What are the methods of degreasing MIM?

What are the methods of degreasing MIM?

MIM degreasing means that in order to make products with complex shapes like plastic injection, we need to add injectable materials, such as wax, or some plastic ingredients, into the metal powder. However, these additives (or binders) are not required for our final products, so we need to remove these binders in some ways after molding. This process is called degreasing (or dewaxing).

Since the emergence of MIM technology, with the different binder systems, a variety of MIM process paths and degreasing methods have been formed. Degreasing time has been shortened from the first few days to the present few hours. In terms of degreasing steps, all degreasing methods can be roughly divided into two categories: one is two-step degreasing method. Two step degreasing method includes solvent degreasing+thermal degreasing, siphon degreasing - thermal degreasing, etc. One step degreasing method is mainly one step thermal degreasing method. At present, the most advanced method is Catamold catalytic degreasing method. Here are some typical MIM degreasing methods.

1 Degreasing by Wiech method

Wiech method is represented by the patent invented by Wiech in 1980 and has been improved several times. They can be called Wiech (1), Wiech (2), and Wiech (3) methods. The binder used by Wiech is the most commonly used wax based binder system in MIM, which contains one or more components. The basic process of Wiech (1) method is: first, place the MIM forming blank in an empty container, heat it to the flowing temperature of the binder or above this temperature, and then slowly add the solvent in the form of gas into the container where the forming is located. The gaseous solvent enters the forming blank to dissolve the binder. When the binder is dissolved to a certain extent, the solvent solution of the binder will seep out from the forming failure. The gaseous solvent can remove most of the binder without cracking or fracture. The preform with most of the binder removed is immersed in the liquid solvent to remove the remaining part of the binder. Due to the pore energy channel formed by gas solvent degreasing, the second step of immersion solvent degreasing is very fast, and no cracks and defects will occur. Finally, the preform is preheated to remove part of the residual binder and part of the solvent, and the finished product is obtained by sintering. The Wiech (1) method only takes 3 days to degrease with gaseous solvent, and the 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: put the MIM preform in an inert gas container, and adjust the temperature and gas flow to make the vapor pressure of the binder in the preform higher than the pressure of the atmosphere in the container, so that the binder can evaporate out of the forming process and enter the atmosphere of the container. An independent part in the container is used to condense and collect the binder, The binder removal speed can be controlled by adjusting the condensation speed. For multi-component adhesives, they can also be selectively and gradually evaporated 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. Siphon degreasing was adopted as the first step. The MIM preform was placed on the siphon material, slowly heated to 200 ℃ for 3h to remove most of the binder, and then the preform was put into the furnace in an atmospheric hydrogen atmosphere at a rate of about 3 ℃/min to about 800 ℃ for further degreasing and pre sintering. The whole degreasing process took about 10h. In this way, Wiech actually uses three forms of two-step degreasing, first solvent vapor degreasing, then evaporation, and then siphon degreasing as the first step, reducing the degreasing time from the first three days to 10 hours. However, other people have some shortcomings. Wiech (1) method is inefficient and the forming billet is easy to deform. The atmosphere pressure in the degreasing furnace of the Wiech (2) method needs to be accurately controlled, and the evaporation method is difficult to work for the binder component with large molecular weight. The Wiech (3) method has the problems that the rainbow response material adheres to the formed blank and pollutes the formed blank.

2 Injectamax degreasing

Johnson of American AMAX metal injection molding company invented the Injectamax method in 1988. The main advantage of this method is that it is fast in degreasing and will not cause cracks. The binder consists of at least two components. When degreasing, a solvent is selected to selectively dissolve the soluble components in the binder first, while the insoluble components are insoluble. In this way, the pore channel is opened, and then the remaining binder is removed by thermal degreasing. The binder used in this method is generally composed of vegetable oil, paraffin and thermoplastic resin, and the oil and paraffin are removed first with trichloroethane solvent. The whole degreasing process takes only 6 hours and is a fast degreasing method. This two-step process of solvent degreasing and thermal degreasing is a production method adopted by most MIM companies and manufacturers at present due to its simplicity, low investment and high efficiency.

