Working conditions of plastic molds
Due to the development of plastic and plastic molding industry, the quality of plastic mold is also higher and higher, so the failure of plastic mold and its influencing factors have become an important research topic. The main working parts of the plastic mold are forming parts, such as punch, concave die, etc., they constitute the cavity of the plastic mold, in order to form a variety of plastic parts of the surface and direct contact with plastic, subjected to pressure, temperature, friction and corrosion.
Failure reason analysis of plastic mold material
General mold manufacturing includes mold design, selection of materials, heat treatment, mechanical processing, debugging and installation processes. According to the investigation, the material and heat treatment are the main factors affecting the service life of the die. From the perspective of total quality management, the factors affecting the service life of the mold can not be measured as the sum of polynomials, but should be the product of multiple factors, so that the advantages and disadvantages of the mold material and heat treatment in the whole mold manufacturing process is particularly important.
From the analysis of the common phenomenon of mold failure, the plastic mold can produce wear failure, local deformation failure and fracture failure in the service process. The important failure modes of plastic moulds can be divided into wear failure, local plastic deformation failure and fracture failure.
Plastics die steel performance requirements
Plastic die steel performance requirements With rapid development of manufacturing, plastic mold is an indispensable tool in the plastic molding processing, in the mold production and the proportion of total increase year by year, with the development of high-performance plastics and continuous production, more and more types of plastic products, expanding use, products to the motors, large-scale and complicated development. Molding production to high-speed development, mold working conditions are becoming more and more complex.
1) Wear and corrosion of the cavity surface
The plastic melt flows in the mold cavity with a certain pressure, and the solidified plastic parts come out from the mold, which causes friction on the mold forming surface and causes wear and tear. The root cause of plastic mold wear failure is the friction between the mold and the material. However, the specific form of wear and wear process are related to many factors, such as the pressure, temperature, material deformation speed and lubrication status in the process of working. When the materials and heat treatment of the plastic mold are not reasonable, the surface hardness of the plastic mold cavity is low and the wear resistance is poor, which is manifested as: the size of the cavity surface is out of tolerance due to wear and deformation; The roughness value becomes higher and the surface quality deteriorates. Especially when the solid material is used to enter the plastic model cavity, it will aggravate the wear of the cavity surface. In addition, chlorine, fluorine and other components in plastic processing are decomposed into corrosive gases HC1 and HF by heat, which causes corrosion and wear on the cavity surface of plastic mold, leading to failure. If there is wear and tear damage at the same time, so that the coating or other protective layer on the surface of the cavity is destroyed, it will promote the corrosion process. Two kinds of damage cross action, accelerate the corrosion – wear failure.
2) Plastic deformation failure
Plastic model cavity surface compression and heating can cause plastic deformation failure, especially when the small mold is working on large tonnage equipment, it is easier to produce plastic deformation overload. The strength and toughness of the material used in the plastic mold are insufficient, and the deformation resistance is low; Another reason for plastic deformation failure is that the hardening layer on the surface of the mold cavity is too thin, the deformation resistance is insufficient or the working temperature is higher than the tempering temperature and the phase transformation softening occurs, so that the early failure of the mold.
The main reason for the fracture is the structural stress and thermal stress produced by the structure and temperature difference or the insufficient tempering. At the service temperature, the residual austenite is transformed into martensite, causing the local volume expansion and the organizational stress generated in the mold.
The working conditions of the plastic mold are different from those of the cold die. Generally, it must work at 150℃-200℃. In addition to being affected by a certain pressure, it also bears the influence of temperature. The same mold will have a variety of failure forms, even in the same mold may also appear a variety of damage. From the failure form of plastic mold, reasonable selection of plastic mold materials and heat treatment is very important, because they are directly related to the service life of the mold.
Therefore, the steel for plastic molds should meet the following requirements:
1) heat resistance
With the emergence of high – speed forming machinery, the speed of plastic products is accelerated. Due to the forming temperature between 200- 350℃, if the plastic fluidity is not good, forming speed and fast, will make the mold part forming surface temperature in a very short time more than 400℃. In order to ensure the precision and small deformation of the die in use, the die steel should have high heat resistance.
2) adequate wear resistance
With the expansion of the use of plastic products, inorganic materials such as glass fiber are often added to plastic to enhance plasticity. Due to the addition of additives, the fluidity of plastic is greatly reduced, leading to wear and tear of the mold, so it is required to have good wear resistance.
