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Analysis of metal injection molding

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Metal injection molding

  Metal injection forming (Metal Injection Molding, abbreviation MIM) is a from plastic injection forming industry in the introduction out to of new powder metallurgical near NET forming technology, known, plastic injection forming technology low of price production various complex shape of products, but plastic products strength not high, to improved its performance, can in plastic in the added metal or ceramic powder to get strength high, and wear sexual good of products. In recent years, this idea has evolved to maximize content of solid particles and then completely remove the binder and sintering process of molding preform densification. This new method of powder metallurgy forming called MIM. Metal injection molding are the basic steps: first select meet the requirements of MIM metal powder and Binder, and then at a certain temperature, the use of appropriate methods of feeding of powder and Binder mixture evenly, after granulation in injection molding, forming billet obtained after sintering densification after degreasing treatment become finished products.

  1.MIM powder and flour milling technology

  MIM materials for higher powder, powder choice to facilitate mixing, injection molding, debinding and sintering, which are often contradictory, studies of MIM feedstock powders include: dust, particle size and particle size composition, surface, table 1 lists the most appropriate for the nature of the raw material powder for MIM.

  Because of MIM feedstock is very fine powder, MIM powder material prices are generally higher, even up to 10 times the price of traditional PM powder, which is now a key factor in limiting MIM technology is used, the current production of MIM feedstock powders methods mainly include Carbonyl, ultra high pressure water spray, high pressure gas atomization method.

  2. adhesive 

  Bond agent is MIM technology of core, in MIM in the bond agent has enhanced liquidity to for injection into cuisine and maintained billet block shape this two most basic of functions, also it also should has easily removal, and no pollution, and nontoxic sexual, and cost reasonable, features, for appeared has various of bond agent, in recent years are gradually from single by experience select to according to on skim method and the on bond agent features of requires, has pointed to design bond agent system of direction development.

  Binders are generally low molecular components polymer components plus some necessary additive compositions. Low molecular weight component low viscosity, fluidity, easy off; high viscosity polymer group, the intensity is high, keeping strength. Both a proper proportion of powder mix for high loading capacity, the resulting high precision and uniformity of product.

  3. mixing 

  Mixing metal powder is mixed with binders get uniform feeding process. Due to the nature of the feeding decided on the final performance of injection molded products, so mixing this process step is very important. This involves the binder and join the manner and order of the powder, mixing a variety of factors such as temperature, mixing characteristics of the device. This process is currently stuck at groping rely on experience level, final evaluation of mixing technology an important indicator is what has been feeding uniformity and consistency.

  MIM feedstock mix is completed under the combined effects of heat and shear forces. Mixing temperature must not be too high or binders may decompose or occur due to viscosity too low powder and Binder phase separation phenomenon, as to the shear size of the mixing way of different changes. Mixing device of the MIM standard double screw extruder, z-shaped impeller mixer, single screw extruder piston extruder, double planetary mixer mixers, double cams, these hybrid devices are suitable for preparation of viscosity in 1-1000Pa · s range of mixtures.

  Mixing methods are supposed to join a high melting point components melts, then cooled, joined the Group of low melting point, and batches to join the metal powder. Gasification of low melting point so as to prevent groups were broken up, batch add metal powder prevents torque surges resulting from cooling too quickly, reduce equipment losses.

  Filling ways for different particle size powders mix, Japan Patent introduction: first join the coarser 15-40um atomized powder bonding agent, then add 5-15um powder, and finally joined 5um powder so that contraction of the final products obtained changes very little. To powder coated evenly around a layer of adhesive, metal powders can also be added directly to a group of high-melting point, then add low-melting component, and finally removal of air. If Anwar will directly added to the stainless steel powder mixing PMMA suspensions, PEG solution is then added, and dry, then stirs edges except the air. O'connor mixed solvent, SA first mixed with the dry powder then add tetrahydrofuran solvent, then add the polymer, THF after Yat in the heat, then add powdered mix, may get a uniform feedstock.

  4. injection molding 

  Objective is to obtain the desired shape of the injection molding defect-free, uniform particle emission by MIM forming green body. As shown in Figure 1, first add the granulated feedstock is heated to a certain high temperature to make it mobile, and then injected into the mold cavity it cools down in the desired shape with a certain rigidity of the body, then remove it from the mold by MIM forming billets. This process is consistent with traditional plastic injection molding process, but because of MIM feedstock with high powder content, making it on the process parameters of injection molding process and significant differences in other, improper control is easy to produce various defects.

  5. skim 

  From MIM technology since the birth, along with different Binder systems, forming a variety of MIM technology path, degreasing method also varies greatly. Skim the first couple of days shortening up to now for a few hours. Skim on the steps all skim can roughly be divided into two broad categories: those two step method of degreasing. Two step degreasing technique including solvent degreasing + hot skim, skim-thermal siphon skim. Heat skim the skim method is primarily a step method, amaetamold method is by far the most advanced. Several typical MIM skim method described below.

  6. sintering

  Sintering is the final step in MIM technology processes, sintering eliminates pores between powder particles. Dense MIM products full or near full density. Thanks to a lot of metal injection molding Binder, sintering shrinkage when very large linear shrinkage rate is generally at 13%-25%, deformation control and precision control so that there is a problem. Especially since most of MIM products are complex shaped pieces, this problem becomes more prominent, even feeding for ultimate control of dimensional accuracy and deformation of sintered products was a key factor. Powder with high-rolling real density reduces the sintering shrinkage and is beneficial and precision control of sintering process. For products such as iron and stainless steel, sintered also has a carbon-potential control issues. Fine powder due to the current high prices and intensifying sintering technology of the research of coarse powder billet blocks is the important way to reduce the production cost of powder injection molding, metal powder injection molding technology is currently an important research study.

  MIM products due to complex shapes, sinter shrinkage, most of the products after finalization required after sintering sintered, including shaping, heat treatment (carburizing, nitriding, carbon and nitrogen infiltration, etc), surface treatment (finishing, ionic nitriding, electroplating, shot peening, etc), and so on.

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