Metal powder injection: taming a tricky process.

Metal Powder Injection Taming a Tricky Process

Advances in processing and materials are overcoming hurdles to rapid growth for this young technology.

Injection molding of mixtures of fine metal powders and plastic binders combines the strength and durability of metal with the design versatility of injection molding. It's finding a place in metal parts with intricate geometries that would cost up to 10 times more to produce by machining, die casting or investment casting. For example, metal injection is thought to have lots of potential to automobiles, medical instruments, and firearms.

The metal injection molding (MIM) market is still small. But Gorham Advanced Materials Institute, a market research firm in Gorham, Me., says MIM could easily grow from about $40 million in 1989 to $1 billion by the year 2000. In 1990, MIM sales reportedly grew 50%. Dr. Andrew Nyce, Gorham's president, says it's clear that MIM has moved beyond the embryonic stage and is on its way to major growth in the 1990s. Other market researchers tend to agree. After about 15 years of sluggish growth and development, MIM seems poised to take advantage of accumulating know-how in materials and processing that could open up new markets and overcome past hurdles to greatly increased sales during the next three to five years. More than 45 companies worldwide (over 30 of them in the U.S.) are engaged in commercial production of MIM parts, and several hundred other companies purchase production quantities of MIM parts.

According to John Popken, v.p. at Advanced Forming Technology, Inc., Longmont, Colo., the largest MIM processor in the U.S., "There are so many applications where MIM has potential that industry couldn't begin to meet the demand if they all came on-line at once."

MIM entails several steps. First, a thermoplastic binder (typically polyethylene, polypropylene, methylcellulose, or an acetate polymer, perhaps mixed with waxes and/or oils) is mixed with up to 80% by weight of extremely fine (10-20 micron) metal powder into a highly filled "feedstock" or molding compound. After that is injection molded, the so-called "green" parts go through a "debinding" stage to burn off the binder in an oven (or wash it out with water, in the case of acetate binder), without disturbing the part's shape. Finally, the new microporous parts are sintered in an oven to produce a solid, densified part.

As with most other relatively new technologies, MIM still has some problems, but progress is being made to solve them. The molders are a highly fragmented group and not well known to the companies that could benefit from MIM parts. The molding process itself is a very sensitive one, requiring tight control. There have been problems with the quality, availability, and some say, the prices of the fine metal powders used in MIM. Many MIM processors are also hoping to see improvements in debinding techniques. And it's hoped that the process can become more continuous--right now it's very batch-oriented. However, of all these difficulties, materials problems are the most fundamental.

PROBLEMS WITH MATERIALS

According to many involved in MIM, a big stumbling block to more widespread use of MIM is the high cost and lack of availability of the fine metal powders needed to make MIM parts. It's one reason why the initial focus of MIM has been on small parts, usually weighing less than 20 g. Cost and availability of powders has long been a bone of contention between MIM processors and powder suppliers.

There are three approaches to formulating the basic metal powder: elemental building, master alloy, and prealloy. The prealloy approach provides a powder with the same metallic composition as the final product, ready to be combined with binder, while the other two approaches require the molder to do his own mixing of different metal powders to produce the right metallurgical composition. Molders say there is ready availability only of pure iron and nickel powders for both the elemental building and master alloy approaches, but there are still shortages of fully formulated stainless- or tool-steel powders (or other complex alloys) for the prealloy approach, which most molders would greatly prefer.

Popken of Advanced Forming Technology disagrees with the common view that the powders are too expensive. "We're selling value added, not material by the pound. MIM offers advantages that other processes can't provide." But Popken says there is still a problem with metal-powder quality, including particle-size distribution, particle shape, and lot-to-lot variation. He says there are also problems with some suppliers' basic chemistry.

According to Dr. Kevin D. Christian, assistant technical director at Amax Metal Injection Molding, Escondido, Calif. (a partnership of metals producer Amax Inc. and RISI Industries, Inc.), metal-powder "prices have been coming down. But prices are also a function of how powders are manufactured [e.g., gas-atomized vs. water-atomized]. So, there's a limit to how low they can get. Even as volumes get higher as the market grows, there will always be a premium compared to metals used in conventional powder-metal techniques."

Christian hasn't had much of a problem yet in obtaining the materials Amax needs to meet demand. However, he notes, "There certainly isn't an oversupply of materials. And if our demand suddenly went up we might get into a bind."

Among those MIM processors who disagree with Christian over powder prices, some have been taking steps to do something about it. They're working to adapt the process to use coarser, less expensive powders, which could also help reduce debinding times, another sore point with some molders.

But a problem with larger particle size is that it can decrease the high sintered densities that are one of the main advantages of MIM. MIM parts can be sintered to 90-99% of maximum density, providing parts with fine spherical pores plus excellent toughness, elongation, and dynamic properties. Companies working with coarser powders believe customers may be willing to accept lower densities in return for decreased cost in applications similar to those that currently use conventional powder-metal (PM) parts.

