Design Guide

MIM Overview


Many engineers think of MIM as a new field. In many regards it is new, but various forms of the technology have been in commercial use since 1975. Over the 40 years MIM has been in practice, some folklore has arisen, leading to some broadly held misconceptions. This section identifies a few of these misconceptions and helps clarify some of the common myths.

Special molding machines are required.
Certain molding machine characteristics are better suited to MIM, especially for higher levels of precision, but many firms are successful with a variety of molding machines. The evidence suggests that there is no single manufacture or design that is necessarily best.

There is easy money to be made in MIM.
With its maturation, MIM has largely remained a healthy technology that continues to grow by satisfying more sophisticated customers. But most of the firms with favorable financial positions tend to have much practical experience, and almost all of the leaders are well over 10 years old. These leaders have established manufacturing systems which are benchmarks for all net-shape manufacturing. To assume that a late entry will jump into this class is not realistic. Yes, there is success if you work long and hard, but MIM is not a route to quick riches.

Only a few firms understand the technology.
There are about 400 firms, several university programs, and a dozen research institutions supporting the technology developments in MIM. None of these has a lock on the technology. The basic concepts are well documented in books, and many of the suppliers will gladly share technology to help create new customers. As is often known as Prado's Principle, 20% of the firms have 80% of the sales, but it is not wise to assume any single firm is smarter or more knowledgeable than another. The field has many successes, failures, and much cross- fertilization, so there is no single pathway or technology that will give success.

A license is required to practice MIM.
Some of the first commercial developments in powder injection molding date from the 1930's and 1940's, so long ago we have trouble reconstructing the history. In the 1960's, Corning used ceramic molding to form tableware, a technology that survives in the ceramic casting core business. In MIM, the original patents were issued in the 1970's. Thus, with the technology aging this much, many of the suppliers provide an excellent support base for start-ups. The generic technology works well and most of the ingredients and equipment can be ordered with help from the equipment manufacturers. So, if a company wants to have its hand held during the start-up phase using a license, that is fine, but to think there is a proprietary or special technology that requires a license does not reflect reality.

Molders that purchase the same feedstock allow for movement of an order between sites.
Clearly, a trend has been to rely on purchased feedstock, but that does not cure the need for similar molding practices, processing equipment, and secondary operations. In general, tool design is highly variable and customized to the shop and molding machine. Hence, moving an order to a second site might encounter problems beyond the feedstock, so there is no assurance of success along these lines.

You need a materials scientist to succeed.
This is a myth that is close to reality in some cases. Several plastic injection molders have successfully entered MIM, but at times they struggle with controlling impurities, optimizing properties, selecting proper heat treatments, and other basic aspects of materials engineering. These can be mastered without full-time staff. One start-up used a skilled metallurgist on an as- needed basis. Another plastic firm initially relied on feedstock and furnace vendors for support. However, to become established at more than say $5 million in annual sales requires mastery of three technologies - materials engineering, manufacturing engineering, and plastic molding, but you can start with only one or two.

Powder costs will continue to fall.
As MIM grows, some of the powders (especially pre-alloyed stainless steel) have declined in cost as consumption increases. But in all cases, consumption has increased faster than the cost reduction. In other words, if powder sales (dollars) have increased 20% per year then powder shipments (tons) went up 40% per year. Along with improved process yields, the increasing tonnage generated a price decrease. For some alloy chemistries this sort of volume growth is not going to be sustained.

Low pressure molding is less costly.
Low pressure MIM machines are used to reduce machine cost and tool wear. They are used by about 15% of the combined metals and ceramics industry. The manufacturing cost is now lower with low pressure molding, since these options lack automation and usually only fill a single cavity. So with a lower purchase price (capital cost) there is a burden of a higher operating cost. For example, in a comparison across the industry, low pressure molding shops show lower sales per employee. More important, without a high molding pressure the components tend to have more internal defects. For surface features (such as in sand blast nozzles and watch cases) there is no problem.

Continuous sintering in hydrogen is a MIM evolution.
The use of hydrogen sintering in a pusher furnace was applied to refractory metals and stainless steel prior to the 1940's. It was widely used for that application at many sites prior to the first commercial MIM use in 1985. Today, continuous furnaces constitute about a quarter of the installed sintering capacity in MIM and prove most productive for long-running components. Thus, they are more common in large Asian shops fabricating consumer products, cellular telephone parts, and watch cases.

Newer binders enable cost-effective production of large components.
The binder is not a barrier to molding large components. Large stainless steel components were formed in the 1980s using MIM, ranging up to 12 kg (26 lb), using a traditional wax-polymer binder. Today large ceramic components are in production at several sites. There are problems with large components, but more significant are the economic barriers. In metals, as the mass increases the raw material cost difference (powder versus casting cost) leaves plenty of margin for machining after casting. Thus, changing the binder will not solve the critical powder cost issue. Indeed several economic problems arise with large MIM components, besides powder cost there are penalties because the molding machines are larger, molding cycles are slower, and debinding times are longer. Accordingly, MIM is less cost-effective as size increases, independent of the binder.

Metal powder injection molding is just another form of powder metallurgy.
Although a few companies practice both MIM and traditional die compaction and sintering (what is generally implied by the term powder metallurgy). The two technologies have little in common other than utilizing metals powders. MIM powders are much smaller, round if not spherical, and sintering temperatures are higher such that the final density and performance are much better. The only thing these two technologies have in common is powdered metal and a sintering process. But the size of the powdered metal particles and the sintering processes vary significantly.