October 27, 1999 United P.M.R. Inc.
The discussion of shop practices is critical to high productivity and quality. Every detail involved with high-quality jewelry manufacturing must be reviewed. This begins with all equipment, materials used and personnel.
Equipment involved in jewelry manufacturing starts with the air compressor, then to the wax pots, then to the burnout ovens, and finally to the casting machine. This would appear simple enough, however in improperly operating air compressors may lead to water or oil becoming trapped in the waxes. Most air compressors use high sulfur oil in the motors. Correct maintenance of an air compressor should be done once a week. This involves draining of the water from the condensate tank. The water should appear either clear or rusty; this is a normal condition. If a gray or black oily film appears floating on the water this means that oil is leaking into the air lines. Proper filtration should be placed on the compressor. A convalescent filter and water filter should each be placed as close to the outlet of the compressor as possible. These filters need to be monitored and cleaned or replaced as necessary. What is also taken for granted is the type of pipe used between the compressor and wax pots. The proper type of pipe is copper. Black or iron pipe will eventually rust and particles of iron may become embedded in the wax. In addition to the water filter many manufacturers use what is known as a desiccated air filter. This filter uses a crystal that changes color when moisture is absorbed. This is an excellent way to monitor the airlines.
The results of having oil reach the wax are residual residues left in the flask during the casting operation. The residue will cause sulfur gas porosity. This residue can be seen when testing a dummy flask. The look is usually a round stain that is black or brown in color. A dummy flask is a flask that has been burned out under normal operating conditions without the metal being poured. The flask is cooled, chipped away and the cavities are inspected under magnification. We always expect a clean white cavity. Sulfur Gas
The results of having iron residue in the wax are iron oxide particles left in the flask during the casting operation. This will cause gas porosity, dark rough surfaces and possible inclusions.
The results of having water in the wax pots will be round clean voids in the castings.
Now we know that the wax is not contaminated by front end equipment. The next area to be aware of is the wax itself. There are many of types of waxes available. They can be normal jewelry wax, industrial waxes used in other industries for investment casting, plastics such as polyethylene, carving wax as well as hybrid wax plastic blends. All these wax materials produced carbon when they are burned. The amount of carbon that will be needed to be reduced in the burnout oven will be determined by the type of wax and the burnout cycle. The least amount of carbon produced will be from normal jewelry wax of light color such as pink or aqua. Hybrid wax plastic blends are the next easiest to burn out. Next comes the carving wax most of which is dark green or purple. Finally we have our plastics usually polyethylene, sometimes polypropylene or polystyrene. One note on these plastics, polypropylene and polystyrene give off toxic fumes when burned. Wax
The reduction of the carbon produced is determined by the amount of carbon, the amount of oxygen and the time at 1350 degrees. What this means is the higher the carbon residue the longer time we need at 1350 degrees. In addition the rate of temperature rise will also determine the amount of carbon residue needed to be burned at 1350 degrees. When dealing with jewelry or hybrid waxes the slow ramp temperatures used by many manufacturers are the preferred method. However when dealing with hand-carved or plastic patterns a rapid ramp temperature may be necessary. At the very least a much longer hold cycle at 1350 degrees will be necessary. This hold time maybe eight to 10 hours. It is also possible that in an oven that cannot ramp rapidly, such as small electric ovens, they may not be able to reduce the carbon fully regardless of time.
The result of not reducing the carbon fully is sulphur gas porosity.
Wax defects can also be caused by mold release sprays and powders. It is recommended to use only pure silicon spray and or jeweler's talc. It is also possible to use cornstarch as a mold release. The use of body or foot powders should not be used. They normally contain magnesium silicate which appears as a salt crystal in the flask cavity.
Storage and handling of the waxes are also very important. Waxes that are stored for future use should be in sealed closed containers. Wax set up areas should be in a clean environment. We have seen situations where polishing compounds were airborne and settling on the waxes before investing. The polishing compounds usually contain iron or chrome which will cause the same problems as iron contamination from the air line pipes.
Some waxes which may be sold as jewelry wax but are designed for industrial applications should not be used in fine jewelry manufacturing. Most of these waxes are dark in color, they may be brown, orange and purple. These waxes contain metallic pigments that when burned leaves the color of the pigment as a residue in the flask cavity. This is a guaranteed method for sulphur gas porosity. If you are not sure who manufacturers your wax you should deal with a reputable jewelry supply house.
Tree wax should always be of a lower temperature than the wax in the flask cavity.
Old or used wax should never be recycled in a fine jewelry application.
Wax inspection should always be done to eliminate rejects.
Now we have a quality wax. The next step in the procedure is to mix the investment. The investment mix should always be to manufacturer's specifications. If the investment does not set up in its normal time period or sets up much too quickly do not proceed with the burnout.
Now we are ready to put the flasks and the oven. A clean properly functioning burnout oven is essential for a quality cast. Thermocouples should be checked periodically and replaced if necessary. As discussed previously, the burnout temperature ramp and hold time at 1350 degrees will be determined by the type of wax. Burnout ovens should be cleaned daily of ash residue. If a gas oven is used it is important to keep the flames at a low height. Gas ovens are neutral or reducing in atmosphere. It would be preferred to have an oxidizing atmosphere. Keeping the flame low will allow more oxygen to be present in the oven.
Now we have a perfect burnout. It's time to cast! There are many methods and machines in which to melt and cast. Depending on the volume of castings needed some may use a torch and a centrifuge machine, a torch and a vacuum machine, a gas furnace and either machine, a bottom pour resistance melter or a bottom pour induction melter. Regardless of the machine the metal should always be taken to its casting temperature. The casting temperature should always be the same with very few exceptions. When hand pouring the metal should always be taken to a liquid state, the surface skimmed before stirring. When a metal has a clear look similar to a mirror it should be poured into the flask. When using a bottom pour furnace exact temperatures need to be set. The length of time it takes to melt will be determined on whether the furnace is resistance or induction. Fluxing is only recommended when hand pouring.
Quenching and breakout will be determined by the type and karat of metal being cast. Please refer to the manufacturer's instructions for these times.
Remelting of sprues and buttons require attention to detail. When remelting, accumulated oxides will rise to the surface of the melt due to it's lower density. It is extremely important to remove these oxides by skimming and not stirring them into the melt.
Failure to remove the oxide will result in oxide inclusions in the cast.
Now we have a perfect cast!
As we are all aware, our most valuable asset is our people. Quality and consistency require that our personnel be diligent and focused.
Proper maintenance schedules and inventory control need to be standard operating procedure. By controlling our equipment and techniques we can diagnose problems much more quickly.
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