1, plastic mold working conditions
Due to the development of plastics and plastic molding industry, the quality requirements of plastic molds are getting higher and higher, so the failure of plastic molds and their factors have become an important research topic. The main working parts of plastic molds are molded parts, such as convex molds, concave molds, etc., which form the cavities of plastic molds to form various surfaces of plastic parts and come into direct contact with plastics, subjecting to pressure, temperature, friction and corrosion.
2, plastic mold material failure cause analysis
General mold manufacturing includes mold design, material selection, heat treatment, machining, commissioning and installation processes. According to the investigation, among the factors of mold failure, the material and heat treatment used in mold are the main factors affecting the service life. From the point of view of quality management, the factors affecting the life of the mold can not be measured as the combination of polynthics, but should be the product of many factors, so that the advantages and disadvantages of mold materials and heat treatment in the entire mold manufacturing process is particularly important.
From the analysis of the general phenomenon of mold failure, plastic mold can produce wear failure, local deformation failure and fracture failure during service. The important forms of failure of plastic molds can be divided into wear failure, local plastic deformation failure and fracture failure.
3, plastic mold steel performance requirements With the rapid development of the manufacturing industry, plastic mold is an indispensable tool in plastic molding processing, the proportion of total mold production increased year by year, with the development and continuous development of high-performance plastics, plastic products are increasing in variety, the use of expanding, products to precision, large-scale, complex development. Molding to high-speed development, mold working conditions are becoming more and more complex.
1) Wear and corrosion of the surface of the cavity
Plastic melts flow within the mold cavity under certain pressure, and solidified plastic parts are removed from the mold, causing friction and wear on the mold molding surface. The root cause of plastic mold wear failure is friction between mold and material. However, the specific form of wear and the wear process are related to many factors, such as the pressure, temperature, material deformation speed and lubrication condition of the mold during operation. When the material used in plastic mold and heat treatment is unreasonable, the surface hardness of the cavity surface of the plastic mold is low, wear resistance is poor, which is manifested as: the size of the cavity surface due to wear and deformation is super-poor; Especially when solid-state materials are used to enter the plastic model cavity, it can aggravate the wear of the cavity surface. In addition, the plastic processing contains chlorine, fluorine and other components by heat decomposition of corrosive gases HC1, HF, so that the plastic mold cavity surface corrosion wear, resulting in failure. If wear is accompanied by wear damage, the coating or other protective layer of the cavity surface is damaged, the corrosion process is facilitated. The intersection of the two types of damage accelerates corrosion and wear failure.
2) Plastic deformation failure
Plastic model cavity surface pressure, heat can cause plastic deformation failure, especially when small molds in large tonnage equipment when working, it is easier to produce overload plastic deformation. The material strength and toughness used in plastic molds are insufficient, the deformation resistance is low, and another reason for the failure of plastic deformation is mainly that the hardened layer of the surface of the mold cavity is too thin, the deformation resistance is insufficient or the operating temperature is higher than the return temperature and the phase softening occurs, which causes the mold to fail early.
3) Break
The main cause of fracture is due to structural stress, thermal stress or insufficient tempering due to structural and temperature differences, which, at the temperature of use, transforms the residual Austella into a marsome, causing local volume expansion and tissue stress generated inside the mold.
The working conditions of plastic molds are different from between cold punches, which generally have to be carried out at 150 degrees C-200 degrees C, in addition to being subjected to certain pressure effects, but also subject to temperature effects. The same mold can have a variety of failure forms, even on the same mold can be a variety of damage. From the failure form of plastic mold can be known, reasonable selection of plastic mold materials and heat treatment is very important, because they are directly related to the life of the mold. Therefore, steel for plastic molds should meet the following requirements:
1) Heat resistance
With the emergence of high-speed molding machinery, plastic products speed up. Because the molding temperature is between 200-350 degrees C, if the plastic flow is not good, the molding speed is fast, will make the mold part of the mold surface temperature in a very short period of time more than 400 degrees C. In order to ensure the precision and small deformation of the mold when used, the mold steel should have high heat resistance.
2) Enough wear resistance
With the expansion of the use of plastic products, in plastic often need to add fiberglass and other inorgeable materials to enhance plasticity, due to the addition of added materials, so that the mobility of plastic greatly reduced, resulting in mold wear, so it is required to have good wear resistance.
