/Injection moulding materials selection

Injection moulding materials selection

Working conditions of plastic injection molding

Due to development of plastic injection molding industry, quality requirements for plastic injection molding are getting higher and higher. Therefore, failure of plastic injection molding and its influencing factors have become important research topics. Main working parts of plastic injection molding are plastic molded parts, such as punches and dies, which form cavity of plastic injection molding to form various surfaces of plastic molded parts and directly contact plastic to withstand pressure, temperature, friction and corrosion.

Analysis of failure causes of injection moulding materials

General mold production includes mold design, material selection, heat treatment, machining, debugging and installation. According to survey, materials used in the mold and heat treatment are main factors affecting service life. From perspective of total quality management, factors affecting service life of mold cannot be measured as sum of polynomials, but should be product of multiple factors. Thus quality of injection moulding materials and heat treatment is particularly important throughout mold production process. From analysis of general phenomenon of mold failure, plastic injection molding can produce wear failure, local deformation failure and fracture failure during service.

Custom plastic molding steel performance requirements

With rapid development of manufacturing industry, plastic injection molding is an indispensable tool in plastic injection molding processing and proportion of plastic injection molding in total mold output has increased year by year. With development of high-performance plastics and continuous production, variety of plastic products is increasing, use is expanding, and products are becoming more sophisticated, larger and more complex. Mold production is developing at a high speed, and working conditions of mold are becoming more complicated.

  • Wear and corrosion of cavity surface

Plastic melt flows in the cavity with a certain pressure, and solidified plastic parts are released from mold, causing friction on the molding surface of mold and causing wear. Root cause of failure of custom plastic molding wear is friction between mold and material. However, specific form of wear and wear process are related to many factors, such as pressure of mold during work, temperature, material deformation speed and lubrication. When material used in plastic mold and heat treatment are unreasonable, cavity surface of plastic injection molding has low hardness and poor wear resistance, and performance is as follows: dimensional difference of cavity surface due to wear and deformation; roughness value becomes higher due to napping, surface quality deteriorates. In particular, when solid material is used to enter mold cavity, it exacerbates wear of cavity face. In addition, plastics contain chlorine, fluorine and other components that are thermally decomposed into corrosive gases HC1 and HF, causing corrosion and wear on cavity surface of plastic injection molding, resulting in failure. If wear and tear are accompanied by wear and damage, coating or other protective layer on the surface of cavity is destroyed, which will promote corrosion process. Intersection of two types of damage accelerates corrosion-wear failure.

  • Plastic deformation failure

Custom plastic molding cavity surface is pressed and heated to cause plastic deformation failure, especially when small mold works on large tonnage equipment, it is more likely to produce overload plastic deformation. Material strength and toughness of plastic mold are insufficient, and deformation resistance is low. Another reason for plastic deformation failure is that hardened layer on the surface of mold cavity is too thin, deformation resistance is insufficient, or working temperature is higher than tempering temperature, and phase change softens, which causes mold to fail early. 3) Main cause of fracture fracture is structural stress, thermal stress or insufficient temper due to structural and temperature difference. At use temperature, retained austenite is transformed into martensite, causing local volume expansion, resulting from tissue stress generated inside the mold. Working condition of plastic injection molding is different from that of cold die. Generally, it must be operated at 150℃-200℃. In addition to being subjected to a certain pressure, it must also withstand temperature effects. Same mold can have multiple failure modes, and multiple damages can occur even on same mold. From failure mode of plastic mold, it is important to select injection moulding materials and heat treatment reasonably, because they are directly related to service life of mold. Therefore, steel for plastic molds should meet following requirements:

  • Heat resistance

With advent of high-speed molding machinery, speed of plastic products is increasing. Since molding temperature is between 200 and 350℃, if plastic fluidity is not good, forming speed is fast, and molding surface temperature of mold portion exceeds 400℃ in a very short time. In order to ensure accuracy and deformation of mold during use, mold steel should have high heat resistance.

  • Sufficient wear resistance

With expansion of use of plastic products, inorganic materials such as glass fiber are often added to plastics to enhance plasticity. Due to addition of additives, fluidity of plastics is greatly reduced, resulting in wear of molds. Therefore, it is required that chess has good wear resistance.

