Metal Injection Molding Is a Natural for Small, Net Shape Parts with Detailed Features
Metal injection molding is best suited to small, complicated parts with numerous features.
Photo courtesy of ASH Industries.
Metal injection molding (MIM) may not be as widely known as its popular counterpart, plastic injection molding, but it offers design engineers some real benefits for smaller parts that would be too costly, impractical, or time-consuming to make via stamping, machining, or casting. One company with a strong handle on the process is ASH® Industries (www.ashindustries.com), a Lafayette, Louisiana-based maker of MIM parts that also offers thermoplastic injection molding, liquid injection molding, and in-house mold making for clients in the medical, defense, electronics, and industrial markets.
Engineering is a valued capability at ASH Industries, which employs several engineers on staff to help fulfill customers’ needs in everything from rapid mold making to production of the finished, molded part. ASH Industries President Hartie Spence, himself an engineer with a degree from M.I.T., fully appreciates the value that engineers bring to the manufacturing process.
“I consider engineers to be problem solvers,” Spence told Design-2-Part Magazine in a recent phone interview. “They have a whole myriad of complexities that they have to organize into consistent lines of thought, and then come up with a list of options to address the customer’s concerns. They then work with the customer, priority by priority, in order to accomplish what the customer has in mind. So engineers are useful in that they solve problems.”
In the course of the interview, Spence pulled back the curtains on the somewhat under-appreciated process of metal injection molding, shining light on its benefits, limitations, and the types of applications for which it’s best and, conversely, least suited. He also discussed the company’s SuperMold™ program, said to reduce the costs and lead times of custom injection molds for manufacturing MIM, thermoplastic, and silicone components. Following are edited excerpts of our conversation.
D2P: Please describe in your own words the basic steps involved in the Metal Injection Molding (MIM) process.
Hartie Spence: Metal injection molding is a unique way to mold a part in its net shape. We do this by molding a combination of metal with a thermoplastic carrier. The thermoplastic carrier is minimized and typically does not exceed 35 percent. The vast majority of the material is metal, so when we melt the material, we’re actually only melting the thermoplastic. We’re using the thermoplastic to convey the metal into a shape.
When that’s done, we have a shape that is called a green part. A green part is typically 30 to 35 percent thermoplastic; the balance of which is metal. The green part is then put through a process where we extract the thermoplastic, leaving a loose, porous matrix of metal. The metal part is then placed in a hydrogen backfilled oven, close to 2400 degrees Fahrenheit, and the metal that is left is coalesced into a solid piece.
The parts have a strength comparable to [a component] that has been cast. Metal injection molding is a distant cousin to powdered metal technology, but is a very good replacement for more durable processes. The reason somebody would use this process is because they have been spending a whole bunch of time in secondary processing parts that had been typically stamped or cast, or maybe they’re machining a part from a block of material, and it takes a whole bunch of steps and a whole bunch of time and is very expensive.
The material itself can be quite expensive, and at some point, the material costs, because of the size of the part can exceed what you would be spending in another process on the secondary machining. So metal injection molding is ideally suited to small, complicated parts with many features, and we mold the parts into the net shape.
D2P: What metal materials do you most commonly use in metal injection molding projects?
Hartie Spence: There are many metal materials; most typically, they are hard metal. Stainless steel is the most common variety of metal injection molded material, but you can even metal injection mold Inconel and titanium, though, once again, the material cost is extremely high, and the applications that are financially beneficial are even more narrowly defined for the very expensive material.
Metal injection molding is generally not typical for brass or copper or aluminum, though technologies to address those metals are developing.
D2P: What would you say are the main benefits of using the metal injection molding process versus competing processes?
Hartie Spence: The main benefits of metal injection molding are that you’re able to get a part that is manufactured consistently in a mold that you have already approved, and that you have the benefit of receiving the net shape. You also have some comfort in the fact that the parts can be very consistent and mass produced cost effectively.
We have little tiny, complicated medical parts that would most likely be machined if they were not metal injection molded because casting is not reasonable for a part with this type of delicate feature. It would not be possible to cost effectively machine these small parts on a mass production basis. You would have nightmarish quality problems, your costs would be through the roof, and, therefore, the project would most likely not exist. So metal injection molding, in this case, makes the part possible.
We have another customer where they were casting the parts and then machining them afterwards, but they were having tremendous problems with consistency, and they spent a whole bunch of time handling the parts in secondary processes. We have eliminated the lack of consistency and we have eliminated the time that they had to invest.
D2P: I noticed your website mentions a couple of design considerations that design engineers or customers should be aware of, which would include wall thickness of at least 0.13mm, and no larger than 12.7mm.
Hartie Spence: Right, and there are always exceptions; these are general guidelines. It’s just like a thermoplastic part: You’d prefer the part not be extremely thick because you want the part to cool consistently. And by having a tremendously thick part, you can cause internal voids or cosmetic abnormalities because a part that is so thick cools inconsistently in molding.
D2P: You also mentioned how metal injection molding is better suited to parts that are smaller in size and more detailed in properties.
