Porosity

There are two main types of porosity seen in jewelry investment casting; Gas porosity and Shrinkage porosity. Gas porosity is spherical in shape and the size can run form microscopic to the size of a pencil point.

Shrinkage porosity (dendritic porosity) is usually angular in shape, can be large or small and is usually seen in patches in the heavier areas of the castings.

Gas porosity :

Check the burn out schedule being used; it may need a longer time at the top end temperature to destroy
carbon residues. (see Wax Burn Out section)

Check the air flow and exhaust on the burn out oven, check flame settings on gas fired ovens. (see Wax Burn Out section)

Check for investment residues left on metal being recast as it will decompose at higher temperatures releasing sulfur compounds.

If torch melting, check the oxygen/gas mix on the torch. Too much oxygen can cause gas porosity.

Check the casting temperature, it may be too hot or you may have a bad thermocouple.

Check the reuse rate on scrap, the metal may have been reused too many times.

Check the flask temperature; it may be much too hot for the designs being cast. If the metal stays molten too long in the flask it may absorb gasses.

Shrinkage porosity:

Check the sprue placement on the casting. Sprues should be placed on the heaviest area of the casting to allow progressive solidification of the metal.

Check the size of the sprues on the castings; they may be undersize for the size and weight of the casting.

Avoid pinched sprues or narrowing of the sprues at the attachment areas of the casting. This is a very common cause of shrinkage porosity.

Check the size of the main sprue rod on the cast trees; it may be too small for the size of the tree being cast. The main sprue rod is a reservoir of fresh metal and heat to feed the castings as they solidify.

Check the flask temperature being used; it may be too hot for the size and weight of the pieces being cast.

Multiple sprues may be needed on some designs to provide a better feed and avoid shrinkage porosity.

Check the quench time on the flasks after casting, quenching too quickly can cause cracking in the castings. 

Check the casting temperature, silicon deoxidized alloys may crack if cast too cold.

 Check the flask temperature, cracking in very thin castings may be caused by too low of a flask temperature being used.

Hydrochloric Acid or Muriatic Acid should not be used in investment remover solutions as it may cause stress corrosion cracking in lower karat alloys.

Inadequate or incorrect placement of sprues on the castings can cause cracking especially at or near the sprue junction with the castings.

Designs with thick to thin sections will often crack on the thin sections. This is due to uneven solidification. Additional sprues or U shaped feeders connecting the heavy sections may help.

Overuse of the metal may cause cracking due to oxide build up in the metal being used for casting.

Contaminated gold or alloy may cause cracking in the castings made with it.

Using a 10k to 14k yellow casting alloy for 18k yellow gold casting will often result in cracking or brittleness.

Casting temperature too cold
Flask temperature too cold.
Insufficient Vacuum for vacuum casting, Check seals, gaskets, filter, vacuum lines and pump.
Wrong speed on centrifugal casting unit.
Inadequate sprues on the castings.
Incomplete burn out can effect air permeability of the investment.

Casting temperature too hot.
Flask temperature too hot.
Rough model, check finish on model.
Rough waxes, check rubber model for excess mold release agents.
Incorrect water to powder ratio on investment mix, usually too wet.
Inadequate set up time before burn out or steam de-waxing.
Steam de-waxing too long, should be 45 minutes to 1 hour.
Flasks heated too rapidly, putting cold flasks into a hot oven.

Incorrect water to powder ratio on investment mix.
Flask heated too rapidly. Insuffcient set up time on the investment before placing flask in the oven.
Flask dried out too much before burn out.
Investing extended past working time.
Using outdated or improperly stored investment.

Inadequate work time on the investment.
Lack of adequate set up time before steam de-waxing.
Steam de-waxing for too long a time.

Investment mix too thick, check water to powder ratio.
Work cycle too long, investment setting in final vacuum cycle.
Weak or faulty vacuum, plugged vacuum lines, bad vacuum seal.
Surface tension can cause air bubbles to stick to the wax during investing; using a wax dip is helpful to break the surface tension.
Lack of vibration in final vacuum cycle.

Flasks overheated during burn out cycle, check oven temperatures.
Metal overheated or overused.
Oxidizing flux used on metal before casting.

Crucible old and deteriorating, replace the crucible.
Slag or oxide build up in crucible, clean or replace the crucible.
Foreign materials or oxides in the metal, refine the metal.
Investment erosion or breakdown, check mixing procedures.
Investment chips or pieces, check wax trees for holes at attachment areas and use great care when removing rubber bases