Mold steel is the core base material that determines the service life, forming quality, production cost and operational stability of injection molds. The rationality of steel selection directly affects polishing effect, corrosion resistance, wear resistance, deformation and cracking probability of molds. In the mold industry, most practitioners rely on experience for material selection and fall into inherent thinking misunderstandings, resulting in premature mold wear, cracking, rust and frequent maintenance. Summarizing typical misunderstandings and forming scientific selection methods can effectively avoid hidden dangers in mold production.
Misunderstanding 1: Blind Pursuit of High Hardness, Believing Higher Hardness is BetterMany practitioners hold the view that mold service life is positively correlated with hardness, and blindly select high-hardness steel above HRC 50 while ignoring the matching of plastic materials, injection temperature and mold structure. High-hardness mold steel usually has low toughness. When applied to complex molds with sharp corners, thin ribs and deep cavities, it is prone to chipping, cracking and deformation during production. Meanwhile, high-hardness steel is difficult for cutting and polishing, extending the processing cycle and increasing mold manufacturing cost. It is also more likely to produce defects in subsequent nitriding and texture etching treatment.
The correct selection principle is to match hardness with working conditions. For ordinary ABS and PP plastic shell molds, pre-hardened steel such as P20 and 718H with hardness HRC 28-35 can meet mass production requirements. For modified plastics added with glass fiber and flame retardant, NAK80 and S136 are preferred to balance polishing performance and corrosion resistance. High-hardness quenched and tempered steel such as SKD61 and DC53 are only selected for long-term mass production and high-precision molds with high wear resistance requirements.
Misunderstanding 2: Selecting Materials Only by Price, Ignoring Full Life Cycle CostMany people simply compare the unit price of mold steel and prioritize low-cost ordinary steel, ignoring hidden costs caused by later mold maintenance, scrapping and production shutdown. Low-cost mold steel has high impurity content and uneven organizational structure, with poor heat treatment stability. Put into production, it is easy to cause cavity scratching, rust and rapid wear, requiring frequent polishing and welding repair. The long-term comprehensive cost of repeated maintenance is far higher than the price difference of high-quality steel. Even for small-batch trial production, mold failure in mass production will cause order delivery losses.
The correct way is to calculate the full life cycle cost rather than only the procurement price. Cost-effective domestic pre-hardened steel can be used for trial molds and small-batch production within hundreds of pieces. For medium and long-term mass production household appliances and daily-use product molds, branded standard mold steel is preferred. It has high material purity and stable heat treatment consistency, doubling the mold service life, reducing shutdown maintenance and rework costs, and achieving higher overall cost performance.
Misunderstanding 3: Using Universal Mold Steel for All Plastics, Ignoring Corrosion and Wear ConditionsIt is common to apply one type of mold steel to all injection raw materials without distinguishing ordinary plastics, corrosive plastics and glass fiber reinforced plastics. Raw materials such as PVC, flame retardant PC and flame retardant ABS will release corrosive gas at high injection temperature, causing cavity rust, surface fogging and peeling of polishing layer for ordinary P20 and 45# steel. PA and PBT filled with glass fiber have strong wear resistance in molten state, leading to rapid cavity wear, dimensional out-of-tolerance and flash of plastic parts when using ordinary pre-hardened steel.
Material selection should be classified according to plastic characteristics. Universal pre-hardened steel 718H and P20 are suitable for non-corrosive and non-glass fiber ordinary plastics. Corrosion-resistant mirror steel such as S136 and STAVAX must be adopted for flame retardant, PVC and PMMA corrosive plastics. Hot work mold steel SKD61 and H13 are used for nylon and long glass fiber reinforced plastics to improve wear resistance and high-temperature fatigue resistance. High-purity NAK80 and S136 are matched for transparent optical products to ensure no impurities and high light transmittance.
Misunderstanding 4: Ignoring Heat Treatment and Subsequent Processes, Thinking Steel Selection is EnoughMany people mistakenly believe that selecting the correct steel model can guarantee mold life and performance, ignoring the importance of quenching, tempering, nitriding, polishing and welding repair. For the same mold steel, regular vacuum heat treatment and simple conventional heat treatment have great differences in toughness and hardness uniformity. Improper heat treatment will leave residual internal stress, causing mold deformation and cracking in later production. Some steels are not suitable for arbitrary nitriding and large-area welding repair, and neglecting subsequent processes in the early selection stage will lead to peeling, cracking and uneven hardness in surface treatment.
The correct selection needs to match processing and heat treatment processes synchronously. In the initial selection stage, clarify whether the mold needs nitriding, texture etching, mirror polishing and other high-standard processes, and prioritize steel with stable heat treatment, excellent polishing performance and compatible surface treatment. Cooperate with professional heat treatment manufacturers for vacuum quenching and tempering to eliminate internal stress and give full play to the material performance of mold steel.
Correct General Selection Method of Mold SteelThree basic conditions shall be clarified before determining steel grade. First, confirm the production batch, select economical pre-hardened steel for trial and small-batch production, and high-quality mirror wear-resistant steel for medium and long-term mass production. Second, distinguish raw material properties, and match corresponding corrosion-resistant, wear-resistant and high-polishing steel for ordinary, corrosive, glass fiber reinforced and transparent plastics respectively. Third, combine product appearance and precision requirements, select pre-hardened steel for ordinary structural parts, mirror stainless steel for high-gloss transparent parts, and high-stability heat-treated steel for precision parts. At the same time, consider mold structure complexity, processing difficulty and cost to realize targeted selection and avoid premature failure and repeated maintenance of molds.
