Characteristics of heat treatment for plastic molds

（1） Heat treatment characteristics of carburized steel plastic molds

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

2. The requirements for the carburized layer are generally 0.8-1.5mm in thickness. When pressing plastic containing hard fillers, the carburized layer thickness of the mold is required to be 1.3-1.5mm, and when pressing soft plastics, the carburized layer thickness is 0.8-1.2mm. The optimal carbon content of the carburized layer is 0.7% to 1.0%. If carbon and nitrogen co infiltration is used, the wear resistance, corrosion resistance, oxidation resistance, and anti adhesion will be better.

3. The carburizing temperature is generally between 900~920 ℃, and small molds with complex cavities can be carburized at a medium temperature of 840~860 ℃. The carburizing insulation time is 5-10 hours, and the specific selection should be based on the requirements for the thickness of the carburizing layer. The carburizing process should preferably adopt a graded carburizing process, that is, in the high-temperature stage (900-920 ℃), the main method is to quickly infiltrate carbon into the surface of the parts; In the medium temperature stage (820-840 ℃), increasing the thickness of the carburized layer is the main method, in order to establish a uniform and reasonable carbon concentration gradient distribution within the carburized layer, which is convenient for direct quenching.

4. The quenching process after carburization varies according to the type of steel. After carburization, the following methods can be used: reheating and quenching; Direct quenching after graded carburization (such as alloy carburized steel); Direct quenching after medium temperature carbonitriding (such as small precision molds formed by cold extrusion of industrial pure iron or low-carbon steel); Air cooling quenching after carburization (such as large and medium-sized molds made of high alloy carburized steel).

（2） Heat treatment of hardened steel plastic molds

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

2. 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. Heating should be done in a protective atmosphere furnace or a salt bath furnace after strict deoxidation. If using a regular box type resistance furnace for heating, protective agents should be applied to the surface of the mold cavity, and the heating speed should be controlled. During cooling, a relatively gentle cooling medium should be selected to control the cooling speed to avoid deformation, cracking, and scrapping during the quenching process. Generally, hot bath quenching is preferred, and pre cooling quenching can also be used.

3. After quenching, it should be tempered 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 size, but it should be at least 40-60 minutes or more.

（3） Heat treatment of pre hardened steel plastic molds

1. Pre hardened steel is supplied in a pre hardened state and generally does not require heat treatment, but sometimes it needs to be forged. The forged die blank must undergo heat treatment.

2. The pre heat treatment of pre hardened steel usually adopts spheroidizing annealing, with the aim of eliminating forging stress, obtaining uniform spherical pearlite structure, reducing hardness, improving plasticity, and improving the cutting performance or cold extrusion formability of the mold blank.

3. The pre hardening process of pre hardened steel is simple, and most of it adopts quenching and tempering treatment. After quenching and tempering, the tempered sorbite structure is obtained. The temperature range of high-temperature tempering is very wide, which can meet various working hardness requirements of the mold. Due to the good hardenability of this type of steel, oil cooling, air cooling, or nitrate graded quenching can be used during quenching.

（4） Heat Treatment of Time Hardening Steel Plastic Mold

1. The heat treatment process of age hardening steel is divided into two basic steps. Firstly, solid solution treatment is carried out, which involves heating the steel to a high temperature to dissolve various alloy elements into austenite, and quenching to obtain martensitic structure after completing the austenite. The second step is to perform aging treatment, using aging strengthening to achieve the final required mechanical properties.

2. Solid solution treatment heating is generally carried out in salt bath furnaces and box furnaces, with heating times of 1 min/mm and 2-2.5 min/mm respectively. Oil cooling is used for quenching, and air cooling can also be used for steel grades with good hardenability. If the final forging temperature can be accurately controlled during forging of the mold blank, solid solution quenching can be directly carried out after forging.

3. The aging treatment is best carried out in a vacuum furnace. If it is carried out in a box furnace, to prevent surface oxidation of the mold cavity, a protective atmosphere must be introduced into the furnace, or aluminum oxide powder, graphite powder, and cast iron chips must be used for aging under box protection conditions. The insulation time for packaging protection heating should be appropriately extended, otherwise it is difficult to achieve the effect of aging.