What are the methods of MIM degreasing?

MIM degreasing refers to the fact that in order to make products with complex shapes like plastic injection, we need to add materials that can be injected into metal powder, such as wax, or some plastic components. But these additives (or binders) are not what we need for our final product, so we need to remove them in some way after we have formed them. This process is called degreasing (or dewaxing).

Since the emergence of MIM technology, with the difference of binder system, a variety of MIM process paths have been formed, and there are various degreasing methods. The skimming time has been shortened from the first few days to the present few hours. From the degreasing step, all the degreasing methods can be roughly divided into two categories: one is the two-step degreasing method. Two - step degreasing method includes solvent degreasing + heat degreasing, siphon degreasing - heat degreasing and so on. One step degreasing method is mainly one step thermal degreasing method. At present, Catamold catalytic degreasing method is the most advanced. The following new materials of guangdong dahong introduced several representative MIM degreasing methods.

1   Wiech defatting

The Wiech act is represented by a patent that Wiech invented in 1980 and has been refined several times. They may be referred to as Wiech (1), Wiech (2) and Wiech (3). The binder used by Wiech is the most commonly used waxy based binder system in MIM, containing one or more components. The basic process of Wiech (1) method is as follows: first, the MIM forming blank is placed in an empty container, which is heated to the flow temperature of the binder or higher than this temperature, and then the solvent is slowly added to the container in the form of gaseous form. The gaseous solvent enters the forming blank to dissolve the binder, which dissolves to a certain extent, and the solvent solution of the binder will leak out from the forming damage. This gaseous solvent can remove most of the binder without cracking or breaking. The preform which has removed most of the binder is then dipped into the liquid solvent to remove the remaining part of the binder. Due to the pore energy channels that have been degreased by the gaseous solvent, the second step immersion solvent degreasing is very fast and does not produce cracks and defects. Finally, preheat the forming blank to remove some residual binder and some solvent, and sintering the finished product. The Wiech (1) method takes 3 days to degrease only the gaseous solvent, and the degreasing efficiency is very low. And because the degreasing temperature is higher than the flow temperature of the binder, the deformation is more serious. Wiech Wiech invented in 1981 (3) method, the basic process is: will the MIM billet in an inert gas in the container, by adjusting the temperature and gas flow rate, the billet in the binder of vapor pressure above the atmosphere pressure in the container, this binder can evaporate from forming a bad atmosphere into the container, the container has a separate part for condensation collection binder, binder removal rate can be controlled by adjusting the cooling speed. For multi-component binders, the temperature and pressure in the vessel can be adjusted to selectively eliminate phase-by-vaporization. This process takes about one day or more. Wiech Wiech invented in 1981 (2) method, using siphon degreasing as a first step, puts MIM figuration billet siphon material, slow temperature to 200 ℃ heat preservation 3 h in removing most of the binder, which is then put into the billet in an atmosphere of hydrogen atmosphere at about 3 ℃ / min rate rose to about 800 ℃ for further degreasing and sintering, the degreasing process about 10 h. In this way, Wiech actually adopted three kinds of two-step method for degreasing, first using solvent steam degreasing, then evaporation method, and later adopting siphon degreasing as the first step, and the degreasing time was reduced to 10 hours from the original 3 days. However, it has some disadvantages. The Wiech (1) method is inefficient and the forming blank is prone to deformation. The ambient pressure in the Wiech (2) degreasing furnace needs to be precisely controlled, and it is difficult for the evaporation method to work for the binder elements with high molecular weight. The Wiech (3) method has the problem of the hongresponsive material adhering to the forming blank and polluting the forming blank.

2   Injectamax method for degreasing

The AMAX metal InjectionMolding company Johnson in 1988 invented Injectamax method, the main advantage of this method is to skim quickly and won't cause crack. The binder consists of at least two kinds of components. When defatting, one solvent is selected to selectively dissolve the soluble components in the removable binder, while the insoluble components are not dissolved. This opens the pore openings and then USES heat degreasing to remove the remaining binder. The binder used in this method is generally composed of vegetable oil, paraffin and thermoplastic resin. The whole degreasing process has a short time, only 6h, which is a fast degreasing method. Due to its simplicity, low investment and high efficiency, this two-step solvent degreasing + heat degreasing method is currently adopted by most MIM companies and manufacturers.

