Heat Treatment Process of Plastic Mould Parts
发布时间：2018-09-05 浏览次数：1064次
Different kinds of steel are used as plastic molds, and their chemical composition and mechanical properties are different, so the manufacturing process is different. Similarly, different types of plastic die steels have different heat treatment processes. This section mainly introduces the manufacturing process of plastic molds and the characteristics of heat treatment process.
First, the plastic mold manufacturing process
1. Mould made of low carbon steel and low carbon alloy steel
For example, the technological route of 20, 20Cr, 20 CRM NTI and other steels is: blanking → forging die blank → annealing → mechanical roughing → cold extrusion forming → recrystallization annealing → mechanical finishing → carburizing → quenching, tempering → grinding and polishing → assembly.
2. High alloy carburized steel mold
For example, the process route of 12CrNi 3A and 12CrNi 4A steel is blanking, forging die blank, normalizing and tempering at high temperature, mechanical roughing, tempering at high temperature, finishing, carburizing, quenching, tempering, grinding and polishing, and assembling.
3. Quenching and tempering steel mold
For example, the technological route of steel such as 45, 40Cr is blanking, forging die blank, annealing, mechanical rough machining, tempering, mechanical finish machining, trimming, polishing and assembling.
4. Carbon tool steel and alloy tool steel molds
For example, T7a - T10a, Cr WMN, 9 SiCr and other steels have the following process routes: blanking, forging into a mold blank, spheroidizing annealing, mechanical rough machining, stress relief annealing, mechanical semi - finishing, mechanical finishing, quenching, tempering, grinding and polishing, and assembly.
5. Pre - hardened steel mold
For example, 5Ni SiCa, 3C R 2 Mo ( P20 ) and other steels. For the direct use of bar stock processing, pre-hardening treatment has been carried out due to the supply status, and it can be directly processed, formed, polished and assembled. For those to be forged into blanks and then processed and formed, the process route is: blanking → forging → spheroidizing annealing → planing or milling six sides → pre-hardening ( 34 ~ 42 HRC ) → mechanical rough machining → stress relief annealing → mechanical finishing → polishing → assembly.
Second, the characteristics of heat treatment of plastic molds
( 1 ) Heat Treatment Characteristics of Carburized Steel Plastic Mould
1. For plastic molds with high hardness, high wear resistance and high toughness requirements, carburized steel shall be selected for manufacturing, and carburization, quenching and low temperature tempering shall be taken as the final heat treatment.
2. For the carburized layer, the thickness of the carburized layer is generally 0.8 - 1.5 mm, the thickness of the mold carburized layer is required to be 1.3 - 1.5 mm when pressing plastic containing hard filler, and the thickness of the carburized layer is required to be 0.8 - 1.2 mm when pressing soft plastic. The carbon content of the carburized layer is preferably 0.7 % to 1.0 %. If carbon and nitrogen infiltration is adopted, the wear resistance, corrosion resistance, oxidation resistance and tack resistance will be better.
3. The carburizing temperature is generally 900 ~ 920 ℃, and the medium temperature carbonitriding can be adopted for small molds with complex cavities at 840 ~ 860 ℃. The carburizing and heat preservation time is 5 ~ 10h, which should be selected according to the requirements for the thickness of the carburized layer. The proper carburizing process is graded carburizing, i.e. the high temperature stage ( 900 ~ 920 ℃ ) is dominated by rapid infiltration of carbon into the surface layer of the part. In the middle temperature stage ( 820 ~ 840 ℃ ), increasing the thickness of the carburized layer is the main step, thus establishing a uniform and reasonable gradient distribution of carbon concentration in the carburized layer and facilitating direct quenching.
4. The quenching process after carburization is different according to the steel grade. After carburization, it can be used separately: reheating and quenching; Direct quenching after graded carburization ( such as alloy carburized steel ); Direct quenching after carbonitriding at medium temperature ( such as small precision die 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 is carried out immediately after rough machining, and then finish machining is carried out to ensure the minimum deformation during heat treatment. For precision molds, the deformation should be less than 0.05 %.
2. The surface requirements of the plastic mold cavity are very strict. Therefore, during quenching and heating, the surface of the mold cavity should not be oxidized, decarburized, eroded or heated. 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, protective agent should be coated on the surface of the mold cavity and the heating speed should be controlled. A relatively mild cooling medium should be selected for cooling to avoid deformation and cracking during quenching and scrapping. Generally, hot bath quenching is preferred, and pre-cooling quenching can also be adopted.
3. Tempering should be carried out in time after quenching, the tempering temperature should be higher than the working temperature of the mold, and the tempering time should be sufficient, depending on the mold material and section size, but at least over 40 - 60 min.
( 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 reformed, and the reformed die blank must be heat treated.
2. The pre-heat treatment of pre-hardened steel usually adopts spheroidizing annealing to eliminate forging stress, obtain uniform spheroidal pearlite structure, reduce hardness, improve plasticity, and improve the machinability or cold extrusion formability of the die blank.
