Which tool steel is right for your plastic injection mold is a complicated system engineering, but it is also pivotal link of plastic molding manufacturing and a major work related to mold life and cost. As following we will analyze in terms of basic function, mold function, metallurgical quality of plastic mold steel, versatility and usage requirements.

 

When selecting molding material, it is necessary to consider that some of basic functions of mold steel, they must be able to conform to needs of manufactured mold. Under normal circumstances, these three functions are most important, that is, steel wear resistance, toughness and hardness. These three functions can comprehensively reflect comprehensive function of tool steel, and should be able to select scale of application at a certain level. Of course, for a mold requirement, one or both of them may be primary, while the other one or two are secondary.

When mold is working, contour often has to be strongly rubbed with workpiece many times. Mold must still be connected to its dimensional accuracy and contour roughness in this case, so as not to fall off early. It is required that tool steel can withstand mechanical wear, and when subjected to heavy load and high-speed friction, mold can be formed into a thin and densely oxidized mold by friction profile, lubrication through series connection, preventing adhesion and welding between mold and contour of workpiece to cause scratching on contour of workpiece, and it can reduce damage caused by further oxidation of contour of mold. In order to improve wear resistance of tool steel, it is necessary to adopt a reasonable production process and disposal process, so that tool steel has both high hardness, reasonable composition, description and distribution of hardened phase for carbide in mold steel. Lubrication situation and contour treatment of mold steel also have a superior effect on improving wear resistance of mold.

For molds subjected to strong impact loads, such as punches for cold-working dies, hammers for hot forging dies, cold boring dies, hot forging dies, etc., toughness of plastic mold steel is a major consideration for identity. Molds that work at high temperatures must also consider their high temperature toughness at operating temperatures. For molds with multi-directional impact loads, it is also necessary to consider their isotropic properties. For chemical composition, grain size, carbide, composition, description, size and distribution of plastic mold steel, metallographic structure, microsegregation, etc., it also has an impact on toughness of plastic mold steel. Purity and forging deformation of steel have a great influence on lateral function. Toughness of molding materials tends to be contradictory to wear resistance and hardness. Therefore, according to specific working conditions of mold, choose reasonable molding materials, and use reasonable essence, hot processing and heat treatment, contour treatment process to make tool steel get the best combination of wear resistance and toughness to meet needs for molds. It is the main growth path for plastic mold steel.

Hardness are the primary technical performance indicators of tool steel. Mold must have high hardness and strength when working, so that it can be connected to its original shape and size. Generally, it is cold worked steel and requires quenching and tempering. Hardness is around HRC60, while hot work tool steel is around HRC 45~50. Also hot work tool steel is required to be connected to a certain hardness at its working temperature. Red hardness is ability of hand-printed steel to cross-link its hardness and structural stability against softening at a certain temperature. It is main performance index for hot work tool steel and partially heavy duty cold worked steel. According to actual working conditions of mold, function of actual requirements should be considered to others, such as hot work tool steel should consider its resistance to cold and heat fatigue, die-casting mold should consider its erosion-resistant metal erosion function For heavy-duty cavity molds, attention should be paid to their isotropic properties; for high-temperature work hot-working molds, anti-oxidation function at working temperature should be considered; for molds working in corrosive media, attention should be paid to their anti-corrosion functions; Molds working under high loads should consider their compressive strength, tensile strength and flexural strength, fatigue strength and fracture toughness.

 

In total manufacturing cost of mold, it is rare for small and delicate molds that cost of molding materials often only accounts for 10% to 20% of total cost, sometimes even less than 10%; Machining, heat disposal, contour disposal, assembly, management and other costs account for more than 80%. Therefore, process function of molding materials becomes a major factor affecting cost of mold. Improving process performance of mold not only makes mold production process simple, easy to manufacture, but also can effectively reduce plastic molding manufacturing cost. Process functions of molding materials are often considered in following ways.
Machinability of molding materials includes cold working functions such as cutting, grinding, polishing, cold extrusion, and cold drawing, and thermal processing function can include thermoplastic and hot plus temperature scale. Die steel belongs to hypereutectoid steel and Leysite steel. Cold working and hot working functions are generally not very good. In production process, process parameters of hot working and cold working must be severely controlled to avoid defects and waste products. On the other hand, it is necessary to improve purity of steel, reduce harmful impurities, improve microstructure of steel, and adopt some measures to improve steel process and reduce manufacturing cost of mold. In order to improve machinability and grindingability of die steel, from beginning of 20th century, we studied addition of an appropriate amount of elements such as sulfur, lead, calcium, rare earth metals or graphitized elements of carbon in tool steel and produced a variety of free cutting tool steel. It is found that after some free-cutting elements be added, some harmful inclusions (such as iron sulfide) will be produced in mold steel, which will make mechanical function of steel especially lateral plasticity and toughness reduced, In late stage of essence, molten steel is subjected to denaturation treatment, a denaturing agent (such as SiCa, rare earth element, etc.) is added to form a calcium-rich sulfide or a rare earth sulfide to spheroidize sulfide, thereby suppressing influence of sulfur on mechanical function of steel, retaining and exerting favorable effect on workability and grinding property of steel, makes free-cutting die steel further grow. Some molding materials, such as high vanadium high speed steel and high vanadium high alloy plastic mold steel, have poor grindingability and low grinding ratio. They are not easy to be ground. In recent years, they have been converted to powder metallurgy to make carbides in steel more small and average, completely eliminating large particle carbides in high-vanadium tool steel produced by popular process, not only greatly improving grinding property of such steel, but also improving plasticity, toughness and other functions of steel, enabling it to promote application in plastic molding manufacturing. Some molds have very low profile roughness requirements, such as mirror-polished plastic molding and cold-worked molds. It is necessary to use a molding materials with good polishing function. These steel types are often refined by electroslag remelting or vacuum arc remelting to obtain high-purity steel to meet requirements of mirror polishing.

