The lost wax casting process, from CAD file to finished piece

Lost wax casting is thousands of years old and still the way almost every piece of fine jewellery is made. What has changed is the front of the process: the pattern now comes from a file rather than a carver's hand. Everything after that is much as it was.

If you commission CAD work, it is worth knowing the whole sequence, because most of the decisions that decide whether a casting succeeds are made before the metal is anywhere near it.

1. The model

The piece is built in CAD as a closed solid. This is the stage where wall thicknesses, prong dimensions, stone seats and shrinkage allowance are settled. Everything downstream inherits what happens here.

2. The pattern

The file is printed in castable resin or wax, or used to cut a master from which a rubber mould is taken. A one-off commission is usually printed directly; a design going into production is usually moulded so the pattern can be repeated.

Each route has a different shrinkage profile, which is why the production route should be known before the model is finished.

3. Spruing and the tree

Patterns are attached by wax sprues to a central wax stem, forming a tree. The sprue is the channel the metal will run down, and where it attaches decides how the piece fills.

Sprue placement is a real skill. Attached in the wrong place, metal reaches part of the piece late, cools early, and leaves porosity or an incomplete fill.

4. Investing

The tree is placed in a steel flask and surrounded with investment — a plaster-like refractory slurry — which is vacuumed to draw out air bubbles and left to set. Any bubble that survives becomes a bump on the finished casting.

5. Burnout

The flask goes into a kiln and is taken up through a ramped temperature cycle over several hours. The pattern melts and burns away, leaving a cavity in the hardened investment — this is the lost in lost wax.

Burnout is where resin patterns are less forgiving than wax. A schedule tuned for wax may leave resin ash behind, and ash in the cavity becomes inclusions in the metal.

6. Casting

Molten metal is introduced into the hot flask, usually by centrifugal force or vacuum assistance. The metal fills the cavity, and the flask is left to cool.

Alloy, flask temperature and pour temperature all interact here. Platinum is poured far hotter than gold and needs different investment entirely.

7. Quenching and divesting

The flask is quenched in water, which shatters the investment and releases the tree. The castings are cut from the sprues and the sprue metal is returned for refining.

8. Finishing

Castings are filed, pre-polished, and the sprue stubs removed. This is where the weight you quoted and the weight the client receives start to diverge, because everything removed here leaves as dust.

9. Setting

Seats are cut or opened, stones are set, and the piece is given its final polish. Setting removes more metal, so the finished piece is lighter again.

Where things actually go wrong

Almost every casting failure traces back to one of a short list.

• Porosity, from metal cooling before the section filled — usually a spruing or temperature problem.
• Incomplete fill, from walls modelled too thin for the alloy.
• Inclusions, from investment breaking down or pattern ash surviving burnout.
• Surface bumps, from air bubbles that survived the vacuum.
• A size that is wrong, from shrinkage that was not allowed for in the model.

Note how many of those start in the file rather than at the furnace.

The cheapest place to fix a casting problem is the model

By the time metal is poured, every decision has been made. A wall that is too thin, a sprue point that was never considered, a size that did not allow for shrinkage — all of them were changeable for free while the file was still open.

We model with the casting route in mind and will talk to your caster if that is useful. Quotations are free, so send the design and tell us who is casting it.