3 Degreasing by water dissolution

The water dissolution 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+thermal degreasing). Due to the problems of toxicity and recovery of chemical solvents, if cheap and pollution-free water can be used as the solvent, the level of MIM process can be greatly improved. Cao has developed a solid polymer solution that can remove 80% of polyethylene glycol by soaking in deoxidized distilled water for about 16h, and then remove the remaining binder by thermal degreasing. Anwar and Yang also did some work with the polyethylene glycol+polymethylmethacrylate binder system. By increasing the water temperature to 60-80 ℃, more than 95% of the polyethylene glycol can be removed at this hour. Bialo has developed another form of water-soluble binder, which uses polyoxyethylene as the water-soluble part. Its binder formula is 76% polyoxyethylene+23% polyethylene wax - 1% stearic acid. Most of the polyoxyethylene can be removed by soaking the preform in water for 60-70 minutes. Because water is cheap, non-toxic and pollution-free, water dissolution is the most economic and environmentally beneficial degreasing method. However, the water soluble binder has the problem of water absorption, which leads to the need for special devices for the storage and transportation of MIM feed. Moreover, there are few polymers compatible with the water-soluble part (such as polyethylene glycol) of the water-soluble binder, which is prone to swelling during mixing, and the feed mixing time is very long. Therefore, although the insoluble method has been introduced for five years, it is still in the laboratory stage and has not been used in actual production.

4 Catamold catalytic degreasing

Catamold method is a one-step degreasing method of MIM developed by Bloemacher of BSAF in Germany in the early 1990s. It is a catalytic degreasing method. The working principle of MIM catalytic degreasing and the main technical feature of this method are that the polyformaldehyde resin is used as the binder and the catalytic degreasing is fast in acid atmosphere. The long chain polyformaldehyde resin is used as the binder, and the polarity of the polyformaldehyde resin is used to connect the metal powder, which can be suitable for a wide range of powder types. Polyaldehyde resin decomposes into formaldehyde under the catalysis of acidic atmosphere. This decomposition reaction occurs rapidly above 110 ℃. It is a direct gas-solid transition, which is conducive to controlling green deformation and ensuring the dimensional accuracy after sintering. The catalytic degreasing is carried out at the interface between atmosphere and binder. There is no gas in the forming blank, and the advancing speed of the reaction interface can reach 1-4mm/h. CREMER of Germany has designed a continuous degreasing and sintering furnace system for Catamold degreasing method. The operation process is: place the MIM forming failure in the first heating zone of degreasing, and heat it to 86 ℃ under nitrogen atmosphere to avoid nitric acid condensation on the billet in the subsequent catalytic degreasing process. Then the preform is moved into the catalytic degreasing area to decompose the polyformaldehyde resin into formaldehyde. After initial degreasing, the billet enters the sintering furnace through the first cleaning chamber, and the residual binder is removed in the first heating zone of the sintering furnace. Subsequently, sintering is carried out under the action of nitrogen, hydrogen, argon, decomposed ammonia and some other mixtures. At present, there are several manufacturers of MIM catalytic degreasing furnace in China. An important feature of Catamold method is that catalyst is used for degreasing, and liquid phase does not appear during degreasing, which avoids the weakness of MIM products that are prone to deformation and difficult to control size accuracy. It is a major breakthrough in MIM industry, and because it is catalytic degreasing, the degreasing time is greatly shortened, thus reducing the cost. And the application of metamold method can produce larger size MIM components. The continuous degreasing and sintering system of CREMER can realize continuous production, making MIM a competitive PM near net forming technology.

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

If you want to know more about metal powder injection molding technology, please consult ZHONGWEI Precision Metal Powder Injection Molding MIM Technology in Qinhuangdao