What Makes a Good MIM Part?

In the world of metal manufacturing, several processes stand out more than others in terms of acceptance and popularity. Machining, stamping, forging, and casting have long been the prominent forms of manufacturing since the inception of the industrial age.  However, in recent years other forms of manufacturing are challenging the dominance of these existing techniques. 3D printing and metal injection molding have quickly gained momentum. Metal Injection Molding is a manufacturing technology well suited for high-volume, complex metal components.  A MIM part can be more cost effective than a similarly complex machined part.  However, it is important to not fall into the trap of “once you have a hammer, everything looks like a nail,” since not all parts are suitable for MIM.  When designing for metal injection molding, there are several characteristics such as materials, mass, size, complexity, and volume to consider that make a good MIM part. 

When evaluating and designing a component for metal injection molding, one of the most common questions asked is “what materials can you MIM”? A good MIM part is typically made from ferrous alloys such as carbon steels, stainless steels, and tool steels. However more exotic alloys can be manufactured using the MIM process. Click here for a full list of available materials. If a material has a high enough melting temperature and the powders are available in the appropriate size it can most likely be utilized by metal injection molding. This rules out non-ferrous alloys such as Zinc and Aluminum.

After material selection, the first characteristic to consider when is designing around metal injection molding is component mass. The mass of a good MIM part is typically under 30 grams with the average being 5-15 grams. However, MIM parts can range from micro medical components with a mass of .030 grams all the way to larger industrial components with a mass of 300 grams.  The rule of thumb for the size of a MIM part is that it should fit in the palm of your hand.  They can vary from 2mm long to 150 mm long - with the average being around 25mm long. 

When we look at design principles for MIM parts, engineers with experience in plastic injection molding will see a number of synergies and similarities. Metal injection molding and plastic injection molding tooling are very similar and are processed using virtually identical molding machines.  One significant and unique difference between plastic and metal injection molding is the MIM processing step of sintering which densifies the component being molded.

Wall thickness can be a larger concern with Metal Injection Molding when compared to other forms of manufacturing.  This is due to alloy material mold flow properties.  Walls much smaller than .010 inches will have a difficult time filling and packing completely.  This is due to high shear forces limiting flow and/or air entrapment.  While on the subject of wall thickness, a good MIM part will also have a relatively uniform wall thickness throughout the part.  If a part has drastically different wall thicknesses, then the thinner features will sinter sooner than the thicker areas – causing distortion.  A way to alleviate this issue is to core out the thicker areas of the part. 

A good MIM part will also have no sharp corners. Sharp internal corners are undesirable because the material can pull a void in those areas.  Sharp external corners in the part require a sharp internal corner in the tooling, which is very difficult and costly to produce.  As a rule of thumb, avoid designing a part with an edge radius smaller than .005 to .006 inches.

Like plastics, a good MIM part can still have undercuts and threads.  However, these do require more complicated tooling.  The tooling will need to have cams or side actions to form undercuts.  They slide into place before the mold closes and slide out of the way before the part ejects from the tool.  As you can expect, this also increases the cost of the tool; however, the return on investment is better than having each part machined secondarily to make the undercut – especially at higher volumes.  Threads can be formed right in the tool, but there needs to be “flats” on the sides of the threads to allow a flat parting line.  

When evaluating metal injection molding as a potential manufacturing solution, cost and return on investment (ROI) must be included in the equation. As mentioned above, those who are used to plastic injection molding will see a similar investment structure with metal injection molding. MIM tooling can cost tens of thousands of dollars, therefore on average production volumes greater than 10,000 units per year are desired to provide adequate ROI. This is where the benefits of metal injection molding are extremely evident. Once tooling is built and qualified, MIM parts can be scaled to high volumes at a much lower per unit cost as compared to other manufacturing processes. 

The goal of metal injection molding is to produce net shape parts that require minimal secondary operations; this is where the true cost benefit comes from. Using metal injection molding as the manufacturing technique for a component with the characteristics laid out above will not only minimize the need for secondary operations but will make for a very cost effective component throughout the life of the program. If you have additional questions and would like to speak to an APP MIM expert, click here.