Best describes the method of MIM degreasing
time：2020-06-01Views：467 Author：Best Seiko
MIM degreasing means that in order to be able to make a variety of products with complex shapes like plastic injection, we need to add injectable materials, such as wax, or certain plastic ingredients to the metal powder. However, these additives (or binders) are not required for our finished products, so we need to remove these binders in some way after molding. This process is called degreasing (or dewaxing). Since the creation of MIM technology, with the different binder systems, a variety of MIM process paths have been formed, and degreasing methods are also diverse. The degreasing time has been shortened from the first few days to the current hours. From the degreasing step, all degreasing methods can be roughly divided into two categories: one is the two-step degreasing method. The two-step degreasing method includes solvent degreasing + thermal degreasing, siphon degreasing-thermal degreasing, etc. The one-step degreasing method is mainly a one-step thermal degreasing method, and the currently advanced one is Catamold catalytic degreasing method. The following Shenzhen Best Editor introduces several representative MIM degreasing methods. 1 Wiech method degreasing: The Wiech method is represented by the patent invented by Wiech in 1980 and has undergone several improvements. It can be called Wiech (1), Wiech (2) and Wiech (3) method. The adhesive used by Wiech is a wax-based adhesive system commonly used in MIM, containing one or more components. The basic process of the Wiech (1) method is: first place the MIM forming blank in an empty container, heat it to the flow temperature of the binder or higher than this temperature, and then slowly add the solvent in the gaseous form to the forming bad Inside the container. The gaseous solvent enters the forming blank to dissolve the binder, and to a certain extent, the solvent solution of the binder will seep out of the forming failure. This gaseous solvent can remove most of the binder without cracking or breaking. The shaped blank from which most of the binder has been removed is immersed in a liquid solvent to remove the remaining part of the binder. Due to the pore channels that have been formed by gaseous solvent degreasing, the second step of immersion solvent degreasing is very fast, and no cracks and defects will occur. Then, the preform is preheated to remove some residual binder and some solvents, and sintered to obtain the finished product. Wiech (1) method of gaseous solvent degreasing takes 3 days, degreasing efficiency is very low. And because the degreasing temperature is higher than the binder flow temperature, the deformation is more serious. Wiech invented the Wiech (3) method in 1981. The basic process is: placing the MIM preform in an inert gas container. By adjusting the temperature and gas flow, the vapor pressure of the binder in the preform is higher than that in the container Atmospheric pressure, so that the binder can evaporate from the formation and enter the container atmosphere. There is a separate part in the container to condense and collect the binder. The binder removal rate can be controlled by adjusting the condensation speed. For multi-component binders, it can also be gradually eliminated by adjusting the temperature and pressure in the container. This process takes about one day or more. Wiech invented the Wiech (2) method in 1981, using siphon degreasing as the first step, placing the MIM forming blank on the siphon material, slowly heating to 200°C for 3 hours to remove most of the binder, and then forming The billet is placed in a furnace in a hydrogen atmosphere at atmospheric pressure at a rate of about 3°C/min to about 800°C for further degreasing and pre-sintering. The entire degreasing process takes about 10 hours. In this way, Wiech actually uses three forms of two-step degreasing, first using solvent vapor degreasing, then evaporation, and then using siphon degreasing as the first step. The degreasing time has also been shortened from the initial 3 days to 10 hour. However, all of them have some shortcomings, the Wiech (1) method is inefficient, and the formed billet is easily deformed. The Wiech (2) method requires precise control of the atmospheric pressure in the degreasing furnace, and for the sale of large molecular weight binder components, the evaporation method is difficult to work. The Wiech (3) method has the problems that the rainbow response material adheres to the forming blank and contaminates the forming blank. 2 Injectamax degreasing: Johnson of AMAX metal injection company in the United States invented the Injectamax method in 1988. The main advantage of this method is that the degreasing speed is fast and does not cause cracks. The binder is composed of at least two kinds of components. When degreasing, a solvent is used to selectively dissolve and remove soluble components in the binder, while insoluble components are not dissolved. This opens the pore channels and then uses thermal degreasing to remove the remaining binder. The binder used in this method is generally composed of vegetable oil, paraffin and thermoplastic resin, and the solvent of trichloroethane is used to remove the oil and paraffin first. The entire degreasing process takes only 6 hours, which is a fast degreasing method. This two-step method of solvent degreasing + thermal degreasing is the production method currently adopted by most MIM companies and manufacturers due to its simplicity, low investment and high efficiency. 