MIM’s the Word

As a solutions-oriented designer, developer, and manufacturer of surgical devices, Pro-Dex, Inc. has the in-house expertise to cost-effective and efficiently machine a broad range of parts for our customers.However, if a project requires higher-volume metal components, we collaborate with industry partners who support our dedication to providing customers with innovative, economical solutions.

February 17, 2012

7 Min Read
MIM’s the Word

As a solutions-oriented designer, developer, and manufacturer of surgical devices, Pro-Dex, Inc. is focused on providing the best options for our customers. In most cases we have the in-house expertise to deliver cost-effective, efficiently machined parts to our customers.

However, when a situation arises where our capabilities might not be the best option for a customer or potential customer, we look to industry partners to help us strategically provide the most innovative, economical solution possible.

This is why at this year’s MD&M West, we reached out to Phillips-Medisize Corporation. Phillips-Medisize was recommended to us by another industry partner in our search to learn more about Metal Injection Molding (MIM) processes.

We spoke with Phillips-Medisize’s Manager of Business Development, Mike Magnine, who thoughtfully walked us through the basics of MIM and when it is the best option for our medical device customers.

PD: What is MIM and how did it come to benefit medical devices?

PM: “MIM” stands for Metal Injection Molding and it’s an effective way to manufacture a variety of metals into small, highly complex geometries without machining. MIM was invented primarily by the gun industry because of the expense of mass machining intricate parts with tight tolerances. Metal Injection Molding was introduced to the medical market about ten years ago but really has grown in the past 2 years as the demand for precise medical device components, as well, disposable surgical instruments, has increased.

PD: What is the Metal Injection Molding process?

PM: There are four primary steps, which utilize four key processes to produce metal injection molded parts with superior quality and dimensional repeatability

1. Feedstock Mixing
Attention to detail at the mixing step is critical to ensure the homogeneity of the feedstock over the long run. MIM feedstock begins with extensive characterization of very fine (less than 22 micron) metal powders. These powders (60% by volume) are carefully hot mixed together with co-polymer binder (40% by volume) to form a uniform mixture. This mixture is then cooled and granulated to form the feedstock for the injection molding machine, similar to a plastoic pellet.

2. Molding
Phillips Medisize specially equipped injection molding machines are designed to mold this metal/polymer feedstock. Using our years of injection molding experience with advanced processing instrumentation and software, ensures tight  control of this process producing consistent components with unvarying density.

3. Catalytic Debinding
The advanced debinding technology used by Phillips is the most efficient form of debinding. Harnessing the power of polymer chemistry, Phillips Medisize introduces a catalyst to remove 90% of the binder from the green part. Because  catalytic debinding occurs at temperatures below the softening point of the binder, parts are processed with excellent shape and dimensional integrity. After the binder has been removed, the result is termed a “brown” part. The brown part consists of a porous matrix of metal powder and a small amount of binder, sufficient to allow the part to retain its shape.

4. Sintering
In the final step, the brown parts are sintered using a temperature and atmosphere profile chosen specifically for the alloy being processed.

PD: So you do you create your own feedstock?

PM: No. we contract with BASF.  Their core strength is creating this feedstock, instead of reinventing the wheel, we partner with an expert to ensure that we’re getting the highest quality feedstock possible.

PD: Why is molding an advantage?

PM: Parts can exhibit complex contours, holes, undercuts small radii, logos, and text, can be all molded in. The molding process creates virtually no waste since runners can be reground and molded again without compromising the  properties of the final part.

PD: What is unique about your molding process?

PM: In the molding area, we incorporate  in-cavity pressure transducers to indicate if the molding cycle spikes out of predetermined limits. If spikes occur, the closed loop feedback system will rejects parts automatically.  Then extensive automation is employed to palletize the only good parts directly onto ceramic setters. This automation eliminates the unnecessary handling of parts, providing consistent and cost-effective means to get parts though our furnaces.

PD: What catalyst is used in the de-bind process and why is it so efficient?

PM: The Green part is introduced into a furnace containing nitric acid. The heat creates a gas that reacts with the polymer (Acetal) then very quickly and precise remove 90% of the co-polymer.  Its efficiency is a result of the nitric acid only attacking the Acetal, or primary binder, leaving the secondary binder untouched, holding the metal particles together to their original shape.  Catalytic Debinding removes much of the guess work, which eliminates slumping and distortion.

PD: So after this step it’s really not pure metal?

PM: Not quite yet. We now take the Brown part and send into a Sintering Furnace where at the lower temperatures of the sintering cycle, the residual co-polymer (10%) binder is removed. As the temperature increases, sintering begins. Neighboring metal particles fuse and bond to one another bringing the structure together and reducing porosity. Ultimately, the required physical properties are obtained and densities between 96-99% of theoretical are achieved. During the densification process, depending on the material being processed, linear shrinkage of 14-20% occurs.

PD: Is 14-20% shrinkage predictable? – how close can tolerances be held?

PM: This shrinkage is predictable.  Each alloy has a specific percentage to compensate for the shrinkage, we simply oversize the mold cavity to yield the correct size. Typical as-sintered tolerances are within ± 0.30 to 0.50% (0.003 to 0.005 inches-per-inch).

PD: What if that MIM tolerance is not tight enough – or need to add special features?

PM: Sintered MIM has the ability to be post machined similar to other machined parts.  Drilling. grinding, lapping, swedging, tapping, are all operations that we can add if the process will not yield the net shapes or meet target tolerances.

PD: The process sounds very involved. How is this process less costly than machining?

PM: It is an involved process. It takes about 24 hours to make a part.  At the same time, throughput can be high, and there’s little to no labor involved, which reduces costs significantly. Our niche is when production volumes are greater than 10,000 parts annually combined with complex part geometries. If you have to go through two or three separate machining steps on a small part, you should consider MIM.

PD: Aren’t the molds a big expense?

PM: Molds for the MIM process are very close in price to what a Plastic Injection Mold costs.  Return on the tooling investment is generally not a problem if your annual volumes are supported, and part detail is demanding.  

PD: Can you add more detail to a part?

PM: Many of the most successful MIM applications are where our Engineers have combined 2 or more machined parts into one consolidated design.  This avoids multiple part numbers and reduces assembly steps, all adding to the total savings.  

PD: What is the optimal sized part for Metal Injection Molding?

PM: MIM is best for highly intricate, small metal parts that are golf ball-sized and smaller. MIM material (feedstock) is more expensive that wrought steel and the process is time-sensitive, so size does matter. Unlike machining, where you only cut away enough material to give you the desired part, with MIM you only want to use enough material to give you the desired part.

PD: Is finishing required on the post-sintered part?

PM: Again, MIM is like any other metal part and all of the typical metal finishes can be applied.  Most Medical parts are made from Stainless Steel which yields a bright finish that customers use as sintered.

Tricia Rodewald, director of marketing and strategic alliances at Pro-Dex Inc.

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