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How to decide how many parts to a mould.

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Hello everyone, 

Hope everyone is doing well.

I want to make a mould for an angular bowl. I have attached a sample picture taken from the internet. I wanted to understand how many parts are needed when making a mould, not necessarily for this bowl but for any other ware. some are 2 parts and some go uptown 4 parts, but how to decide how many parts are needed and why they are needed. 

Thanks a lot. 


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By far the best book I've seen on this topic is plaster mold and model making by Chaney & Skee
You can get a 2nd hand copy in the UK for £15 https://tinyurl.com/hkjbe5dy
Nepal is probably more difficult, but it's listed from £22 https://tinyurl.com/pdvr3t6b

Oh dear, the rest sounds pretentious, but I'll leave it in anyway ...

Obvious criteria are
- there are no undercuts preventing movement when the casting and mold[part] are separated.
- the natural shrinkage of the casting should be away from the mold, not shrinking on to part of it. [mold interior should be mainly convex]
- the drying process should tend to free the casting from the mold, and permit movement [i.e. so you don't need air-pressure release]

I suspect that your example can be cast in a one-piece mold, but the angle of the more vertical surfaces surfaces needs checking. Also curved surfaces have a natural strength, while flat ones can distort readily if removed clumsily or too soon.  [It's probably been designed for a drop-out mold, but maybe needs a certain thickness of wall for stability.]

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To decide how many pieces a mould needs.......

Look at an object, say an apple, orange or banana.

The orange, if no dimple only needs 2 pieces.

The apple, with a dimple both ends could look like it needs more pieces, but if you turn the apple sideways, and each half of the mould contains a dimple, then it too might only need 2 pieces.

Same for the banana.

A blackberry, bunch of grapes, or a pineapple - each need many pieces, as the undercuts will get stuck.


I find this is the hardest part of mould-making.  I've heard that if you shine a torch on one side of an object, and look at the shadows cast on the other side that can tell you, but it doesn't work for me.


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The main reason you use a mold, to form a ceramic piece, is to reduce the effort necessary to make the piece. One impact, of the molding process, is that the cast piece will have a seam where the molds parts come together. If the seam, is placed well, the cleaning process will be minimal. If the seam is not placed well, the cleaning process will be time consuming, and the seam may be evident after cleaning. 

The form you picture may come out of a one piece mold but its hard to tell without seeing the form from the backside.

per Chilly's comment about using a light. When customers have a hard time envisioning where the seam might be I suggest they do the same. In a darkened room turn on one light source. Take a pencil, and draw a line, where the shadow ends, on the piece, What this tells you is everything that is up to that pencil line, will "pull" in that direction. Anything that is in shadow, on that side, is undercut and needs to be filled in. You can then turn the piece so one shadow lines meets the edge of the next shadow. Then draw another line and this will show you the parting line of the second piece.

By hiding the seams well you make the cleanup process a breeze. The quicker a person, can clean a clay object, the less time they have to handle the clay object. The less time they handle the object, the less chance they have, of deforming it/distorting it in anyway.

A little abstract but the basic idea behind good mold design.

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On 6/7/2021 at 3:02 AM, Jeff Longtin said:

reduce the effort necessary to make the piece

How many items are you planning to make?  That answer could help with mould decisions.  Also, just re-looked at that bowl.  Could you make a hump or slump mould and use slabs of clay?

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The best way to determine how many parts your mold should have is to figure out the technical minimum number of parts in which the mold can be made due to the objects form. And then to figure out if you can make your own life easier by adding more.

Side note: I know you're not necessary worried about the number of molds needed to make the bowl in the photo. But I'm going to guess that the bowl in the photo was made using a press. So it was likely made with 2 molds.

For slipcasting that bowl, you could rationally break it down into it 1, 3, 5, or 11 parts. Depending on what you were going for. 

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You could absolutely do 2 parts. I personally just don’t see any benefit to a 2 part mold instead of doing either a 1 or 3 part.


A 1 part mold is the lowest complexity and therefore the easiest to make. Which gives it some added benefits that multi part models don’t have. Like the fact that its never going to spring a leak if you don’t put it back together properly. And the lack of seam lines. (If that’s your aesthetic)


A 3 part mold (made from left side / right side / bottom) adds complexity to the mold design itself, but is hardly more complex than a 2 part mold. Since you’re essentially making a 2 part mold + a base.

But this gives you the ability to take the sides off the mold, without having to actually touch the piece itself, while also giving you a way to move/relocate the piece until it stiffens up some more. Since it’s still on the base of plaster.

A 3 part mold also gives you the added con that there are more places to spring a leak, but other than that the pros and cons are the same for a 2 part vs 3 part system. 

Of course, things are a bit different if you do a 2 part system where it’s split Base /bottom + Top. But that sort of goes along the lines of what I described as a 3 part system  


So overall, that’s the long way saying I excluded the 2 part solely because I personally prefer to work in a manner where I do as little clay-touching as possible. And because of that, I’d go with either a 1 or 3 and not a 2. 

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8 minutes ago, StonedStudio said:

A 3 part mold (made from left side / right side / bottom) adds complexity to the mold design itself, but is hardly more complex than a 2 part mold. Since you’re essentially making a 2 part mold + a base.

Right, thanks. I was thinking that you were meaning a 120° split.

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You could do it that way too, there is no right or wrong answer. After the technical limitations based on undercuts/etc, it really all just comes down to preference and what you're trying to accomplish and your own personal system of working. 

Edited by StonedStudio
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