3) Excellent machining
Most plastic molding molds, in addition to electric spark processing also need to carry out certain cutting and fitting. To extend the life of the cutting tool, the machining hardening is small during the cutting process. In order to avoid mold deformation and affect accuracy, it is hoped that the residual stress of processing can be controlled to a small limit.
4) Good thermal stability
Plastic injection mold part shape is often more complex, hardening is difficult to process, so should try to choose a good thermal stability of the material.
5) Mirror processing performance
The cavity surface is smooth, the molding surface requires polishing into a mirror surface, and the surface roughness is lower than Ra0.4 m to ensure the appearance of the plastic press and facilitate release.
6) Heat treatment performance
In mold failure accidents, accidents caused by heat treatment are generally 52. 3%, so that heat treatment in the entire mold manufacturing process occupies an important position, heat treatment process has a greater impact on the quality of the mold. Generally requires small heat treatment deformation, wide quenching temperature range, low sensitivity to overheating, especially to have greater hardening and quenching and so on.
7) Corrosion resistance
In the forming process may be released corrosive gas by heat decomposition of corrosive gases, such as HC1, HF and other corrosion molds, sometimes at the air flow port to make the mold rust and damage, so the mold steel is required to have good corrosion resistance.
4, new plastic mold steel
General plastic molds are often made of normal 45 steel or 40Cr steel by tempering. Plastic molds with high hardness requirements are made of steel such as CrWMn or Crl2MoV. For plastic molds with higher operating temperatures, you can choose to use high toughness heat as mold steel. In order to meet the higher requirements of plastic cavity for dimensional accuracy and surface quality, a series of new mold steels have recently been developed.
1) Carbon-penetrating plastic mold steel
Carbide-type plastic mold steel is mainly used in cold extrusion molding cavity complex plastic molding, such steel has a low carbon content, often add elements Cr, while adding the appropriate amount of Ni, Mo and v, the role is to improve quenchability and carbon seepage capacity, in order to facilitate cold extrusion forming, this type of steel in the de-ignition state must have a high plasticity and low deformation resistance, anti-ignition hardness ≤1 00HBS. After cold extrusion forming, carbon seepage and quenching re-ignition are carried out, and the surface hardness can reach 58—62HRC. There are special steel types abroad, such as Sweden’s 8416, the United States P2 and P4. Domestic often use 12CrNi3A and 12Cr2Ni4A steel, 20Cr2Ni4A, good wear resistance, no collapse and surface peeling phenomenon, mold life increased. The elements cr, Ni, Mo and V in steel increase the hardness and wear resistance of the carbide layer and the strong toughness of the heart.
2) Pre-hard plastic mold steel
This type of steel has a carbon content of 0.3% -O.55%, commonly used alloy elements such as Cr, Ni, Mn, v, etc. In order to improve its cuttingability, add s, ca and other elements. Through the development, introduction and development of several typical plastic mold steel Y55CrNiMn-MoVS (SMI) is developed in China with S-series easy-to-cut plastic mold steel, characterized by pre-hard state delivery hardness of 35-40 HRC, has better cutting machining, after processing is no longer hot treatment, can be used directly. Adder Ni solid solubility strengthens and increases toughness, adds Mn and S to form easy-to-cut phase MnS, adds Cr, Mo, V, increases the quenchability of steel 8Cr2S steel is enough to belong to easy-to-cut precision mold steel.
3) Time-hardened plastic mold steel
A low cobalt, cobalt-free, low-nickel Mars esophor steel has been developed, and MASI is a typical Mars esophor steel. After 8150C solid solubility treatment, hardness of 28-32HRC, ding for mechanical processing, and then by 4800C estration, erration time fold out Ni3Mo, Ni3Ti and other intermetallic compounds, so that the hardness reached 48-52 HRC. Steel’s strong toughness is high, the time-available size change is small, the welding performance is good, but the steel is expensive and not very popular in China.
4) corrosion-resistant plastic mold steel
Plastic products made from PVC and ABS plus anti-combustion resins decompose to produce corrosive gases during forming, which can corrode molds. Therefore, plastic mold steel is required to have good corrosion resistance. Foreign commonly used corrosion-resistant molded steel has marzizi stainless steel and hardened stainless steel. Foreign countries such as Sweden’s ASSAB STVAX (4Crl3) and A SSAB 8407.