Originally Published MDDI November 2002

MOLDING

As the number of biocompatible metals available for injection molding increases, the process is becoming a practical option for manufacturers of implants and surgical instruments.

John L. Johnson

As labor costs for medical manufacturing continue to rise, reductions in component manufacturing costs become ever more important for controlling overall costs. Many medical devices, especially instruments and implants, are produced from difficult-to-machine materials such as stainless steels, cobalt-chromium alloys, and titanium alloys. A relatively new process to reduce the costs of fabricating components from these materials is metal injection molding. This is a net-shaping process in which metal powder is mixed with a thermoplastic binder and molded into a cavity. The molded part is then thermally processed, or sintered, to remove the binder and produce a high-density metallic component.

Injection molding can be used to form complex parts as easily as simple ones. The process is generally best suited to parts measuring less than 6 mm thick and weighing less than 100 g. Newer binder removal techniques, however, have enabled the processing of cross sections above 12.5 mm and up to 400 g. For all size ranges the process can usually achieve tolerances within 0.3 to 0.5%. Higher tolerances are best met by machining critical dimensions after sintering.

Metal injection molding is used increasingly within the medical device industry to produce a variety of components. The technology has matured to the point where quality and delivery can be assured through the ISO 9002 and QS-9000 certification of metal injection molding suppliers. These companies can make components from many of the alloys used for medical devices with properties comparable to those of wrought and cast materials. Surgical instruments and implants are two types of medical devices for which this process is particularly well suited. As will be discussed below, there are now a large number of materials with a variety of desirable properties for these applications that can be used in metal injection molding.

MEDICAL INSTRUMENTS

Surgical instruments, are generally produced from stainless steels because of these materials' strength, hardness, corrosion resistance, and ease of sterilization. Many grades of stainless steel are available, depending on the exact properties required. For instance, a martensitic stainless steel, such as 420, may be preferred for applications that require increased wear resistance to maintain a sharp cutting edge.

Examples of the types of stainless steels used for various cutting and noncutting instruments are given in Table 1.¹ Many of these components are traditionally produced in high volume by machining them from wrought material. Indeed, sulfur is often added to stainless steel to improve its machinability for high-volume production. Such high-volume, machined stainless-steel components are excellent candidates for metal injection molding.

The use of metal injection molding for stainless-steel medical instruments has been steadily increasing. The initial use and broad success of metal-injection-molded orthodontic brackets in the 1980s demonstrated the biocompatibility and corrosion resistance of injection-molded stainless steels and led to early instrument applications, including scalp-el handles, bipolar forceps, and jaws and clevises for biopsy forceps (see Figures 1, 2, and 3).

New applications for metal-injection-molded components are trending toward smaller, more-complex devices for minimally invasive surgery, especially laparoscopic instruments for grasping tissue, cutting, and suturing.² Such devices are being designed for greater freedom of movement, which has increased the numbers of metal parts used in the assembly. Metal injection molding has provided the design freedom to be able to produce such parts cost-effectively. A new area of exploration for the process is the production of microsized parts, which should help meet future medical needs as parts continue to shrink for minimally invasive surgery.

Mechanical Properties. Various grades of stainless steel are commonly available for metal injection molding, with lower costs for the more common grades. Generally, austenitic alloys such as 304 and 316 are only used in their low-carbon forms, i.e., 304L and 316L. The reason for this is that the injection molding process works best with minimal carbon, which also gives reduced susceptibility to sensitization and improved corrosion properties. Still, for martensitic alloys that require carbon for high hardness, such as 420 and 440C, carbon levels can be precisely controlled.

Mechanical properties of commonly available stainless steels for injection molding are well established and are very competitive with wrought materials, as shown in Table II ^3–5. These properties are sufficient to meet the requirements for medical instruments as given in ASTM F899-95. Mechanical properties can be further modified through additional heat treatments or atmosphere changes during thermal processing.

Owing to the similarities of some of the compositions, the steels for injection molding listed in Table II cover many more of the applications listed in Table I than may appear at first glance. For example, 304L should be suitable for all applications that normally use 302 and 303. The only difference between 304 and 302 is that 304 has a lower maximum carbon level, which is beneficial to corrosion resistance. Type 303 is a free-machining grade of 304 that contains sulfur, which is not needed for metal injection molding.

Likewise, 410, 416, and 420 are all very similar. Type 416 contains sulfur for machinability. Carbon content distinguishes the other grades of 410 and 420. XM-16 is a precipitation-hardening stainless steel that is only used for applications for which a more common alloy for metal injection molding is used. However, if specifically required, alloys such as XM-16 can be processed by metal injection molding. In addition, improved or even unique compositions are possible. For example, in addition to low-carbon, low-sulfur forms of stainless steel, nickel-free and vanadium-free versions can be produced to reduce the likelihood of allergic reactions. This flexibility ensures that injection molding alloys will continue to be able to meet the requirements of new applications.

