Metal injection molding (MIM) can be a cost-effective alternative to EDM'ing, casting, or machining for relatively small medical parts. The process combines powder metallurgy technology with the complex shape-making capabilities of plastic injection molding. "MIM part sizes are usually described by weight," explains Director of Operations Mike Stucky of NetShape Technologies Inc (netshapetech.com) in Solon, OH. "The typical weight range is about 5 to 25 grams, which translates to about 1/4-in. long × 0.020 in. diameter up to about 4 × 3.5 × 3 in. But parts can go down to tenths of grams and up to around 100 grams."

MIM materials

Among the advantages of MIM is that it handles a wide variety of materials in low, or high-volume runs. "Medical parts such as debridement tips and dental bracket hooks are often made from 17-4 stainless steel," says Stucky. "But with the right equipment, shops can MIM tool steels, tungsten heavy alloys, magnetic materials, and even titanium."

MIM also lets designer pick material properties, such as strength, hardness and corrosion-resistance. "In addition, the process eliminates worrying about issues such as machinability and weldability," says Stucky. "MIM can build complex, high-tolerance parts containing small, precise features. Designers often look at die casting for complex shapes and high-production rates. However, die casting is typically restricted to low-temperature alloys such as magnesium and aluminum alloys. MIM provides better material properties without the expense of machining or casting and then machining."

According to Stucky, MIM tooling is similar to that used in plastic injection molding. "In our case, the shops that make our tools are primarily plastic-injection mold makers," he says. "There are minor differences in vent depths and gating schemes, but otherwise, the basic construction is identical."

In addition to metal powder, an important component of MIM is the polymer binder system, which mixes with the powder to create a feedstock. The type of binder system used dictates whether molds incorporate hot or cold runners. NetShape Technologies uses hot runners systems, whereas other shops might use cold runners.

"The kind of equipment a shop already has often dictates what kind of binder it selects, so the choice is mostly a matter of convenience, "says Stucky. "The binder a shop chooses has a lot to do with when the company came into the MIM game, whether the shop is going to compound its own feedstock, and where the shop learned the technology — did it license the technology or pick up the information from an academic paper? However, binders don't really play a role in part design."

Again, designers need to be concerned with selecting the correct alloy with the needed strength, hardness, and corrosion resistance. However, MIM shops can blend metal powders to create materials with very specific properties. "For example, should a designer want a 17-4-like stainless-steel alloy with more corrosion-resistance than the base alloy, it is possible to adjust the individual alloy element ratios," says Stucky. "Plus, there are a lot of commercially available elemental powders that let shops, say, buy iron powder and then blend in amounts of nickel, chromium, and molybdenum. There are even alloys available intended to be tailored. When working with magnetic alloys, designers are often looking for a specific magnetic response. Shops can adjust the alloy elements to get that response."

17-4 stainless steel is a very good alloy for medical parts because it has good corrosion-resistance and it is very strong and can be heat treated, says Stucky. "316 stainless steel has even better corrosion resistance but not quite the strength of 17-4. However, for a lot of applications it's an excellent choice."

"We don't handle implantable alloys, but there are a lot of companies that do," says Stucky. "Those alloys are specified by medical standards to ensure they are biocompatible. Examples are titanium and cobalt-chrome alloys for hip implants. But usually, alloys are not tailored for a specific application. Instead, they are designed to meet an acceptable standard of either the medical industry or some other governing bodies."

Relative to highly machined parts — those undergoing either multiple CNC operations or EDM'ing — MIM is considered high volume. "For medical, the range is around hundreds of thousands a year," explains Stucky. "However, MIM can handle a million parts a year. What really defines the quantities is the application and the willingness of the customer to pay the price that's needed to make the part viable for the producer. For some parts, we produce a thousand a year, while for others we might produce a hundred thousand a month."

