David S. Hotter
Some may remember the original style of orthodontic braces made from welded bands that slid over individual teeth. The braces, prolific up to the early 1970s, gave real meaning to the term “metal mouth.” Powder-injection molding (PIM), along with new adhesives, brought dentists smaller brackets that were also more soothing to the eye. Single-piece designs, backed with a waffle pattern to promote adhesion, replaced the two-piece designs with welded mesh backs.
Now, more than 100,000 PIM brackets are produced each day, with production costs reaching $15 million/ yr. This is just one example of small, complex components used in the medical industry.
Recent developments are focusing on instruments used to perform endoscopic and laproscopic procedures. As a result, engineers must find ways to maintain precision while boosting strength, wear and chemical resistance, and biocompatibility. PIM gives them the ability to form intricate parts with tight tolerances in materials that don’t traditionally have the processing ease associated with injection-molded thermoplastics. This technology is reemphasizing metal and ceramic components that are all too often overshadowed in a materials world burgeoning with plastics.
PIM is not a new technology, though it didn’t receive a lot of commercial attention until the 1980s. Recent developments include advanced feedstocks with controlled shrinkage and stateof- the-art processing equipment that precisely control molding, debinding, and sintering.
PIM was first developed in the 1920s and used to mold ceramic spark-plug bodies. In the 1950s it saw limited use in shaping carbide and ceramic components using epoxy, wax, and cellulose binders. Attention to PIM grew in 1979 as two aerospace applications received design jet and a thrust chamber and injector for a liquid-propellant rocket engine. Today, the PIM market has yearly worldwide sales reaching $400 million and continues to grow more than 20%/yr.
PIM begins with fine powder materials consisting of nearly spherical particles ranging in diameter from 0.1 to 20 μm. To prepare them for injection-molding machines, engineers blend powders into a feedstock by compounding them with traditional binders consisting of waxes and thermoplastic resins. Recent developments give engineers more options including binder systems based on polyacetals and polysaccharide.
The binder’s role is to lubricate the powder mixture so it flows easily through molding cavities. As a result, binders also fill the voids between metal or ceramic particles and add integrity to molded shapes so they can be handled during processing. Polymeric binders comprise up to 40% of the mixture by volume. The final step in compounding solidifies the mixture and chops it into easy-to-handle pellets, similar to that of plastic feedstocks.
After compounding their own resin or buying material from a supplier, manufacturers form components at relatively low temperature and pressure in standard plastic injection-molding equipment. Molding temperatures range from 150 to 260°C, which softens binders to a pastelike consistency that will flow through mold cavities.
Once cooled, parts go through a debinding process using heat, chemicals, or even water to remove most of the binder. In thermal debinding, microproccessor- controlled, low-temperature ovens, analagous to convection ovens, sweep air over components and collect condensates. A small amount of binder is left behind so that workers can move parts to sintering furnaces without destroying them. Chemical methods leach binders through a catalytic process.
Sintering, the next step, binds particles together at an atomic level by diffusing atoms at temperatures approaching 85% of melting point. Sintering removes the voids left by binders which boosts density and shrinks parts. Shrinkage is uniform and isotropic, however, so engineers can compensate for it by designing molds approximately 20% larger than final dimensions. Sintering metal parts in a controlled atmosphere or vacuum keeps them from oxidizing. Another method used is single-step debinding/ sintering, where debinding and sintering take place in the same furnace. This further protects surfaces from oxidation since parts aren’t handled between steps, leading to improved surface finish, manufacturers report.
Sintered components are typically 96% of theoretical density, with properties approaching that of wrought material. Density is further increased using hot and cold-deformation methods such as hot-isostatic pressing.
While injection molding is generally associated with high production volumes, PIM is economical for high and low-volume rates, from as many as 100,000 parts/day to as few as 5,000 parts/yr. From a production perspective, PIM often eliminates operations such as grinding, machining, drilling, and boring. Manufactures also save money by recycling feedstock from runners, sprues, and damaged parts, using nearly 100% of raw materials.
While PIM works for nearly any shape that can be formed by plasticinjection molding, it does have drawbacks and limitations. For example, the process isn’t competitive with traditional screw machining or die compacting for simple, axial-symmetric shapes; the cost of materials, tooling, and processing equipment limits the size of components; and manufacturers set limits on thickness from 10 to 50 mm because it lengthens the debinding step.
