General hints

The three dimensional expansion of the investment material must compensate the thermal contraction of the alloy after the cast, which is 2.2% of the total volume for CoCr based alloys. The casting parameters must be considered in view of the alloy properties. An overheated metal causes a changed metal structure, rough surfaces and tension inside the cast.

Casting methods

Usually the alloys are cast with:

• gas burner
• high frequency centrifugal casting
• vacuum pressure unit

Cooling

If the muffle is put back into the furnace chamber directly after the cast, the tension inside the cast of long span bridges, implant supra-structures or one-piece casts will be reduced. The oven is switched off and the door slightly opened. After one hour, the temperature decreases to 600 °C / 1112 °F. Take the ring out of the chamber and let it cool down to room temperature. In general, never quench the muffle in cold water!

Spruing system and cooling fins

It depends on the geometry of the casting object which kind of spruing system is used, or whether one works with direct feeder sprues or runner bars. When casting CoCr based alloys, the casting reservoir should not be attached too close to the casting object because of the short solidification phase of these types of alloys. Consequently, the reservoir should be placed in the heat centre of the muffle to avoid shrinkage porosities. Cooling fins should effect a controlled solidification of the melt for implant suprastructures, large bridges with bulky pontics and casting objects with very different volumes. Wax grids can be attached at the thickest part of the wax frame.

Influences on the surface quality

When casting CoCr based alloys, a prolonged stirring time (120 seconds) and the reduction of the preheating temperature to 800 °C / 1472 °F induces very smooth surfaces of massive casting objects such as customised bars in addition to the fine graininess of the investment material. However, this procedure depends on the casting method of the respective lab.

Overheating the alloy causes always rough casting surfaces.

Fit of the crown

The required expansion is achieved by the concentration of the mixing liquid. Copings made by thermoforming have a tighter fit as those made of dipping wax. The hard plastic copings result in a decreased setting expansion of the investment dies.

Fit of the bridge

An average value between single fit and total fit must be found in order to compensate the contraction of different bridge spans. Crowns and pontics should be connected with a modeling wax of low shrinkage (see remanium® Kompendium 1/ crowns and bridges). Wax sticks of extra-hard quality provide for the stability of the wax frames. The runner bar can be separated in small segments to avoid tension inside the bridge. An optimal fit of the bridge is achieved by the “core investing procedure” (see chapter 4).