With recent high and currently uncertain precious metal prices, many manufactures who cast using the lost wax process are turning to non-precious, copper based alloys to produce high quality casting. Similarly, many costume manufacturers are beginning to look closely at the possibility of moving from white metal spin casting to lost wax casting for greater quality and strength in their product.
The continuing growth of non-precious metal usage by the lost wax casting industry for jewerly manufactuing has nearly doubled in the past five years, and there is no slow-down in sight. Those maufacturers producing jewelry in base meals as an alternative to gold and silver are finding a ready market for products whose value is tied more and more to artistry, creativity and innovation than to metal value.
Total use of a variety of copper-base alloys by investment casters continues to rise dramatically, and in the absence of published figures, it is estimated that upwards of a half million pounds will be cast this year. Does this mean that precious metal castings are on the way out? Not by a long shot, but the pendulum has been swinging toward base metal usage. With the economic downturn, the pendulum is picking up speed.
There is new interest in alternative alloys as manufacturers struggle to remain competitive, seek to reduce labor costs and search for equipment to speed up the manufacturing process. it is more imporntant than ever for investment casters to learn about what metals are available and how to get products ready for the marketplace. It’s past the time to get down to the basics of exactly what is being used by the majority of casters; rather, we must delve into the characteristics of alloys.
With some 30 different bronze and brass alloys available, the selection of the right metal can become expensive and time-con- summing. Most of the alloys were originally intended for industrial casters employing sand, shell, ceramic and permanent mold techniques, not lost wax methods. Instead of covering such a wide spectrum, it would be better to pinpoint the half-dozen copper base metals that have gained the widest acceptance. In general, these metals are a combination of varying amounts of copper, zinc and manganese, with some of them containing additives of antinomy, tin and aluminum.
Free cutting brass
The metal first used as an alternative to precious alloys and still popular -is yellow free cutting brass. Long before the advent of the more sophisticated metals now marketed, casters found yellow brass a low-cost, readily available substitute. originally produced to serve the screw machine industry, free cutting brass rod lengths were easily adapted by chopping the quarter or half-inch round stock into the configuration needed to fit the crucible. The fumes and high smoking characteristics of free cutting brass have always been difficult to deal with, but the metal casts to a pleasant yellow color, has good solderability, and plates well. Because it often contains 3% to 4% lead, many casters are not satisfied with the surface quality of the castings.
Lead has a tendency to remain as unalloyed particles in most copper alloys, thus affecting the cast surface structure. Use of yellow free cutting brass may be further complicated by its relatively high zinc content. Wherithe sprue and gate design restrict the escape of zinc vapor, the castings exhibit unacceptable porosity. Yellow brass poured at excessively high temperatures results in zinc vapor collecting on the mold wall, and castings will have a worm type surface defect. In casting yellow free cutting brass – as with all copper base alloys strict temperature control is paramount.
The use of silicon bronze has seen a quantum leap in popularity because it has proven to be a versatile and forgiving alloy. Ring, earring and pendant manufacturers are the largest users. Silicon bronzes cast extremely clean, without the fuming and smoking found in free cutting brass. Castings have a rich gold color, requiring minimum finishing. Dross formation is extremely low. Silicon bronzes have excellent fluidity, and they produce a fine-grained surface that replicates and holds sharp detail. Another plus is that there is no significant change in chemical composition during melting, and the metal can be remelted a number of times without the,addition of new metal. The general practice is to use a mix ratio of 50% recycled gates and sprues to 50% new alloy. This method does not affect castability, and it represents important savings in metal cost.
In those applications where metal malleability is important, silicon bronzes may not be the proper alloys because of its ascast hardness. For example, in manufacturing costume rings where prongs are to be bent over stones, fracturing can occur.
Users of silicon bronzes report that no deoxidizer is needed, and castings are solderable and take excellent plating.
