德语
A mold is a tool for metal or non-metal molding. On the one hand, the mold is subject to friction from metal or non-metal, and on the other hand, the extruded material brings a relatively large compressive stress to the mold. Therefore, the mold should have high strength and hardness, and at the same time have a certain toughness. In mold manufacturing, each part has complexity and speciality, and has an overall coordination between them, which brings different requirements to the mold design and processing technology.
1. Mold hardness: soft mold, hard mold
1) Soft mold: The mold steel has been pre-hardened and does not require quenching treatment, but the hardness is low, the HB is below 400, the production life is within 500,000 times, and it is widely used.
First-class mold quality
1. Mold: It must be able to open a mold of one million or more times.
This is a first-class mold, and the customer requires that the best materials and accessories must be made together.
The best mold.
2. The requirements for first-level molds are as follows:
(1) Detailed mold design (connected to computer pictures and materials);
(2) The mold embryo must have at least HB280;
(3) The mold core must be hard at least, and all row positions and accessories must be hardened;
(4) You must hold the edge of the tube to the thimble;
(5) There must be wear-resistant sheets on the line position;
(6) The temperature control monitor must be installed in the mold, brother or position according to feasibility;
(7) It is recommended to use all cooling waterways to make nickel plating (ELECTROLESS NICKEL PLATING) to prevent rust and easy cleaning;
(8) Positioning locks are required for the split mold line.
2) Hard mold: The mold steel is annealed and the processing is divided into two rough processing and finishing processes. After rough processing, the hardness is above HRC48°. After rough processing, the processing allowance of about 0.2 must be retained, because the workpiece will deform after quenching treatment to prevent the finishing process from meeting the workpiece requirements. The production life is more than 500,000 to 1 million times, and small molds are most used.
Secondary mold quality
1. Mold: It must be able to open molds of 500,000 or more.
This is a secondary and high-quality mold. Good materials and accessories must be used, and there are certain standards for the tolerances of the mold (dimensional accuracy). This mold also requires a high quality.
2. The requirements for secondary molds are as follows:
(1) It is recommended to do detailed mold design
(2) The hardness of the mold base must be at least HB280
(3) The mold core must be at least hard HRC48, and all positions and accessories must also require heat treatment.
(4) The temperature control monitor must be installed in the mold, brother or row position according to feasibility.
(5) Positioning locks must be added to the parting line
(6) The following requirements will be established upon individual request and at the time of quotation.
Level 3 quality mold
1. Mold: It must be able to open molds for 250,000 or more times. This is a general mold requirement, and the production is also a medium production mold.
2. The requirements for level three molds are as follows:
(1) It is recommended to do mold design
(2) The hardness of the mold base must be at least HB165
(3) The mold core must be at least HB280
(4) In addition to the above three basic requirements, all others are regarded as optional additional requirements.
Level 4 quality mold
1. Mold: Approximately 10,000 shots are required. This is a low-production mold. Generally speaking, there are no special requirements, but the mold quality still needs to be good and accepted by customers.
2. The requirements for level four molds are as follows: Mold Man Magazine WeChat is focused and professional@@
(1) It is recommended to do mold design
(2) The mold base may be ordinary copper or aluminum
(3) The mold meat can be made of aluminum or steel with the customer’s consent.
(4) In addition to the above three basic requirements, all others are regarded as optional additional requirements.
4. Mold difficulty: Level A, Level B, Level C, Level D
Grade A: Molds with multiple row positions, multiple partings on the sloped top, core pulling and rotating core pulling, etc., are very complex structures.
Grade B: There are multiple (two to four) rows of inclined roofs, two to three partings, and a drawn core. The mold structure is relatively complex.
Grade C: A simple mold with a fine nozzle for glue inlet, a mold with one or two rows, a sloping top and other general structures.
Grade D: Dashuikou mold, two-plate mold, mold with simple structure such as no rows and no inclined roof.
5. Mold size: extra large, large, medium, small
Extra large: mold with mold width above 800mm.
Large: Molds with a mold width between 600~800mm (excluding 800).
Medium size: molds with mold widths between 350 and 600 (excluding 600).