3. The pre-hardening process of pre-hardening steel is simple, most of them are quenched and tempered, and tempered sorbite structure is obtained after quenching and tempering. The temperature range of high-temperature tempering is very wide and can meet the requirements of various working hardness of the die. Due to the good hardenability of this kind of steel, oil cooling, air cooling or saltpeter grading quenching can be used for quenching.
( 4 ) heat treatment of aging hardened steel plastic molds
1. The heat treatment process of age-hardening steel is divided into two basic steps. Firstly, the solution treatment is carried out, that is, the steel is heated to a high temperature so that various alloy elements are dissolved into austenite and the martensite structure is obtained after the austenite is quenched. In the second step, aging treatment is carried out and aging is used to strengthen the mechanical properties to meet the final requirements.
2. The heating of solution treatment is generally carried out in salt bath furnace and box furnace. The heating time can be 1 min / mm and 2 ~ 2.5 min / mm respectively. Oil cooling is used for quenching. Steel with good hardenability can also be air cooled. If the final forging temperature can be accurately controlled when forging the die blank, solution quenching can be directly carried out after forging.
3. The aging treatment should preferably be carried out in a vacuum furnace. If it is carried out in a box furnace, in order to prevent oxidation of the surface of the mold cavity, a protective atmosphere must be introduced into the furnace, or the aging treatment should be carried out with alumina powder, graphite powder and cast iron scraps under the condition of packing protection. Packing protection heating should appropriately extend the holding time, otherwise it is difficult to achieve the aging effect.
Third, the plastic mold surface treatment
In order to improve the wear resistance and corrosion resistance of the plastic mold surface, it is often properly surface treated.
1. Chrome plating on plastic mold is the most widely used surface treatment method. Chrome plating layer has strong passivation ability in the atmosphere, can maintain metallic luster for a long time, and does not undergo chemical reaction in various acidic media. The hardness of the coating reaches 1000 HV, so it has excellent wear resistance. Chrome plating layer also has high heat resistance, and its appearance and hardness remain unchanged when heated to 500 ℃ in air.
2. Nitriding has the advantages of low treatment temperature ( generally 550 ~ 570 ℃ ), little deformation of the mold and high hardness of the infiltration layer ( up to 1000 ~ 1200 HV ), so it is also very suitable for surface treatment of plastic molds. Steel grades containing alloy elements such as chromium, molybdenum, aluminum, vanadium and titanium have better nitriding performance than carbon steel, and nitriding treatment can greatly improve wear resistance when used as plastic molds.
Surface treatment methods suitable for plastic molds include nitrocarburizing, electroless nickel plating, ion plating of titanium nitride, titanium carbide or titanium carbonitride, PVD, CVD deposition of hard films or superhard films, etc.
First, the plastic mold manufacturing process
1. Mould made of low carbon steel and low carbon alloy steel
For example, the technological route of 20, 20Cr, 20 CRM NTI and other steels is: blanking → forging die blank → annealing → mechanical roughing → cold extrusion forming → recrystallization annealing → mechanical finishing → carburizing → quenching, tempering → grinding and polishing → assembly.
2. High alloy carburized steel mold
For example, the process route of 12CrNi 3A and 12CrNi 4A steel is blanking, forging die blank, normalizing and tempering at high temperature, mechanical roughing, tempering at high temperature, finishing, carburizing, quenching, tempering, grinding and polishing, and assembling.
3. Quenching and tempering steel mold
For example, the technological route of steel such as 45, 40Cr is blanking, forging die blank, annealing, mechanical rough machining, tempering, mechanical finish machining, trimming, polishing and assembling.
4. Carbon tool steel and alloy tool steel molds
For example, T7a - T10a, Cr WMN, 9 SiCr and other steels have the following process routes: blanking, forging into a mold blank, spheroidizing annealing, mechanical rough machining, stress relief annealing, mechanical semi - finishing, mechanical finishing, quenching, tempering, grinding and polishing, and assembly.
5. Pre - hardened steel mold
For example, 5Ni SiCa, 3C R 2 Mo ( P20 ) and other steels. For the direct use of bar stock processing, pre-hardening treatment has been carried out due to the supply status, and it can be directly processed, formed, polished and assembled. For those to be forged into blanks and then processed and formed, the process route is: blanking → forging → spheroidizing annealing → planing or milling six sides → pre-hardening ( 34 ~ 42 HRC ) → mechanical rough machining → stress relief annealing → mechanical finishing → polishing → assembly.
Second, the characteristics of heat treatment of plastic molds
( 1 ) Heat Treatment Characteristics of Carburized Steel Plastic Mould
1. For plastic molds with high hardness, high wear resistance and high toughness requirements, carburized steel shall be selected for manufacturing, and carburization, quenching and low temperature tempering shall be taken as the final heat treatment.
