How to use MIM in the production of metal parts

Metal Injection Moulding (MIM) is a suitable technology to fabricate small or medium-sized pieces with complex geometry. A variety of materials can be used (stainless steels, low-alloy steels, soft magnetic, tooling steel or ceramic materials). MIM combines the technique and versatility of plastic injection with sintering in order to fabricate metal pieces with high density and narrow geometric tolerances.  

The following image shows the MIM process.  

First steps. Fabrication of the mold: The molds employed in the MIM process are made of highly resilient steel, of complex geometry and more challenging than the molds used for plastic injection. When fabricating the hollow form, it is important to fator in the contraction of the material that will be injected, which means the hollow form has to be bigger than the actual desired piece. It is vital to stick to the design rules of the technology in this step in order to obtain a stable and immaculate product.  

The number of hollow forms in the mold depends on the geometry and dimensions of the esired piece and can vary between one and ten hollow forms in total. The usual number of hollow forms lies between two and four, production time can vary between 15 and 60 seconds.  

Feedstock or primary material: The primary material consists of a metal powder with a particle size of maximum 32 micrometers (at least 80% of the material must be smaller than 22 micrmeters), mixed with binders, and normally comes in the shape of pellets. The binders are made of thermoplastic, waxes, polymeres and other substances.  

The characterization and control of the feedstock is crucial in order to calibrate the parameters of the following processes, especially the injection and sintering, and obtain the desired tolerances and repeatability.  

Injection: Now, the feedstock is injected into the mold. The most important parameters in  this step are precision, the flow volume and temperatures (of the injection head and the mold). The parameters of the injection have to be adapted to the geometry of the desired piece. The manufacturer’s know-how is vital for obtaining intact parts without internal defects. The piece obtained in this production step is called the “green part”. During the extraction from the hollow form, the material normally contracts for the first time (0.7 – 0.9% when using carbon or low-alloy steels, about 0.4% when using stainless steels).  

De-binding: The binder that was necessary for the injection process now has to be removed. This de-binding can take place in several different ways: 100% thermical, water-based, with solvents or through a catalytic reaction. At Ecrimesa Group, we use catalytic feedstock, because it offers several advantages compared to other methods – especially the speed of the process. The catalytic reaction is produced under controlled conditions with N2 and nitric acid with a temperature of 120ºC. During this proces,s the main component of the binder, POM, passes from a solid to a gaseous state and leaves the green form in a very fragile state, called the “brown” state.  

The de-binding process can be carried out in vaccum batch furnaces or continuous furnaces. The advantages of using continuous furnaces are that manual handling is avoided and the brown state can be transferred directly to sintering. During this process, the piece is especially unstable and very susceptible to deformation and other damages, which makes the absence pf manual handling an important factor for the reliability of the process. Ecrimesa Group have both types of facilities at our disposal.  

During this phase, a small part of “secondary” binders remains in the brown state, keeping it in its shape. These secondary binders are removed an the begining of the sintering process.

Sintering: The sintering is the last steps of the process, during which the desired density is aquired by interdiffusion of the metal particles due to thermal reactions and fase (H2 or N2, depending on the type of alloy). The pieces can be sintered in vacuum batch furnaces (especially suitable for small production volumes or special materials) or in continuous furnaces (more suitable for bigger production volumes). At Ecrimesa Group, we have 3 continuous furnaces and  vacuum batch furnaces at our disposal. The positioning of the parts during the de-binding and sintering can alter the dimensional results, which is why the phase of definition and industrialisation is very important to obtain a robust and reliable process.   

During the sintering, the second and definite cotraction occurs, resulting in the final dimensions defined before the beginning of the process. With carbon and low-alloy steels, the contraction factor between mold and final piece amounts to 1.2165, and 1.205 between the green form and the final piece. With stainless steels, the contraction factor is 1.166 between mold and final piece, and 1.160 between green part and final product.  

Last steps and termination: After sintering, the product might require further operation of machining in ordr to meet the requirements of the client. Ecrimesa Group owns a modern facilty for heat treatment, allowing us to offer completely finished products. The machining can be processed in-house as well, for instance when pickling or polishing should be needed.  

Our ample experience of more than fifty years in this area enables us to find synergies between the MIM and investment casting process.