a proven industrial process
suitable for 3D printing
PiM feedstock fabrication
Compared to the forge or the foundry of metal parts, MIM (Metal injection moulding) is very recent, this technique belongs to the family of PIM's (Particle Injection Moulding).
Work was initiated in the United States in the 1920's on ceramic (CIM - Ceramic injection Moulding). The original idea was to plasticize powders, that is, to give them plastic properties by coating them with organic matter. This thermoplastic behavior "composite" is subsequently formed using a press and an injection mould to obtain a more or less complex part. During the Second World War, work was extended on metallic powder, it was not until the 1970s that industrial applications of the MIM process surfaced.
Since then, the MIM market has grown enormously and covers a wide range of industrial applications, such as the automobile, watchmaking, defense, aerospace, medical (prostheses, implants, probes, etc.), connectors, etc. In short, we all cross paths in our daily lives and without being aware of the parts resulting from the MIM processes.
PiM process is an indirect part manufacturing process, that is to say; yo obtain a 100% metal or ceramic part it is necessary to post-treat the formed parts. Overall, the PIM process is broken down into three main stages.
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The first step in the manufacture of PiM parts is done either by a conventional process by injecting a feedstock into a mould under pressure using an injection press, or by using compatible metal additive manufacturing technologies with these materials.
The part thus produced is a preform, called "green part". The level of binder contained in the green parts varies according to its chemical nature and the powders used; the percentage of binder is generally between 35 and 50% by volume.
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This first post-process is a key operation of the PIM process, it makes it possible to prepare the parts for the sintering cycle and consists in removing approximately 98% of the organic binders contained in the “green part”. The residual binder is necessary for handling the debinded piece, called "brown part" and allows it to ensure good stability during the sintering process.
The quality of this operation is fundamental so as not to cause physical (cracking) or chemical (carburetion) degradation to the part. A very large part of the defects which appear after sintering is generated by unsuitable debinding.
Depending on the chemical nature of the binder, debinding can be carried out catalytically, thermally or by solvent. At the end of the debinding cycle, the part has not undergone any dimensional change (shrinkage).
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A pre-sintering cycle, also called 2nd debinding, is generally carried out before sintering. Its objective is to remove the residual organic binder retained at the end of the debinding cycle. This is followed by a heating cycle, called sintering, allowing the metal particles to be welded together, it is during this stage that the mechanical properties are given to the final part, called "sintered part". This operation takes place at a temperature close to the melting point of the treated metal, under a controlled atmosphere and sometimes under vacuum.
The gaseous mixture composing the sintering atmosphere is specific for each metal to be sintered, it aims to reduce, among other things, the oxide present on the surface of the powders. The shrinkage accompanying the sintering is important, controlled and isotropic.
Frequently Asked Questions
Note that almost all the alloys that can be atomized in powder form can be treated with MIM and therefore with PAM.
These alloys can be classified into four categories:
- Ferrous alloys: steels, stainless steels, tool steels, magnetic iron-nickel alloys and special ferrous alloys;
- Tungsten alloys: heavy alloys of tungsten and tungsten-copper;
- Hard materials: cemented carbides (tungsten carbide) and cermets (composite material composed of a ceramic reinforcement (Cer) and a metallic matrix (Met));
- Special materials including precious metals (silver, copper, gold, etc.), titanium alloys, aluminum, chromium-cobalt, nickel, nickel-based superalloys, molybdenum, and particulate composites.
In short, PiM offers a very wide range of usable materials and they are compatible with PAM. For this, the user can adjust the machine parameters specific to the chosen PiM feedstock and make the production of reproducible parts.
Conversely, the use of a so-called “open” system on materials and compatible with industrial granules will allow the user to benefit from the entire MIM material catalog already available, at its lowest cost and not be dependent on a supplier, etc.
What are the main advantageous to use MIM-Like 3D printing technologies rather than conventional powder bed laser fusion processes?
Indeed, the handling and the operability of FDM systems are very accessible. The initial investments, maintenance and operating costs are relatively low compared to DMLS / SLM solutions. In addition, the powder being plasticized, it is not volatile. The powder therefore does not represent any health and safety risk unlike conventional solutions with laser fusion on a powder bed, where it is "free".
Compared to a DMLS / SLM solution, a Pam Series M machine, dedicated to MIM-Like applications, represents:
- an initial investment 5 to 10 times lower,
- the lowest consumable cost on the market, on average 10 times lower,
- no investment in specific equipment and infrastructure (clean room, safety suits, ventilation, etc.).
From an investment point of view, it is of course necessary to consider the installations already in place, indeed, the production of MIM parts requires post-processing equipment. For a company that already has this equipment, investing in a Pam Series M is particularly competitive. Conversely, investments in a debinding station and a sintering furnace can be expensive and require expertise to qualify them in relation to the applications and materials envisaged. Even if there are so-called "office" equipment, representing moderate investments; they remain limited and do not allow optimum processing of the materials.
- Wall thickness: it is advisable to design parts with uniform wall thicknesses in order to avoid distortion and cracks. Sudden variations in these thicknesses can cause variations in shrinkage during sintering, making dimensional control difficult. The greater the wall thickness, the longer and more uncertain the debinding cycle is.
- Support structure: it is preferable for PIM parts to design a flat surface on which they can be positioned for sintering and to avoid their collapse. Support structures can be created to minimize sagging of the parts during the sintering process.
- Shrinkage: the "brown part" obtained after the debinding cycle has a porous structure, due to the elimination of the organic binder. During the sintering process, the powder particles are brought to a temperature close to their melting points allowing the powders to get closer to each other, thus reducing the quantity of pores present in the part. Simultaneously, a dimensional shrinkage occurs, it is due to the elimination of the porosity in the powder agglomerate. This shrinkage is linear, that is to say identical in the three directions, its value is a function of the initial volume content of organic binder, it is between 12 and 20%. The shrinkage coefficient is a value communicated by the PIM materials suppliers. It has to be anticipated during the part design phase by applying this multiplying coefficient to the preform.