Mold steel is a popular choice for injection molding dies. It is strong and wear-resistant, making it ideal for high-volume production. This guide will discuss the different types of mold steel available and their respective advantages and disadvantages. Whether you are just starting in plastic injection molding or are looking to upgrade your current die selection, this guide will surely help!
What is molds steel?
Mold steel creates molds, such as cold-punching molds, hot-forging dies, and die-casting molds. The mold is the primary processing tool used to fabricate radiometers, motors, and other types of machinery.
The Ultimate Guide To Mold Steel
What are the points to consider when selecting mold steel?
Resistance to wear
Wear resistance is one of the most important characteristics of die steel. Hardness is a key factor determining wear resistance. High die hardness, minimal loss, and superior corrosion resistance are desirable qualities. Wear resistance varies with carbide type, number, and shape.
Durability
The mold’s working pressure is extremely high and needs to support a heavy impact load. Therefore, handling such demanding working conditions must be very strong and resilient. The amount of carbon and the texture affect how tough it is.
Fatigue resistance
Molds that are operated under such high pressure always experience fatigue fractures. The characteristics that affect the fatigue limit are the material’s strength, toughness, hardness, and quantity of inclusions.
Features of high-temperature
Working at a high temperature will decrease the mold’s hardness and strength, causing it to distort and fail. The mold steel must have excellent tempering stability to guarantee that the mold has great strength and hardness when working at high temperatures.
Corrosion protection
Because the plastic contains chlorine, fluorine, and other elements, thermal decomposition will produce HCl, HF, and other etching gases. These gases will damage and harden the mold cavity’s surface, increasing the likelihood of the mold breaking.
The following video will help you better understand the basics of mold steel and what to consider when selecting mold steel that meets your specific requirements.
What are the different types of molding materials?
There are many different plastic mold material series available because of the variety of working conditions (processing objects), manufacturing processes, precision, and requirements for the durability of plastic molds. Steel, copper, aluminum, and zinc alloys are some materials used to make plastic molds.
Mould steel
Steel is used in the majority of plastic molds. The two main types of steel are carbon steel and alloy steel, with varying chemical compositions. Heat treatment can also alter steel`s strength, plasticity, and toughness. The steel of ordinary carbon composition is typically marked with sulfur and phosphorus content depending on the letter A, B, C, or D; for instance, steel of grade A is stamped with yield strength and the number Q, with the Q indicating the yield point size, and the number for the symbol Q plus the number. Letters F and b go behind the grade for boiling steel, respectively; letters F and b do not go behind the grade for sedimentation steel.
Can steel be injection molded?
This process is called Metal Injection Molding (MIM) or steel injection molding. Laser technology allows it to produce intricate parts made from metals such as steel, stainless steel, or even high-temperature alloys such as nickel and titanium. Industries such as medical, electronics, aerospace, etc., now use this process.
Tables 1 and 2 provide information on common carbon structural steel’s mechanical properties, chemical composition, and grade.
