In an age where productivity and consistency can determine whether a jewelry piece is profitable or not, many manufacturers are discovering a technique first explored during the rubber shortages of world war 2 – the use of metal instead of rubber to create molds.
“We’ve used it especially for promotional color and diamond ladies’ rings, where we’re being hard pressed to hit a price point for the retailer,” says Jonathan Seidel, general manager of Kelman Casting in Long Island City, New York. “Rubber molds tend to expand, so pieces start getting heavier and heavier. With metal molds, you don’t get the same expansion-you’re working with a hard surface, so you can inject at a much higher pressure without fear of gaining weight.”
In fact, metal molds can withstand pressures ofbetWeen 3,000 psi and 6,000 psi, compared to the 30 psi to 40 psi common among most wax injectors. Such high pressure provides another benefit: Pieces fill better, which permits the creation of designs with both very fine detail and thin walls, giving a big look at a lower price. “At low pressure, you can’t get molds to fill [on very thin pieces], but at high pressure you can overcome the friction,” explains Chuck Wolfmueller, director of manufactUring for Rio Grande in Albuquerque, New Mexico. “Plus, if you put high pressure on a rubber mold, it causes compression of the rubber, so your part [not only] gets bigger [but] also distorts.” Stuller Settings in Lafayette, Louisiana, uses metal molds for another good reason: increased productivity.
“A good operator shooting quality waxes of a similar product [in a rubber mold] might produce 500 pieces a day,” says Greg Gilman, the company’s technical and teams training consultant. “With metal molds, we’ve had people produce 2,500 pieces a day.
“The big difference is that rubber molds have personalities-mostly contrary personalities,” Gilman explains. “One minute they work beautifully, and the next minute you can’t get a good injection no matter how hard you try. With metal molds, once you set up and fine-tune [the process], you can inject all day without the mold overheating or changing temperament for some reason. It’s just so consistent: The last [wax pattern] looks exactly like the first.”
Finer detail, better consistency, lower price points, higher productivity…all fine reasons for replacing rubber and moving to metal. However, before switching any part of their casting production, manufacturers must also be aware of certain drawbacks. For example, not every piece of jewelry can be made with a metal mold, and even those that can will often raise labor costs. As with any change, the proof lies in the details-and metal molds have many.
Soft-Pressed Versus Hard Metal
There are two primary types of metal molds: soft-pressed molds and molds milled ftom harder metals, such as brass, bronze, aluminum, and steel. Soft-pressed metal molds are made ftom bismuthbased alloys with melting points of 115°F to 500°F, which are poured over and around a model and pressed in a vulcanizer to create an injectable mold. In hard metal molds, the mold cavity is cut directly into the metal by means of a computer-driven milling machine.
This technology has made the creation of hard metal molds much simpler than in the past. “When we first began making metal molds, we were using hand-traced etchings on acetate, and using that as a pattern with a pantograph to machine the molds,” says Gilman. “That was as lowtech as could be: A guy would take an 8 by 10 sheet of acetate and lay it out with a compass and protractors and a French curve.”
Stuller’s early approach was not only low-tech, but also time-consuming and labor-intensive. Today, the company uses an advanced CAD/CAM system. “We can get a two-dimensional drawing from [the design department], scan it in, convert it to a 3-D pattern, and look at it as wire frame, solid modeled object, or tool path [code],” says Gilman. “We can run the program and see if the tool is going to crash, or see if our programming will cause [the milling machine] to plunge too deep. Then we simply send that file to the mill, set it up with a pre-machined block [of metal], hit ‘run,’ and it starts machining.”
The CAM system produces precise mold pieces that fit together without so much as a visible parting line, he adds. In addition, the same files can be used to cut carbon fixtures for oven soldering that will precisely fit the mold’s product, speeding final assembly.
Not all CAD/CAM systems will work for milling hard metal molds, however. “When you make a metal mold it has to be right on, because you have to do a right and a left and put them together,” says Wolfmueller. “You have to have good milling machines with good tolerances that can put the two together without a parting line. The little inexpensive milling machines are good for carving wax, but they don’t hold tolerances for metal molds.”
And that technology comes with a hefty price tag. “If you’re just talking about making the molds and injecting them, you’re talking about $30,000 to $40,000 to get into it,” says Wolfmueller. Plastic injection equipment alone can cost up to $8,000 for a top-of-the-line model, adds Alex Benedict of AB Jewelry Machinery in Montreal, a manufacturer of plastic injectors. Compare this to the $500 price tag of equipment for rubber molds-as quoted by Timo Santala, director of manufacturing for Jacmel Jewelry in Long Island City, New York and the benefits ofhard metal molds take on a new perspective.
For manufacturers who want a less expensive alternative, soft-pressed metal molds may be the answer. Their manufacture does require some special equipment, such as a custom-machined case to hold the metal as it is pressed in a vulcanizer, and a temperature-controlled melting pot. It also requires a real rather than a virtual model for shaping the mold, and that model must be carefully finished for best results. However, there is no need for expensive CAD/CAM equipment.
The downside to soft-pressed molds is that they are more fragile than those made of hard metal, and they require careful handling. “Soft metal molds are real delicate: If you drop them and put a ding in [the surface], it’s all done,” says Wolfmueller. “If you’re real careful with metal molds, they’ll last a long time. But if someone is sloppy with them, it’ll eat your lunch.”
