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Metal Injection Molding Introduction
Metal Injection Molding (MIM) is powder metallurgy near-net forming technology derived from the plastic injection molding industry, it is known that plastic injection molding technology low price to produce a variety of complex shape products, but plastic products are not high strength, in order to improve its performance, you can add metal or ceramic powder to plastic to In order to improve their performance, metal or ceramic powders can be added to plastics to obtain products with higher strength and good wear resistance. In recent years, this idea has evolved to maximize the content of solid particles and completely remove the binder, and densify the forming blank during the subsequent sintering process. This new method of powder metallurgy forming is called metal injection molding.
Metal Injection Molding is used in a wide variety of automotive, medical, electronic, industrial, and consumer industries for products such as automotive parts, aerospace equipment, cell phones, dental instruments, electronic heat sinks and sealing packages, electronic connector hardware, industrial tools, fiber optic connectors, spray systems, disk drives, medical devices, handheld power tools, surgical instruments, and sports equipment.
Metal Injection Molding Process
The basic process steps of metal injection molding are firstly, the metal powder and binder are selected to meet the requirements of MIM, then the powder and binder are mixed into a homogeneous feed at a certain temperature using appropriate methods, after granulation in injection molding, the resulting molding blank is sintered and densified after degreasing treatment to become the final product.
1: MIM powder and powder making technology
MIM raw material powder requirements are high, the choice of powder to facilitate mixing, injection molding, degreasing, and sintering, which is often contradictory, the study of MIM raw material powder including: powder shape, particle size and particle size composition, specific surface, etc., Table 1 lists the most suitable for MIM with the properties of raw material powder.
As MIM raw material powder requirements are very fine, MIM raw material powder prices are generally higher, some even reach 10 times the price of traditional PM powder, which is a key factor limiting the widespread use of MIM technology, the production of MIM with raw material powder methods are mainly carbonyl method, ultra-high pressure water atomization method, high-pressure gas atomization method, etc.
Binder is the core of MIM technology, in MIM binder has to enhance the fluidity to suit the injection molding and maintain the shape of the blank, the two most basic functions, in addition it should also have easy to remove, non-pollution, non-toxic, reasonable cost and other characteristics, for this reason there are a variety of binder, in recent years is gradually selected from experience alone to the method of degreasing and the requirements of the binder function, targeted The direction of the design of the binder system.
The binder is generally composed of low molecular components and high molecular components plus some necessary additives. Low molecular component viscosity, good fluidity, easy to remove; high molecular component viscosity, high strength, to maintain the strength of the forming blank. The appropriate ratio of the two together to obtain high powder loading capacity, and finally get high precision and high uniformity of the product.
Mixing is the process of mixing metal powder with binder to get uniform feeding material. Since the nature of the feed material determines the performance of the final injection molded product, the mixing process step is very important. It involves various factors such as the way and sequence of binder and powder addition, mixing temperature, and the characteristics of the mixing device. This process step has remained at the level of experience, and ultimately, an important indicator to evaluate the mixing process is the uniformity and consistency of the resulting feed.
The mixing of the MIM feed is done under the combined effect of heat and shear. Mixing temperature cannot be too high, otherwise the binder may decompose or due to the viscosity is too low and the separation of the two phases of powder and binder, as for the size of the shear force varies depending on the mixing method. MIM commonly used mixing devices are double screw extruder, Z-shaped impeller mixer, single screw extruder, plunger extruder, double planetary mixer, double cam mixer, these mixing devices are suitable for Preparation of viscosity in the range of 1-1000Pa-s mixer.
The method of mixing is generally to add high melting point components to melt first, then cool down, add low melting point components, and then add metal powder in batches. This can prevent the vaporization or decomposition of the low melting point group element and adding metal powder in batches can prevent the torque surge caused by cooling down too quickly and reduce equipment losses.
For the way of adding different particle size powder when matching, the Japanese patent introduces: the course 15-40um water atomized powder is added to the binder first, then the 5-15um powder is added, and finally the powder ≤5um powder is added, so that the final product obtained has little change in shrinkage.