Special yellow brass
Still another metal used in large quantity over them is a special yellow brass that has gone several steps beyond free cutting brass in chemical composition. Producers of these alloys lowered the zinc content, while adding varying amounts of tin, nickel and silicon to improve fluidity and eliminate the smoking and fuming. Such alloys, have found acceptance in the ring, charm and fashion buckle industries. These alloys melt very clean, cast items have high ductility, and are easily finished to an attractive pale yellow gold color
These metals may be recycled using the same ratio mix recommended for the silicon bronzes. No de oxidizer is needed, and there is no problem with gas absorption, provided the molten metal is not heated too far above the melting point. Solderability is good, finishing time is minimal and castings plate well.
Manufacturers of recognition and award products, as well as casters of heavy belt buckles, after experimentation, have been turning to a metal excellent for items with more density than usually seen in jewelry products. It is silicon brass which provides the same strength and hardness of silicon bronzes while yielding what they claim is a finer surface finish. The alloy has outstanding fluidity, does not require a deoxidizer and captures and holds sharp detail. It cleans easily to a true bronze color. This metal recycles well for cost savings. It is zinc-free, and has additives of both manganese and silicon.
At this point, a few words of caution are appropriate. Most metal users seeking to produce a cast item of specific color or ductility should bear in mind that mixing of various alloys to achieve their goals can be risky. Many alloys are simply not compatible, and the possiblity of metal contamination is always present. The result of such experimentation is often castings that are extremely hard or brittle. The search for color and ductility is best accomplished with the help of a metal supplier’s research facility.
Casters seeking an alloy that produces a white, silvery finish need not go on an exhaustive hunt. There are a number of manganese bronzes containing a mix of copper, manganese, zinc and aluminum. Manganese gives this metal a color equal to that of nickel silver, while providing a greater versatility due to low casting temperatures. Castings have excellent strength and ductility, solder well, and finish to a high silvery lustre.
Casting manganese bronze requires close attention to temperature control. The high manganese content will turn the molten metal gummy if the melt is held too long at high temperatures. The metal should be melted as rapidly as possible to the desired range, avoiding superheat, and then poured as soon as possible. Agitation of the melt must be minimal, and that rule applies to all copper base alloys. Many users of manganese white bronze say they have greater success when they use a de oxidizer.
Possibly the least understood of copper alloys are the beryllium copper metals. Prospective users are often not aware of the many advantages offered by this alloy. These metals provide precise replication of the most intricate detail, while producing castings of exceptional strength. Key features of beryllium copper alloys are the high degree of fluidity and low freezing teniperatures. Many casters use small amounts of beryllium copper to sweeten their other copper-base alloys to improve fluidity and attain improved cast surface finish. Several large school ring manufacturers produce their masters in beryllium copper to capture and hold minute detail. The use of this particular alloy in jewelry manufacturing appears to be far more popular in Europe than in this country.
Strict plant hygiene must be practiced when using berylliumbearing alloys. This is a toxic substance and extreme care is needed in the casting, grinding and polishing areas to restrict worker intake of fumes or particles. Adequate ventilation is a must. Manufacturers may request a complimentary copy of an excellent publication on the handling of beryllium copper (see end of article ).
College and high school ring producers, as well as firms marketing recognition and award devices, are now casting a “stainless” type alloy that finishes with the appearance of white gold. They have found that using these new metals has enabled them to hold their market share with a low-cost substitute for precious metals. The metals being used are primarily a combination of nickel and chromium, along with additives which improve casting characteristics and reduce the melting point of the alloy. The metals are castable at under 24000 F. Jewelry, art and dental castings are now being produced in these alloys. They require no plating, and finish to a durable and lustrous white gold look. School rings cast in these metals often bear somewhat exotic-sounding names, and the lower price tag has made the product attractive to students. Users of these metals should insist on knowing the exact composition and whether there are any health hazards that may be encountered in their usage.
Certain alloys serve well for one application but not for another. The selection process does not have to be frustrating or costly.
The first step should always be to discuss specific applications with the metal supplier. Order small sample lots of recommended metals and cast them. Castability, ease of finishing and the quality of the finished product will provide the answer. Patience, a willingness to experiment with flask and casting temperatures, and the guidance of the metal supplier are the necessary ingredients during the initial trial stage.