2. For the carburized layer, the thickness of the carburized layer is generally 0.8 - 1.5 mm, the thickness of the mold carburized layer is required to be 1.3 - 1.5 mm when pressing plastic containing hard filler, and the thickness of the carburized layer is required to be 0.8 - 1.2 mm when pressing soft plastic. The carbon content of the carburized layer is preferably 0.7 % to 1.0 %. If carbon and nitrogen infiltration is adopted, the wear resistance, corrosion resistance, oxidation resistance and tack resistance will be better.
3. The carburizing temperature is generally 900 ~ 920 ℃, and the medium temperature carbonitriding can be adopted for small molds with complex cavities at 840 ~ 860 ℃. The carburizing and heat preservation time is 5 ~ 10h, which should be selected according to the requirements for the thickness of the carburized layer. The proper carburizing process is graded carburizing, i.e. the high temperature stage ( 900 ~ 920 ℃ ) is dominated by rapid infiltration of carbon into the surface layer of the part. In the middle temperature stage ( 820 ~ 840 ℃ ), increasing the thickness of the carburized layer is the main step, thus establishing a uniform and reasonable gradient distribution of carbon concentration in the carburized layer and facilitating direct quenching.
4. The quenching process after carburization is different according to the steel grade. After carburization, it can be used separately: reheating and quenching; Direct quenching after graded carburization ( such as alloy carburized steel ); Direct quenching after carbonitriding at medium temperature ( such as small precision die 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 is carried out immediately after rough machining, and then finish machining is carried out to ensure the minimum deformation during heat treatment. For precision molds, the deformation should be less than 0.05 %.
2. The surface requirements of the plastic mold cavity are very strict. Therefore, during quenching and heating, the surface of the mold cavity should not be oxidized, decarburized, eroded or heated. 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, protective agent should be coated on the surface of the mold cavity and the heating speed should be controlled. A relatively mild cooling medium should be selected for cooling to avoid deformation and cracking during quenching and scrapping. Generally, hot bath quenching is preferred, and pre-cooling quenching can also be adopted.
3. Tempering should be carried out in time after quenching, the tempering temperature should be higher than the working temperature of the mold, and the tempering time should be sufficient, depending on the mold material and section size, but at least over 40 - 60 min.
( 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 reformed, and the reformed die blank must be heat treated.
2. The pre-heat treatment of pre-hardened steel usually adopts spheroidizing annealing to eliminate forging stress, obtain uniform spheroidal pearlite structure, reduce hardness, improve plasticity, and improve the machinability or cold extrusion formability of the die blank.
3. The pre-hardening process of pre-hardening steel is simple, most of them are quenched and tempered, and tempered sorbite structure is obtained after quenching and tempering. The temperature range of high-temperature tempering is very wide and can meet the requirements of various working hardness of the die. Due to the good hardenability of this kind of steel, oil cooling, air cooling or saltpeter grading quenching can be used for quenching.
( 4 ) heat treatment of aging hardened steel plastic molds
1. The heat treatment process of age-hardening steel is divided into two basic steps. Firstly, the solution treatment is carried out, that is, the steel is heated to a high temperature so that various alloy elements are dissolved into austenite and the martensite structure is obtained after the austenite is quenched. In the second step, aging treatment is carried out and aging is used to strengthen the mechanical properties to meet the final requirements.
2. The heating of solution treatment is generally carried out in salt bath furnace and box furnace. The heating time can be 1 min / mm and 2 ~ 2.5 min / mm respectively. Oil cooling is used for quenching. Steel with good hardenability can also be air cooled. If the final forging temperature can be accurately controlled when forging the die blank, solution quenching can be directly carried out after forging.
3. The aging treatment should preferably be carried out in a vacuum furnace. If it is carried out in a box furnace, in order to prevent oxidation of the surface of the mold cavity, a protective atmosphere must be introduced into the furnace, or the aging treatment should be carried out with alumina powder, graphite powder and cast iron scraps under the condition of packing protection. Packing protection heating should appropriately extend the holding time, otherwise it is difficult to achieve the aging effect.
Third, the plastic mold surface treatment
In order to improve the wear resistance and corrosion resistance of the plastic mold surface, it is often properly surface treated.
1. Chrome plating on plastic mold is the most widely used surface treatment method. Chrome plating layer has strong passivation ability in the atmosphere, can maintain metallic luster for a long time, and does not undergo chemical reaction in various acidic media. The hardness of the coating reaches 1000 HV, so it has excellent wear resistance. Chrome plating layer also has high heat resistance, and its appearance and hardness remain unchanged when heated to 500 ℃ in air.
2. Nitriding has the advantages of low treatment temperature ( generally 550 ~ 570 ℃ ), little deformation of the mold and high hardness of the infiltration layer ( up to 1000 ~ 1200 HV ), so it is also very suitable for surface treatment of plastic molds. Steel grades containing alloy elements such as chromium, molybdenum, aluminum, vanadium and titanium have better nitriding performance than carbon steel, and nitriding treatment can greatly improve wear resistance when used as plastic molds.
Surface treatment methods suitable for plastic molds include nitrocarburizing, electroless nickel plating, ion plating of titanium nitride, titanium carbide or titanium carbonitride, PVD, CVD deposition of hard films or superhard films, etc.
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