Corrosion Properties. Metal-injection-molded parts made of stainless steel have been subjected to numerous tests of general, pitting, and intergranular corrosion. Many standards exist for testing corrosion properties in various media, but even within these standards some variables are left up to the tester. Thus, comparing corrosion properties among reported values is difficult unless test conditions are identical. Still, several conclusions from previous studies can be summarized.^6–10

Corrosion resistance is largely a function of composition. It can be affected by trace elements, so even within the compositional specification for a given stainless steel, corrosion resistance can vary. Injection-molded stainless steels generally perform as well as wrought materials in general corrosion tests. Pitting corrosion, however, is related to surface roughness; as-sintered injection-molded stainless steels often show reduced pitting corrosion resistance in comparison with wrought materials, if not sintered to a closed surface porosity (a density above 7.6 g/cm³). The surface finish for as-sintered stainless steel is typically 0.8 µm Ra. This can be improved by mechanical polishing or electropolishing. Polishing removes surface defects that serve as pit sites. Pitting corrosion resistance can be further improved by passivating the surface. Passivation involves subjecting the steel to nitric acid for 30 minutes to produce a protective film.

Medical instruments must meet ASTM F1089-87 corrosion requirements. One part of this test involves boiling the instruments in water and letting them air dry to see if they rust. The second part involves submerging the instruments in a copper sulfate solution, rinsing them, and inspecting them for copper plating. Tests on injection-molded stainless steels confirm their ability to meet ASTM F1089-87 criteria without additional polishing or passivation treatments.¹¹

In addition to providing corrosion properties comparable to those of wrought materials, metal-injection-molded components can significantly reduce corrosion in cases in which wrought parts are brazed or welded together. Often, metal injection molding can produce the desired component as a single part, thus eliminating the need for brazing or welding and the consequent reduction in corrosion resistance at the weld.

IMPLANTS

Metal injection molding is also suitable for the production of components for medical implants; however, these components are subject to much more stringent standards than medical instruments due to the more severe in vivo environment. Type 316L stainless steel has been widely used for many implants, but is now generally restricted to temporary implants, owing to problems with pitting and fretting corrosion.

Materials that are more biocompatible, such as cobalt chromium or titanium alloys, are used for permanent implants. Tantalum also has both excellent corrosion properties and biocompatibility, but its mechanical properties have limited its use. These implant materials all have individual ASTM specifications for chemical, mechanical, and metallurgical requirements.

A list of materials used for medical implants is given in Table III, along with example applications ¹². These alloys are less widely available for metal injection molding than are stainless steels. The Co-28Cr-6Mo alloy has been successfully injection molded, but its use to date has been limited. Titanium for metal injection molding is commercially available but is generally used for moderate- to low-stress applications such as surgical tools, golf club putters, and watch cases and bands. There are no published reports of tantalum being injection molded, even though tantalum powders are widely used to make components in the electronics industry. Tantalum could be injection molded for medical applications using those powders. Biomaterials for metal injection molding continue to be the subject of active research, and much progress has been made in meeting many of the standards for cast and wrought implants.

Mechanical Properties. Some examples of recently reported results for the mechanical properties of biomaterials for metal injection molding are compared with the ASTM specifications for cast and wrought materials in Table IV ^{1, 3, 5, 13–16}. These data indicate that 316L stainless steel, Co-28Cr-6Mo, unalloyed titanium, and Ti-6Al-4V can all be processed by metal injection molding with mechanical properties comparable to their cast and wrought counterparts.

Carbon and oxygen control are critical to achieving sufficient ductility for metal injection molding Ti-6Al-4V. The lowest reported oxygen content for sintered injection-molded Ti-6Al-4V is 0.27%.¹⁵ This is still slightly above the ASTM F1108 chemical requirements, but low enough to give suitable mechanical properties.

In addition to the property requirements summarized in Table IV, many implants must meet component-specific requirements. For example, femoral hip prostheses must meet specific fatigue property requirements as described in ASTM F2068-00. Since fatigue properties are highly sensitive to porosity, metal-injection-molded components must often be hot isostatically pressed to eliminate any remaining porosity to meet these requirements. Potential particulate inclusions may also reduce a material's fatigue resistance, especially for titanium and titanium alloys.

At the same time, metal injection molding offers opportunities for unique design solutions. For example, an implant with a porous coating for bone in-growth can be manufactured by metal injection molding as a functionally graded device with controlled surface porosity surrounding a fully dense core. The process can also produce composites of titanium and hydroxyapatite; such materials are advancing to animal studies.

