Saturday, August 27, 2011

Calculating Shrinkage Revisited

I will be the first one to admit that I am not very clever at doing math, and that is one reason I studied Art at University, and not Science. It isn't that I don't like numbers, I do! I have a calculator, and I use it all the time. Of course, the trick to using a calculator is knowing which keys to punch in to do what you want to do. Fortunately, most of the practical math I do in my studio is adding, subtracting, multiplying and dividing.

Shrinkage is one of those things that should be thought of at the design stage of BJD development. This is when the working drawings are made. The working drawings should be full size drawings that show the front and side views of the doll, at the very minimum. These drawings are very useful for making the armature for the doll's original sculpt.

If there is any shrinkage, in any of the materials used to make the doll, the original sculpt of that doll must be larger by the total amount of shrinkage of all the materials that have shrinkage. So the one of first things to know, when designing the doll, is which materials are going to be used, and what their rates of shrinkage are.

I am going to use Aalish as an example.
Aalish is being modeled in brown sculpture wax to make the original sculpt.
The brown sculpture wax is being molded in moulage molds, and cast in carving wax.
The carving wax is refined, and molded in plaster slip casting molds, and cast in CompoBell CS-1000 doll composition slip.

The brown sculpture wax used for the original sculpt does not shrink. 0% shrinkage.
The carving wax shrinks 2.78%.
The CS-1000 slip shrinks 6%.
The total shrinkage of all the materials is 8.78%, which I round off to 9%.

First of all, I subtract the percentage of shrinkage from one (1.0).
Next I divide the height I want my doll to be, when finished, by .91 (91%).
60cm / .91
The answer is how tall my original sculpt must be to be 60cm when finished.
65.93406, can be rounded off to 66cm.

I will want to make my working drawings of the doll, 66cm tall.

Previously, I had figured that I should make Aalish 63cm tall to end up 60cm when finished. Now I see that Aalish will probably end up being more like 58cm tall when finished because I did not figure in the shrinkage for the carving wax into my original calculations.

Yeah, well, I am too far along at this point in time to quibble over 2cm, so this is going to have to be a lesson learned. I will know better when I make the second BJD.

Many thanks to dollutional at Enchanted forum for teaching me this way of calculating shrinkage.

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