Shrinkage Rate

What is "Shrinkage Rate? "  Is it important?

Shrinkage rate is the amount a cast will deviate usually in a descending format from the original master.   When you make a cast inevitably it will shrink.  Engineers and scientists and people with lots of signed pieces of paper on their walls have determined that there is roughly  a  3% -5%  Shrinkage Rate for resins.

And I say "POPPYCOCK!!!"   I dispute this based on 1 thing and 1 thing only...imperfection.

Not to be arrogant and not to cause trouble.  Ever here of a joke that goes...what 10 000 lawyers at the bottom of the ocean?  A good start.   Well next in line are Engineers.  Why?  Because 99% of Engineers have never been outside of their office and seen people putting their calculations to work.  In todays imperfect world their is only one constant.  Their is only one thing in life that IS perfect.  Its math.  Numbers are beautiful.  There is no grey.  Only black and white.  Right or wrong.  And you can make numbers do whatever you want them to.  Their only flaw is the people using them.

On paper...ON PAPER...everything looks good.  Paper doesnt have any factors.  No outside influences or interferences.  Paper doesnt have  "variables".    In the real world, there is EVERYTHING fighting against you.  I did construction.  Sheetmetal.  Man...  Everyones blueprints were based on them being the first ones on site.   Well now I have to work around the electricians work or this beam got missed and is in my way...lots of little things.  According to the blueprint everything fit fine.  Um...yeah.

Engineers are the pain in my ass.

The same applies to casting.   You can say there is a 3%-5% shrinkage rate.  Sure.  Based on WHAT?  Is that for 2 tablespoons of mixed resin or 2 cups or 2 gallons?  Whats the temperature outside?  Hows the humidity?  How big is the actual piece you are making?   AND THE BIG QUESTION>>> How much pressure are you putting on the mold halves when casting?  How far above sea level are you if at all?  What method(s) are you using to cast? Smush?  Sprue?

These are soft silicone molds...not steel injection molds.  There is give and play in the mold.  You can squeeze and flatten the mold if too much pressure is used.  The mold can separate and the pieces can ACTUALLY GET BIGGER if not enough pressure is put on the mold halves.  So where is the shrinkage rate then? 

These are important things to consider.   And thats why sometimes the pieces need to be fitted.  Because shrinkage rate is based on perfect conditions.  We live in a imperfect world.