Grades of standard carbon structural steel and its chemical makeup in Table 1
| Chemical composition (mass fraction,%) | ||||||||
| Brand | C | Si | Mn | P | S | Ni | Cr | Cu |
| ≤ | ||||||||
| 08F | 0.05 ~ 0.11 | ≤ 0.03 | 0.25 ~ 0.50 | 0.035 | 0.035 | 0.25 | 0.1 | 0.25 |
| 10F | 0.07 ~ 0.14 | ≤ 0.07 | 0.25 ~ 0.50 | 0.035 | 0.035 | 0.25 | 0.15 | 0.25 |
| 8 | 0.05 ~ 0.12 | 0.17 ~ 0.37 | 0.35 ~ 0.65 | 0.035 | 0.035 | 0.25 | 0.1 | 0.25 |
| 10 | 0.07 ~ 0.14 | 0.17 ~ 0.37 | 0.35 ~ 0.65 | 0.035 | 0.035 | 0.25 | 0.15 | 0.25 |
| 15 | 0.12 ~ 0.19 | 0.17 ~ 0.37 | 0.35 ~ 0.65 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 20 | 0.17 ~ 0.24 | 0.17 ~ 0.37 | 0.35 ~ 0.65 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 25 | 0.22 ~ 0.30 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 30 | 0.27 ~ 0.35 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 35 | 0.32 ~ 0.40 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 40 | 0.37 ~ 0.45 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 45 | 0.42 ~ 0.50 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 50 | 0.47 ~ 0.55 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 55 | 0.52 ~ 0.60 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 60 | 0.57 ~ 0.65 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
| 65 | 0.62 ~ 0.70 | 0.17 ~ 0.37 | 0.50 ~ 0.80 | 0.035 | 0.035 | 0.25 | 0.25 | 0.25 |
Carbon steel is typically used without heat treatment. Q195, Q215, and Q235 steels are known for their low carbon content, superior welding characteristics, and strength. Steel grades Q255 and Q275 are stronger, more flexible, and tougher when welded. Ensure high-quality carbon structural steel’s chemical structure and mechanical properties. The steel grade is a two-digit number that represents its carbon content. Carbon structural steel is crucial to the creation of plastic molds. Tables 3 and 4 describe premium carbon structural steel chemically and mechanically. Based on its many uses, steel can also be classified as structural steel, tool steel, carburized steel, stainless steel, spring steel, bearing steel, and heat-resistant steel.
Table 3 High-quality carbon structural steel has the following chemical makeup:
| Brand | Sample | Recommended heat treatment/℃ | Mechanical properties | Hardness of steel in delivery state HBW | |||||||
| Blank | ≤ | ||||||||||
| Dimensions/ | Normalizing | Quenching | Tempering | σ b/MPa | W | AK/J | |||||
| Mm | σ/MPa | 5 | |||||||||
| Not less than | Untreated | Annealed steel | |||||||||
| 8 | 25 | 930 | 295 | 175 | 35 | 60 | 131 | ||||
| 10 | 25 | 930 | 315 | 185 | 33 | 55 | 137 | ||||
| 8 | 25 | 930 | 325 | 195 | 33 | 60 | 131 | ||||
| 10 | 25 | 930 | 335 | 205 | 31 | 55 | 137 | ||||
| 15 | 25 | 920 | 375 | 225 | 27 | 55 | 143 | ||||
| 20 | 25 | 910 | 410 | 245 | 25 | 55 | 156 | ||||
| 25 | 25 | 900 | 870 | 600 | 450 | 275 | 23 | 50 | 71 | 170 | |
| 30 | 25 | 880 | 860 | 600 | 490 | 295 | 21 | 50 | 63 | 179 | |
| 35 | 25 | 870 | 850 | 600 | 530 | 315 | 20 | 45 | 55 | 197 | |
| 40 | 25 | 860 | 840 | 600 | 570 | 335 | 19 | 45 | 47 | 217 | 187 |
| 45 | 25 | 850 | 840 | 600 | 600 | 355 | 16 | 40 | 39 | 229 | 197 |