As far as casting, there are a lot of cast 17-4 parts that could do well with MIM for the right size part, says Stucky. "MIM supplies better material properties and equivalent or better shape capabilities, and it does finer detail than casting. We do see a lot of parts that were cast and machined, and we can MIM the part and eliminate the machining operation."

When it comes to tolerances, most MIM companies quote ± ½%. "For example, when the part's longest dimension is 1-in. long, we quote ± ½%, or 0.005 in. As the dimensions get smaller, MIM holds even tighter dimensions. Large dimensions get worse because of the effects of gravity during sintering, distortions due to nonhomogenous molding or feedstock properties. ± ½% is a common number to quote to."

MIM tolerances and flow simulation

Quite often, NetShape is asked to quote a part where it can't hold a certain tolerance. "But we can offer a near-net blank to be machined, sometimes a much lower cost option than machining the whole part from solid stock," says Stucky. "In general, though, part designers must worry about the same things they do when designing for plastic injection molding, such as draft angles and thin walls."

NetShape uses a basic mold-flow simulation module in its CAD software to verify gate placement and gate design. "I've been in industry a long time and have yet to see a software analysis program that gives accurate and repeatable results for MIM," says Stucky. "Simulating MIM materials is difficult because the high solid loading of the metal generates very different thermal properties and flow characteristic than a plastic would," he says. "Most mold flow software was designed for plastics because it's such a huge industry. The programs don't do a very good job of modeling MIM results unless you go through the effort of having your feedstock characterized very extensively — a very expensive proposition to get all the thermal and rheological properties to build an accurate model. However, simulating MIM mold flow is becoming more commonplace and there is a continuous efforts to improve the modeling software appropriate for MIM."

MIM, step-by-step

"The first step is feedstock production. "In our case, we buy all the metal powders individually — either free alloy, mass alloy, or elemental, and we buy individual binder components," says Stucky. "We then batch-up the feedstock batches. We start with a basic binder formulation — most of our feedstock is made up of the same basic binder constituents. Then we add the metal powders. A twin-screw extruder compounds the material by heating up the plastic under high-sheer conditions. This incorporates the metal powder into the plastic and produces a nice homogenous blend."

MIM parts are designed for a certain shrink rate that will produce the correct geometry and dimensions. Thus, molds are designed with a certain oversize factor. The blend used for a particular part is determined by the tool and the alloy the designer specified. A metallurgist develops each mix to ensure correct shrinkage rates.

Once the feedstock is compounded, NetShape tests it. "We do what's called a green density check to check the density of the feedstock," says Stucky. "That determines whether there is the right ratio of metal powder to plastic. We use a pycnometer to test the granulated feedstock powder. The device uses the gas laws of pressure/volume to find to find the total volume of the part. There's an empty chamber. You put the feedstock in, pressurize the chamber a certain amount, and the volume of gas used shows the part density. It's a very accurate check and commonly used in industry. That way we are pretty sure that when the feedstock goes in the molding machine, the parts will come out the proper size."

The company also checks the alloy chemistry of a test piece with a small, hand-held- X-ray analyzer, a device used a lot in scrap yards to identify which metal is steel, which is aluminum, and which is stainless steel.

Once the feedstock is approved, it goes to the floor to get molded. The feedstock goes into the molding-machine hopper and gets metered into the screw and barrel, just like with plastic injection molding. "We have larger and smaller machines — 88 ton and 100 ton, but all of our tools can run in an 88-ton machine," says Stucky. The company usually uses molds with four to eight cavities, as well as single cavity and dual cavity tool. "This is mostly determined by the number of parts the customer needs and what they are willing to pay for the tool," he says.

How extensively a company is automated depends on how many parts it is making. "We have automation that removes parts from the mold and sets them on the conveyor," says Stucky. "There are way more expensive automated methods using sophisticated robots, but that approach is for companies that run millions of parts a year for many years." NetShape can run all the parts it needs to on the equipment it has. "In our case, the machines heat the stock to about 350º F. Again, the binder system determines the temperature used," he says.