  • Excellent machinability

Most plastic molding dies require a certain amount of cutting and fitter repair in addition to EDM. In order to extend service life of cutting tool, work hardening is small during cutting process. In order to avoid deformation of mold and affect accuracy, it is desirable to control residual stress to a minimum.

  • Good thermal stability

Shape of parts of plastic injection molding is often complicated and difficult to process after quenching. Therefore, materials with good thermal stability should be used as much as possible.

  • Mirror surface processing performance

Surface of cavity is smooth, molding surface is required to be polished into a mirror surface, and surface roughness is lower than Ra 0.4 μm to ensure appearance of plastic pressed part and facilitate demolding.

  • Heat treatment performance

In the mold failure accident, accident caused by heat treatment is generally 52.3%, so that heat treatment plays an important role in the whole mold production process, and quality of heat treatment process has a great influence on mold quality. Generally, heat treatment deformation is small, quenching temperature range is wide, and heat sensitivity is small, in particular, there is a large hardenability and hardenability.

  • Corrosion resistance

During forming process, corrosive gas may be released and decomposed by a corrosive gas, such as HC1, HF, etc., sometimes mold is rusted and damaged at air flow passage, so mold steel is required to have good resistance. Corrosive.

New plastic mold steel

General plastic injection molding are often used in normalized 45 steel or 40Cr steel after quenching and tempering. Plastic molds with higher hardness requirements are made of steel such as CrWMn or Crl2MoV. For plastic molds with higher working temperatures, hot work steel with high toughness can be selected. In order to meet higher requirements of dimensional accuracy and surface quality of plastic cavities, a series of new mold steels have been newly developed.

  • Carburized plastic mold steel

Carburized custom plastic molding steel is mainly used for cold extrusion molding of complex plastic molds. This type of steel has low carbon content, often adding element Cr, and adding appropriate amount of Ni, Mo and v. Effect is to improve hardenability and carburizing ability to facilitate cold extrusion, such steel must have high plasticity and low deformation resistance in the annealed state, annealing hardness less than 1 00HBS. After cold extrusion, carburizing and quenching and tempering are performed, and surface hardness can reach 58—62HRC. Such steels have special steel grades abroad, such as 8416 in Sweden, P2 and P4 in United States. 12CrNi3A, 12Cr2Ni4A steel and 20Cr2Ni4A are often used in China mold maker, which have good wear resistance, no collapse and surface peeling, and injection moulding die life is improved. Elements cr, Ni, Mo, and V in steel increase hardness and wear resistance of carburized layer and toughness of core.

  • Pre-hardened plastic mold steel

Carbon content of such steel is 0.3% -0.55%, and commonly used alloying elements are Cr, Ni, Mn, v and so on. In order to improve its machinability, add elements such as s, ca. Several typical plastic mold steels have been developed through the development and introduction. Y55CrNiMn-MoVS (SMI), is a S-based free-cutting plastic mold steels developed in China mold maker. It is characterized by pre-hardened delivery hardness of 35_40 HRC. It has good machinability and can be used directly after processing without heat treatment. Adding Ni solid solution strengthening and increasing toughness, adding Mn and S to form a free-cutting phase MnS; adding Cr, Mo, V, increasing hardenability of steel 8Cr2S steel is a steel for easy cutting precision injection moulding die.

  • Age-hardening plastic injection molding steel has developed low-cobalt, cobalt-free, low-nickel maraging steel.

MASI is a typical Martens-age steel. After 8150C solution treatment, hardness is 28-32HRC. After mechanically processed, and then subjected to 4800C aging, intermetallic compounds such as Ni3Mo and Ni3Ti are precipitated to achieve a hardness of 48-52 HRC. Steel has high toughness, small dimensional change during aging, and good weld repair performance, but steel is expensive and not very popular in China mold maker.

  • Corrosion-resistant plastic injection molding steel

Plastic products made of polyvinyl chloride (Pvc) and ABS plus flame retardant resin will decompose and produce corrosive gas during forming process, which will corrode mold. Therefore, plastic mold steel is required to have good corrosion resistance. Commonly used corrosion resistant plastic mold steels in foreign countries include martensitic stainless steel and precipitation hardened stainless steel such as STVAX (4Crl3) and A SSAB-8407 from Sweden ASSAB.