By Alex Dumas © 2009

For my Swift project I wanted to use as most of the original wiggits (model kit parts) used to detail the studio model. The main problem I was facing though is that I was building a much smaller scale replica at 1/87th, than the studio Swift model which is estimated at 1/32th (almost 3 times larger than my kit...)

Flying-Sub hatch reduced to 30% of its initial size after 7 cycles. More parts top-right.

The Shrink-Ray!

The solution was then to either find equivalent parts on smaller scale kits - assuming they have the same parts breakdown and details - or to reduce them to the needed scale. That means reducing these parts to 30% of their original size (a 70% reduction rate if you prefer the other way around) and all this without loosing any detail.

The way to achieve this is by making molds that shrink while curing. How to do that? Well, simply by adding a solvent to the silicon mix that will evaporate while curing. I use Smooth-On silicon (Mold Max 30) and their technical support suggested mineral spirit as a compatible solvent. They didn't have any ratios to suggest so I have tried a few to come up with this formula which I have been using over the last few months:

STANDARD MIX OF SILICON/HARDENER + 50% SOLVENT (BY WEIGHT) = 85% OF THE PART (15% REDUCTION)

The solvent will cause the silicon mix to be a lot more fluid (less viscous) so be sure to have a well sealed form, otherwise silicon will leak before hardening. The curing time is also much longer. The mold will start solidified within the normal curing time BUT it will shrink slowly over a period of 1 to 2 weeks (maybe more). REMOVE the mold from the pattern the day after to allow it to shrink freely. To speed up shrinking once I removed the mold from pattern I put it in an oven at a 100F for 4 to 6 hours (you will smell the solvent evaporate...)

Some math

Reducing parts to 80% of their initial size was obviously not enough for my needs so I had to do a few reducing cycles to get to 30% or so of the original size:

Initial part size = 100%
After 1 cycle = 85%
After 2 cycles = 72%
After 3 cycles = 61%
After 4 cycles = 52%
After 5 cycles = 44%
After 6 cycles = 38%
After 7 cycles = 32% (enough for my needs)

I have experimented with higher solvent ratios to found out that too much solvent results in moulds deformations (warping) once cured. Therefore, a 50% ratio of solvent in the mix of silicon will keep all details without loosing shape.

Even though I haven't tried this technique with 2 parts molds yet, logic dictates that the same approach should work.

Hope this helps in anyway!

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Alex Dumas
scihighmodels.com