| 50 | 25 | 830 | 830 | 600 | 630 | 375 | 14 | 40 | 31 | 241 | 207 |
| 55 | 25 | 820 | 820 | 600 | 645 | 380 | 13 | 35 | 255 | 217 | |
| 60 | 25 | 810 | 675 | 400 | 12 | 35 | 255 | 229 | |||
| 65 | 25 | 810 | 695 | 410 | 10 | 30 | 255 | 229 | |||
Table 4 High-quality carbon structural steel’s mechanical characteristics
| Steel grade | C | Si | Mn | Cr | W | Mo | V | Others | |
| Carburized plastic die steel | 20Cr | 0.17 ~ | 0.17 ~ | 0.50 ~ | 0.80 ~ | ||||
| 0.24 | 0.37 | 0.8 | 1.1 | ||||||
| 12CrNi2 | 0.10 ~ | 0.17 ~ | 0.30 ~ | 0.60 ~ | Ni1.50 ~ 2.00 | ||||
| 0.17 | 0.37 | 0.6 | 0.9 | ||||||
| 12CrNi3 | 0.10 ~ | 0.17 ~ | 0.30 ~ | 0.60 ~ | Ni2.75 ~ 3.25 | ||||
| 0.17 | 0.37 | 0.6 | 0.9 | ||||||
| 20Cr2Ni4 | 0.17 ~ | 0.17 ~ | 0.30 ~ | 1.20 ~ | Ni 3.25 ~ 3.75 | ||||
| 0.23 | 0.37 | 0.6 | 1.75 | ||||||
| 20CrMnTi | 0.17 ~ | 0.17 ~ | 0.80 ~ | 1.00 ~ | Ti 0.04 ~ 0.10 | ||||
| 0.23 | 0.37 | 1.1 | 1.3 | ||||||
| Pre-hardened plastic die steel | 3Cr2Mo | 0.28 ~ | 0.20 ~ | 0.60 ~ | 1.40 ~ | 0.30 ~ | |||
| 0.4 | 0.8 | 1 | 2 | 0.55 | |||||
| 3Cr2MnNiMo | 0.28 ~ | 0.20 ~ | 1.20 ~ | 1.40 ~ | 0.30 ~ | Ni0.85 ~ 1.15 | |||
| 0.4 | 0.4 | 1.5 | 2 | 0.55 | |||||
| 5CrNiMnMoVSCa | 0.50 ~ | 0.20 ~ | 0.85 ~ | 1.00 ~ | 0.30 ~ | 0.10 ~ | Ni0.85 ~ 1.15 Ca, S | ||
| 0.6 | 0.8 | 1.15 | 1.3 | 0.6 | 0.3 | ||||
| 8Cr2MnWMoVS | 0.75 ~ | ≤ 0.40 | 1.30 ~ | 2.30 ~ | 0.50 ~ | 0.10 ~ | S0.06 ~ | ||
| 0.85 | 1.7 | 2.6 | 0.8 | 0.25 | 0.1 | ||||
| Age-hardening plastic die steel | 25CrNi3MoAl | 0.20 ~ | 0.20 ~ | 0.20 ~ | 1.20 ~ | 0.20 ~ | Al1.00 ~ 1.60 Ni2.50 ~ 3.00 | ||
| 0.3 | 0.5 | 0.5 | 1.8 | 0.4 | |||||
| 1.30 ~ | 0.90 ~ | 0.08 ~ | TiAl | ||||||
| 06Ni6CrMoVTiAl | ≤ 0.06 | ≤ 0.06 | ≤ 0.50 | 1.6 | 1.2 | 0.16 | 1.0/0.5 | ||
| Ni 5.50 ~ 6.50 | |||||||||
| Ti0. 30/0.50 Al0. 05/0.15 Ni17. 5/18.5 Co7. 00/8.00 | |||||||||
| 18Ni (250) | ≤ 0.03 | ≤ 0.10 | ≤ 0.10 | 4.25 ~ | |||||
| 5.25 | |||||||||
| Corrosion-resistant plastic die steel | 4Cr13 | 0.35 ~ | ≤ 0.06 | ≤ 0.80 | 12.00 ~ | ||||
| 0.45 | 14 | ||||||||
| 9Cr18 | 0.90 ~ | 0.50 ~ | ≤ 0.80 | 17.00 ~ | |||||
| 1 | 0.9 | 19 | |||||||
| Cr14Mo | 0.90~ | 0.30~ | ≤0.80 | 12.00~ | 1.40~ | ||||
| 1.05 | 0.6 | 14 | 1.8 | ||||||
| Cr18MoV | 1.17~ | 0.50~ | ≤0.80 | 17.50~ | 0.50~ | 0.10~ | |||
| 1.25 | 0.9 | 19 | 0.8 | 0.2 | |||||
| 1Cr17Ni2 | 0.11~ | ≤0.80 | ≤0.80 | 16.00~ | Ni1.50~2.50 | ||||
| 0.17 | 18 | ||||||||
| Integral hardened plastic mould steel | 9Mn2V | 0.85~ | ≤0.40 | 1.70~ | 0.15~ | ||||
| 0.95 | 2 | 0.25 | |||||||
| CrWMn | 0.90~ | ≤0.40 | 0.80~ | 0.90~ | 1.20~ | ||||
| 1.05 | 1.1 | 1.2 | 1.6 | ||||||
| 9CrWMn | 0.85~ | ≤0.40 | 0.90~ | 0.50~ | 0.50~ | ||||
| 0.95 | 1.2 | 0.8 | 0.8 | ||||||
| Cr12MoV | 1.45~ | ≤0.40 | ≤0.40 | 11.00~ | 0.40~ | 0.15~ | |||
| 1.7 | 12.5 | 0.6 | 0.3 | ||||||
| 4Cr5MoSiV1 | 0.32~ | 0.80~ | 0.20~ | 4.75~ | 1.10~ | 0.80~ | |||
| 0.45 | 1.2 | 0.5 | 5.5 | 1.75 | 1.2 | ||||
Molds made of CrWMn, Cr12MoV, and other alloy tool steels have poor machine performance, have a hard time processing complex cavities, and have short mold lives. Moreover, they cannot deal with the issue of heat treatment deformation. In recent years, several new plastic mold steels have been introduced. There are four basic types of steel:
