Metal Casting Processes: Understanding Shrinkage, Expansion, and Compensation Techniques

Metal Casting Processes

Following are the differences between normal and hygroscopic setting expansion:

All castings will shrink as the metal transitions from a liquid state to a solid state. As a casting cools there are three phases of shrinkage, liquid shrinkage, solidification shrinkage, and solid shrinkage.

• Liquid shrinkage is where the melt cools and as it is cooling the melt is changing volume in its liquid form. Solidification shrinkage is an effect of the phase change from liquid to solid.
• This is just the same as water changing phase from liquid to ice. Solid shrinkage is a stage of shrinkage where the temperature of the solidified casting is decreasing causing the casting volume to decrease also.
• Liquid shrinkage and solidification shrinkage are the primary cause of shrinkage defects in castings.
• The design of the part will determine the likelihood that a shrinkage defect will occur. Gating and risering are employed in such a way as to reduce the amount of shrinkage or move it into a non-critical area of the casting.
• The third phase, solid shrinkage, occurs after the metal has solidified. This shrinkage is accounted for in the pattern sizing.

Metal Shrinkage

• At the time of casting, the molten metal is forced into an investment mold to occupy the space occupied by the wax pattern before. As the molten metal cools at room temperature, it shrinks dimensionally this is known as casting shrinkage
• The casting shrinkage of gold alloy is about 1.25–1.65% and for cobalt-chromium alloys, it is about 2.25%.
• High shrinkage is due to high fusion temperature.
• Silica or phosphate-bonded investments are used to compensate shrinkage for cobalt-chromium alloys.
• Casting shrinkage can be compensated by the expansion of mold investments.
• The setting, hygroscopic, and thermal expansion can be controlled by varying P/L ratios of investments.
• Increasing the burn-out temperature and water bath temperature increases the expansion and vice versa.
• In the controlled water-added technique, water is added during the setting of investment to compensate for the casting shrinkage.

Shrinkage Occurs in three Stages:

• Thermal contraction of liquid metal between temperature to which it is heated and the liquidus temperature.
• The contraction of the metal in heat its changes from a liquid to a solid state.
• The thermal contraction of the solid metal occurs on further cooling to room temperature.

For Phosphate-bonded Investments:

• The expansion of mold is desirable to compensate for casting shrinkage. The expansion of phosphate-bonded investments depends on the thermal expansion of the material. At higher temperatures, melting alloys have high contraction at the time of solidification which necessitates greater expansion of investment. This is done by thermal expansion of investment and inversion of silica.
• Replacement of liquid components from water to colloidal silica increases hygroscopic expansion. So the expansion of investment varies as per the ratio of colloidal silica to water. As the ratio of colloidal silica to water increases, the expansion increases.
• So when the ratio of colloidal silica to water is 1:3 expansion of phosphate investment material is 1.3% to 1.5%; the ratio of 1:1 leads to an expansion of 1.5 to 1.7% and the ratio of 3:1 leads to an expansion of 1.7 to 1.9%.

Expansion of phosphate investment occurs from three sources, i.e.

• Wax pattern expansion: Heat during setting allows significant expression of wax pattern.
• Setting expansion: It is 0.7 to 1%.
• Thermal expansion: It is 1 to 1.5%.