Many of the most common defects observed in casting copper base metals are simply caused by trying to introduce too much molten metal into the casting through too small a gate area.
There is always understandable concern about time spent grinding and finishing the gate areas. This often results in a practice of pinching off the gate area -and the end product is a poor casting. Gating practices in lost wax casting fly in the face of methods used by foundrymen where large gates and risers are the standard. Trapped gases in the mold must be removed or they will be absorbed by the metal, cause misruns, or be trapped as bubbles. A close working relationship between the model maker and the head caster are important, and the head caster’s experience in proper gating is invaluable.
After metal selection is made, it becomes vital for the head caster to establish standard operating procedures for flask and casting temperatures. There is always a suggested casting range for alloys. These ranges provide a high and low side, with lighter castings poured at the higher temperatures. Care must be taken to avoid overheating the metal; further, the molten metal must not be held at high temperatures longer than necessary before casting. Without the use of adequate temperature controls, the production of consistently fine quality castings can often become a case of hit or miss. Anyone who has been exposed to the lost wax industry for years will not debate the head casters who insist that the “eyeball” method of gauging casting temperatures serves them best. However, if the manufacturer can use equipment that accurately gauges temperatures on every shot, the end results will be worth the effort and expense.
Proper flask temperatures best used with copper-base metals are a matter of diverse opinions, and each caster usually develops his own method. Many casters use a starting point of having the flask come out of the oven approximately 800°F to 900°F below metal casting temperature. Working from that point, it then becomes necessary to do some experimenting, moving the flask temperature up or down until the desired results are achieved. Casters who insist on employing standard operating procedures say that they usually run a reject rate of below 5% on copper-base alloys.
Casters who rely on the “eyeball” method should refrain from stirring up the molten metal in the crucible. A probe to ensure full melt is fine, but agitation prior to casting must be avoided. Shops using a centrifugal machine need not worry about how the molten metal is introduced into the flask -the machine handles that operation. Careful pouring practices for vacuum assist table shops. however, cannot be overstressed. Molten metal must be as free from turbulence as possible. The lip of the crucible or ladle should be brought as close as possible to the top of the sprue to minimize turbulent pouring that creates dross and causes misruns. The stream of molten metal being poured should remain constant and be of such a size that it can be controlled without splashing, while ensuring that a full sprue is poured.
Quench time after casting has always been an area that needs closer attention. Quenching the flask too soon means harder castings and increased difficulty in removing gates and increased finishing time. Once again, some experimentation is needed to determine exact time frames for quenching, depending on the degree of hardness and workability desired in castings. Many casters feel that quenching somewhere between 10 and 20 minutes provides the best results.
Once the proper time frame has been established for certain products it should become standard procedure to stay with that time frame for uniformity. It makes little sense to monitor flask and casting temperatures closely while ignoring the correct flask quench time.
Before removing castings from the sprue, most shops use a glass bead blasting operation to clean up the dry sprue. Small, dissimilar size grains of glass beads give the castings a fine microfinish while removing investment residue and surface oxides. There are a number of different size machines on the market, and price is predicated on the size and whether or not the user wants to recycle the glass beads or capture the investment dust in the process.
General practice in removing castings from the sprue is still the time-consuming, one-at-a-time method using a benchmounted cutter. Some innovative casters use a square-shaped sprue and remove the castings with a bench-mounted saw. Recently, an ingenious machine designer developed equipment that will remove all castings from the sprue in less than 40 seconds – a tremendous labor saving.
Jewelry producers have or are now turning to vibratory finishing equipment. Many plants employ a chemical clean-up operation before they vibratory finish. They say that this multi-step process results in surface improvements when the pieces are then vibratory finished. I won’t discuss this process because it doesn’t fall strictly within the topic of this article, but interested manufacturers may obtain information on this procedure by writing me.
