Common Defects in lost wax casting / POROSITIES

Gas porosities occur when gas remains inside a casting metal due to excess gas contained in casting metal or/and substitution between internal gas and pouring metal does not performed smoothly during casting.
Degassing of casting metal before casting or forced degassing during melting may be required sometimes.
In some cases, it would be necessary to check burn-out cycle for furnaces in order to confirm there is no residue in casting molds.

Cross-sectional image of a gas porosity

Cause 1 / Internal gas

There is a case casting metal itself may contain gas.
When reusing buttons that has been used for casting once, if investment powder adheres to the buttons, the investment powder (gypsum component) will thermally decompose and generate gas. Thoroughly wash away investment powder adhering with an ultrasonic cleaner.
Even virgin metal may contain gas. Precious metal is guaranteed purity when it is sold but not guaranteed to be free of inclusions that would adversely affect casting. In other words, new metal doesn't mean it doesn't have gas.
It is possible to forcibly expel gas with adding deoxidizers such as zinc, silicon, and phosphorus into the melted metal. Deoxidizing material should be put in from 1/1000 to 2/1000 (weight ratio) of the dissolved amount of metal. Stir well the melted metal containing deoxidizer.
When borax is desired to use, never add borax first but after degassing process.
If you don't want to mix the deoxidizing agent into casting metal, solidify melted casting metal into a rod shape and beat or stretch the ingot with a roller or hammer to physically remove internal gas.
In the case of platinum, it can often be degassed by remelting it in a crucible. Let platinum solidify in a crucible after complete melting. Observe the top shape of the solidified platinum. If the center of the metal is bulge, the platinum alloy definitely has gas inclusion. Continue this heating and solidification process until the canter of the metal is flattened or dented.
This confirmation work is highly recommended before casting.

Cause 2 / Gas generated by crucibles

Since the raw material of the crucible is natural, gas may be generated from a crucible depending on raw material.
Even if material is carbon or silica, it is recommended to replace a gas-producing crucible to a new one immediately.

Cause 3 ／ Remaining gas due to lack of air permeability of casting molds

In vacuum suction casting and pressure casting other than centrifugal casting, part of gas in a mold is released out of a casting mold through gaps between the investment powder particles. If air permeability of a casting mold is obstructed, gas remaining in a casting mold may not be able to be discharged efficiently. This resulting in gas porosities or/and small dents appearing in castings.
It is necessary to check mixing ratio of powder / water or/and mixing ratio with binder.
For silica system investment powder, if powder is heated at a sintering temperature (900°C (1650F) or higher) for a long time (5 hours or longer), sintering of binder will proceed and air permeability of casting molds will be hindered.

Cause 4 / Gas entrainment due to turbulence

It is one of typical porosities that occurs when solidification of molten metal is completed in the state of dendrite (needle-like crystals like conifer leaves) due to insufficient temperature.
It occurs on entire surface layer of a cast but it is often difficult to repair the porosities because it is unpredictable how many cavities there are inside and how deep it is.
To solve this problem, metal temperature at casting or/and flask temperature should be increased.

Although the limiting temperature changes depending on original temperature that related to shape or/and volume of a cast , it is recommended to try increasing casting temperature in increments of 50C.(120F)

Cross-sectional image of porosities by dendritic crystal

As "directive solidification" cannot be secured when a casting solidifies, it occurs when supply of melted metal that matches volume of solidification shrinkage of the metal is obstructed.
Generally speaking, it occurs at thick parts of castings. Metals with high melting points and metals that shrink violently have a high risk of shrinkage porosity.
It is a porosity tend to be found in pressure casting. But in any case, with directive solidification should be in mind, try to supply melted metal that compensates for metal shrinkage by adjusting attaching position and thickness of gate sprues.
For “directive solidification”, see (Link: "Cracks in Castings | Causes of Casting Defects and Countermeasures ④")

Cross-sectional image of shrinkage porosities

Countermeasures for porosities are summarized blow.
To prevent hotspot that would create shrinkage, refer to “Hot Spots”Where areas get hot in casting and its countermeasures”

There might be more than one cause. Check all items again and see what happens.