Some plastic molded products are required to have a grain pattern, a decorative pattern or a contour of character. In order to produce these plastic molded products, it is required to produce a corresponding clear pattern and pattern in contour of mold for pressing these products. Processing of these patterns and skin textures is generally carried out by a chemical etching process, which requires mold material to conform to chemical etching process, skin contours and patterns with clear patterns and clear texture can be obtained in mold contour.

In order to simplify production process, domestic and foreign companies have been working in recent years to grow and use forging process to directly produce forged blanks close to shape of finished mold. For example, China has studied use of forging technology to produce a part of cold work molds, hot work molds and glass forming molds. Correspondingly, some steels for forging molds have been grown, and such materials are required to have superior forging process functions such as fluidity and shortening rate.

Some molds require special wear-resistant or corrosion-resistant materials to be welded in the most demanding parts of working conditions. Some molds are expected to be renewed after being repaired by surfacing process during use. For such molds, it is required to use a molding materials with good weldability to simplify welding process, which can avoid or simplify pre-weld preheating and post-weld disposal processes, better meet needs of welding process, and grow a batch of superior mold welding materials.

In order to simplify process and improve manufacturing efficiency of mold, some of cavity molds produced in batches are pressed by a cold extrusion process, and cavity of mold is directly pressed out by a hardened punch. Steel material must has excellent cold deformation function, such as low carbon and silicon steel in plastic molding steel, which has excellent cold deformation function.

In order to facilitate the production, we hope that quenching temperature of molding materials is wider. It is rare that some molds require flame heating for local quenching, it is difficult to accurately measure and control temperature, and mold steel is required to be wider quenching temperature scale. When mold is in heat treatment, requires deformation level to be small. It is rare that some molds with complicated shapes are difficult to be trimmed in the future, deformation level of quenching and tempering is more serious. Deformed die steel are used to manufacture.

Hardenability depends mainly on carbon content of steel. Hardenability depends mainly on chemical composition of steel, content of alloying elements and microstructure of steel before quenching. For most cold working molds requiring high hardness, hardening property is required to be high. For most hot working molds and plastic molds, hardness requirements are not too high, and hardenability is often considered more; In large-section deep cavity mold, in order to obtain superior structure and average hardness of mold core, it is required to select a mold steel with good hardenability. For other molds with complex shapes, high precision requirements and prone to heat treatment deformation, in order to reduce heat treatment deformation, it is often possible to use quenching medium with cooling capacity (such as oil cooling, air cooling, pressure quenching or salt bath quenching). It is necessary to use a molding materials having better hardenability to obtain a quenching hardness and a hardened layer depth of crucible.

If oxidation and decarburization phenomenon occurs during heating process, shape and function of mold will be changed, which will affect hardness, wear resistance and service life of mold, which will lead to early fall of mold. Some tool steel with high molybdenum content have been restricted for popularization and application due to their easy oxidation and decarburization. Until heat treatment process equipment grows, special heat treatment processes (such as vacuum heat treatment, controlled atmosphere heat treatment, salt bath heat treatment) are adopted, it is possible to avoid oxidation and decarburization, and such tool steels have been promoted and applied. Although molybdenum-based alloys have extremely excellent high-temperature functions, they are extremely susceptible to oxidation at high temperatures, which severely limits their application scale.

Metallurgical quality also has a great influence on function of plastic mold steel. Only with excellent metallurgical quality can various functions of molding materials be fully utilized. It is necessary to select suitable materials with excellent metallurgical quality for molds of prudence, complexity and with long life. Leading metallurgical production departments at home and abroad have also made every effort to improve metallurgical quality of tool steel.

