Defects in Investment Casting – Part 2
Every problem has a solution and the same philosophy can be applied to Investment Casting. As discussed previously, due to the complexity of the process there are numerous opportunities for problems to present themselves, resulting in casting defects. Understanding the causes of the defect is essential to managing casting quality and taking remedial actions.
One of the most common defects found in castings is shrinkage, which can undermine the integrity of the entire part and may eventually cause it to break under stress. One of the causes of shrinkage is the mould or metal pouring temperature being too high at the foundry stage. Although obvious, the solution is to reduce the temperature, or ensure constant control of the temperature by taking readings using the pyrometer, both of which can be done by adjusting the power accordingly. Constant control of the temperature is crucial, as low temperature can result in other defects occurring such as misruns or cold shuts, which occur when the material being poured freezes before it has filled the mould.
Experience and good housekeeping play a fundamental role in preventing defects which may present themselves. For example, inclusions can be formed as a result of ceramic debris entering the mould after dewaxing, good housekeeping such as wiping the rim of the pouring cup and top of the crucible before turning up-right is one preventative solution but in some cases an inclusion may be caused by the design of your part.
Small sharp internal corner with no radii on the part introduce small sharp external edges within the shell after dewax. These features can be washed away if the metal enters the mould too rapidly or if the strength of the shell in these small areas is compromised. A good example of a design issue that could cause this would be debossed lettering, we would propose to use embossed lettering within a recess, this reduces sharp internal corners in the shell and therefore reduces any likelihood of the above occurring.
Gas Porosity is another defect we discussed previously, one of the most effective ways to prevent this is to melt the material in a vacuum, however there is a much higher investment in terms of foundry equipment and may not be a viable option for foundries without vacuum melt equipment already in place. Alternatives include melting in an environment of low-solubility gases such as nitrogen or argon shield. One of the more common methods would be through precipitation, this involves adding an element which forms a compound, this then forms a dross which floats to the surface of the melt and can be removed.
A valuable tool to help understand where defects may or may not occur is the use of a simulation software package, these can simulate flow and solidification of metal to aid investigations into any potential issues that could occur. At Investacast we work with our customers to remove the likelihood of casting defects through good casting design in the early stages of development.