Manufacturers outside the northeast area continuously inquire about the chemical makeup of “Brite-Dip.” The use of this chemical process has its advocates, and an equal number of manufacturers who feel that the process results in attacking the surface finish of cast items. A five-gallon solution requires a mixture of three gallons sulfuric acid, two gallons nitric acid, and one to two ounces of hydro-chloric acid. Consult the chemical supplier for specific information on proper containers and safe handling procedures, and, of course, on how to dispose of such chemicals. The immersion of items in the solution is normally for a few seconds, followed by a thorough rinsing in cold running water.
Vibratory finishing equipment in a wide range of sizes is being used increasingly to give excellent pre-hand polishing or preplating finish to castings. Users report that they often eliminate hand polishing entirely due to the excellent surface attained with this equipment. A variety of media sizes and composition are tailored for almost every type of jewelry configuration. Where hand polishing is the final step, vibratory finishing reduces the time necessary for the operation.
The topic of manufacturing jewelry in copper-base alloys is not complete without some discussion of plating steps. Copperbase alloys do not resist tarnishing because of the very nature of chemical composition. The producers who do not plate their product line are in the minority. Whether the manufacturer plates in his own shop or farms it out, it is important that certain initial steps are understood and followed. All copper-base alloys must be scrupulously clean before plating. Cleaning solutions secreted anywhere in the cast surface may result in a bleedthrough of the plated item.
The first step of the process is a relatively heavy copper strike, followed by nickel plating, then on to gold, silver or rhodium. When there is bleed-through of plated items, the problem may usually be traced back either to improper cleaning techniques or failure to put that initial heavy copper strike on the piece.
There is an excellent publication for those interested in the basics of cleaning and finishing metals. The book covers cleaning, finishing, shot peening, heat treating and plating. The title is “Metals Handbook,” V olume 2, and it is available from the American Society for Metals, Metals Park, Ohio 44073. The publication is in its eighth edition, and well worth the price of $62 to non-members; it may be available in local libraries.
Members of the lost wax casting industry are justly proud of the skill, creativity and innovation long associated with investment casting. They are well aware that the quality of their craftsmanship transcends the low cost of thE) alloys used when they go into copper-base metals. Investment casters must spend a great deal of time learning their trade; and then they must spend a great deal of money for equipment long before a finished item is ever produced. Manufacturers thinking in terms of establishing in-plant casting operations have to ponder long and hard before taking that step. They often find that without the expertise in the craft or the volume to sustain them, they may be better advised to seek out and utilize the services of job shops and freelancers.
Manufacturers already casting precious metals know they can make the transition into base metals using the techniques and equipment already in place. Many precious metal producers are already manufacturing a line of castings in low-cost alloys to capture both ends of the market. Others have found that substituting copper-base metals for their sample lines has resulted in tremendous dollar savings. Not only do they eliminate the use of operating capital in sample lines, they avoid the serious losses suffered if salesmen’s samples are lost or stolen. For the firm or individual looking to produce lost wax casting jewelry items, there remain many questions about types of equipment and startup costs. Many experienced casters prefer a centrifugal machine, while an even greater number endorses the vacuum assist tabl~ method. Judgement is always influenced by individual experience, and many times the final decision is based on how much capital is available. Those preferring the vacuum assist table method point to its lower initial cost and high rate or productivity. The choice requires careful research and shopping.
If someone outside the industry guessed that the investment casting field derived its name from the dollars needed to venture into the business, they wouldn’t be far off the mark. The basic cost of one each of a vulcanizer, air pressure pot, prcision waxer, investment mixer, vacuum air pressure pot, precision waxer, investment mixer, vacuum table for investment, burnout oven, programmer temperature control, washout box, glass bead machine and vibratory finishing equipment means an initial outlay of close to $14,000. That does not include cost of crucibles, flasks, wax, investment and possibly an electric dewaxing oven. Then one must measure the difference in price of a centrifugal casting machine at about $14,000 against that of a vacuum assist table system and gas crucible furnace costing about $3500.
The numbers of lost wax casting manufacturers grows larger each year, and so does the total usage of alternative alloys. The quality of the work being done is impressive. Jewelry artisans have relied on copper-base alloys for thousands of years, and there is no doubt that these alloys will increase in popularity, and continue to playa vital part in the jewelry industry.
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