Box-shaped MIM process

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Metal Powder Injection Molding (MIM for short) is a new type of powder metallurgy near-net-shape technology formed by introducing modern plastic injection molding technology into the field of powder metallurgy. MIM technology can produce high-performance products with complex shapes at a lower cost. Due to the use of a large amount of binder as a means of enhancing flow, various complex metal parts can be arbitrarily formed. Since injection molding is a near-net-shape process, it basically does not require subsequent processing, which greatly reduces the cost of parts manufacturing. In addition, due to the uniformity of flow filling of the cavity during injection molding, the density of the MIM product is uniform throughout, avoiding the inevitable density non-uniformity in the PM molding process. Due to the use of fine powder, the product can reach a high density after sintering, and the mechanical properties of the product are generally better than those of molded and precision cast products.

Housing1530 is a three-dimensional box-shaped part on semiconductor products, the material is iron-nickel alloy (Fe54%, Ni46%). Its shape is complex, the wall thickness is 1.5mm, and the side wall has two through grooves with high dimensional accuracy, the groove width is 15∙3±0∙03mm, and the groove height is 1∙63±0∙03mm. If the part is processed by conventional stamping, machining and casting, it usually requires more than a dozen processes, which is time-consuming, labor-intensive, and material-intensive, and it is difficult to achieve the shape and dimensional accuracy of the product. Save about 90% of the cost.

Process flow: First, Fe, Ni powders and organic binders are uniformly mixed. After granulation, the granules are injected into the mold cavity by an injection molding machine to be condensed and formed in a heated state. The binder is removed after sintering, and the product is densified by sintering, and the sintered product can meet the requirements after a small amount of post-treatment and processing.

The metal powders used in the MIM process are Fe and Ni alloys, in which the volume content is Fe54%, Ni46%, and the powder particle size is 0∙5~20μm. The finer the particles, the larger the specific surface area, which is easy to form and sinter. The traditional powder metallurgy process uses coarser powders larger than 40 μm. The metal powder and the organic binder are uniformly mixed together to make a mixture for injection molding. The homogeneity of the mixture directly affects its fluidity, which in turn affects the injection molding process parameters, as well as the density and other properties of the final material.

The function of the organic binder is to bind the metal powder particles, so that the mixture has rheological properties and lubricity when heated in the barrel of the injection machine, that is, the carrier that drives the powder flow. In addition, the binder can maintain the shape of the green body after injection molding and during debinding. Therefore, the choice of binder is the key to the whole powder injection molding. Requirements for binders: 1) Less dosage, better rheological properties of the mixture can be produced with less binder; 2) No reaction, it has nothing to do with the metal powder in the process of removing the binder. Chemical reaction; 3) Easy to remove, no carbon remains in the product.

The metal powder and the organic binder are uniformly mixed together to make a mixture for injection molding. The homogeneity of the mixture directly affects its fluidity, which in turn affects the injection molding process parameters, as well as the density and other properties of the final material. The mixing of the MIM feed is accomplished under a combination of thermal and shear forces. The mixing temperature should not be too high, otherwise the binder may decompose or the two phases of powder and binder may separate due to too low viscosity.

The injection molding process is the same in principle as the plastic injection molding process, and the equipment conditions are basically the same. In the injection molding process, the mixture is heated in the barrel of the injection machine into a plastic material with rheological properties, and injected into the mold under appropriate injection pressure to form a blank. The injection-molded blank should be microscopically uniform so that the product shrinks evenly during the sintering process. The possible defects of MIM products are basically problems such as holes, short shots, jetting, flash, surface blistering, cracking, formation of weld lines and surface depressions, bending and poor dimensional accuracy control during the injection molding process. These defects are often not discovered until after debinding and sintering, after the stress caused by injection is released, so the control of the injection molding process is very important to improve product yield and material utilization.

Sintering is the result of high temperature activation of atomic movement, usually diffusion processes dominate. Many factors affect the sintering rate, including material, starting density, powder particle size, sintering atmosphere, temperature, time and heating rate. Sintering can shrink and densify the porous degreasing blank into a product with certain structure and properties. After degreasing, the binder usually remaining in the billet must be evacuated with air before densification, otherwise carbon ash will contaminate the billet and furnace. The sintering atmosphere is vacuum, hydrogen and nitrogen. Because the vacuum is clean, reproducible, and relatively easy to control, vacuum sintering avoids the reduction of oxides at low partial pressures of oxygen for many metals. Also involving highly reactive materials, high temperature materials, hydride elements or corrosion resistant materials, vacuum is the most reliable atmosphere.

The sintering process of this process is as follows: under the condition of vacuum atmosphere, the workpiece is heated to 650℃ in the MIM sintering furnace; then under the condition of hydrogen atmosphere, the workpiece is heated to 800℃; then under the condition of vacuum atmosphere, the workpiece is heated to 1280°C, and then cooled to 1200°C; then, under high pressure nitrogen atmosphere, slowly cool the workpiece to 1000°C, stop heating, and cool to 700°C; finally, quickly cool it to 40°C in nitrogen.

Post-sintering treatments include: assembly, densification (recompression), trimming, and heat treatment. Because the flatness of the bottom plane of the box-shaped Housing1530 is required to be high, the flatness is 0∙05mm. After sintering, it is difficult to meet the requirements of the product. Therefore, the final sintered product needs to be leveled. After pressing and leveling with a simple tool on the leveling machine, the flatness of the bottom plane is guaranteed to be below 0∙05mm. The perpendicularity of the two small protrusions on the side to the bottom plane is also not guaranteed, and it needs to be machined to meet the requirements.

The RVS series MIM sintering furnace produced by SIMUWU is a high-quality product that can meet the relevant process. The high temperature uniformity and high temperature control accuracy of the furnace determine its excellent sintering quality. Good after-sales service can meet various needs of customers, and production is very convenient.