Heat Treatment of Plastic Mould

Manufacturing Technology of Plastic Mould

1. Dies Made of Low Carbon Steel and Low Carbon Alloy Steel

For example, the technological routes of 20, 20Cr, 20CrMnTi and other steels are as follows: blanking forging die blank annealing mechanical roughing cold extrusion forming recrystallization annealing mechanical finishing carburizing quenching, tempering abrasive polishing assembly.

2. Die Made of High Alloy Carburized Steel

For example, the technological route of 12CrNi3A and 12CrNi4A steel is as follows: blanking forging die blank normalizing and high temperature tempering mechanical roughing high temperature tempering finishing carburizing quenching, tempering grinding and polishing assembly.

3. Tempered and tempered steel die

For example, the technological route of 45, 40Cr and other steels is as follows: blanking forging die blank annealing mechanical roughing conditioning mechanical finishing dressing, polishing assembly.

4. Dies Made of Carbon Tool Steel and Alloy Tool Steel

For example, the technological routes of T7A-T10A, CrWMn, 9SiCr and other steels are as follows: blanking forging die blank spheroidizing annealing mechanical roughing stress relief annealing mechanical semi-finishing mechanical finishing quenching, tempering abrasive polishing assembly.

5. Pre-hardened steel die

For example, 5NiSiCa, 3Cr2Mo (P20) and other steels. For those directly processed with bars, Pre-hardening treatment has been carried out due to the state of supply, which can be directly processed, polished and assembled after forming. For those to be changed into blanks and then processed, the technological route is as follows: blanking forging spheroidizing annealing planing or milling six sides Pre-hardening (34-42HRC) mechanical roughing stress relieving annealing mechanical finishing polishing assembly.

Heat Treatment Characteristics of Plastic Moulds

(1) Heat treatment characteristics of carburized steel plastic moulds

1. For plastic moulds with high hardness, high wear resistance and high toughness, carburized steel should be selected to manufacture, and carburizing, quenching and low temperature tempering should be taken as final heat treatment.

2. For the requirement of carburizing layer, the thickness of carburizing layer is 0.8-1.5mm in general, 1.3-1.5mm in mould when pressing plastics with hard fillers, and 0.8-1.2mm in soft plastics. The best carbon content of carburized layer is 0.7%~1.0%. If carbonitriding is used, the wear resistance, corrosion resistance, oxidation resistance and stickiness resistance will be better.

3. The carburizing temperature is generally 900-920 C, and the small-scale die with complex cavity can be carburized at 840-860 C. The holding time of carburizing is 5 to 10 hours, which should be selected according to the requirement of the thickness of carburizing layer. The optimum carburizing process is graded carburizing, i.e. the high temperature stage (900-920 C) is to rapidly infiltrate the carbon into the surface of the parts; the medium temperature stage (820-840 C) is to increase the thickness of the carburizing layer, so that a uniform and reasonable gradient distribution of carbon concentration can be established in the carburizing layer to facilitate direct quenching.

4. The quenching process after carburizing is different according to the steel type. After carburizing, the following methods can be adopted: reheating quenching; direct quenching after grading carburizing (such as alloy carburizing steel); direct quenching after medium temperature carbonitriding (such as small precision die formed by cold extrusion of industrial pure iron or low carbon steel); air quenching after carburizing (such as large and medium dies made of high alloy carburizing steel).

(2) Heat treatment of hardened steel plastic mould

1. For the die with complex shape, heat treatment should be carried out after rough processing, and then finish processing, so as to ensure the minimum deformation during heat treatment. For the precision die, the deformation should be less than 0.05%.

2. The surface requirement of plastic model cavity is very strict, so it is necessary to ensure that the surface of the cavity is not oxidized, decarbonized, corroded or overheated during quenching and heating. It should be heated in a protective atmosphere furnace or in a salt bath furnace after strict deoxidation. If ordinary box resistance furnace is used for heating, the protective agent should be applied on the surface of the die cavity, and the heating speed should be controlled. When cooling, a relatively mild cooling medium should be selected to control the cooling speed so as to avoid deformation, cracking and scrap in the quenching process. Generally, hot bath quenching is preferred, and pre-cooling quenching can also be used.

3. Tempering should be timely after quenching, tempering temperature should be higher than the working temperature of the die, tempering time should be sufficient, depending on the die material and section size, but at least 40-60 minutes.

(3) Heat Treatment of Pre-hardened Steel Plastic Mould

1. Pre-hardening steel is supplied by pre-hardening, generally without heat treatment, but sometimes need to be forged, the forged die must be heat treated.

2. Spheroidizing annealing is usually used in pre-heat treatment of pre-hardened steel to eliminate forging stress, obtain uniform spherical pearlite structure, reduce hardness, improve plasticity, and improve cutting performance or cold extrusion formability of die blanks.

3. The Pre-hardening process of pre-hardened steel is simple. Most of the pre-hardened steel is tempered to obtain tempered sorbite after tempering. The temperature range of high temperature tempering is wide enough to meet various hardness requirements of the die. Because of the good hardenability of this kind of steel, oil-cooled, air-cooled or nitrate step quenching can be used in quenching.

(4) Heat Treatment of Age Hardening Steel Plastic Mould

1. The heat treatment process of aging hardening steel consists of two basic steps. Firstly, the steel is heated to high temperature to dissolve various alloying elements into austenite, and then quenched to obtain martensite structure. In the second step, aging treatment was carried out to strengthen the mechanical properties to meet the final requirements.

2. Solution treatment heating is usually carried out in salt bath furnace and box furnace. The heating time is 1 min/mm and 2-2.5 min/mm respectively. Oil-cooled quenching is used for quenching, and air-cooled steel with good hardenability can also be used for quenching. If the final forging temperature can be accurately controlled during forging, solid solution quenching can be directly carried out after forging.

3. Aging treatment is best carried out in vacuum furnace. If it is carried out in box furnace, in order to prevent the surface oxidation of the mould cavity, protective atmosphere should be introduced into the furnace, or alumina powder, graphite powder and cast iron chips should be used to age under the condition of packing protection. It is necessary to prolong the holding time properly for the protective heating of packing, otherwise it is difficult to achieve the aging effect.

Surface treatment of plastic moulds

In order to improve the wear resistance and corrosion resistance of the surface of plastic moulds, proper surface treatment is often carried out.

1. Plastic mould chromium plating is one of the most widely used surface treatment methods. The chromium plating layer has strong passivation ability in the atmosphere, can maintain the metallic luster for a long time, and has no chemical reaction in various acidic media. The hardness of the coating reaches 1000HV, so it has excellent wear resistance. Chromium plating layer also has high heat resistance, and its appearance and hardness remain unchanged when heated to 500 C in air.

2. Nitriding has the advantages of low treatment temperature (generally 550-570 C), slight deformation of the die and high hardness of the nitriding layer (up to 1000-1200HV), so it is also very suitable for surface treatment of plastic mould. Steel grades containing chromium, molybdenum, aluminium, vanadium and titanium have better nitriding properties than carbon steel. Nitriding treatment can greatly improve wear resistance when used as plastic moulds.

Surface treatment methods suitable for plastic moulds include nitrocarburizing, electroless nickel plating, ion plating of titanium nitride, titanium carbide or titanium carbonitride, and deposition of hard or superhard films by PVD and CVD.

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