- Pre-hardened plastic mold steel is forged steel pre-heated to the desired hardness before it is used to create molds. In our alloy tool steel standard, two of the most popular China mold steel are P20 mold steel and 718 mold steel, which increases processing quality and mold life.
- China has developed sulfur-containing free-cutting pre-hardening plastic mold steel. To increase machinability, S, Pb, Cs, etc., are added to pre-hardened plastic mold steel. Large and medium-sized plastic items requiring longer life, complex shapes, and high precision are usually cast in pre-hardened plastic mold steel.
- China has developed age-hardening plastic mold steel in recent years. When quenching and heat-treating steels, the deformation is minimized, as the steel is designed to help prevent deformation.
- Cold extrusion molding of steel and plastic is a cost-effective processing technique for producing complex plastic mold cavities since it does not require cavity trimming before pressing the hardened products.
Injection mold steel materials
420 stainless steel, a high-quality steel with 13% chrome, is preferred for custom injection molding in China. Upon hardening, the high carbon content provides maximum corrosion resistance. The edge retains its shape and wears well.
Alloy of aluminum
Pure aluminum is insufficient for plastic molds. However, magnesium, zinc, copper, silicon, and other alloying elements can improve aluminum’s strength and casting qualities. Aluminum also has high corrosion resistance, wear, thermal, and electrical conductivity. Plastic molds may change shape and dimensions under higher temperatures and stresses.
Cobalt alloys
Copper alloys can be classified into three main categories: brass, bronze, and white copper. Alloys with nickel, including white copper, are made from copper. Due to their low melting points, zinc alloys are suitable for making plastic molds using casting techniques. Zinc alloys are often strengthened and corrosion-resistant by adding trace amounts of magnesium.l
Making the right choice for your mold – A guide to steel selection
The rapid development of plastic products makes it possible to categorize plastics into two categories: thermosetting and thermoplastics. Plastic molding molds and the creation of mold materials provide various needs due to the varied types of diverse polymers, various qualities, and the size, shape, complexity, size accuracy, surface roughness, and production batch of various plastic goods. Therefore, a choice of plastic mold material should be based on some general guidelines, roughly according to the processing method, the service conditions, the product quality, and the batch selection of the material.
When choosing mold materials, it is important to consider the circumstances of the plastic mold service and the quality of the product.
1) Carburized plastic mold steel is best for mold cavities requiring high hardness and wear resistance while maintaining a reasonable level of toughness. Carburizing plastic with steel that has a low hardenability is ideal for injection molds with simple cavities and small sizes.
