Analysis of common defects in metal powder injection molding

Analysis of common defects in metal powder injection molding

Metal powder injection molding (MIM) is a kind of plastic mixture of binder and metal powder mixed into a specific mold injection molding process method, is a combination of powder metallurgy and polymer injection molding development of a new molding technology. Especially suitable for mass production of small, complex shape and large amount of metal products, especially complex structure, small volume, special requirements of careful metal parts, difficult to use traditional processing means processing and processing costs are high, metal powder injection molding can reduce processing costs.

The production process of metal powder injection molding (MIM) products is: powder + binder mixing granulation injection molding degreasing (MIM degreasing furnace) sintering (MIM sintering furnace) follow-up treatment of formed products. In the process of producing MIM cemented carbide parts, improper material selection and operation control of any link may cause defects in cemented carbide parts, so how to avoid this defect?

1, powder selection link. MIM carbide powder metallurgy in addition to meet its particle size distribution, particle size and other basic requirements, but also requires the purity of the powder to be high, can not use the powder with impurities, if the powder is included in sulfur, phosphorus, silicon and other elements, the sintering process of these substances will form pores, resulting in product defects.

2, feeding production link. Cemented carbide powder in mixing need a suitable binder, mixing the cemented carbide powder and binder fully mixed, mixing process must be strictly controlled the temperature, to avoid volatile binder and uneven distribution of the situation, so that the mixture made of feeding has good rheological properties and viscosity values, to avoid defects in the future.

3, forming green links. This is also the key link in the production of cemented carbide parts, to avoid product defects, it is necessary to pay attention to the reasonable control of mold temperature, feeding amount, injection pressure, pressure holding pressure, pressure holding time, injection speed, etc., in the injection process, can effectively avoid the defects of injection green billet.

4. Degreasing link. In the degreasing process, if the degreasing furnace temperature is too fast, it will cause cracks in the cemented carbide parts, and the method of step-by-step temperature rise can be used to degreasing.

5, sintering link. Tungsten carbide density is large, liquid phase sintering due to its own gravity, the product is easy to produce deformation. Appropriate support devices can be used, and materials with comparable shrinkage can be selected as support plates for larger products, and the liquid phase sintering time should be shortened as much as possible.

Metal powder injection parts common defects are as follows.

1, underinjection: Underinjection refers to the insufficient pressure of the injection molding machine used or the poor fluidity of the injection material used and other factors caused by the material fails to fill the entire mold cavity, so that the injection product is incomplete. The reasons for underinjection are poor material flow, too small wall thickness, too low mold temperature and too short injection time, which can be solved by improving material flow or changing materials, increasing wall thickness, increasing mold temperature, increasing injection pressure and extending injection time.

2, weld mark: the material is divided into several strands of material flow in the cavity during the injection process and then joined together, and there may be linear traces at the junction, that is, weld marks, affecting the appearance quality and even mechanical strength of the product. The reasons for the formation of welding marks are too low injection pressure, too slow injection speed, too low feed temperature and too many feed flow, which can be solved by increasing the injection pressure, speeding up the injection speed, increasing the feed temperature and die temperature and appropriately reducing the split flow.

3. Air pockets. During the injection process, the air in the mold cavity is too late to discharge, and is enclosed by the material or compressed to the inner wall of the mold to form an air pocket, causing the surface of the product to underfill, affecting the appearance quality and even mechanical strength of the product. The primary points of forming air holes are bad exhaust, improper gate position, too fast injection speed and excessive thickness change of products. The corresponding measures can be taken to add or deepen the exhaust hole, change the gate position, appropriately slow down the injection speed, extend the pressure holding time and avoid the drastic change of product thickness.

4. Deformation. Deformation refers to the bending or distortion of the molded product after cooling, which directly affects the appearance and dimensional accuracy of the product, and even causes the product to be scrapped. The causes of deformation are uneven cooling of the product, too hot and premature topping of the product, too thin and unreasonable structure of the product and the residual stress inside the product, which can be solved by corresponding measures such as uniform cooling of the mold channel, extending the pressure holding time and timely ejecting the product, improving the wall thickness and structural design of the product and improving the forming conditions.

Metal powder injection molding (MIM) can form complex shapes of cemented carbide products, with the problem of defect control, the gradual improvement of MIM technology, so that the application of cemented carbide gradually expanded, greatly promoted the development of the entire cemented carbide industry.