Small: molds with a mold width less than 350mm.
Selection of plastic mold steel
In recent years, while introducing foreign plastic mold steel, our country has independently researched and developed a series of new special steels for plastic molds, forming a series of plastic mold steels with Chinese characteristics.
1. Application of carbon plastic mold steel
Because carbon plastic mold steel has the advantages of good processing performance, low price, and convenient sources of raw materials, it is widely used in small molds with simple shapes or molds with low precision and long service life.
YB/T074-1997 lists steel grades such as SM45, SM50 and SM55. Compared with high-quality carbon steel, this type of steel has lower S and P content in the steel. The purity of the steel is good, the fluctuation range of carbon content is narrow, and the mechanical properties are more stable.
For thermosetting plastic molds with higher hardness requirements and smaller sizes, they are generally made of high-carbon carbon tool steel with wC=0.7﹪~1.3﹪, such as T7, T8, T9, T10, T11, T12 and other steels. This type of steel can obtain high hardness and high wear resistance after heat treatment.
2. Application of carburized plastic mold steel
Some complex plastic mold cavities are cold extruded and directly pressed through hardening, eliminating the need for cutting the cavity. This is a very economical processing method for batch-produced molds. After carburizing, quenching, and low-temperature tempering, the mold has a surface with high hardness, high wear resistance, and a core structure with good toughness. It can produce various molds that require high wear resistance on the surface and good core toughness. This requires the use of carburized plastic mold steel.
The carbon content of carburized plastic mold steel is very low, generally 0.1﹪~0.2% by mass, and the plastic deformation resistance is very small. The hardness after softening and annealing is ≤160HBW, and the complex cavity is ≤130HBW to facilitate cold extrusion of the cavity. In the past, my country generally used low carbon steel and low carbon alloy steel, such as 15, 20, 20Cr, 12CrNi2, 12CrNi3, 20Cr2Ni4 and 20CrMnTi steels. my country has developed special steel LJ (0Cr4NiMoV) for cold extrusion plastic molds in recent years. After cold extrusion, this steel has been carburized, quenched and low-temperature tempered. Its surface hardness reaches 58~62HRC, and its core hardness is 28HRC. The mold has good wear resistance and no collapse or surface peeling. It can be used to manufacture plastic forming molds with complex shapes and high loads.
3. Application of pre-hardened plastic mold steel
For large and medium-sized plastic molds with complex and precise shapes, in order to avoid deformation during the quenching heat treatment process, mold steel is supplied to the market in a pre-formed state.
This type of steel is generally medium-carbon low-alloy steel. It is forged in a steel plant and made into modules. It is pre-heated to reach the hardness when the mold is used, thereby avoiding mold deformation and cracking problems caused by heat treatment, thereby ensuring the manufacturing accuracy of the mold.
Pre-hardened plastic mold steels that have been included in national standards include: SM3Cr2Mo steel and SM3Cr2MnNiMo steel. The hardness is generally 28~36HRC, which is suitable for manufacturing large and medium-sized and high-precision plastic molds.
In order to improve the processing performance of pre-hardened plastic mold steel, easy-cutting elements S, Ca, Pb, Sb, etc. are often added to the pre-hardened steel. Our country has developed some easy-cutting pre-hardened plastic mold steels, such as 5CrNiMnMoVSCa steel, 8Cr2MnWMoVS steel, 4CrMnVBSCa steel, 5CrNiMnMoVS steel, etc., which have significantly improved the cutting performance of steel under higher hardness. Pre-hardened free-cut plastic mold steel can be used not only to manufacture large and medium-sized precision injection molds, but also to manufacture sophisticated and complex cold work molds.
