How to solve heat treatment in the selection of injection molding mold materials
Edit time: 2021-11-11         source: Xiaxin official website         Views: 435

The injection molding mold is subjected to the combined action of force and heat when it is working. Its working conditions are different from the cold-punching mold. The force received includes the pressure when the mold is closed, the pressure of the melt in the cavity, and the pulling force when the mold is opened. The pressure of the melt is the main factor, and the injection molding mold generally works at a certain temperature, and the temperature fluctuates.


In addition, injection molding molds are also subject to plastic friction and may be corroded by corrosive gases when working. The working conditions of injection molding molds are relatively harsh. To ensure that the mold has a certain life, the injection molding mold materials, especially the molded parts materials, must Meet certain requirements.

  

First: Excellent machining performance: Choose steel grades that are easy to cut and can be processed with high precision. Medium-carbon steel and medium-carbon alloy steel are the most commonly used. This is especially important for large molds. For parts that require EDM, It is also required that the burn hardened layer of this steel is thinner.


Second: Excellent polishing performance: Most of the working surface of the molded parts needs to be polished to reach the mirror surface, Ra<0.05um, and the hardness has exceeded 40hrc. However, a too hard surface will make polishing difficult. The microstructure of the steel should be uniform and dense, with more impurities Few, no blemishes and pinpoints.


Third: Good wear resistance and fatigue resistance: The cavity is not only scoured by the high-pressure plastic melt, but also affected by the temperature stress of alternating cold and heat. The selected steel grade should enable the injection molding mold to reduce the number of polishing and repairing molds. , It can maintain the dimensional accuracy of the cavity for a long time, and meet the requirements of the injection mold for the service life of mass production. It is especially important for the injection molding production of more than 300,000 injections and fiber-reinforced plastics.


  Fourth: Corrosion resistance: For some plastics, such as polyvinyl chloride and flame-retardant plastics, steel types with corrosion resistance must be considered.

  

The structural shape of the stamping die parts, the difference in cross-sectional dimensions, the heating and cooling rate in different heat treatment processes, the combination of thermal stress and structural stress and the phase change of the volume change, cause part of the volume to expand or contract, such size and shape deviations , Deformation, and even cracking, there are many reasons that affect the heat treatment deformation and cracking of the stamping die, mainly related to the chemical composition of the steel and the structure of the original structural cross-sectional size and shape, components and heat treatment processes, etc., heat treatment deformation is not Avoided, but can prevent the occurrence of cracks.


Preliminary heat treatment: stamping die forgings, eutectoid steel should be normalized, and then spheroidizing annealing to eliminate the forging in the secondary cementite network, refine the grains, eliminate internal stress, and prepare the organization for subsequent treatment. Cold tempering should be performed before stamping. For some more complex shapes and high-precision molds, quenching and tempering treatment should be performed before rough machining is completed to reduce quenching deformation, avoid cracking tendency, and prepare for the final heat treatment.


Optimization of quenching and tempering process: protection of quenched parts: quenching and tempering are important factors that affect the deformation or cracking of stamping parts by heat treatment. For parts that are easy to deform or crack, effective protective measures must be taken. Injection molding can make the shape of the part. , The cross-section is symmetrical, and the internal stress is equalized. Commonly used protection methods are: package method, completion method, and block method.

  

Improvement of heating method: For some small compact stamping or slender cylindrical parts, preheat it to 520-580℃, and then put it into a medium temperature salt bath furnace to heat the quenching temperature, which is significantly less deformed than directly using an electric furnace or reverberatory furnace heating and quenching parts. It is small and can control the cracking tendency, especially for high-alloy steel mold parts. The correct heating method is: preheat and then rise to the quenching temperature. During the heating process, the high temperature period should be shortened to reduce quenching deformation and avoid small cracks.


Measurement of heating temperature: Excessive quenching temperature will make the austenite grains coarser, leading to oxidation and decarburization, and increasing the tendency of parts to deform and crack. Within a certain heating temperature range, the decrease of the quenching temperature will cause the parts to shrink. The hole diameter becomes smaller, so the upper limit of the heating temperature should be adjusted. In the case of alloy steel, the heating temperature is high, which causes the inner hole to expand and the hole diameter becomes larger. Therefore, the lower limit of the heating temperature should be selected.


The choice of alloy steel coolant, the best way to reduce quenching deformation is to use potassium nitrate and sodium nitrite constant temperature quenching or intermittent quenching, which is especially suitable for stamping molds with complex shapes and precise dimensions, and some porous mold parts (such as porous molds) , The austempering time should not be too long, otherwise the hole or hole spacing will become larger. If cooling shrinkage oil is used, it is reasonable to use the cooling expansion characteristics of nitrate and dual-medium quenching to reduce the deformation of the parts.


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