Importance of waxes in investment casting processes

SUMMARY

The wax is the starting point in order to obtain a high quality product by investment casting. For this reason, it is fundamental to pay attention both to the quality of waxes used and their preparation, by avoiding practices potentially dangerous for the articles to produce.

PROBLEM

In the case at issue, a 14 ct red gold sample produced by investment casting was affected by porosity of modest sizes, but which offered a visual sensation of rugosity. Such defect had a regular morphology and was spread all over the sample's surfaces. After analyzing it by metallographic microscope, a deeper analysis on the waxes used to produce that kind of model was carried out.

In this way we managed to come back to the problem's origins. First of all, the examined waxes had several air bubbles entrapped under its first surface. These air bubbles can break during mould's preparation, by being filled with investment and then replied in negative in the precious item as regular shaped porosities. Besides, from the exchange of information intercourred with the sample's producer, it came out he was used to use talc powder to facilitate the detachment of waxes by their mould. This practice, although, could lead to a talc deposit in investment's mould and cause possible inclusions or porosities, when this is removed only during the casting phase.

SOLUTION

To limit porosity phenomena due to problems of waxes, it is necessary to follow some advices:

1. As first thing, it is fundamental to make sure to always use high quality waxes, which do not display then superficial irregularity or air bubbles entrapped;
2. always try to use fresh waxes and avoid stocking them for a too long time; in any case, stock them in clean places and not dusty;
3. to avoid problems due to air bubbles, we suggest you to daily degas the used wax, so to completely evacuate the air entrapped inside it;
4. as far as the waxes' assembly onto the tree, we remember the importance of avoiding leaving weak areas, such as sharp edges between the main sprue and the item;
5. the use of talc powder is not recommended; this last one, in fact, could deposit inside the mould and cause inclusions or porosities, by compromising the piece's superficial quality. In case you could not avoid the use of talc, it is advisable to try to reduce its amount, little by little, and to pay attention to not reintegrate the wax inside the injector with wax dirty of talc.