Medical Device Link.

MANUFACTURING

Delivering efficiency

A common problem when developing a medical device can be that the machined prototype meets all the stated design goals, but the manufacturing cost for each device is too high. Precision turning, machining and electrodischarge machining often require manual setups, tool replacement and extended machining times. All of these, together with the inherent scrap involved in machining, drive up cost and can delay the schedule for production. One solution to this problem is to use metal injection moulding (MIM).

In fact, it would be less expensive to use standard polymer injection moulding. But strength and sterilisation requirements often drive a design towards metal. Because MIM, sometimes called powder injection moulding, is actually a combination of injection moulding and powder-metal sintering, it delivers the same time and cost savings as injection moulding, while retaining robust material properties.

A variety of metals can be used as the metal powder that is sintered into the final part, including stainless steels and titanium as well as metal-ceramic hybrids. Many of these materials are approved for use in medical devices. Once fully sintered, the resulting parts have properties similar to those of the wrought version of the same material.

Metal injection moulded parts can be complex and capture small features with tight tolerances. A common application is endoscopic and laproscopic mechanisms where pins and walls are 0.25 mm +/- 0.01 mm or less to fit in the small diameters they must pass through to enter the body. The advantage of the process is that the complexity and required precision has to be captured only once, during manufacture of the mould rather than in each part made.

The MIM process

The first step in creating a part is to design tooling to serve as a mould. This mould is then injected with a heated feedstock made of metal powder combined with a polymer binding agent. Once cooled, the binding agent hardens and creates a solid, but fragile green part that is ready to undergo a debinding and sintering process. This final step uses a numerically controlled oven to apply a precise heating profile that dissipates the binder and causes the metal particles to sinter to one another.

The biggest challenge with this process is the fact that the dissipation and sintering step causes the green part to shrink by up to 20% in all directions. The tool designer must be able to accurately calculate the shrinkage and create a mould that is capable of producing parts that deliver the desired size with as much accuracy as possible.

Designing for MI moulded parts

The most important issue to remember when specifying a part that will be made by MIM is to design the part for injection moulding. Parts must be shaped so that

• they can be filled completely with the heated fill stock
• draft is sufficient to pull the mould apart and eject the part
• the part has no undercuts, or that any undercuts can be captured through the use of simple slides or inserts.

In addition, the way in which the mould fills with the feedstock can cause nonuniform shrinkage during sintering and must be considered in the design. An injection mould tool designer with MIM experience should be consulted early in the design process and he/she should review the design for mouldability at every stage of the development process.

The other critical design consideration for a metal injection moulded part is to take into account the fact that the part will go through a debinding and sintering process. Someone with sintering experience should be consulted to make sure that basic design rules about part size, thickness and support are considered. An experienced manufacturing engineer with this type of experience can greatly increase the yield and reduce the cycle time by suggesting relatively small changes to the design.

Rapid MIM

A rapid MIM process is available that is ideal for the relatively low volumes required for many medical devices, especially during the clinical testing period. By combining rapid prototyping technology such as stereolithography with epoxy moulded tools, a tool can be created directly from CAD software in a matter of days. A team with expertise in working with hand load/unload tooling will also avoid the delay of designing and building automatic slides and inserts to capture complex features. Another advantage is if the metal injection moulder has a partnership with a sintering vendor who understands how to work with the green parts created with MIM. Putting these elements all together allows production of several hundred metal parts in the time usually required to create the tooling for the traditional MIM approach.

Getting started with MIM

Finding a dependable MIM vendor who is willing to participate in the design process is the first and most important step in using the process for a medical device. Working with the vendor, the design team must make material choices such as deciding between titanium, stainless steels or ceramic-metal hybrids based on their design’s particular need for strength, chemical resistance and weight. This choice will then allow the team to establish essential design constraints such as wall thickness ranges, feature sizes and support needs.

Once this critical foundation is established, the design team should be able to move forward with the development process. Thus, next time a new device design begins, wise companies will avoid the cost and time required to precision machine by designing for MIM from the outset.

Eric Miller is Director of Analysis & Design Technologies at Phoenix Analysis and Design Technologies Inc., 7755 S. Research Drive, Suite 110, Tempe, Arizona 85284-1803, USA, tel. +1 480 813 4884, e-mail: eric.miller@padtinc.com.