PIM works best for complex components where the process is competitive with other forming processes such as investment casting and machining. “Highly complex parts are difficult and expensive to manufacture using investment casting and machining methods, particularly for high volumes,” explains Paul Hauck, director of design engineering at Kinetics Inc., Wilsonville, Oreg. “Manufacturers can save anywhere from 20 to 70% by switching to PIM by eliminating machining, whether it be the primary process or used to add fine details to investment castings. Avoiding machining also eliminates the possibility of imparting stress on parts.”
PIM is used for both metals and ceramics. Current medical applications for PIM include surgical tools, fastener hardware for infant incubators, and instrument handles, finger and thumb loops, and blades.
Stainless steels such as 316L and 17-4 PH are the most widely used materials for these applications. “We use 17-4 when components need superior hardness or wear resistance, such as on the cutting edges of instruments, while 316L protects components subjected to more corrosive conditions,” says Mark Rasmussen, president of Powder Metal Molding, Menomonie, Wis., a division of Phillips Plastics Co.
Other PIM materials include low-alloy steels such as Fe-2% Ni and Fe-8% Ni. Low-alloy steels are used for wear resistance and high strength in sealed, oiled environments such as drug-delivery systems, where corrosion isn’t a problem and there is no direct contact with skin tissue. Titanium and cobalt-chrome alloys, known for their corrosion resistance and biocompatibility, are also under research for use in implantable orthopedic devices.
Ceramics have few applications in the medical industry, besides dental implants, because of their brittleness. Potential applications include zirconia scalpels and surgical tools. With superior wear qualities, ceramics would be ideal for applications such as instrument cutting edges. One barrier to growth in this area, however, is whether the demand for reuse is high enough to justify using such advance technology.
With health-care costs under constant scrutiny, medical-device manufacturers must slash costs while remaining profitable. The real driver for PIM is getting the net shape for less money, while meeting performance requirements such as strength, ductility, and corrosion resistance.
Randall German, a professor specializing in powder metallurgy at Penn State University, explains, “For PIM to be the best process, manufacturers must see a significant cost reduction.” The trend towards selling products through buying pools or distributors has provided a means of accomplishing this goal. “One medical-component manufacturer told us that it could discount products up to 40% because it no longer deals directly with surgeons and hospitals. These discounts force manufacturers to cut costs, since they can’t count on selling products at trade prices anymore,” says German.
Surface finish is still a barrier to wider use of PIM, as well as other powder metallurgy methods. “The medical field is very finicky about quality, especially when it comes to aesthetics and surface finish,” says German. “There is more concern with surface appearance than other markets such as the automotive industry. Automotive engineers care about tight tolerances, not whether a gear turns black or blue during sintering.”
There are some exceptions, however, when device manufacturers take advantage of duller surface finishes, such as the ends of laproscopic and orthoscopic instruments. “Very-fine surface finishes on instrument tips reflect fiber-optic light back into surgeons’ eyes, making it difficult to perform procedures,” explains Robert Merhar, a consultant for the PIM industry. “However, brighter finishes are still required on areas such as thumb loops and handles where it gives the impression of a clean, sterile surface.”
Some manufacturers rely on different material grades to achieve smoother surface finish. “We offer two different grades of 316L,” says Powder Metal Molding’s Rasmussen. “One uses a finer powder to produce a smoother surface finish — in the range of 20 to 35 μin. as sintered; the other grade has a 40 to 50-μin. surface finish. We can also polish parts for a finer finish, but most customers are satisfied with the matte finish given in the 20 to 35-μin. range.”
To achieve a balance between part complexity and costs, engineers should initially design PIM components with a simple shape and then adjust dimensions according to processing constraints and part performance. The same design features apply to PIM as with conventional plastic injection molding or die casting. Some guidelines for dimensional constraints include:
• Draft angle: A draft angle between 0.5 and 2.0° helps components release and eject from molds. The amount of draft required increases with part complexity or multiple cores and should be uniform on all design features.
• Wall thickness: Uniform wall thickness helps prevent distortion, internal stresses, voids, cracking, and sink marks. Thickness variations cause nonuniform shrinkage which makes it difficult to hold tight dimensions. Ideal wall thickness ranges from 1.3 to 6.5 mm.