First few aspects are as follows:

For high-quality mold steel, extra-furnace essence, vacuum treatment, vacuum smelting, dusting treatment, electroslag remelting and other processes are adopted at home and abroad to reduce harmful elements, oxygen, hydrogen and inclusions in steel content, fine-tuning of chemical composition and casting temperature. Electroslag remelting can also usefully improve low-fold structure, density and improve isotropic properties of plastic mold steel. Test of some domestic production plants shows that lateral impact toughness of 4Cr5MoSiV1 steel produced by electric arc furnace is only equivalent to 31% of longitudinal direction. Transverse impact toughness value of 4Cr5MoSiV1 steel after electroslag remelting can be equivalent to 70% of longitudinal direction, more than doubled. For tool steel with rare requirements, powder high-speed steel and powder high-alloy tool steel produced by powder metallurgy process can better improve structure and function of steel.

In root pedicle which guarantees a certain forging ratio, upsetting and cross-rolling processes of upsetting and lengthening are used as much as possible to improve isotropic property of mold material. In order to reduce processing allowance and increase monopoly rate of materials, we use precision forging machines, fast forging hydraulic presses, and high-precision continuous rolling mills to produce high-precision steels to meet needs of plastic molding manufacturing.

Heat treatment of hot forging and rolling materials should be controlled by controlled atmosphere or vacuum heat to avoid oxidative decarburization. Some plastic mold steel and hot work tool steel should be pre-hardened by metallurgical department. For some hot-working molding materials with high requirements, metallurgical department should organize and refine materials in advance to eliminate coarse carbides and chain-like carbides in steel, and obtain fine and evenly distributed carbide structures to further improve various types of functions of steel. According to some foreign reports, some hot work tool steel are subjected to electroslag remelting-multidirectional forging (rolling)-tissue refinement to produce high-quality and high-grade mold steel with transverse impact toughness values. It can be equivalent to more than 90% of longitudinal direction. Many steel mills have named brand name of steel produced by this process, such as ISODISC of Austria Bole Steel Works; ISOTROPY of today's Metals Corporation, MICROFINE of Japan High Frequency Steel Company. Many metallurgical production departments in China are also working hard for this work. Rest of steel from metallurgical quality of various parts of steel, mold should be made to focus on mold's primary working surface (such as cavity or blade) that close to contour position of steel; because in general, contour of steel is clean reference of steel. Core of steel is the area where steel is low-fold defect control concentration; it is rare that in large-section Leysite steel, eutectic carbide in middle part of steel will be more than 2 to 3 grades than contour part. In addition, primary purpose of mold to bear load is preferably selected in accordance with purpose of deformation target of steel to reduce influence of anisotropy of steel on helium of mold.

Thermal conductivity is also one of primary performance indicators of some hot work tool steel and plastic mold steel. Tool steel with good thermal conductivity can agile heat generated in processing and heat transferred from workpiece, and avoid overheating of working contour of mold to improve working condition of mold. For some thermoplastic molding molds and some die-casting molds, in order to speed up production rhythm, pressed workpieces are desperately cooled and demolded to exceed yield. In order to familiarize with this problem, sometimes some heat conductivity is better than steel. Good molding materials, such as high-strength copper alloys, high-strength aluminum alloys, etc.

In order to shorten manufacturing cycle of mold, plastic molding manufacturing department should select concentrate and products when purchasing tool steel, such as fine steel by stripping, cold drawing or grinding, through roughing plastic molding manufacturing department monopolizes these fines and products for processing with standard formwork. It can not only shorten plastic molding manufacturing cycle usefully, but also meet needs of mold use department. Because production of concentrates and products in previous stage is produced in high efficiency and quantity in metallurgical plant, it can reduce production cost and increase material monopoly rate.

 

Other factors to be considered in selection of plastic mold steel, according to use conditions and requirements of mold, in addition to above various types of identity must be considered, it is rare that primary function of tool steel must be in line with requirements of mold conditions. It is also necessary to consider cost and versatility of selected mold steel. Under normal circumstances, when batch size of produced workpiece is large and size of mold is small, proportion of tool steel in plastic molding manufacturing cost is small, and cost of tool steel can not be regarded as primary consideration, and steel of high-level combination can be selected as much as possible. For large or extra large molds with simple shape, since cost of tool steel will account for a large share of total cost of mold, it is possible to select a lower cost mold steel according to batch size of produced workpiece, or low-cost mold steel for mold body. In working part of mold hub, such as cavity or edge, high-grade mold steel is mounted by inserting or surfacing, which can improve service life of mold. It can also reduce cost of materials. Versatility of mold steel is also a factor that must be considered when selecting tool steel. Tool steel is generally used in small quantities, and there are many varieties and specifications. In order to facilitate procurement and preparation of materials in the market, versatility of materials should be considered. In addition to rare requirements, a large number of general-purpose mold steels are produced as much as possible. Due to maturity of general-purpose mold steel technology, production and use experience of savings accumulation is more, functional data is also complete, which is convenient to design and manufacturing process for reference. The rest use universal tool steel, which can facilitate procurement, preparation and material management work.