Different types of steel are used as plastic molds, their chemical composition and mechanical properties are different, so manufacturing process routes are different; similarly, different types of plastic mold steels use different heat treatment processes. This section mainly introduces manufacturing process route and characteristics of heat treatment process of plastic mold.

(1) Concept: Heat treatment process of heating steel to an appropriate temperature, keeping it for a certain period of time, and then slowly cooling (usually cooling with furnace).
(2) Purpose:
a. Increase hardness of low steel to increase its plasticity (to facilitate cutting);
b. Refine grains, even structure and composition of steel;
c. Eliminate residual internal stress in steel, prevent deformation and cracking
(3) Classification: complete annealing; spheroidizing annealing; stress relief annealing

(1) Concept: Process method of heating steel to 30～500C above Ac3 or ACcm, holding it for an appropriate time, and cooling it in air.
(2) Purpose: It is basically same as purpose of annealing, but normalizing cooling rate is slightly faster than annealing, so pearlite structure obtained after normalizing is relatively fine, strength and hardness are higher than that of annealed steel.
Normalizing has a shorter production cycle than annealing, has low cost and easy operation. Therefore, normalizing is preferred when possible, annealing should be used for complex parts.

(1) Concept: Heat treatment process of heating steel to a certain temperature above Ac3 or Ac1, holding it for a certain period of time, then cooling at an appropriate speed to obtain martensite or bainite structure.
(2) Purpose: In order to obtain martensite, improve strength and hardness of steel.

(1) Concept: a heat treatment process in which quenched steel is heated to a temperature below Ac1 point, held for a certain period of time, then cooled to room temperature.
(2) Purpose:
a. Eliminate internal stress;
b. Obtain required mechanical properties (improve toughness of steel, appropriately adjust strength and hardness of steel);
c. Stable organization and size

 

For example, process route of 20, 20Cr, 20CrMnTi and other steels is: blanking→forging die blank→annealing→mechanical rough machining→cold extrusion forming→recrystallization annealing→mechanical finishing→carburizing→quenching, tempering→grinding and polishing →assembly.

For example, process route of 12CrNi3A and 12CrNi4A steel is: blanking→forging die blank→normalizing and high temperature tempering→mechanical rough machining→high temperature tempering→finishing→carburizing→quenching and tempering→grinding and polishing→assembly.

For example, process route of 45, 40Cr and other steels is: blanking→forging die blanks→annealing→mechanical rough machining→tempering and tempering→mechanical finishing→dressing, polishing→assembly.

For example, process route of T7A～T10A, CrWMn, 9SiCr and other steels is: blanking→forging into die blanks→spheroidizing annealing→mechanical rough machining→stress relief annealing→mechanical semi-finishing→mechanical finishing→quenching, tempering→grinding Polishing → assembly.

For example, 5NiSiCa, 3Cr2Mo (P20) and other steels. For those directly processed with bar stocks, they have been pre-hardened due to supply status, can be directly processed and formed, then polished and assembled. For those that need to be forged into blanks and then processed into forming, process route is: blanking→forging→spheroidizing annealing→planing or milling six sides→pre-hardening treatment (34～42HRC)→mechanical roughing→stress relief annealing→ mechanical finishing → polishing → assembly.

1. For plastic molds with high hardness, high wear resistance and high toughness requirements, carburized steel should be used for manufacturing, carburizing, quenching and low temperature tempering are final heat treatment.
2. Requirements for carburized layer, generally thickness of carburized layer is 0.8 ~ 1.5mm, when pressing plastics containing hard fillers, mold carburized layer thickness is required to be 1.3 ~ 1.5mm, when pressing soft plastics, carburized layer thickness is 0.8-1.2mm. Carbon content of carburized layer is preferably 0.7% to 1.0%. If carbon and nitrogen co-infiltration are used, wear resistance, corrosion resistance, oxidation resistance and anti-sticking properties will be better.
3. Carburizing temperature is generally 900～920℃, small molds with complex cavities can take 840～860℃ medium temperature carbonitriding. Carburizing heat preservation time is 5-10h, which should be selected according to requirements of carburized layer thickness. Carburizing process is suitable to adopt hierarchical carburizing process, that is, high temperature stage (900～920℃) is mainly to quickly infiltrate surface of part; medium temperature stage (820～840℃) is mainly to increase thickness of carburized layer. A uniform and reasonable carbon concentration gradient distribution is established in carburized layer, which is convenient for direct quenching.
4. Quenching process after carburizing is different according to 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 small precision molds formed by cold extrusion of industrial pure iron or low carbon steel); air quenching after carburizing (such as large and medium-sized molds made of high-alloy carburized steel).

1. For molds with more complex shapes, heat treatment should be performed after rough machining and then finish machining to ensure the smallest deformation during heat treatment. For precision molds, deformation should be less than 0.05%.
2. Surface requirements of plastic mold cavity are very strict, so during quenching and heating process, it is necessary to ensure that surface of cavity is not oxidized, decarburized, not corroded, overheated, etc. It should be heated in a protective atmosphere furnace or in a salt bath furnace after strict deoxidation. If a common box-type resistance furnace is used for heating, a protective agent should be applied to surface of mold cavity, heating rate should be controlled at the same time. When cooling, a relatively gentle cooling medium should be selected and cooling rate should be controlled to avoid deformation, cracking and scrap during quenching process. Generally, hot bath quenching is better, and pre-cooling quenching can also be used.
3. After quenching, it should be tempered in time, tempering temperature should be higher than working temperature of mold, tempering time should be sufficient. Length depends on mold material and section size, but at least 40-60min.

1. Pre-hardened steel is supplied in a pre-hardened state, and generally does not require heat treatment, but sometimes it needs to be modified forging, and die blank after modified forging must be heat treated.
2. Pre-heat treatment of pre-hardened steel usually adopts spheroidizing annealing, purpose is to eliminate forging stress, obtain uniform spherical pearlite structure, reduce hardness, increase plasticity, and improve cutting performance or cold extrusion forming performance of die blank.
3. Pre-hardening process of pre-hardened steel is simple, most of which adopt quenching and tempering treatment, tempered sorbite structure is obtained after quenching and tempering. High temperature tempering has a wide temperature range that can meet various working hardness requirements of mold. Due to good hardenability of this kind of steel, oil cooling, air cooling or nitrate-salt graded quenching can be used during quenching. Table 3-27 shows pre-hardening process of some pre-hardened steels for reference.
Table 1 Pre-hardening process of some pre-hardened steel

1. Heat treatment process of age hardening steel is divided into two basic steps. First, solution treatment is carried out, that is, steel is heated to a high temperature, so that various alloying elements are dissolved in austenite, and after austenite is completed, martensite structure is obtained by quenching. Second step is aging treatment, and aging is used to strengthen mechanical properties that meet final requirements.
2. Solution treatment heating is generally carried out in a salt bath furnace or a box furnace. Heating time can be respectively: 1min/mm, 2~2.5min/mm, quenching adopts oil cooling, and steel with good hardenability can also be air-cooled. If final forging temperature can be accurately controlled when forging die blank, solution quenching can be performed directly after forging.
3. Aging treatment is best carried out in a vacuum furnace. If it is carried out in a box furnace, in order to prevent surface of mold cavity from being oxidized, a protective atmosphere must be passed into furnace, or aluminum oxide powder, graphite powder,cast iron filings must be used for aging under conditions of box protection. Packing protection heating should appropriately extend heat preservation time, otherwise it will be difficult to achieve aging effect. Refer to Table 2 for heat treatment specification of partially age-hardening plastic mold steel.
Table 2 Heat treatment specifications for some age-hardened steels