As the medical profession increases its demands for small, intricate, precision metal components to be used in increasingly sophisticated procedures, the process of metal injection molding (MIM) has provided an alternative for the manufacture of these small, complex parts. MIM is sometimes referred to in its broader sense as “powder injection molding”–a term comprising several processes that offer the freedom of design associated with close- tolerance injection molding of plastics but form components in a wide range of metal and ceramic compositions. This article will provide an introduction to metal injection molding by briefly reviewing the basic process steps, the competitive position of MIM in relation to other metalworking technologies, the relative economics of the process, and material and part characteristics.
While MIM is certainly a new technology when compared with traditional metalworking processes, it has been a commercial reality for about 30 years. During that time, the technology has spawned an industry that is now international in scope. The Metal Injection Molding Association (MIMA) was formed in 1987, under the auspices of the Metal Powder Industries Federation (MPIF); current membership includes representatives from six countries. In 1993, a guide to specifying MIM materials, “Materials Standards for Metal Injection Molded Parts,” was published and is available from MPIF.
Any industry requiring small, highly configured metal components represents a potential market for metal injection molding. The medical and dental industries clearly fit this description, and are indeed among the primary market segments currently being served. Components for the medical market are commonly specified in applications such as endoscopic and laparoscopic devices. Performance characteristics of MIM components include wear and chemical resistance, structural strength, biocompatibility, and controlled expansion.
The Mim Process
Metal injection molding uses very fine metal powders, generally in the range of 2 to 20 µm. At the lower limit, powders are so fine as to approach the “dust” category. Alloy compositions are developed through blends of elemental powders, or supplied directly as prealloyed powders. In the case of prealloyed powders–which are generally used for stainless-steel compositions–each powder particle is in reality an ingot of wrought material.
The metal powders are thoroughly mixed with various waxes, thermoplastics, and other ingredients, and the blend is granulated to form a feedstock. The polymeric binder materials may comprise as much as 40% of the mixture by volume. The feedstock is then fed into a conventional injection molding machine, with molding temperatures generally ranging from 300° to 500°F (149°260°C). Once the feedstock has attained a toothpaste-like consistency, it can be injected into cavities to form precision components. As with plastic injection molding, multiple-cavity tooling can be used to reduce manufacturing costs.
After the parts are removed from the mold, they exhibit excellent strength, with a consistency similar to that of wax crayons. Most, but not all, of the binders and additives are then removed from the molded (green) components by a low-temperature thermal treatment or solvent extraction–or combination of both–in a process known as debinderizing. The exact method chosen depends on the binders used and the cross sections of the part. Binder ingredients are designed to separate sequentially, without disturbing part geometry; just enough binder is left so that the shape integrity of the part is maintained. Following debinderizing, parts are thermally processed (sintered) in either atmosphere or vacuum furnaces, during which the remaining binder is removed and the metal particles bonded into a coherent mass. Sintering temperatures are usually in excess of 2300°F (1260°C). The very fine powders used for MIM provide a strong thermodynamic driving force, and parts undergo shrinkage of as much as 20%. Repeatability is achieved through close control over the feedstock and molding variables. Final densities are generally from 95 to 98% of theoretical projections, with interconnected porosity of less than 1%. If necessary, parts can be processed to be pressure-tight and to satisfy hermeticity requirements.
MIM offers a significant variety of metal compositions. An abridged listing of metals being processed comprises a number of nickel-iron alloys (ranging from 2% nickel for structural applications to 3642% nickel for glass/metal sealing and 50% nickel for soft magnetic requirements); alloy steels 4340 and 4650; 3% silicon-iron alloys; and stainless steels, including 304L, 316L, 410, and 17-4PH. Medical applications in current production are generally produced from 316L or 17-4PH stainless.
