Platinum Do’s and Don’ts For Wax Model Preparation
In order to help you get the best possible platinum casting from Au, we have prepared the following list of suggestions to help you with the mold making process and to prepare the wax for casting:
- If you are using any wax cleaner or polishing solution consider eliminating it from your routine. The residual solution remaining on the wax can cause investment breakdown and consequently yield a very poor casting. It is very difficult for us to tell if these solutions have been used on the wax. If you do use it, clean the wax extremely well with soap and water. The use of these cleaners is even more detrimental if the piece is going to be cast in gold.
Another concern is that of silver/aluminum paint on the wax that allows the customer to see approximately what the finished piece will look like. This application must be completely cleaned off of the wax prior to investing. Residual amounts of this paint will cause casting difficulties as well as excessive finishing labor. We do not invest waxes with the paint remaining on them.
- As pieces increase in weight, they become more difficult to cast. This is due to the mass of material and the attendant shrinkage that has to be fed during the very short solidifications window. A few things to consider would be to relieve areas under very heavy shoulders and don’t assume that the same designs you’ve been using for gold will work equally well for platinum. In many cases the thick-to-thin designs cannot be successfully cast without the addition of more and larger gates to minimize shrinkage porosity. In some cases the pieces should be redesigned into smaller parts to avoid these problems and appropriately meet the manufacturing challenges. Component pieces would necessarily have to be soldered into the finished piece after casting.
A similar situation exists with large thin pieces, only in this case the piece becomes more difficult to fill because of both the short liquid window of the metal and the added length of a very thin cavity. Although each piece requires an evaluation on its own characteristics, as a general rule, these pieces should be fabricated.
- If possible, fill as many holes as you can. Some pieces have holes drilled for channel set and burnished in diamonds. These leave a column or other similar shaped quantity of investment in the cavity that will be the finished piece once the platinum is cast. Because of the mass and density of this metal, it tends to break these “columns” free and in so doing, it causes them to flow to some other location inside the mold pattern. They cannot escape, because the cavity is a closed system. Therefore, the small pieces of investment that are broken away, when the superheated metal is thrown, with very high force, into the cavity can cause “pits” in the finished piece. These pits must either be burnished away or platinum must be fused into them. This requires additional work and cost to be incurred. This, of course, is not only caused by holes, it can be any fine detail that is delicate enough to be broken away when the casting is done. These designs should be given close attention when the piece is being created to avoid post casting and finishing difficulties.
Although not as obvious, when two or more pieces, such as when a machined wax bezel is joined to a carved ring shank, these is almost always a thin space somewhere in the joint that does not get filled and consequently will form a shelf of sorts when invested. This shelf (or whatever form it takes on) will always be very thin and subject to the same forces of the superheated platinum, when cast. Again, the results are the same; free investment in a closed system that will surely result in pits due to the trapped pieces. The guideline here is to seal all joints that are created when two or more pieces of wax model are joined prior to casting or alternatively, to do this after casting and finishing with some sort of metallurgical bond.
- Don’t be tempted to sprue the piece yourself. We take the design into account in a technical fashion and place the gates in relation to the mass and shape of the piece.
- If you have a mold, send it along with the order. If that is not possible, send more than one injection. This is an additional precaution to ensure timely completion of your order. With all of the technology we employ to assure you of high quality castings, there is always the possibility of something that Murphy has a hand in. If we claimed to be perfect, then you would have something to be concerned about.
Make sure to check out our Jewelry Casting section for more information about our casting services. If you have any questions concerning the above suggestions, or we can help you with any piece, please contact us. We will be happy to assist you.
