Making :(


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Can anybody recommend a good book/tutorial/anything that I can read? Made a few one part moulds and two part moulds with some success so I thought lets try a three part mould. Lots of things went wrong, for one I don't think the soap I used was that good as I came to open the two parts today and it was fused shut  :blink: Also failed at estimating the plaster I needed and ended up with too little for one half.

 

Here is my attempt 

 

post-23281-0-35214500-1416575351_thumb.jpg

post-23281-0-35214500-1416575351_thumb.jpg

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Thanks for the pdf :)

 

I was using hand soap but after a quick visit to the library it seems my process was wrong. In the book it said to apply soap then sponge down 3 times to build up a protective layer. I think I watered it down too much as I have had success before but next time I will try the soap and sponge.

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There are a few books I'd recommend, try checking out your library service, that’s where I found most of these.  

 

The Clay Lover's Guide to Making Molds, published by Lark Ceramics - Some really good info, tables of which model materials need a mould release, and which don't.  I don't like the example of multi-piece moulds, but maybe that's just me.  I have this one in my hands at the mo.

 

Slip Casting, by Sasha Wardell - again some good info but not enough on it's own.

 

Essential Guide to Mold Making & Slip Casting, by Andrew Martin, Lark Ceramics -

 

One book I had from the library was much thicker than all the others, and had the most info, but I can't find it again.  I'm sure it was a Lark Ceramics publication, so possibly was the Essential Guide.

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Yea they broke. I was a bit stupid and used a cylindrical handle with no curve on the end so I wasn't surprised. Still, it had snapped in two before I noticed

 

 

Do you thing that this was enough to cause your mould to snap when trying to separate them?

 

BTW twisting a coin (halfpenny bit) or a teaspoon to make a rounded notch works well and avoids undercuts.

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Just had another look at your photos.  Does your model for the pour spout contain any undercuts.  It needs to be as smooth as possible, so you can cut round it with your plastic (lucy) tool.

 

One of those books had a really good section on pour spouts.  About how they were undercut or overcut giving different pros and cons.   Thinks out loud " Did I scan that page?"  Have to go and look on the other computer...........

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Three or four keys would have been good enough for registering the two mold halves. You want to make your keys bigger then they will be stronger.

It's not really mentioned much, or the information is glazed over in books, but Plaster expands slightly as it sets. If your keys are rounded or have a heavy tapper they will release better. A spade bit that is ground round at the tip and used in a cordless drill makes quick divots in your plaster for registration. If you use a clay wall for your first mold section, use balls of clay cut in half or heavily flat topped pyramid tapered strips as keys.

 

Use Mold soap full strength 100% do not thin. I use Murphy oil soap, Brush it on leave it dry, wipe it down with a damp sponge then repeat 2 more times this should get any missed spots. You will want to finish the mold right away because mold soap will naturally break down if given enough time, and your mold parts will stick. I'm not quite sure what is in Murphy soap , but it leaves a waxy residue. The soap can be removed from the mold with white vinegar.

 

Green soap has been mentioned in a recent post on the forum. It is basically the same as home made liquid soap made with KOH, Potassium Hydroxide. Not Sodium Hydroxide... Lye makes hard soap. I guess the Potassium Hydroxide does something to the plaster and it makes a good mold soap. I have yet to try this and see if it works better than Murphy soap, but it is on my list of things to do. You may love soap made with Potassium Hydroxide.

 

Wet sanding your mold is often under rated. Sureform to the rough shape then I use 180 grit to sand molds then jump to 600 grit to finish the mold under running water. You can make a really professional mold with good sanding technique.

 

I have read a few books on mold making, and I like the Andrew Martin book. He offers seminars from time to time, but they are way more than I would want to pay. He makes single and multiple piece molds in his book and gives a very good overall look at how you might do the same.

 

One last thing to add. Keep an extra batch of plaster weighed out ahead of time, It's not ideal, but if you run short you can mix quick and add a little more if needed.

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