6.2.1. General aspects

MIM is an innovative technology which allows the manufacturing of small sized and highly complex parts in a wide variety of materials. This technology combines the possibility of achieving highly complex parts, typical of the plastic injection moulding process, with the mechanical properties of the metal parts. Therefore, it can obtain simultaneously the advantages of both the injection moulding process and the powder metallurgy methods. Additionally, since it is a near-net-shape process, final machining operations are reduced or eliminated, with the associated economical advantage. The process can be applied to a wide variety of materials (low alloy steels, stainless steels, heat resistant steels,…).

This technology, which stages are shown in Figure 6.2, is based on the mixture of metal powder with a binder system that acts as a transportation means for the metal powder and allows injection moulding to obtain the desired shape of the part.

The so-called green part is as complex as it is allowed by the mould design. After injection, it is necessary to remove the binder without producing defects. This extraction process is called debinding, and it can be carried out in different ways, as it will be explained below. The part obtained after debinding process is called brown and it is composed by the metal powder and a minimum amount of binder, maintaining the geometrical shape achieved after injection moulding.

Then, the part undergoes a thermal sintering process to increase density and improve mechanical properties and hardness. This process determines the final dimensions of the part, which undergoes a contraction process that has to be previously determined.

The process can reach tolerances in the range of ±0.5% with a final density between 95 and 98%.

Basically, the process is divided in the following stages:

Therefore, in the MIM process, the following parameters must be taken into account

6.2.2. Feedstock components

For metal injection moulding, the main characteristics of the metal powder should be the following: powder particle size below 20 µm, spherical shape and a minimum density of 50% [74]. The majority of metals that can be atomized, can be processed by MIM. Aluminium is an exception, due to the oxide surface layer that makes atomization difficult. Metals that can be processed by MIM include low alloy steels, stainless steels, superalloys, intermetallics, magnetic alloys, etc

Characteristics of the  binder are critical both in the injection moulding and debinding processes, since it is the way used to maintain metal particles bound in the shape of the part. Binder characteristics to be considered include viscosity, contraction upon cooling and the possible contamination of the metal part that can be produced. The vast majority of binders are multi-component, since it is easier to produce a step-by-step debinding.

The amount of binder varies between 15 and 50% volume fraction depending on the size, shape and size distribution of the metal powder particles and the binder used. Normally, the critical amount is calculated as shown in Figure 6.3. The optimal amount is normally 2-5% below the critical amount..

Many binders have been historically used for the industrial metal injection moulding process. Basically, they can be divided in 5 types, being polymer systems the majority of them: thermoplastic systems, thermo-stable systems, water-based-systems, gel containing systems and inorganic systems.

Thermo-plastic systems have been preferentially used for industrial production, including commercial polymers like poly-etilene, poly-estirene, poly-propilene... There are many patents describing the use of these binders. As an example, J. Huggins et al. [75] describe the utilization of thermoplastic binders for ceramic injection moulding, K. Menke et al. [76] describe the utilization of a mixture of two thermoplastic polymers as a binder for metal and ceramic injection moulding, and K.F. Hens et al. [77] describe a two-component binder, with one of them being water soluble. In this case, debinding is carried out in two steps; first the water soluble component and then, the other one.

Thermo-stable systems require long time for complete polymerization; therefore, they have not been employed for industrial production. On the other hand, the green parts produced with this binder show a high mechanical strength, which allows additional mechanical operations before sintering.

6.2.3. Mixture and rheology of the mixture

Mixture is the first stage in the feedstock preparation to allow the subsequent injection moulding. The industrial process is carried out in industrial mixers working in continuous or discontinuous regime. Since it is important to control the homogeneity and viscosity of the mixture, it is very useful to determine the torque generated during the mixing process, and a capilar rheology study is recommended [78].

6.2.4. Injection moulding

Conventional plastic injection machines are used for the injection moulding process. A specific hardening treatment of different components of the machine is needed to avoid wear. Feedstock shows a different rheological and thermal behaviour than conventional polymers, thus, the following factors must be taken into account to optimize the process:

•    Viscosity of the mixture is different than the viscosity of the polymer used as binder, due to the effect of the metal powder, thus, a rheological study is a key factor.
•    Thermal conductivity of the metal powder is much higher than the one of the polymer.
•    Density of the metal is higher than density of the polymer, thus, metal powder is more sensitive to gravitational and centrifugal forces.

Due to these differences in the behaviour of a polymer and a metal-polymer mixture, injection moulding process requires an accurate optimization of the parameters (pressure, temperature, etc). Finally, since the green compact is more fragile than an injected plastic, de-moulding and handling of the parts show additional difficulties.

6.2.5. Debinding process

This is a critical stage that can be accomplished step by step or in a single stage, depending on the binder used. The most usual debinding systems are the following:

Optimization of this stage is important to avoid the presence of remains of binder which could affect the sintering process and the final characteristics of the material. When considering stainless steel, the total elimination of the binder is mandatory, since any remain of carbon could produce the sensibilization of the steel during sintering. In some other cases, the presence of extra carbon could improve the sintering process.

6.2.6. Sintering

Sintering process is based on heating the parts inside a furnace with controlled atmosphere to a temperature lower than the melting point of the parts. Since it is mandatory to avoid oxidation of the parts, reductive atmosphere is commonly used by means of nitrogen, hydrogen, argon or vacuum, depending on the material to be sintered. The appropriate atmosphere is normally selected as a function of the desired final carbon content in the part.

The aim of the sintering process is to increase density and mechanical behaviour of the parts, as well as to adjust the chemical composition. Since starting powder particles contained in the feedstock used as raw material are very small, the obtained density is higher than the one achieved with conventional powder metallurgy processes (typically, around 97%). Linked to this high density, contraction is very homogeneous, which allows tolerances around 0,1% [83].

Basically, sintering process can be divided in the following stages (Figure 6.5):

Figure 6.5 shows the equipment used for the process: injection, debinding and sintering.

(b) 

(c) 

6.2.7. Advantages and disadvantages of MIM process

MIM process shows important advantages when compared to conventional manufacturing processes like investment casting, machining or sintering.

Generally speaking, MIM offers lower design limitations, better dimensional tolerances, lower surface roughness, higher automation capacity and lower lead time than the investment casting process. With regards to machining processes, MIM costs are lower for high volume series or highly complex parts. Finally, MIM allows lower porosity than conventional sintering, reaching densities in the order of 95-98%, whereas sintering processes allow density values below 80-85%.

Therefore, MIM process is specially suitable for the production of big series of highly complex parts with accurate dimensional tolerances and good surface quality. From this point of view, since the most important limitation of the technology is related to the size of the parts and the need of a minimum volume of the series that justifies the process costs, alternative technologies are suitable when bigger parts or lower volume series are to be produced. Table 6.1 shows a comparison of the characteristics of these processes.