3   Water solubilization degreasing

The water solubility method is based on the water-soluble binder developed in the 1990s, which is similar to the direct development of Injectamax two-step method (solvent degreasing + heat degreasing). Due to the toxicity and recycling problems of chemical solvents, the MIM process level can be greatly improved if the solvent can be used as cheap and pollution-free water. Cao developed a solid polymer solution in deoxygenated distilled water for about 16 hours to remove 80% of polyethylene glycol, followed by thermal degreasing to remove the remaining binder. Anwar and Yang also USES polyethylene glycol + polymethyl methacrylate adhesive system to do some work, by means of improving the water temperature to 60-80 ℃, can be in the h in addition to more than 95% of polyethylene glycol (peg). Bialo developed another form of water-soluble binder, which USES polyethylene oxide as the water-soluble part. The binder formula is 76% polyethylene oxide +23% polyethylene wax - 1% stearic acid. As water is cheap, non-toxic and pollution-free, water solubilization is an economical and environmentally friendly main degreasing method. However, water-soluble binder has water absorbing problems, which leads to the storage and transportation of MIM feedstock requires special equipment, and the polymers compatible with water-soluble parts of water-soluble binder (such as polyethylene glycol) are rare and tend to swell when mixed, so the mixing time of the feedstock is very long. So even though it's been around for five years, it's still in the lab, it's not actually used.

4   Catamold method catalyzes degreasing

The Catamold method is a MIM one-step degreasing method developed by Bloemacher of BSAF in Germany in the early 1990s, which is a catalytic degreasing method. The principle of MIM catalytic degreasing and the main technical features of the process are the rapid catalytic degreasing in an acidic atmosphere using polyaldehyde resin as the binder. The long chain polyaldehyde resin can be used as the binder to connect metal powder with the polarity of polyaldehyde resin. Atmosphere aldehyde resin in acidic catalysis decomposition of formaldehyde, the decomposition reaction occur at 110 ℃ or more fast, is a direct air - solid transformation, is beneficial to control the deformation of green, ensure that the size precision of the sintering. Catalytic degreasing is carried out at the interface of atmosphere-binder, no gas is present in the forming blank, and the reaction interface can advance at a rate of 1-4mm/h. German CREMER company for Catamold degreasing method to design a continuous degreasing and sintering furnace system, operation process is: put the MIM forming bad skim the first heating zone, and heated to 86 ℃, under nitrogen atmosphere in order to avoid in the process of the subsequent catalytic skim nitric acid condensation on the blank. The forming blank is then moved into the catalytic degreasing area and the polyaldehyde resin is decomposed into formaldehyde. After preliminary degreasing, the blank enters the sintering furnace through the first cleaning chamber and removes the remaining binder in the first heating zone of the sintering furnace. It is then sintered under the action of nitrogen, hydrogen, argon, ammonia decomposition and some other mixtures. There are also several domestic manufacturers of MIM catalytic degreasing furnace. One of the important features of the Catamold method is the use of catalyst defatting, which avoids liquid phase when defatting, and avoids the weakness of MIM products that are prone to deformation and difficult to control size accuracy. It is a major breakthrough in the MIM industry, and as a result of catalytic defatting, the defatting time is greatly shortened, thus reducing the cost. And the metamold method can produce MIM parts of larger size. With the continuous degreasing and sintering system of CREMER, the continuous production can be realized, making MIM truly a competitive PM near-net forming technology.

Catamold method is the most advanced MIM degreasing method in industrial production. However, this method has some problems such as acid atmosphere corrosion equipment and waste gas treatment, and the cost of equipment investment is higher than other methods.

For more information about powder injection molding technology, please consult guangdong da-hong metal powder injection molding MIM technology.