To evenly coat a layer of binder around the powder, the metal powder can also be added directly to the high melting point component, then add the low melting point component, and finally remove the air can be. Such as Anwar PMMA suspension directly into the stainless-steel powder mix, then add the PEG aqueous solution, dry, and then remove the air while stirring. O’Connor use solvent mixing, first SA and powder dry mix then add tetrahydrofuran solvent, and then add the polymer, tetrahydrofuran in the heat to escape, and then add the powder mix, you can get a uniform feeding.
4: Injection molding
The purpose of injection molding is to obtain the desired shape of defect-free, uniform particle row by the MIM forming blanks. First, the granular feed is heated to a certain high temperature to make it flowable, then it is injected into the mold cavity and cooled down to get the desired shape of a certain rigidity of the blank, and then it is removed from the mold to get the MIM molding blank. This process is the same as the traditional plastic injection molding process, but due to the high powder content of the MIM feed, making its injection molding process in the process parameters and some other aspects are very different, improper control is prone to a variety of defects.
Since the emergence of MIM technology, with the different binder systems, the formation of a variety of MIM process paths, degreasing methods are also varied. Degreasing time has been shortened from a few days at the beginning to a few hours. All degreasing methods can be roughly divided into two categories in terms of degreasing steps: one is the two-step degreasing method. The 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, and the most advanced one is a meta mold method. The following are a few representative MIM degreasing methods respectively.
Sintering is the last step in the MIM process, sintering eliminates the porosity between the powder particles. MIM products to achieve full dense or nearly full dense. Due to the large amount of binder used in metal injection molding technology, the shrinkage during sintering is very large, and the linear shrinkage generally reaches 13%-25%, so there is a problem of deformation control and dimensional accuracy control. Especially because most of the MIM products are complex shaped parts, this problem becomes more and more prominent, and uniform feeding is a key factor for the dimensional accuracy and deformation control of the final sintered products.
High powder shaking density can reduce sintering shrinkage and facilitate the sintering process and dimensional accuracy control. For products such as iron-based and stainless steel, there is also a problem of carbon potential control in sintering. Due to the high price of fine powder, the study of enhanced sintering technology for coarse powder blanks is an important way to reduce the production cost of powder injection molding, and this technology is an important research aspect of metal powder injection molding research.
Due to the complex shape of MIM products and large sintering shrinkage, most of the products still need post-sintering treatment after sintering, including shaping, heat treatment (carburizing, nitriding, carbonitriding, etc.), surface treatment (fine grinding, ion nitriding, electroplating, shot hardening, etc.), etc.
The reference design of MIM net molding accuracy is usually ± 0.5% of the size. Certain characteristics of net molding can reach ± 0.3%. As with other technologies, the higher the accuracy requirement the higher the cost, so moderate relaxation of tolerance requirements is encouraged where quality allows. Tolerances that cannot be achieved with MIM primary molding can be achieved with surface preparation.
Weight and Size
MIM is particularly suitable for parts weighing less than 100 grams, with less than 50 grams being the most economical. However, parts weighing up to 250 g can also be handled. the main cost of the MIM process is the raw material, so MIM uses new technology to reduce the weight of the part as much as possible. As with plastic products, the weight of parts can be reduced through cores and brackets without compromising the integrity of the product. mim excels in very small and micro parts, and weights of less than 0.1 grams are feasible. Weight is not a limiting factor, and products longer than 250mm can be handled.
Wall thicknesses of less than 6 mm are optimal for MIM. Thicker outer walls are also possible, but costs will increase due to longer processing times and the addition of additional material. Also, very thin walls below 0.5 mm are achievable for MIM, but have high design requirements
MIM is a highly flexible process and can be achieved very economically with an annual demand of several thousand to several million. As with cast and injection molded parts, MIM requires customer investment in tooling and tooling costs, so for small quantities, this usually affects cost estimates.