Other MIM molders have turned to diluting powders. According to market researcher Leander Pease III of PowderTech Associates, Inc., North Andover, Mass., several MIM shops are diluting $2.70/lb gas-atomized carbonyl iron with up to 60%-325-mesh, water-atomized iron, costing about 40 [cents]/lb.

Another problem has been lot-to-lot variances in the quality of the powders. This can lead to defects or variations in parts as they leave the mold. If there are variations in metal/binder ratio or segregation of metal and plastic, there will be resulting dimensional variations as full density is approached through sintering. And while material variances lead to problems with part quality, they can also apparently cause other problems. Dr. Andrew P. Plochocki, research professor of Chemistry and Chemical Engineering at Stevens Institute of Technology in Hoboken, N.J., says that variances in quality can lead to tooling problems. "People have a set of molds suitable for a given compound. Then they realize they have to change that tool because of mixing problems [that cause compound inconsistency]," he says. On the positive side, Plochocki says the current trend away from batch mixing of metal powder with binder toward continuous, twin-screw compounding is "vastly improving the quality of feedstock and reducing the quantity of rejects in molding and especially in sintering."

At Advanced Forming, Popken feels there are still some problems with variances in materials quality, but progress has been made during the last couple of years. When it became apparent that some lots worked better than others, specifications were written around the good lots. Popken says these powders work well every time.

One reason for lot-to-lot variations is the ready availability of powders for the elemental building and master alloy approaches. When molders have to mix their own batches it complicates the process. Popken thinks there should be industry standards for powder grades. Professional societies within the metal industry, including the Metal Injection Molding Association in Princeton, N.J., are making progress in this direction, and it's hoped that standards specifically for MIM will speed widespread acceptance of the process.

WANTED: FAST, SIMPLE DEBINDING

Processors long for a faster, simpler, less expensive debinding process. Debinding has widely been considered a stumbling block to growth of MIM. The problem goes back to the conflicting demands on a plastic binder material: it must have good "green strength," yet be easily removed from the part in debinding. The earliest MIM processes used thermal debinding, and debinding times of three days or more were common. Despite relatively long cycle times, thermal debinding at 445-500 F is still widely used. Ovens can handle 1000-2000 small parts at a time. But other processes, such as solvent extraction, wicking, and two-stage debinding (a combination of solvent extraction and thermal debinding), are reducing debinding times and production costs. Chemical debinding processes often use modified solvent degreasing units.

As on the issue of price, Popken of Advanced Forming disagrees with the common perception: "It's not true that debinding takes too long and that it's too expensive. It isn't a labor-intensive process. And the unit you do the debinding in isn't expensive. The parts just sit in it. That doesn't require any labor. So the idea that debinding times need to be shortened is false if you're looking at it from a cost perspective."

Amax has patented a solvent debinding process that can be completed in a few hours for most parts, instead of a few days, as is still common to many MIM processors. According to Amax's Christian, the company's proprietary technique also leads to better part quality, eliminating surface formation of metal oxides that result from thermal debinding in air.

Amax's process (available for licensing) uses a special feedstock whose binder is readily removed with a particular solvent. The feedstock consists of around 60% metal and 40% binder. It's said to enable a 3/8-in.-thick part to be debound in 3 hr, removing about 60% of the binder. The remainder is removed in a vacuum furnace, where the vaporized binder is trapped before it reaches the vacuum pump.

Christian says the dimensional distortion that sometimes occurs during debinding and subsequent sintering can be blamed mainly on incomplete, inefficient, or problematic binder removal. If residual binder is left in a "green" part, evolution of organic gases will occur during sintering at higher temperature, resulting in cracks, voids, and dimensional variation.

R&D to improve debinding is being done at Rensselaer Polytechnic Institute in Troy, N.Y., whcih has a corporate-sponsored MIM research program, where Christian studied. "RPI is looking at several different systems and ways of debinding," he says.

A DELICATE MOLDING JOB

MIM molding is more complex than most plastics injection molding. It calls for presses with abrasion-resistant barrels, and precise injection and clamping units to maintain tight tolerances and prevent defects. Most presses being used for MIM have closed-loop control of injection speed, holding pressure, and backpressure. Robotics are common too. MIM parts in the "green" state when they exit the press are fragile, requiring very sensitive handling.

Current MIM technology limits applications to smaller injection molding machines. Advanced Forming uses presses from Battenfeld of America, West Warwick, R.I., which has sold machines for MIM from 11 to 85 tons. Popken says, "We're now running 11 Battenfield BA-CD injection machines with Unilog 4000 control systems, and we have more on order. Closed-loop controls are very important to the process. We've concluded that there are over 90 critical variables that we must control precisely for the process to be successful."