3 Degreasing by water dissolution method: The water dissolution method is based on the water-soluble binder developed in the 1990s. It is similar to the direct development of the Injectamax two-step method (solvent degreasing + thermal degreasing). Due to the problems of toxicity and recycling of chemical solvents, if cheap, pollution-free water can be used as a solvent, the MIM process level can be greatly improved by one step. Cao developed a solid polymer solution in deoxygenated distilled water and soaked it for about 16 hours to remove 80% of polyethylene glycol, and then used thermal degreasing to remove the remaining binder. Anwar and Yang have also done some work using the polyethylene glycol + polymethyl methacrylate binder system. By increasing the water temperature to 60-80°C, more than 95% of the polyethylene glycol can be removed in this h. Bialo has developed another form of water-soluble binder, which uses polyethylene oxide as the water-soluble part, and its binder formula is 76% polyethylene oxide + 23% polyethylene wax-1% stearic acid, forming The billet only needs to be immersed in water for 60-70 minutes to remove most of the polyethylene oxide. Because water is cheap, non-toxic, and non-polluting, water dissolution is an economical and environmentally friendly main method of degreasing. However, the water-soluble binder has a problem of water absorption, which causes the storage and transportation of the MIM feed requires special equipment, and the polymer compatible with the water-soluble part of the water-soluble binder (such as polyethylene glycol) is few and mixed. Swelling tends to occur during refining, and the mixing time of the feed is very long. So although the insolubilization method has been available for five years, it is still in the laboratory stage so far, and has not been used in actual production. 4 Catamold method catalytic degreasing: Catamold method is a one-step MIM degreasing method developed by Bloemacher of German BSAF company in the early 1990s, which is a catalytic degreasing method. The working principle of MIM catalytic degreasing and the main technical characteristics of the method are the use of polyacetal resin as a binder and rapid catalytic degreasing in an acidic atmosphere. The use of long-chain polyacetal resin as a binder, the use of polyacetal resin to connect metal powder, can be suitable for a wide range of powder types. The polyacetal resin decomposes into formaldehyde under the catalytic action of an acidic atmosphere. This decomposition reaction occurs rapidly above 110 ℃. It is a direct gas-solid transformation, which is beneficial to control the deformation of the green body and ensure the dimensional accuracy after sintering. Catalytic degreasing takes place at the interface between the atmosphere and the binder. There is no gas inside the preform, and the advancing speed of the reaction interface can reach 1-4 mm/h. The German CREMER company designed a continuous degreasing and sintering furnace system for the Catamold degreasing method. The operation process is: the MIM forming is placed in the first heating zone of the degreasing and heated to 86 ℃ under a nitrogen atmosphere to avoid subsequent During the catalytic degreasing process, nitric acid condenses on the billet. Then move the preform into the catalytic degreasing zone to decompose the polyacetal resin into formaldehyde. After preliminary degreasing, the billet enters the sintering furnace through the first clean room, and the residual binder is removed in the first heating zone of the sintering furnace. Subsequently, it is sintered under the action of nitrogen, hydrogen, argon, decomposed ammonia, and some other mixtures. There are currently several domestic manufacturers of MIM catalytic degreasing furnaces. An important feature of the Catamold method is the use of catalyst for degreasing. No liquid phase appears during degreasing, which avoids the weakness of MIM products that are easy to deform and difficult to control dimensional accuracy. It is a major breakthrough in the MIM industry, and because it is catalytic degreasing, it is greatly shortened. Degreasing time, thereby reducing costs. And the application of metamold method can produce larger size MIM parts. The continuous degreasing and sintering system of CREMER can realize continuous production, making MIM truly a competitive near-net forming technology for PM. The Catamold method is a very advanced MIM degreasing method currently used in industrial production. However, this method has the problems of acidic atmosphere corrosion equipment and exhaust gas treatment, and the equipment investment cost is higher than other methods.