Views: 44 Author: Site Editor Publish Time: 2021-09-06 Origin: Site
The heat treatment hardness of injection molding tooling is a very important mechanical performance index, and unqualified hardness is a very serious defect. Insufficient or uneven hardness of custom plastic injection molds after heat treatment will reduce the wear resistance and fatigue strength of the injection molding tool, leading to early failure of the mold and severely reducing the service life of custom plastic injection molds.
Cause
1) The cross-section of the mold is large, and the hardenability of steel is poor. For example, a steel with low hardenability is selected for large-scale molds.
2) The carbide segregation in the original structure of the die steel is serious or the structure is coarse, and there is segregation and aggregation of graphite carbon and carbide in the steel.
3) The die forging process is incorrect, and the spheroidizing annealing is not performed well after forging, resulting in poor spheroidizing structure of the die steel.
4) The surface of the mold has not been cleaned of the decarburized layer produced during annealing or quenching heating.
5) The mold quenching temperature is too high, and the amount of retained austenite after quenching is too much; or the quenching temperature is too low, and the heating and holding time is insufficient, so that the phase transformation of the mold steel is incomplete.
6) After the mold is quenched and heated, the cooling rate is too slow, the grading and isothermal temperature is too high or the time is too long, and the quenching cooling medium is improperly selected.
7) The water in the alkaline bath is too little, or the quenching cooling medium contains too much impurities, or the quenching cooling medium is aging.
8) After the mold is quenched and cooled, the temperature of the quenching cooling medium is too high and the cooling is insufficient.
9) Insufficient tempering and too high tempering temperature.
Countermeasure
1) Select mold steel types correctly, and large-scale molds should use high-alloy mold steels with high hardenability.
2) Strengthen the inspection of raw materials to ensure that the raw materials meet the standards. Reasonable forging and spheroidizing annealing treatment of poor raw material steel to ensure a good structure. Carbon tool steel is not easy to be annealed many times to prevent graphitization.
3) Strictly implement the forging process and the spheroidizing annealing process to ensure a good preliminary heat treatment organization.
4) Before heat treatment, the rust spots and scales on the mold surface should be thoroughly removed, and the protection during heating should be paid attention to. Vacuum heating quenching or protective atmosphere heating quenching should be used as much as possible, and good deoxidation treatment should be carried out during salt bath heating.
5) Correctly formulate the mold quenching and heating process parameters to ensure sufficient phase change, and perform rapid cooling at a cooling rate greater than the critical cooling rate to obtain a qualified metallographic structure.
6) Correctly select quenching cooling medium and cooling method, and strictly control the classification and isothermal temperature and time.
7) It is necessary to strictly control the moisture content of the alkaline bath. The quenching cooling medium used for a long time should be filtered and regularly replaced, and kept clean, and its quenching cooling characteristic curve should be checked regularly.
8) For large-sized molds, properly extend the time of immersion in the quenching cooling medium to prevent the mold from exiting the quenching cooling medium at an excessively high temperature.
9) After the mold is quenched, it should be tempered promptly and fully, and the tempering temperature should be prevented from being too high.
10) Cryogenic treatment (such as -110～-196℃) can be used for injection molding tooling with high hardness requirements.
11) Perform surface strengthening treatment.