A robot arm travels over the top of the machine, picks the molded part out of the tool, and drops the part on the conveyor, says Stucky. "The runner-system pieces fall down a chute on the other side of the machine. We regrind and reuse the pieces. From a material standpoint, that's another advantage of MIM. The re-use of expensive material gives high yields. It's not like cutting a part and then tossing-out the scraps."

Part of the efficient use of materials comes from hot-runner systems, which help keep the material molten as it travels into the mold. "All our machines are hot-runner, single-drop machines," says Stucky. "A hot runner on one side extends to a central drop. Sub-runners take it from the central drop and into the cavities. Hot runners are good for expensive metals, such as tungsten. We pay about $50 a kilo for tungsten. Compare that to 17-4 stainless steel at $20 a kilo."

When some parts come out of the machine, they still have the gate on them. Operators remove the gates by hand and set the parts on a tray. Other parts might come out with tiny vestiges. These parts drop into a filter basket, which the operator shakes, thereby removing the vestiges. "Again, this is very similar to what you see in plastic jobs, maybe not quite as automated as some of the high-volume plastic shops," says Stucky.

NetShape runs 24/7. "Technicians just shut the machines off for cleaning, maintenance, and mold changes," says Stucky. "A lot of machines nowadays have pre-heat software that will run the hydraulic fluid through a small orifice, heating the machine up to running temperature in 15 to 30 min. Older machines used to take hours. Newer machines run much more quickly and are much more accurate."

After the parts are molded, they go through the initial stage of debinding. NetShape uses a water bath heated to 60º C, while some companies debind parts thermally or catalytically, depending on the binder. "The binder system we use has a minor and a major component, necessitating a two-phase separation process," says Stucky. "The water removes the minor component to allow some part porosity. This leaves space for the major binder to move into when the part is sintered. Without this space, parts would distort and blister."

After the parts come out of the water bath, operators place them on trays and into a dryer. From there, the parts go into what's called a "boat" with a special lid that keeps the parts within the correct work envelope. NetShape has continuous furnaces, which means that each boat pushes the one in front of it. The furnace gradually heats up parts to remove the rest of the binder and then heats them up to the sintering temperature where the parts become solid. The last stage cools the parts down and they exit the furnace.

Sintering puts enough energy into the parts that the atoms in the metal move and migrate and diffuse, but not enough to actually melt. It's a solid-state defusion of the individual atoms. "Because we have a continuous furnace, it would be have been highly inefficient to change the temperatures for each alloy, so we designed temperature profiles, each of which can run multiple alloys," says Stucky. "There are all kinds of furnaces. In addition to continuous, there are batch, hydrogen, and vacuum furnaces. Our furnace handles a fairly wide range of alloys. To be more competitive, we can run several alloys using a single temperature profile."

After the boats come off the conveyor, parts are boxed up, matched with their lot ticket, and move on to the next operation, which could be anything from visual inspection to machining, plating, or heat treating. "We do visual and dimensional checks on the parts and if they are going to get heat treated, we do some sort of surface treatment such as tumbling or bead glass," says Stucky. "Heat treating manipulates the properties such as of a 17-4 from condition A — the base condition of 17-4 — to increase the hardness and strength of the material. It is possible, for instance, to take a 17-4 to a high low 40s RC Hardness with an H 900 heat treat process and still maintain its corrosion resistance."

Lastly, operators visually inspect parts and use functional gages to check max and min tolerances. "For example, the distal tip has a special gage that shows the channel width and the height," says Stucky. "We design the fixturing and gaging in-house and have them built outside. Most of our secondary operations such as plating, machining, and heat treating are shipped outside."

MIM developments over the last five years are largely focused on materials, says Stucky. "There is a lot of work being done in implantable materials. And a group is working on a new ASTM standard for implantable materials. Micro MIM is getting bigger, but it is still kind of on the fringes of medical. There is also a lot of pure R&D being done, which eventually will filter down to industry."