1. Heat treatment characteristics of carburized steel plastic molds

A. For plastic molds with high hardness, high wear resistance and high toughness requirements, carburized steel should be selected for manufacturing, and carburizing, quenching and low temperature tempering should be used as the final heat treatment.

B. Requirements for the carburized layer: Generally, the thickness of the carburized layer is 0.8–1.5 mm. When pressing plastics containing hard fillers, the required thickness of the carburized layer is 1.3–1.5 mm; when pressing soft plastics, the thickness is 0.8–1.2 mm. The carbon content of the carburized layer is preferably 0.7%–1.0%. If carbon and nitrogen co-carburizing is used, the wear resistance, corrosion resistance, oxidation resistance, and non-stick properties will be even better.

C. The carburizing temperature is generally between 900 and 920℃. For small molds with complex cavities, a medium-temperature carbonitriding temperature of 840 to 860℃ can be used. The carburizing holding time is 5 to 10 hours, depending on the required carburized layer thickness. A staged carburizing process is recommended, where the high-temperature stage (900–920℃) focuses on rapidly penetrating carbon into the surface layer of the part; the medium-temperature stage (820–840℃) focuses on increasing the thickness of the carburized layer. This establishes a uniform and reasonable carbon concentration gradient distribution within the carburized layer, facilitating direct quenching.

D. The quenching process after carburizing varies depending on the steel grade. The following methods can be used after carburizing: reheat quenching; direct quenching after graded carburizing (e.g., alloy carburizing steel); direct quenching after medium-temperature carbonitriding (e.g., small precision molds cold-extruded from industrial pure iron or low-carbon steel); and air-cooling quenching after carburizing (e.g., large and medium-sized molds made of high-alloy carburizing steel).

2. Heat treatment of hardened steel plastic molds

A. For molds with complex shapes, heat treatment should be carried out after rough machining, followed by fine machining, in order to ensure minimal deformation during heat treatment. For precision molds, the deformation should be less than 0.05%.

B. The surface requirements for plastic mold cavities are extremely stringent. Therefore, during the quenching and heating process, it is crucial to ensure that the cavity surface does not oxidize, decarburize, corrode, or overheat. Heating should be performed in a protective atmosphere furnace or a strictly deoxidized salt bath furnace. If a conventional box-type resistance furnace is used, a protective agent should be applied to the mold cavity surface. Simultaneously, the heating rate must be controlled, and a relatively gentle cooling medium should be selected during cooling to control the cooling rate, thus preventing deformation and cracking during quenching, which could lead to scrapping. Hot bath quenching is generally preferred, but pre-cooling quenching can also be used.

C. After quenching, tempering should be carried out in a timely manner. The tempering temperature should be higher than the working temperature of the mold, and the tempering time should be sufficient. The length depends on the mold material and cross-sectional dimensions, but it should be at least 40 to 60 minutes.

3. Heat treatment of pre-hardened steel plastic molds

A. Pre-hardened steel is supplied in a pre-hardened state and generally does not require heat treatment. However, sometimes it needs to be re-forged, and the re-forged mold blank must be heat treated.

B. Preheat treatment of pre-hardened steel usually involves spheroidizing annealing, which aims to eliminate forging stress, obtain a uniform spheroidal pearlite structure, reduce hardness, improve plasticity, and improve the machinability or cold extrusion forming performance of the blank.

C. The pre-hardening process for pre-hardened steel is simple, mostly involving quenching and tempering to obtain a tempered sorbite structure. The high-temperature tempering temperature range is wide enough to meet various working hardness requirements of molds. Due to the good hardenability of this type of steel, oil cooling, air cooling, or salt-pepper quenching can be used. Table 3-27 shows the pre-hardening processes for some pre-hardened steels for reference.

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