Hartie Spence: Think about a massive funnel, a giant funnel. The small part of the funnel is pointing down, the big part of the funnel is pointing up. The parts that fill the big part of the funnel are typically smaller because in the small part, the material costs can almost be inconsequential. If you’re talking about a micro part for some hearing aid battery, the material cost is almost non-existent because it’s so light and so small. If you’re talking about a larger part for a gun, it’s going to be in the bottom end of the funnel, where there’s a lot more material. So the material cost has driven the part cost to be higher, and there are only a very narrow number of applications where metal injection molding makes sense with larger parts.
I’ve seen Inconel parts that were massive—I’m talking 10 inches tall and 8 inches across, that were metal injection molded, but they were almost entirely hollow with relatively thin walls. The reason it made sense was because to machine a part in Inconel of that size, you would have to start out with almost a solid block and machine away 90 plus percent of the material, and so the machining costs and the material loss were astronomical.
D2P: Could you give, in general terms, an example of an application where metal injection molding was the right process to use and contributed to the success of the project?
Hartie Spence: Sure. We had a customer who has a very specific optical device—a medical optical device for glaucoma—and they had been machining parts in prototyping. We met at a Design-2-Part tradeshow, discussed their part, and it was very obvious that they were spending tremendous amounts of money machining these small parts. It was very questionable whether that was sustainable in production.
The part itself very easily lent itself to metal injection molding, and it would be very easy for us to make 95 percent of the features on the part. They had some very exact tolerances on a couple of the features—overall length, and a couple of smaller features—that required us to do a one-step secondary operation. So we molded the part, we sintered the part, putting it through the hydrogen oven to get a 100 percent stainless steel part. And then we built specialized fixtures in order to hold the part and just ‘kissed’ the end of the part in order to bring it within the tenths tolerance that the customer required over the full length of the part.
So now, we have a product, which management did not think would be possible for the engineers to bring to market, that is very possible—and, in fact, is in production today—because of the metal injection molding process.
D2P: Besides medical, are there any other industries where you see metal injection molding as more likely to be used?
Hartie Spence: Really, any industry. We have focused on a type of customer, not a specific product line. The engineers we deal with have the same focus and same concerns, whether they be medical, industrial, consumer related, or military. They all have very general concerns that are very consistent, and the processes that we offer to these customers provide them with solutions in whatever realm they’re in.
D2P: Aside from the cost of larger metal parts, what are some of the limitations of MIM in certain situations?
Hartie Spence: When the features of a MIM part are extremely small and delicate, during that sintering process, they can deform under heat. And so, often, as a secondary step, we might need to press or coin those metal features back to where they need to be. If you had a 0.020 (20 thousandths) wall thickness that would deform off to the left or right, it may need to be pressed back into shape. We have some parts that have, percentage wise, massive amounts of material to the left and right, and are very, very thin walled in the center. And so we have added extra material to the part in the center to help propagate the flow of the material, and then, as a secondary process, we machine away that extra material, so that the customer receives a part that is net shape and is exactly what they need for their process.
So because of the delicate nature of some features, because of the potential deforming effects of the sintering process, it may be necessary to create secondary processes to remove excess material or to coin the parts into shape. You may need to produce some ceramic fixtures that hold the parts during sintering.
D2P: What would you say are the main benefits that your in-house mold making team can bring to a customer’s molding project?
Hartie Spence: The best benefit is that everything is contained in house. There’s nobody else to blame. We have one team working together to get a customer the parts they need, whether those parts are thermoplastic, silicone, or metal injection molded. So we’re all working together to consistently deliver components. And so, with mold making here on site, we’re able to make sure that we’re communicating thoroughly to make that a reality.
Secondary to that, we’re able to react very quickly, whether it be a mold maintenance issue or an engineering change, or a customer has decided to add a feature, or a customer would like to examine existing tooling for the addition of more cavities or more capability. We’re able to do that quickly with the input of the production people.
D2P: I understand you offer the SuperMold service. What exactly goes into the SuperMold program?
Hartie Spence: The SuperMold service was a unique way to address a market characteristic. There are certainly providers out there that offer inexpensive molds quickly, the problem being that those inexpensive molds would deteriorate over time and may offer limitations on configuration, material choices, and tolerances. In the SuperMold program, we offer a tool steel mold that is cost effective and can get a project started. And I’ll give you a good example.
We have a customer that we met at a Design-2-Part tradeshow up in New England. They challenged us with a very complicated part that we were able to prototype for them in the SuperMold process in a tool steel mold that was suitable for early production. We ran that part successfully and solved a number of technical issues. We had to change materials and change configurations, and it was not at all known whether this was going to be a success. After proving the concept, the material, and the configuration successful, we now are going to build a multi-cavity mold in order to get the customer into long term, low cost production.
The SuperMold is a real mold. It’s not aluminum and it’s not cheap. They have a lifetime guarantee. We’re not going to limit the material that the customer uses. It can be modified. While the basic program is basic tolerances, plus or minus five thousandths (+/- 0.005), we do run the SuperMold in tight tolerance varieties for an additional fee.
So the SuperMold has proven to be an excellent way to address the majority of the market, and flexible enough that we can use it in very specific, very tight tolerance, and highly demanding roles.
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