SUMMARY
To realize a flat item with wide surfaces by investment casting can often be a risk for the success of the casting itself. In fact, if it is not properly fed, or if not sufficient attention to each phase of the production cycle is paid, different problems may occur, such as the shrinkage porosities.
PROBLEM
Some 925 millesimals silver manufactured items, characterized by wide flat surfaces, were affected by several rather evident cavities on their surface, as you can see in the next two photos.
The analyses we carried out by means of the scanning electron microscope proved that it was mainly a matter of two kinds of defect, i.e. shrinkage porosity and cavities imputable to investment inclusions. The sample we analyzed was composed, in addition to the above cited wide flat surface, by very thin thicknesses. These characteristics make its production by investment casting very difficult and require closer attention to the phases of wax patterns preparation and tree s assemblage.
Very thin surfaces are critical as for the shrinkage porosity. In particular, they tend to solidify before the thicker surfaces, by stopping the passage of the molten metal towards these last ones. Or they may solidify starting from the outside, entrapping in the middle of the item some metal still molten. This involves a block of the metal flux to the liquid zones (the last to solidify). There, as a consequence, some cavities take shape in the areas remaining without molten metal they are the so-called shrinkage porosities and look like those represented in Photos 3 to 5. These shrinkage cavities take place because of a physic issue, that is the metal, transforming liquid to solid, notably reduces its volume. If some other molten metal is not available, to compensate for this volume reduction, the shrinkage porosity occur.
Another difficulty can be faced producing by investment casting this kind of very thin items in this case they are rich in details, too. It is the preparation of the flasks. In fact, if the investment quality is not excellent, or if the flask is not prepared in the proper way, some problems such as breakings or defects caused by investment inclusions may occur. For example, in the case of particularly thin sections, these may flake off during casting, by causing inclusions or porosities.
SOLUTION
Some suggestions to reduce or even solve this kind of problem:
- first of all, in case of items with wide and flat surfaces, it would be better to produce them by mechanical working (e.g. stampato) rather than by investment casting;
- anyway, if you intend to produce this kind of items by investment casting, you should try to avoid using flat feeders: round-section ones are much better, even though this involves the junction of the feeder onto the flat surface - so, a notable work during the polishing phase;
- besides, we suggest you to try and add a feeder in each area affected by the defect, to compensate for possible metal lacks during solidification and then avoid the onset of the shrinkage porosities;
- avoid to have particularly weak sections of investment, such as sharp edges in the junction between the items and the main sprue; in fact, in these areas, the molten metal flux may easily break some portions of investment and take them inside the item;
- Finally, it is a good rule to pay the closest possible attention during the flasks preparation. A properly prepared flask, with a correct water-powder ratio, produced exactly following the burnout cycle suggested by the investment supplier, allows avoiding breakings of the investment itself and the possible inclusion of material inside the casting.