3) excellent machinability
Most plastic molding die, in addition to EDM also need to carry out certain cutting processing and fitter repair. In order to prolong the service life of cutting tools, the work hardening during cutting is small. In order to avoid mold deformation and affect the accuracy, it is hoped that the machining residual stress can be controlled in the minimum limit.
4) Good thermal stability
The shape of the parts of plastic injection mold is often complex, and it is difficult to process after quenching, so the material with good thermal stability should be selected as far as possible.
5) mirror processing performance
The cavity surface is smooth, and the forming surface should be polished to mirror surface with surface roughness less than Ra0.4μm to ensure the appearance of the plastic pressed parts and facilitate demold
6) Heat treatment performance
In the mold failure accidents, the accidents caused by heat treatment are generally 52.3%, so that heat treatment plays an important role in the whole mold manufacturing process, and the quality of heat treatment technology has a great influence on the quality of the mold. General requirements of heat treatment deformation is small, quenching temperature range wide, small overheating sensitivity, especially to have greater hardenability and hardenability and so on.
7) Corrosion resistance
In the forming process, corrosive gas may be released and decomposed by heat to produce corrosive gases, such as HC1, HF and other corrosion molds, which are sometimes damaged by corrosion at the mouth of the air channel, so the mold steel is required to have good corrosion resistance
New type of plastic die steel
Generally, the plastic mold is usually made of normal 45 steel or 40Cr steel after quenching and tempering. The plastic mold with higher hardness requirements is made of steel such as CrWMn or Crl2MoV. For the plastic mold with high working temperature, hot die steel with high toughness can be selected. In order to meet the higher requirements of dimensional precision and surface quality of plastic cavity, a series of new die steels have been developed recently.
1) carburized plastic die steel
Carburizing type plastic die steel is mainly used for cold extrusion molding cavity complex plastic mould, this kind of low carbon steel, often add element Cr, at the same time adding suitable amount of Ni, Mo and v, role is to improve the hardenability and carburizing ability, to facilitate the cold extrusion forming, this kind of steel in the annealed state must have a high plasticity and low deformation resistance, annealing hardness 1 00 HBS or less. After cold extrusion, the surface hardness can reach 58- 62HRC after carburizing and quenching and tempering. This kind of steel abroad has special steel, such as Sweden’s 8416, the United States P2 and P4, etc. 12CrNi3A and 12Cr2Ni4A steel, 20Cr2Ni4A steel are often used in China, which have good wear resistance, no collapse and surface peeling phenomenon, and improve the life of the mold. The elements cr, Ni, Mo and V in steel increase the hardness and wear resistance of the carburized layer and the strength and toughness of the core.
2) pre-hardened plastic die steel
The carbon content of this kind of steel is 0.3%-O.55%, and the common alloying elements are Cr, Ni, Mn, v, etc. In order to improve its machinability, s, ca and other elements are added. Several typical plastic die steels Y55CrNiMn-MoVS(SMI) are developed in China, which contain S-series free cutting plastic die steels. The characteristics of SMI are that the hardness of pre-hard delivery is 35_40 HRC, it has good machinability, and it can be used directly without heat treatment after processing. Adding Ni solution to strengthen and increase toughness, adding Mn and S to form free-cutting phase MnS; Add Cr, Mo, V, increase the hardenability of steel 8Cr2S steel is enough to belong to the free cutting precision die steel.
3) age hardening plastic die steel
Maraging steel with low cobalt, no cobalt and low nickel has been developed. MASI is a typical maraging steel. After 8150C solution treatment, the hardness is 28-32HRC, Ding mechanical processing, and then 4800C aging, aging fold Ni3Mo, Ni3Ti and other intermetallic compounds, so that the hardness of 48-52 HRC. Steel has high strength and toughness, small size change when aging, good welding performance, but the price of steel is expensive, not very popular in China.
4) corrosion-resistant plastic die steel
Plastic products made of polyvinyl chloride (Pvc) and ABS plus anti-combustion resin as raw materials decompose corrosive gases in the forming process, which will corrode the mold. Therefore, the plastic die steel is required to have good corrosion resistance. Martensitic stainless steel and precipitate hardening type stainless steel are commonly used abroad. Foreign companies such as Sweden ASSAB STVAX (4Crl3) and ASSAB A 8407 and so on.