Metal Injection Molding: advantages and disadvantages

During the last decades, MIM or “Metal Injection Moulding” has been confirmed as a very competitive technology for the manufacture of small steel parts of high precision and complexity, which would be very costly to produce by other processes.

Explanation of the MIM process for metal fabrication

• The feedstock is a material composed of very fine steel powder and a polymer that acts as a binder.

• This material can be injected into a metal mold as if it were a plastic, obtaining the “green piece“, which has the same shape as the piece we want to achieve, but is 20% larger in all its dimensions, since it still retains the plastic component. 

• By attacking the green part with a catalyst, a phase called “debanding”, we cause the sublimation of the polymer, which is expelled in the form of gas. We call this intermediate state, in which the part maintains its initial dimensions but is porous and has very low mechanical strength, the “brown part”. 

• During the last phase of the process, the temperature is raised (always below the melting temperature of the steel) in a protective atmosphere, activating sintering by mass attraction. The part decreases in size as its density increases, reaching values above 97% for the final dimensions of the “MIM part“, which often coincides with the finished part. 

• Depending on technical requirements, MIM parts can be subjected to any type of subsequent treatments (thermal, surface, straightening, machining, polishing, welding, etc.). 

• In 1993, ECRIMESA was the first company in history to develop this process in a continuous furnace, having at present 3 fully operational lines. Thanks to a wide range of automatic injectors with robotic handling systems that include artificial vision systems, high levels of productivity and quality are achieved. 

Advantages and Disadvantages of MIM. Comparison with other technologies

• Generally speaking, any part that can be manufactured by plastic injection molding could be produced in steel by MIM, but there are some limitations or rules to keep in mind: 

• Although at ECRIMESA there are parts in production up to 350 gr (“record” of the technology, still in force worldwide), MIM is a recommended process for parts under 100 gr and especially competitive below 40 gr. The weight reduction increases the precision of dimensional tolerances and reduces raw material consumption, which has a great influence on the total cost. 

• The maximum allowable radius is recommended for all edges, especially internal edges, since sharp edges can be sources of potential cracks. 

• Section changes should be gradual, facilitating the flow of material during injection and avoiding isolated masses. 

• A flat face will facilitate the palletizing of the part inside the MIM furnace. The more stable this support is, the smaller the deformations will be and the more precise the dimensional tolerances will be.    

Advantages and Disadvantages of MIM: Comparison with other technologies

• MIM is essentially a manufacturing technology for precision steel parts, with complex three-dimensional shapes, fine details, good aesthetics, demanding mechanical requirements and high consumption.  

• It is colloquially known as “the enemy of machining“, because the tight geometric tolerances (±0.5%), low roughness (between Ra0.8 and Ra1.2) and high finishing quality (possibility of including threads, knurling, logos, etc. in the mold) greatly reduce the need for second operations to obtain a finished part ready for the corresponding assembly line.  

• When the shape of the part in question allows it to be manufactured by traditional powder metallurgy (single shaft pressing + sintering), MIM may often appear uncompetitive. However, these types of parts are much more porous and fragile, so it is common for the application to force a switch to higher strength components such as MIM parts. 

To learn more about MIM materials: