Side effects of casting from resin patterns

SUMMARY

The employment of resin patterns is gaining ground in investment casting processes. But this involves an increase of the attention required by the investment burnout cycle, to avoid the appearance of defects such as holes or superficial porosities on the final jewels.

PROBLEM

A 925 silver specimen produced by investment casting was affected by surface cavities similar to small holes.

The piece was then analysed by the use of the scanning electron microscope, to better understand the origin of the defect. We discovered that it consisted in a series of porosities characterized by a rather regular morphology, particularly evident in the last two photos.

Considered the fact that this customer was used to prepare the flasks starting from resin patterns, we hypothesized that the cause of those porosities, spread all over the sample surface, was right due to the use of the resin itself.

The evacuation of the wax from the flask during burnout cycle is relatively easier as regards the ejection of the resin. If this last one was not completely eliminated from the flask, e.g. due to a non ideal tree's architecture, it could lead to the formation of gasses for its decomposition when it gets into contact with the molten metal; the resin could also act as catalyst for an investment decomposition, in case casting and flasks temperatures were (too) high; some portions of resin could remain entrapped in the item surface as well, by causing the formation of porosities. For this reason, the use of resin patterns requires the maximum attention during the preparation of the flask and during the burnout cycle as well.

SOLUTION

Some advices to solve this kind of problem:

1. First of all, it is important to use a type of investment suitable for casting from resin patterns; on the market different types of investment are available, studied on purpose to avoid the formation of the above cited defects;
2. It is necessary to pay close attention during the tree's preparation; in fact, its architecture has to favour as much as possible the resin elimination during the investment burnout cycle, so that no residuals remain inside the mould;
3. In case, it is right to check the temperatures of the investment and flasks and to avoid keeping them too high.