1. Wiech process
The Wiech process is represented by a patent invented by Wiech in 1980 and has been improved several times. The basic adhesive system of Wiech process is composed of paraffin wax and thermoplastic resin. The binder and powder are mixed on a “Z” or “Σ” type blade shear device. The initial degreasing process is a two-step process in which the MIM forming billet is first placed in a vacuum container and heated to or above the flow temperature of the binder, and then the solvent is slowly added in gaseous form to the container in which the forming billet is located. The gaseous solvent enters the forming billet to dissolve the binder, and when dissolved to a certain extent, the solvent solution of the binder will exude from the forming billet. The forming blank with most of the binder removed is immersed in a liquid solvent to remove the remaining binder. Finally, the forming billet is preheated to remove some of the residual binder and solvent, and the finished product is sintered. The process takes 3 days to degrease the gaseous solvent alone
The efficiency of degreasing is very low. As the degreasing temperature is higher than the flow temperature of the binder, the deformation is serious. The process was later changed to all degreasing by thermal decomposition and the degreasing time was much reduced. In the initial development of MIM industry, most companies adopted Wiech process. Until now, some companies still use Wiech process for production. However, Wiech process has problems such as high stress of injection green blank, easy to crack and deformation, and it is difficult to produce parts with larger size.
2 Injectamax process
Johnson of AMAX Metal Injection Molding USA invented the Injectamax process in 1988. The main advantage of the process is that its binder consists of paraffin, vegetable oil, and polymer. Vegetable oil is liquid when injected and cooled, which makes the volume of the forming billet change little before and after injection forming, and alleviates the internal stress in the forming billet. When degreasing, a solvent such as trichloroethane is used to selectively dissolve vegetable oil and paraffin wax first, while insoluble components are not dissolved. This opens the pore channel and then removes the remaining binder by thermal degreasing. The whole degreasing process time is short, only 6h, is one
A faster method of degreasing. Due to its simplicity, low investment and high efficiency, the solvent degreasing and thermal degreasing two-step process is currently adopted by most MIM companies and manufacturers.
3 Metamold method
Metamold process is a MIM catalytic degreasing process developed by Bloemacher equal of BSAF in Germany in the early 1990s. The main technical characteristic of this method is that it uses polyether resin as binder and catalyzes degreasing quickly in acidic atmosphere. The use of long chain polyether resin as binder, using polyether resin polarity to connect metal powder, can be suitable for a wide range of powder types. Formaldehyde is decomposed into formaldehyde under the catalysis of acid atmosphere. This decomposition reaction occurs rapidly above 110 ℃, which is a direct gas-2 solid transition. BASF initially used nitric acid as a catalyst, which catalyzes degreasing at a lower level than polymerization
The softening temperature of aldehyde resin avoids the formation of liquid phase, which is beneficial to control the deformation of green billet and ensures the dimensional accuracy after sintering. The catalytic degreasing is carried out at the interface of the binder in atmosphere 2. There is no gas in the forming billet, and the advancing speed of the reaction interface can reach 1 ~ 4mm/ h. BASF later developed a new method of using oxalic acid as a catalyst, which can be applied to hard alloys and ceramics, expanding the application of Metamold process. An important feature of Metamold method is that it adopts catalyst degreasing, and no liquid phase appears during degreasing, which avoids the weakness that MIM products are prone to deformation and dimensional accuracy control is difficult. It is a major breakthrough in the MIM industry, and because it is catalytic degreasing, the degreasing time is greatly shortened and the cost is reduced. Large size MIM parts can be produced by Metamold method. With CREMER’s continuous degreasing and sintering system, continuous production is possible, making MIM a truly competitive PM near-clean forming technology. However, this method has some problems such as acid atmosphere corrosion equipment and waste gas treatment, and the cost of primary equipment investment is relatively high.
Hens of the Us-Based company Thermal Precision Technology has developed a new process called precision metal powder injection moulding (PPIM). The company claims that using this process will propel MIM technology into a new era of low cost, high precision, high yield and environmental adaptation. The main technical characteristics of PPIM process are embodied in two aspects, one is the use of water-soluble binder. The binder consists of polyethylene glycol (PEG) as the first element and a crosslinked polymer such as polyvinyl butyral (PVB) as the second element. So degreasing can be divided into two steps, the first is removed by water dissolution methodPEG, where PVB remains in a cross-linked solid state. On the other hand, the PPIM process adopts the metal powder composed by the deployment of wide particle size, using coarse powder, fine powder collocation, so that the loading of powder in the feed is increased, for stainless steel powder can reach 74% (volume fraction), greatly reducing the size shrinkage. Due to these two improvements, the dimensional accuracy of the product can reach ±011% by using PPIM process, which is the highest among all the MIM processes at present.
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