Why the cast piece comes out smaller than the CAD file

A ring modelled at size N comes back from the caster at something closer to M. Nothing went wrong. Metal and wax both shrink as they cool, and the total loss between a CAD file and a finished piece is predictable enough to design around — provided you know where it comes from.

Shrinkage happens three times, not once

It is tempting to treat shrinkage as a single number applied at casting. In practice the model passes through three separate contractions, and they stack.

• The pattern. A resin or wax pattern grown from your file shrinks slightly as it cures and cools. Modern resins are stable, but it is not zero.
• The mould. A vulcanised rubber mould taken from a master shrinks as it cures, and every generation taken from that mould inherits the loss.
• The metal. The casting itself contracts as it solidifies and cools to room temperature. This is the largest of the three.

A piece printed directly and cast once passes through one contraction. A piece cut from a mould taken from a master that was itself cast has passed through several, and the cumulative difference is large enough to change a ring size.

Roughly what to expect

Figures vary by alloy, by investment, and by how your caster runs their burnout. As a starting point rather than a guarantee:

• Silver and gold alloys: broadly 1 to 2 per cent linear contraction from mould to finished casting.
• Platinum: more, and less forgiving, because it is poured far hotter.
• Rubber moulds: another 1 to 3 per cent depending on the rubber and the cure.
• Direct resin print to casting: the smallest total loss, which is why it is now the default for one-off commissions.

The number that matters is your caster's, not a published average. Ask them what they see across a run, because they are the only person measuring it on your work.

Linear shrinkage is not volume shrinkage

This is the part that catches people out. A 2 per cent linear contraction is not a 2 per cent loss of metal. Volume falls by roughly three times the linear figure, because the piece shrinks in all three directions at once.

So a 2 per cent linear shrink costs you about 6 per cent of volume, and therefore about 6 per cent of the weight you quoted. On a heavy 18ct piece that gap is real money, and it moves the wrong way.

Where it shows first

Shrinkage is not evenly annoying. Some features tolerate it and some do not.

• Ring size. The most visible casualty, and the one clients notice. A half size is well within the range that stacking contractions can take.
• Stone seats. A seat cut to fit a 6.5mm stone in CAD may not accept one after casting. Setters expect to open seats, but they should not be rescuing a size error.
• Fine detail. Millgrain, engraving and thin bezels lose crispness with every mould generation.
• Mating parts. A ring and a fitted wedding band that were modelled to sit flush will not, if only one of them was recast.

How to design around it

There are only a few sensible levers, and they are all cheaper than a recast.

• Model the ring a fraction over size when the piece is destined for a mould rather than a direct print.
• Cut stone seats after casting rather than modelling them to final dimension, unless the piece is printed and cast once.
• Keep a master pattern, not just a master casting, so later runs do not inherit another mould generation.
• Ask for direct printing on anything where size or detail is critical.

Ask the question before the model is finished

Whether a piece will be printed and cast once, or moulded for a production run, changes what the file should be. That is a decision to make while the model is still editable — not after the first casting comes back a half size small.

We ask how a piece will be produced before we finish the model, and adjust the file to suit. Quotations are free, so it costs nothing to tell us the route and find out what the file should allow for.