• Holes: Coring holes cuts raw material usage, boosts wall-thickness uniformity, and reduces or eliminates machining steps. It is better to position holes perpendicular to the parting line. Through holes help support cores at two ends. It is possible to connect holes internally, but they should be perpendicular to each other with a D-shape on one hole to help seal the core pins.
• Ribs and webs: Ribs and webs reinforce thin walls to avoid using thick sections. They also improve material flow and limit distortion. Rib thickness should fall between 0.50 to 0.70 of the thickness of adjoining walls.
• Fillets and radii: Fillets and radii reduce stresses at edges and also eliminate sharp corners that cause cracking and mold erosion. A common range for fillet and radii dimensions is 0.4 to 0.8 mm.
• Bosses and studs: Bosses and studs form attachment points. Stud length should be no more than five times the thickness of the adjacent wall. A boss with a blind hole or through hole requires a diameter twice that of the hole and its wall thickness shouldn’t exceed that of the adjoining surface. Therefore, hole diameters can’t be more than twice the thickness of adjoining walls.
• Threads: Both internal and external threads are manufacturable with PIM, but tapped internal threads are more precise and eliminate expensive core rods. External threads should be located on mold parting lines to avoid having to unscrew mold sections from each other. Adding flats to threads on the parting surface permits up to 0.05 mm of flash, improves thread quality, and reduces mold maintenance.
• Undercuts: External undercuts should be formed on a parting line for the same reasons threads are. But undercuts increase costs when used to mold critical surfaces, such as O-ring grooves, which require machining to remove the parting line. Internal undercuts are formed using collapsible and soluble cores, but are also costly. With soluble cores, manufacturers insert mold a polymer part onto cores. The core then creates a footprint in the molded component.
Developments in PIM feedstocks
BASF Corp., Wyandotte, Mich., has developed a feedstock, called Catamold, with a polyacetal binder that uses catalytic debinding which is 10 to 20 times faster than conventional thermal methods. In the process, the binder quickly escapes from molded parts without leaving a residue, while parts retain modest structural properties. The polar structure of polyacetal attaches to metal and ceramic powders to promote uniform distribution. Catalytic debinding cleanly decomposes polyacetal into its monomer units through an endothermic reaction which unzips the polymer backbone.
BASF uses its own blend of polyacetal resin because commercial grades don’t meet PIM processing requirements. Modifications to the polymer structure include additives that reduce viscosity, high-temperature stabilizers, and powder dispersants.
One of the significant differences between conventional and polyacetal-based feedstocks is the way they debind. Conventional binders consist of a polyolefin and wax. Waxes and oils slightly speed debinding, yet the process still takes more than 24 hr for a 6-mm-thick component. Green, or nonsintered parts, soften and deform or slump during the debinding/solvent-extraction step because waxes and oils liquefy.
During catalytic debinding with Catamold, an acid vapor creates a reaction which removes the resin at temperatures below its melting point. The low debinding temperature avoids a liquid phase and prevents distortion.
One advantage of BASF feedstocks is that different material grades within a material family, such as the stainless steels, have the same shrinkage factor. This gives engineers the ability to make tooling without deciding which material will work best for an application, and they can use the same tooling to mold and test prototype parts made from different powders.
Powder Metal Molding’s Mark Rasmussen explains, “Steels in the stainless category, such as 17-4, have the same shrinkage as the low-alloy steels. This lets us zero in on the best material by testing prototypes as long as a customer has a target material going into development.”
Engineers discover another advantage to using polyacetalbased feedstocks during full-run production. “When a manufacturer using an Fe-Ni 2% needs more corrosion resistance for a certain segment of its market, it can simply change over to a stainless steel with the same shrinkage rate,” says Dave Krueger, PIM product application specialist, BASF, Wyandotte, Mich. Medical-instrument makers rely on this flexibility to customize components for different environments. Catamold feedstocks come in a wide range of metal alloys, as well as ceramics such as alumina and zirconia.
AlliedSignal Inc., Morristown, N.J., has developed waterbased ceramic and metal-powder feedstocks using polysaccharide binder. The water-based binders reportedly mold like thermoplastics and produce complex components with both thick and thin sections. Molding temperatures are as low as 80°C and drying times are as short as 1.5 hr with no binder burnout required. Currently the company offers the technology in ceramic powders, including alumina and yttriastabilized zirconia. Stainless steels will be available mid next year.