Notes Battenfeld product manager Heinz Rasinger, "Because of high shrinkage rates, fragility of green parts, and other variables, there are special equipment considerations in MIM. We had to work closely with Advanced Forming to overcome these obstacles." One problem is a 20-25% shrinkage rate, compared with 3-4% typical in thermoplastic molding. Such high shrinkage magnifies any part flaws during the debinding and sintering processes. To compensate, precise injection and shot-size control are all the more critical. Battenfeld's Unilog 4000 closed-loop system uses a precise servovalve to control injection parameters and maintain part-weight consistency to within 0.02 g.

Wear and tear on the barrels from the abrasive metal powders is also a consideration. According to Rasinger, "This can normally be compensated for by selecting a special screw/barrel alloy, such as CPM-10-V," a vanadium alloy from Crucible Specialty Metals, Syracuse, N.Y. (see PT, Feb. '88, p. 35).

Because there are so many critical process variables to control, Advanced Forming has a comprehensive quality-control lab to monitor part structure and dimensions. The lab includes a light microscope to reveal structural voids, and other instruments for gas analysis, surface measurement, and automatic evaluation of part shape. Any parts found to be out of tolerance go to a coining operation, where they are stamped into correct tolerance.

One molder advises that there are a few things to remember about MIM that differ from conventional plastics molding. This firm uses a standard Boy 50T machine with a modified screw tip. "You've got to watch the tips, as a few thousandths' wear can affect the part," warns this source. "Also, remember that mold gating is slightly different. The sprues and runners are larger. Mold temperature control requires more attention, too."

Mold filling is affected by the high heat-transfer rate of the highly metal-filled compounds, which speeds cooling. According to Dr. Plochocki of Stevens Institute, molders should be careful of packing. "Filling is easy from the standpoint of viscosity," he says. "But packing can be a problem. Suitable viscosity lasts only a few seconds."

For that reason, every one of the 50-plus molds at Advanced Forming uses a hot-runner system or hot sprue bushing. Tooling manager Fred Stone says, "It's extremely important that the bushing or hot-runner system has uniform temperature. If there are hot spots in the manifold or anywhere along the length of the bushing, the metal powders can actually weld to the hot surface of the runner wall. The metal then needs to be scraped off." Advanced Forming has had no problems of this sort, using heat-pipe hot-runner components from Kona Corp., Gloucester, Mass. A Kona spokesman notes that it has equipped 16-cavity hot-runner molds for another large MIM molder, Flowmet, sub. of Metal Powder Products Inc., Deland, Fla.

According to officials at SHI Plastics Machinery, Inc., Norcross, Ga., which offers Sumitomo presses configured for MIM, you need a machine with high injection pressure. They say that because cooling and solidification characteristics differ from standard injection molded plastics, molding the very thin parts common in MIM requires very high injection pressure. Sumitomo's SG series machines include an accumulator as standard equipment. For MIM, Sumitomo also provides specially designed, super-wear-resistant screws and Sycap closed-loop speed and pressure controls.

With regard to wear protection, Don Zeiger, president of Zeiger Industries, Canton, Ohio (formerly Mallard Machine Co.), reports that he has sold several Mallard nonreturn ring valves to metal-powder injection operations on the West Coast, mostly for use on Boy and Kawaguchi machines under 100 tons. The key to its wear resistance is a carbide facing on mating surfaces of the replaceable front seat and check ring. This reportedly unique design was originally developed for fast-cycle molding with high screw rpm, but it has also proved successful with abrasive glass-filled and metal-powder compounds, Zeiger says.

WHAT'S AHEAD FOR MIM?

MIM molders are confident of the market's future. "MIM is only in the adolescent stages," says Popken. "The automotive industry is very interested in utilizing MIM technology, and we are working closely with some manufacturers to make this possible. While we have had many success stories, there has been a lot of development work involved, and there is much more still to learn about improving the cost efficiency of the process."

Part of the entails developing a more continuous process with less labor. Says Popken "Currently, we use two batch processes: debinding and sintering. We'd like to combine the molding, debinding and sintering into one continuous process.

PHOTO : Most metal-powder injection molded parts are very small, like fiber-optic connectors (right), firearm hammers (left), and computer disk-drive components (middle)--all less than 1/2-in. long. Extremely consistent compounding and molding is critical. (Photo: Advanced Forming Technology)

PHOTO : These 3/16-in.-long stainless-steel MIM fiber-optic connectors (left) molded by Advanced Forming Technology are staged on a ceramic fixture to prevent distortion during sintering in a vacuum furnace (above). A furnace can hold over 20,000 parts per batch, processing over 100,000 parts a week. Processors hope that in the future it will be possible to combine molding, debinding and sintering into one continuous process.

PHOTO : Nissei Plastic Industrial Co. of Japan (Nissei America, Inc., Fullerton, Calif., in the U.S.) demonstrated metal injection molding at the JP 90 fair on this 44-ton PS40E5ASE model with new NC-9300 electronic controller. At the show, parts were reground and mixed with virgin for remolding.