In addition to these standard materials, there are several other material families that are undergoing developmental work to determine their appropriateness for MIM applications. The first group comprises cobalt alloys, and especially cobalt chrome, an implantable biomaterial that is of special interest to designers of orthopedic devices. A significant amount of experimental work has also been done toward the goal of producing titanium parts with MIM.
MIM offers the freedom of design associated with the injection molding of plastic parts, or with investment- or die-casting techniques. As with any manufacturing process, however, the limits of applicability need to be understood. First, MIM processing is best suited for relatively small parts. On a volume basis, parts typically should be smaller than approximately the size of a tennis ball; the process is generally most cost-effective when parts are smaller than a golf ball. Irregularly shaped parts can be produced with a major axis measuring up to about 100 mm (3.9 inches) in length.
The cross section or wall thickness of a part actually has a greater influence on determining feasibility for MIM production than does overall size, as the maximum thickness of a component must be kept relatively low to allow for effective removal of the binders. It is generally recommended that parts have cross sections of less than 6 mm (0.24 in.), although components with cross sections up to 10 mm (0.39 in.) are commonly in production. A practical lower limit is 1 mm (0.040 in.). Still thinner sections are possible, but depend on the flow length or height of the segment. Thin-walled tubular sections are not feasible with MIM.
The weight of a part will obviously be related to its size and section thickness–in other words, to the volume of material present. Components that benefit the most from MIM appear to be those weighing less than 100 g (with a lower limit of less than 1 g). Parts of up to 200 g, or even greater, can be produced, although technically feasible large parts may not be cost-effective. The majority of parts produced are probably within a range of 1 to 50 g.
MIM processing is normally carried out to a tolerance of ±0.3%, but specific dimensions may be held to accuracies of ±0.1%. In order to achieve tighter tolerances, it is desirable to keep cross sections as uniform as possible. The surface finish of sintered MIM parts is typically 32 µin. MIM parts also exhibit those surface features often associated with the injection molding process. Gate vestiges, parting lines, and knockout or ejection-pin marks will sometimes be present and should be taken into account when considering part function. However, many secondary machining operations can be eliminated because of the dimensional accuracy, high density, and surface finish of MIM parts.
Experience in the marketplace has shown MIM to be competitive with investment casting and discrete machining within the component size and weight range described previously. Whereas the price of an MIM part is often equivalent to that of an investment casting, the process’s similarities to injection molding result in parts with better surface finish, closer tolerances, greater freedom of formed holes and–in some instances–sections with thinner walls. Conventional multicomponent assemblies can often be formed as one piece with MIM, reducing material, labor, and inventory costs.
The process compares favorably with conventional powder metallurgy (P/M) when machining of a P/M part is required to generate additional geometry. Within the perspective of a wider range of metalworking techniques, MIM normally will not compete in cost with drop-off screw machining, stamping, or die casting (see Figure 1).
An economic benefit of MIM is that the process often employs multi -cavity tooling, which makes it suitable for meeting medium- to high-volume fabrication requirements (see Figure 2). Most MIM projects are probably within the 20,000 to 200,000 pieces/year range, though there are numerous applications demanding annual production runs in excess of one million parts. Lower-run quantities, which are typically specified during the development phase of a medical device, are also common.
Metal injection molding offers an effective alternative for manufacturing the kind of small, intricate, precision metal components typically required by the medical device industry. The process is well suited to the high-volume production of geometrically complex parts, and offers a freedom of design equivalent to that of injection molded plastic parts or investment castings. Parts can be produced from a wide selection of alloy compositions, including medical-grade stainless steels. Close-tolerance production often permits the formation of net-shaped parts directly by the MIM process.
With any manufacturing technology, the greatest benefit is derived when parts are designed from their inception to be produced by that particular process. Such is certainly the case with metal injection molding. Medical product engineers should “design for the process” whenever possible, since converting from existing designs may not permit full exploitation of the advantages of MIM.
For more information on metal injection molding, or any other powder metallurgy process, or to find a powder metallurgy parts fabricator, visit www.finemim.com .