Casting technique — from wax model to metal-cast crown

Investment casting – often called the lost wax technique – is one of the oldest techniques in metalworking and is still used in dental technology for metal cores under ceramics, for complete metal crowns and bridges and for prosthetic bases. The principle is simple to explain but requires great care in practice: you form what you want in wax, make a negative mold of it in a heat-resistant material, burn away the wax and fill the resulting void with molten metal.

§Building in wax

Work starts with the dental technician modeling the restoration in wax directly on a refractory model or the original plaster model. The wax mimics the final crown or bridge's shape exactly - including thickness and transitions - because the metal subsequently assumes the wax model's form to the letter. If the wax buildup is too thin somewhere the metal will be too and it can result in a weak structure.

§From wax to cavity: casting

The wax model is provided with one or more feeding channels (called sprue) which later will guide the molten metal into the mold. The model is then mounted in a casting ring which is filled with a heat-resistant casting compound — typically phosphate-bonded for most metal alloys. The mass is poured liquid around the wax model and hardens to a hard porous block that closely reflects every detail of the model's surface.

§Burn-out and the actual casting

The casting ring is placed in a burn-out furnace where the temperature is gradually raised until the wax melts and finally burns completely away. This leaves a precise cavity with exactly the same shape and volume as the original wax model — hence the name lost-wax technique. After this, the metal, typically a cobalt-chromium alloy or a precious metal alloy, is melted at high temperature and pressed or flung into the still-glowing cavity using either centrifugal force or vacuum-pressure casting, so it reaches the finest details before it solidifies.

• 011. Wax build-up on model - the replacement's shape and thickness are formed precisely.
• 022. Attachment of inlet channels (sprue) for later metal supply.
• 033. Embedding in heat-resistant, phosphate-bonded mass.
• 044. Burnout, where wax is completely removed from the mould.
• 055. The actual casting with molten metal under centrifugal or vacuum force.
• 066. Removal of casting compound and post-processing of the cast structure.

Although many laboratories today mill metal structures digitally from a solid block casting technique is still thoroughly taught in the education's basic course. It provides a very concrete understanding of how metal behaves under heating cooling and solidification — an understanding that is just as useful when you later need to assess a milled structure or troubleshoot work that does not fit.

“You don't really understand metal until you've seen it melt and solidify in your own hands.”