Failure forms and causes of plastic molds
plastic deformation
It means that the plastic mold fails due to local plastic deformation under the action of continuous heat and pressure. The main reasons are: the material strength and toughness of the mold used is insufficient; the mold is overloaded; the hardened layer on the surface of the mold cavity is too thin and the deformation resistance is insufficient; the mold is insufficiently tempered; the mold softens because the working temperature is higher than the tempering temperature, causing surface wrinkles, depressions, pitting, edge collapse (collapse), etc.
fracture
Due to the complex shape of the plastic mold, there are many edges, corners, thin walls and other parts, which are prone to stress concentration when the mold is working. When the stress value of these parts exceeds the fracture strength of the mold material, fracture failure will occur. In addition, when plastic molds made of alloy tool steel are not sufficiently tempered, they are prone to breakage and failure during use.
surface corrosion
This is due to the presence of chlorine, fluorine and other elements in solid fillers in thermosetting plastics and some thermoplastic plastics. When heated, they decompose and emit highly corrosive gases such as HCl and HF, which corrode the surface of the mold cavity and aggravate its wear and failure. Surface corrosion will cause the surface quality of the mold cavity to decrease and the dimensions to be out of tolerance, reducing the life of the mold.
surface wear
The severe friction of the thermosetting plastic on the mold surface causes scratches (roughening) on the mold surface, which affects the appearance quality of the pressed parts. After repeated polishing and repair, the mold cavity will fail due to out-of-tolerance dimensions. The solid additives contained in the thermosetting plastic will also intensify the wear on the mold cavity, which will not only increase the surface roughness value of the mold cavity rapidly, but also make the mold cavity size out of tolerance. When the materials and heat treatment used for the mold are unreasonable, the surface hardness of the plastic mold cavity will be low, which will also make the mold wear resistance worse.
fatigue and thermal fatigue
During the working process of the plastic mold, due to the cyclic mechanical load, the cavity surface of the mold is subjected to pulsating tensile stress, which causes the damage of the mold, which is called fatigue failure. The plastic mold also bears cyclic thermal loads during the service process. Under repeated heating and cooling conditions, the cavity surface can cause thermal fatigue cracks to sprout at the stress concentration of the mold cavity. In addition, the pulsating tensile stress on the mold cavity surface causes the thermal fatigue cracks to expand in depth, eventually causing the mold to break.
Reason for failure:
The mold life is less than 2,000 pieces, and its main failure modes are roughening (scratches) on the cavity surface and edge collapse. The hardness test of the failed mold revealed that the hardness of the cavity surface and edges was 56~58HRC, indicating that the mold had a certain degree of hardness reduction during use. Its metallographic structure was tempered martensite + granular cementite + a small amount of retained austenite.
Through the analysis of the hardness reduction of the mold, it can be seen that the service temperature of the mold is higher than the tempering temperature (200°C), which is also related to the insufficient tempering of the mold, which causes the mold to continue to be tempered after being heated during service, resulting in the decomposition of martensite and the transformation of retained austenite, resulting in "phase transformation superplastic" flow under pressure, causing roughening of the surface of the mold cavity and collapse of the edges.
Failure forms and causes of cold work molds
wear and tear
The material loss caused by the friction between the working part of the cold working die and the processed material can cause the shape and size of the working part (edge, punch) to change and cause failure, such as the cutting edge of the blanking die becoming blunt, the working surface of the cold heading die having grooves, etc.
Wear failure includes normal wear failure and abnormal wear failure. For punch dies and cold extrusion dies, if the working part is worn to the point of being irreparable without breaking, it is normal wear; abnormal wear is when the working surface of the die and the material being processed are engaged under the action of local high pressure, causing sudden changes in the shape and size of the blank surface, or serious scratches on the product surface, leading to failure. Such defects are prone to occur in cold drawing dies, bending dies and cold extrusion dies.
fracture
Cold work molds suddenly crack or break during use and fail. According to the damage situation, they can be divided into local damage (such as peeling, chipping, tooth loss, etc.) and overall damage (such as chipping, fracture, swelling, splitting, etc.). Their common feature is that most damage occurs at the working part that bears the greatest stress, or at the stress concentration point where the cross-section changes.
