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Vacuum heat treatment process of plastic mold parts

Plastic mold is a tool that is matched with plastic molding machines in the plastic processing industry to give plastic products a complete configuration and precise size. Due to the variety of plastic varieties and processing methods, and the complex and simple structures of plastic molding machines and plastic products, the types and structures of plastic molds are also diverse.

The heat treatment process plays an important role in the processing of plastic molds. Depending on the mold, the heat treatment process mainly includes two main processes, carburizing and quenching.

The process route of low carbon steel and low carbon alloy steel molds such as 20, 20Cr, 20CrMnTi and other steels is: blanking → forging die blanks → annealing → mechanical roughing → cold extrusion forming → recrystallization annealing → mechanical finishing → infiltration Carbon → quenching, tempering → grinding and polishing → assembly.

The process route of high alloy carburizing steel molds such as 12CrNi³A and 12CrNi4A steel is: blanking → forging blanks → normalizing and high temperature tempering → mechanical roughing → high temperature tempering → finishing → carburizing → quenching, tempering → Grinding and polishing → assembly.

Heat treatment characteristics of carburized steel plastic mold

For plastic molds with high hardness, high wear resistance and high toughness requirements, carburizing steel should be used to manufacture, and carburizing, quenching and low temperature tempering should be used as the final heat treatment.

For the requirements of the carburized layer, the thickness of the carburized layer is generally 0.8 to 1.5 mm. When pressing plastics containing hard fillers, the thickness of the carburized layer in the mold is required to be 1.3 to 1.5 mm. When pressing soft plastics, the thickness of the carburized layer is 1.3 to 1.5 mm. 0.8～1.2mm. The carbon content of the carburized layer is preferably 0.7% to 1.0%. If carbon and nitriding are used, the wear resistance, corrosion resistance, oxidation resistance and anti-stickiness are better.

The carburizing temperature is generally 900~920℃, and the small mold with complex cavity can take 840~860℃ medium temperature carbonitriding. The carburizing holding time is 5-10h, which should be selected according to the requirements of the thickness of the carburizing layer. The carburizing process is preferably a graded carburizing process, that is, the high temperature stage (900 ~ 920 ℃) is mainly to rapidly infiltrate the surface of the part; the medium temperature stage (820 ~ 840 ℃) is mainly to increase the thickness of the carburized layer, so that in the A uniform and reasonable carbon concentration gradient distribution is established in the carburized layer, which is convenient for direct quenching.

The quenching process after carburizing is different according to the steel type. After carburizing, it can be used separately: reheating and quenching; direct quenching after graded carburizing (such as alloy carburized steel); direct quenching after medium temperature carbonitriding (such as using industrial pure iron) or small precision molds formed by cold extrusion of low-carbon steel); air-cooled quenching after carburizing (such as large and medium-sized molds made of high-alloy carburized steel).

Quenching and tempering heat treatment

For molds with complex shapes, heat treatment is performed after rough machining, and then finishing is performed to ensure the smallest deformation during heat treatment. For precision molds, the deformation should be less than 0.05%.

The surface requirements of the plastic mold cavity are very strict, so during the quenching and heating process, it is necessary to ensure that the surface of the cavity is not oxidized, decarburized, corroded, or overheated. It should be heated in a protective atmosphere furnace or a salt bath furnace after strict deoxidation. It is best to use a vacuum heat treatment furnace for heat treatment. At the same time, the heating speed should be controlled. Deformation, cracking and scrapping occur during the quenching process. Generally, hot bath quenching is preferred, and pre-cooling quenching can also be used.

After quenching, it should be tempered in time. The tempering temperature should be higher than the working temperature of the mold. The tempering time should be sufficient. The length depends on the mold material and section size, but it should be at least 40 to 60 minutes.

Surface treatment of plastic molds

In order to improve the wear resistance and corrosion resistance of the plastic mold surface, it is often subjected to appropriate surface treatment.

Chromium plating of plastic molds is the most widely used surface treatment method. The chrome plating layer has strong passivation ability in the atmosphere, can maintain the metallic luster for a long time, and does not react chemically in a variety of acidic media. The hardness of the coating reaches 1000HV, so it has excellent wear resistance. The chromium plating layer also has high heat resistance, and its appearance and hardness still have no obvious change when heated to 500 ℃ in the air.

Nitriding has the advantages of low processing temperature (generally 550~570℃), little deformation of the mold and high hardness of the infiltrated layer (up to 1000~1200HV), so it is also very suitable for the surface treatment of plastic molds. Steel grades containing alloying elements such as chromium, molybdenum, aluminum, vanadium and titanium have better nitriding properties than carbon steel, and nitriding treatment when used as a plastic mold can greatly improve wear resistance.

Suitable surface treatment methods for plastic molds include: nitrocarburizing, electroless nickel plating, ion plating titanium nitride, titanium carbide or titanium carbonitride, PVD, CVD deposition hard film or super hard film, etc.

SIMUWU focuses on the production of equipment suitable for vacuum heat treatment processes. Including RVC vacuum carburizing furnace, RVGQ vacuum air quenching furnace and VOGQ vacuum oil quenching furnace, as well as tempering furnace and annealing furnace, etc., are widely sold all over the world. It has the characteristics of high degree of automation, excellent heating performance, affordable and durable. SIMUWU has nearly 20 years of experience in vacuum furnace manufacturing and sales, and is equipped with a professional after-sales team of engineers, which can effectively meet the processing and production needs of major companies.