Duplicate images are often difficult to reproduce accurately for the mold maker. This problem can often be overcome by making one half larger than intended and make a mold of it. Make two patterns and either join them in the wax or cast them separately and assemble them later. The intricacy of the piece will help determine which technique is the best. Keep in mind, too, that higher degrees of surface finish can often be achieved in tight recesses prior to assembling the piece.Getting Great Details
The amount of detail that can be achieved in the casting process is incredible. More often than not this fact is not fully utilized in the master preparation, particularly in the development of the wax model. Most model makers feel that the best detail can be produced in metal and do not attempt to achieve the same level of detail in wax. Due to the metal’s resistance to deformation and abrasion it offers a firmer surface that provides greater control for most people. As a result it is easier for them to achieve a higher level of detail. But this is not due to the materials. Wax on the other hand does not offer the same resistance, which for many means less control and less detail.Details can be placed in wax more easily and quickly than they can in metal, and the errors are more readily corrected. The quality of the detail will vary with skill level. Whatever your skill level may be at this time, through practice it can be developed further when working with wax masters.
The more work you can do in the wax the less you have to do in metal. This may allow you to complete 90-95% of the detail in wax and the remaining 10-5% in metal.Model Metal
The metal most often chosen for models is sterling silver. It provides an excellent compromise between ductility, durability, malleability, cuts easily and solders together and maintains detail well. In some instances it may be too malleable to hold its shape due to its size. In instances such as these other metals can be used instead of sterling. Fine pierced filigree detail that can be easily deformed in sterling becomes much more rigid when made in a white gold alloy. This will effect the cost of the master but given the life of the master relative to the cost of repairing it, the cost is nearly insignificant.
Ideal Master Surface
The quality of finish on the model will affect the quality of the casting. Some may believe that the surface of the model does not have to be any finer than the investment, because the investment controls the surface quality of the casting. That might be true is the model isn’t used to product the pattern—but it is.
The surface quality of the model has a direct affect on the quality of the pattern. It also affects the amount of surface resistance between the mold and injection wax. The casting cannot be any better than the pattern, making surface quality an important issue. Masters should be brightly polished in all areas.
Another point to keep in mind is that the surface of the mold and pattern affect the ease or difficulty in removing the pattern from the mold. The greater the resistance the more likely a distorted or a broken pattern is to result. If it is difficult to remove patterns from a mold, the mold will be worked harder and the life of the mold will be shortened. Fine parts of the mold are likely to tear from repeated stretching as well as suffer a loss of memory that results in alignment problems.
During a discussion concerning the need to polish the master a point was brought up that if the surface is intended to have a matte finish it doesn’t need to be polished. It makes a good argument until the primary purpose of the model was considered. The model is used to make a cavity in an injection mold to produce a pattern. Rough surfaces on a pattern can contribute to turbulence during casting. Surface finishes are one of the last things applied to the casting during the finishing process so they do not have to be present when the pattern is made.
Accounting For Stones
Building a model to accept a specific stone size is a common request. The request may be in reference to a stone weight (.10 carat diamond) or dimensionally (3 mm).When calculating for shrinkage, there are a few things to be aware of to get to the desired size:
- First is that unless calibrated stones are used each and every time, the intended stone size is relative. A 3mm seat may be expected to hold a range of stone sizes anywhere from 2.8mm to 3.2mm if the stones are not adjacent to one another.
- For stones that are adjacent to one another the range may narrow slightly to 2.8mm through 3.1mm.
- If the seat is made exactly to size, 3mm, it would be unable to accommodate any stone larger without overlapping.
- Generally, allow for more shrinkage when designing seats for adjacent stones. Try add 12-15% when building a metal model. This will make the seats 3.36mm-3.45mm. So if you have 8% shrinkage the seats end up being 3.1mm-3.17mm.
- Say you know what your actual shrinkage rate is; then use that over the maximum desired size. In our previous example that would be 3.1mm.
- If your shrinkage factor was 8% then your seat will be 3.35mm. This will allow the cast mounting to accept the desired range of stone sizes.
Flexibility Is Key
One valuable feature of a good master model is flexibility. It should look like the intended design, while at the same time allow for the degree of variation that is common when dealing with stone sizes. Occasionally a master model is made to such precise standards that, unless the stones are carefully calibrated, they simply will not fit. This can be problematic in a production environment. We must always keep in mind that it is always important to make what the customer wants and produce pieces they can use.
[Adapted from Model Making-Creating Master Models]