Corrosion Properties. The corrosion requirements for metallic implants are much more stringent than for medical instruments. Limited testing of metal-injection-molded parts has been reported under conditions that simulate the salty, 37°C environment of the human body. ASTM F746-87 establishes a procedure for determining pitting or crevice corrosion of metallic surgical implants. Since pitting corrosion is unacceptable for metallic implants, additional polishing or passivation treatments should be expected for 316L stainless-steel implants. Injection-molded titanium and Ti-6Al-4V have been shown to resist pitting and crevice corrosion in various media, including artificial saliva, artificial sea water, 22% NaCl, and 6% FeCl₃.¹⁴ The performance of metal injection molded Co-28Cr-6Mo alloy is expected to be similar.

Although metal-injection-molded biomaterials can meet corrosion requirements, additional qualification barriers must also be overcome. These include the effort of confirming biocompatibility and conducting clinical trials. Additional success in these areas can help metal-injection-molded implants gain acceptance by the medical industry.

CONCLUSION

Metal injection molding is fully capable of meeting the dimensional and material property requirements of medical instruments and has many demonstrated applications. It also shows the potential to produce implant materials; recent work has demonstrated its ability to meet most requirements for the chemical, mechanical, and corrosion properties required of such applications. More trials are needed for metal-injection-molded biomaterials to gain acceptance as implants. Besides making manufacturing of current medical devices more affordable, metal injection molding can enable the cost-effective production of novel designs, including microsized and functionally graded devices. Such developments may enable new solutions to current healthcare problems.

REFERENCES

1. Annual Book of ASTM Standards, vol. 13.01 (West Conshohocken, PA: American Society for Testing and Materials, 2001).

2. TA Tomlin, "Metal Injection Molding: Medical Applications," International Journal of Powder Metal 36, no. 3 (2000): 53–57.

3. Product literature, Advanced Materials Technologies (Singapore).

4. JR Davis, ed., ASM Specialty Handbook: Stainless Steels (Materials Park, OH: ASM International, 1994).

5. Product literature, BASF (Ludwigshafen, Germany).

6. SR Collins, "Corrosion Behavior of Metal Injection Molded 316L Stainless Steel" (paper presented at the International Conference of Powder Injection Molding of Metals and Ceramics 2002, San Diego, March 2002).

7. H Wohlfromm et al., "Novel Stainless Steels for Metal Injection Molding," in Proceedings of the 1998 PM World Congress [compact disc], (Granada, Spain: European Powder Metallurgy Association, 1998).

8.H Wohlfromm et al., "Corrosion Resistance of MIM Stainless Steels," in Advances in Powder Metallurgy and Particulate Materials—1999, vol. 2, ed. CL Rose and MH Thibodeau (Princeton, NJ: Metal Powder Industries Federation, 1999), 6.27–6.38.

9. R Tandon et al., "Mechanical and Corrosion Properties of Nitrogen-Alloyed Stainless Steels Consolidated by MIM," International Journal of Powder Metallurgy 34, no. 8 (1998): 47–54.

10. MK Bulger and AR Erickson, "Corrosion Resistance of MIM Stainless Steels," in Advances in Powder Metallurgy and Particulate Materials—1994, vol. 4, ed. C Lall and AJ Neupaver (Princeton, NJ: Metal Powder Industries Federation, 1994), 197–215.

11. Material Standards for Metal Injection Molded Parts, MPIF Standard 35 (Princeton, NJ: Metal Powder Industries Federation, 2000).

12. D Williams, ed., Concise Encyclopedia of Medical and Dental Materials (Cambridge, MA: MIT Press, 1990).

13. R Tandon, "Net-Shaping of Co-Cr-Mo (F-75) via Metal Injection Molding," in Cobalt-Base Alloys for Biomedical Applications, STP 1365, ed. JA Disegi, RL Kennedy, and R Pilliar (West Conshohocken, PA: American Society for Testing and Materials, 1999), 3–10.

14. H Wohlfromm, M Blömacher, and D Weinand, "Metal Injection Molding of Titanium and TiAl6V4," in Powder Injection Molding Technologies, ed. RM German, H Wiesner, and RG Cornwall (State College, PA: Innovative Material Solutions, 1998), 339–348.

15. K Kato, "Effect of Sintering Temperature on Density and Tensile Properties of Titanium Compacts by Metal Injection Molding," Journal of the Japan Society of Powder and Powder Metallurgy 46 (1999): 865–869.

16. H Wang, SHJ Lo, JR Barry, "Development of High Density (99%+) Powder Injection Molded Titanium Alloys," P/M Science & Technology Briefs 1, no. 5 (1999): 16–18.

John L. Johnson, PhD, is R&D manager for AMTellect Inc., in State College, PA.

Copyright ©2002 Medical Device & Diagnostic Industry