The MIM ProcessA schematic representation of the MIM process is given in Fig. 1. It may be broken down into the following steps:
- Selection of the powders and binders and mixing of the feedstock
- Making of the mold and injection molding of the part
- Removing the primary binder(s)
- Removing the secondary binder(s) and sintering the part
To make consistent parts, the feedstock must be consistently reproducible. This means the powder, the binder and the ratio between the two must be the same. All of the major MIM manufacturers in the U.S. historically made their own feedstock, and they were very secretive regarding their process.
Making consistent feedstock is very difficult, and this has been the stumbling block for many former entrants into this field. Today, however, there are several feedstock manufacturers that sell consistent feedstock, which may be easily processed by following the feedstock manufacturers’ instructions on molding, debinding and sintering. This development makes it relatively easy for new participants to enter and be successful in the field of manufacturing MIM parts.
Molds and Molding
Mold makers and molders have adapted to MIM relatively easily. The feedstock is like a highly filled plastic. Many molders have experience with filled plastics, and they readily adapt to the MIM feedstock. Minor modifications are required to the molding equipment, and there are several molding-machine manufacturers with a large amount of experience in this area. As more plastic injection-molding and investment-casting houses move their manufacturing to low-labor-cost countries, a large number of mold makers and molders are looking for new work, and there is a surplus of talent in this area.
Techniques to remove the primary binder depend on the binder system being used to manufacture the parts. There are three basic techniques for removing binders:
- Thermally debind the parts to evaporate or burn the primary binder at a low temperature.
- Thermally debind the parts using a catalyst to disassociate and break down the primary binder to smaller chains that volatilize easily.
- Dissolve the primary binder in a solvent, which may be a halogenated solvent, a hydrocarbon solvent or an aqueous solvent, depending on the binder system used.
Secondary Debinding and Sintering
In the original MIM process – after removing the primary binders by a thermal debinding process – the parts would be debound and pre-sintered in a hydrogen furnace to about 1100°C (2012°F). They are then removed and placed in a vacuum furnace that reached the sintering temperature around 1300°C (2372°F). The secondary binder had to be cleaned out from the binder traps after every couple of runs in the pre-sintering furnace. The complete debinding and sintering process took around three days, depending on the size of the parts.
The process was reduced to about two days by using a bell-shaped, Inconel retort furnace where debinding and sintering took place. The complete cycle could be performed under hydrogen, but this furnace was temperature limited by the Inconel retort to less than 1250°C (2282°F). Graphite furnaces are used, but these are limited to vacuum or partial pressures of nitrogen and cannot process stainless steels to their optimum properties. Furnace builders have adapted batch refractory-metal sintering furnaces for MIM, but these have poor temperature uniformity or poor gas flow. Also, the traps for all of these furnaces require frequent cleaning.
For these reasons, material properties were poor – comparable to cast parts – at best. There were also wide dimensional variations. It was said that the sintering process was too complicated, and it was deemed suitable for only small parts with low tolerance requirements. Since the sintering furnace is the largest investment ticket and sintering is “difficult to understand,” newcomers are hesitant to enter this technology.
New Developments in MIM Furnace TechnologyJust as the availability of consistent feedstock from reliable suppliers has simplified the molding process, one furnace manufacturer makes the sintering process as simple as following a set of rules. This batch furnace has the following attributes:
- The work zone is made from refractory metals, which permits the use of hydrogen, nitrogen, argon or vacuum, or a combination of these atmospheres during the different steps using various partial pressures. This gives the processor the flexibility to process iron and steels, stainless steels, copper alloys, nickel- or cobalt-based alloys and titanium-based alloys in the same furnace.
- It is completely computer controlled. The sintering programs are established by using an Excel spreadsheet. Once established, they may be recalled at will. Figure 2 shows the main computer control screen.
- Laminar gas flow is used inside the retort to distribute the gases uniformly throughout the work zone and pull the effluents into the binder trap. Laminar gas flow also results in a more uniform temperature inside the work zone.
- The use of a dry pump and a special binder trap allow the binder to be easily collected in the traps. A special program heats the binder to drain out of the traps as needed (sidebar). This eliminates the need for vacuum leak checks after opening and cleaning the binder traps, and it reduces maintenance downtime.
- A special center thermocouple is used to determine when the furnace temperature is equated and to time the debinding and sintering events.
- Calibrated thermocouples are used, and the special Accutemp program uses the calibration data to correct the temperature reading displayed. This eliminates the temperature differential caused by thermocouple inaccuracies.
- Service technicians from the furnace-manufacturing company can verify operations remotely, using a modem, if the furnace is connected to a dedicated phone line.
ConclusionsTwo major developments have made it possible to keep the MIM industry growing at double-digit rates. One is the availability of ready-to-mold, premixed feedstock of consistent quality that can be processed per the manufacturer’s instructions. The other is a batch furnace with sophisticated computer control of the major sintering parameters and process functions. It is projected that this industry will continue to grow at 8-15% a year in most areas of the world. IH
For more information: Claus J. Joens is president of Elnik Systems, Div. of PVA MIMtech, LLC, 107 Commerce Road, Cedar Grove, NJ 07009, USA. Tel: 973-239-6066 ext 12; fax: 973-239-3272; e-mail: email@example.com; web: www.elnik.com
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