According to the characteristics of the fracture process, it can be divided into two forms: brittle fracture and fatigue fracture. Brittle fracture is mainly caused by metallurgical defects, process defects in the mold, or overloading due to improper operation; fatigue fracture is mainly caused by cyclic stress, which is common in various heavy-duty molds, such as cold heading dies and cold extrusion dies.
plastic deformation
Cold work molds undergo plastic deformation and lose their original geometric shape during use. This usually occurs in molds with low hardness or too thin hardened layers. The specific manifestations include upsetting and bending of the punch, sinking and collapse of the concave mold cavity, collapse of edges and corners, and expansion of the die hole.
bite
When the material to be processed comes into contact with the working part of the mold, under high pressure friction, the lubricating oil film breaks and bite occurs. At this time, the metal of the workpiece is "cold welded" to the surface of the mold cavity, causing scratches on the surface of the product to be processed. Occlusion failure is common in bending, deep drawing, cold heading, cold extrusion and other processes.
Cause analysis
The die is used to process FAW "Volkswagen" A4 and "Jetta" 5V pedal mechanism assemblies and other products. When the die is processed by wire cutting, the problem of die cracking (see Figure 2) often occurs. After analysis, the main reasons for mold wire cutting cracking are: the presence of quenching residual internal stress and low material toughness. When the mold is large in size, the mold will generate internal stress after heat treatment (tensile stress on the surface and compressive stress on the inside). When the two internal stresses cancel each other out to maintain the internal stress balance, the mold will not easily crack. When the mold is processed by wire cutting, the tensile stress will increase, which will destroy the internal stress balance. In addition, the high carbon steel mold material has low toughness, which will easily cause the mold to crack. Statistics show that this situation mostly occurs when the mold thickness exceeds 50mm.
Attached factors affecting mold failure
1. Influence of mold structure
Mold structure includes mold geometry such as fillet radius, geometric angle (end face shape of punch, change in taper and cross-section of female die), structural form (integral structure, combined structure, prestressed nesting structure), mold clearance, structural stiffness, etc.
The rationality of the mold structure has a great impact on the load-bearing capacity of the mold. A reasonable mold structure can make the mold evenly stressed during operation, less likely to be unbalanced, have less stress concentration, and less likely to cause early failure; an unreasonable mold structure may cause severe stress concentration or high operating temperatures, thereby deteriorating the working conditions of the mold and leading to early failure of the mold.
2. Influence of mold material
1) Mechanical performance indicators of mold materials. Mold materials should usually meet the mold's performance requirements for plastic deformation resistance, fracture resistance, fatigue resistance, hardness, wear resistance, and hot and cold fatigue resistance. If the requirements cannot be met, early mold failure will occur. For example, under cyclic loading, if the fatigue resistance of the mold material is low, fatigue cracks may initiate after a certain stress cycle and gradually expand until the mold breaks and fails.
2) Metallurgical quality of mold steel. The metallurgical quality of mold steel has a great influence on the failure mode of the mold. Defects such as non-metallic inclusions, segregation, loose centers, and white spots in steel can reduce the strength, toughness, fatigue, and hot and cold fatigue resistance of the steel, causing early failure of the mold, such as cracking, splitting, breaking, and dents on the working surface.
3. Influence of mold manufacturing process
The mold manufacturing process includes forging, machining, heat treatment, etc. Some mold processing requires all of the above processes. The processing quality of each process will affect the damage process and failure mode of the mold to varying degrees.
If the forging process is unreasonable, it will not achieve the purpose of breaking grains, improving directionality, increasing the density of steel, etc., and may even cause defects such as forging cracks; if there are sharp corners or high surface roughness values during mechanical processing, leaving machining knife marks, fatigue cracks will easily sprout in these parts. Improper grinding can easily burn the mold surface or produce grinding cracks; improper heat treatment may cause heat treatment defects. For example, if the quenching temperature is too high, it will cause the steel to overheat or even overburn. The mold is prone to chipping or early fracture. If the quenching temperature is too low, it is difficult to ensure that sufficient alloy elements are dissolved in the matrix, and the mold is prone to early deformation, collapse or thermal fatigue cracks. If the quenching cooling rate is too fast, quenching cracks are prone to occur, and early fracture is prone to occur during the use of the mold.
4. Influence of mold working conditions
The working conditions of the mold are related to the following factors: the characteristics of the equipment, the material, shape, accuracy and temperature of the blank to be processed, the cooling and lubrication conditions of the mold, the preheating and working stress relief of the mold, etc. These factors all have an impact on the failure mode of the mold.
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