Gary Ayvazian, president of Unimold Cast Ltd. in New York City, estimates that most of Unimold’s soft metal molds will produce an average of 5,000 pieces before the mold needs to be refteshed. “But the life of the mold can be much longer if it’s properly maintained,” he adds. “I have had molds do more than 50,000 pieces.”
Seidel, on the other hand, finds that figure a bit high: His soft-pressed molds produce closer to 500 pieces before they need refurbishing. Because metal molds are often made in multiple sections that lock together, “all you need is for someone to sneeze while they’re assembling and the whole mold is off,” he says. “Then you’re back to reworking the entire mold.”
Deciding whether to use soft-pressed or hard metal is just one of the choices manufacturers must make. In addition, they must choose which material will be injected into the mold: wax or plastic. Both have their advantages and disadvantages.
“Wax is a lot easier to clean up and handle, but it doesn’t store [or] travel nearly as well,” says Seidel. “The plastic pieces tend to be very pliable, so they have a much higher storage life. They spring back and have a lot of [memory]. But it takes a lot longer to clean up plastic. [Plastics can require aggressive solvenrs.] We tend to use a plasticized wax for most of our domestic production, just because it’s easier to work with.”
Gilman, on the other hand, considers the option of using plastic to be one of the main advantages to metal molds. “In plas- tic, the replica is just so precise, and there is no distortion,” he says. “The plastic has tremendous memory; you can squeeze it and bend it, and it returns to its original shape instead of breaking, as a wax pattern would.”
Plastic also offers productivity pluses. “The plastic cools faster, and you can handIe it much faster than wax because it’s not brittle and it doesn’t tear as a wax does,” says Benedict.
Despite these differing characteristics, production procedures are nearly the same for both wax and plastic. “The only difference [berween wax injection and plastic injection] is that with wax injectors you can hold the mold manually or use a clamp,” says Benedict. “With the plastic injector, you have to use a clamp.” Plastic can also be invested and burnt out using normal casting procedures, with the exception of steam dewaxing, which isn’t effective on plastic. “Very frequently, we have wax and plastic injections on the same tree, and they’re burnt out side by side in the same flask,” says Gilman. However, an investment with a higher ratio of calcium sulfate is often needed to prevent such problems as spauling, even though this investment makes breakout more difficult.
The Minuses of Metal
While metal molds may offer many advantages, they do have their limirations-and companies must consider them before making any final decisions. Not every manufacturer sees big productivity gains, for example. In fact, some companies have found their production actually dropped with the introduction of metal molds.
“With rubber molds, you just fire away,” says seidel. “With metal molds, you have to assemble them. They can be in four, five, even six sections, so it takes a little more time [for production] .” seidel says that his company’s waxers normally produce 200 to 350 pieces a day with metal molds, compared to 400 to 500 with rubber molds.
Metal molds also take longer to produce, and as a result cost more. seidel estimates that while a rubber mold takes an hour and a half, a complicated metal mold can take a couple of days. “One of the things we’ve found is that metal mold makers come in two types,” he adds. “Some are fast but may not be as careful, so they turn out a usable metal mold that requires more repairs. Then there are other mold makers who take a little longer, but [the mold] comes out right the first time.”
Ayvazian agrees that metal molds take longer to create- “A regular [metal] mold takes about 15 hours for a specialist to do, much longer than a rubber mold,” he says. However, he believes the added time and expense are worth it, because the improved consistency results in lower gold loss: “After 25 to 50 pieces, the mold is paid for.”
Perhaps the most important consideration is that not every piece of jewelry can be reproduced in a metal mold. Undercuts, for example, pose special problems. While rubber or silicone will flex to release an undercut piece, a metal mold doesn’t yield, making such a piece difficult if not impossible to remove. This problem can be avoided by creating a mold with more sections, enabling it to come apart and release the pattern more easily. However, the greater the number of undercuts, the more sections that will be needed. “Unless you want to make a puzzle with a bunch of parts, you can’t have any undercuts,” says Wolfmueller.
So while in theory almost any piece of jewelry can be made if the mold has enough parts, metal molds are most useful for symmetrical pieces, such as pendants or charms.
“The simplest mold we have is a two-piece mold, with two halves that just split the product,” says Gilman. “If you have [a piece of jewelry with] a more complex shape, you might have the mold split in four directions or six directions or eight directions. Some of them are like Chinese block puzzles, or a Rubik’s cube.”
Seidel also advises manufacturers to remember that not every model successfully created in a metal mold will be successful in the next steps of production. “Don’t get overly ambitious in terms of what you think you can do with [metal molds],” he says. “A common weakness is thinking that you can make the pieces as thin as you want and you’ll have a very lightweight piece. But there are a lot of production problems that go along with that. You can make a ring very thin, but just try to produce the ring without denting it.”
Given the variables, then, should companies move at least some of their production into metal molds? That’s a decision that can be made only by individual manufacturers. “We’re continually trying to expand as many of our items into metal molds as possible, just because of the productivity advantage,” says Gilman. “But there are some things that, in my opinion, won’t be put into metal because they’re too complex and the mold would have so many parts it would lose its productivity.
“I would say to do your research,” he concludes. “Look at what your product is. If you’re making fashion mountings or something like that, it’s really not going to work for you. But if you’re into findings or fairly simple, straightforward pieces, it might be just the ticket.”
AJM, August 2000
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