2) To ensure the mold’s accuracy and performance, it is recommended that pre-hardened plastic mold steel be used for complex mold cavities with a high degree of precision and long mold life requirements. This prevents cavity cutting and formation due to excessive cavity deformation and ensures a long mold life.
3) Molds of thermoplastic or thermosetting polymer products containing glass fiber as filler or compression molds containing thermosetting polymers have extensive requirements for hardness, wear resistance, compressive strength, toughness, polishability, and electric processability.
4) The use of PVC, polystyrene, and ABS as raw materials for plastic products will release corrosive gases during the molding process, resulting in mold failure. Therefore, it is vital that the mold material is corrosion-resistant; corrosion-resistant plastic mold steel can be used to make molds.
5) Plastic mold steel that hardens over time is recommended for long-section plastic molds with high surface roughness in the cavity, high brightness, precise size and shape, and corrosion resistance. It should also be used for products with high wear resistance and transparency. As stated in Table 4, various plastic mold types require different materials.
Table 4: Material choices for plastic molds
| Use of plastics or products | Representative raw material | Typical product | Die requirements | Applicable die materials | |
| General thermosetting and thermoplastics | General | ABS | Radiator grille | High strength and high wear resistance | P20, 4Cr5MoV1Si, Cr12MoV, 5NiSCa, 8Cr2MnWMoVS |
| Polypropylene | Fan, blade, bucket | ||||
| Decorative piece | ABS | Automobile inner panel | High strength, high wear resistance, decorative pattern processability | P20, SM2 | |
| Transparent part | Propylene | Three-dimensional dustproof automobile taillight | High strength, high wear resistance, and good abrasiveness | 8Cr2MnWMoVS, 5NiSCa, 18Ni maraging steels | |
| Thermosetting plastics and thermoplastics filled with glass fibers, etc. | Thermoplastics | Polyacetal nylon, polycarbonate resin | Engineering plastics products, automobile interior panels, electric tool housings | High abrasion resistance | Cr12MoV, CrWMn, 9Mn2V, 4Cr5MoSiV1, 8Cr2MnWMoVS, 5NiSCa, 7CrSiMnMbV |
| Thermosetting plastics | Phenolic epoxy resin | Engineering plastics parts of gears | High abrasion resistance | ||
| Optical lens | Propylene material, polypropylene vinyl | Camera lens, Fresnel lens | High mirror machinability, high strength, and rust prevention | 18Ni maraging steel, 3Cr13, 4Cr13 | |
| Flame retardant parts | ABS | Television housing, cathode ray tube housing, radio base | Appropriate strength and corrosion resistance | 17-4pH, 3Cr13, 4Cr13, P20 chromium plating | |
| Polyvinyl chloride | PVC | Telephone, sink pipe, joint | Appropriate strength and corrosion resistance | 17-4pH, P20 chromium plating | |
| Polytetrafluoroethylene | Polytetrafluoroethylene | Sealing gasket, wire coating material | Corrosion resistance | Ni-based superalloy containing tungsten and molybdenum | |
Carbon steel is typically used without heat treatment. Q195, Q215, and Q235 steels are known for their low carbon content, superior welding characteristics, and strength. Steel grades Q255 and Q275 are stronger, more flexible, and tougher when welded. Ensure high-quality carbon structural steel’s chemical structure and mechanical properties.
The steel grade is a two-digit number that represents its carbon content. Carbon structural steel is crucial to the creation of plastic molds. Tables 3 and 4 describe premium carbon structural steel chemically and mechanically. Based on its many uses, steel can also be classified as structural steel, tool steel, carburized steel, stainless steel, spring steel, bearing steel, and heat-resistant steel.
If you have any further questions about steels for injection moulds, please contact us! At Go4mould, we have over 20 years of experience in the industry, and our team of experts can help you create the perfect mould for your needs, ensuring that your product is manufactured with precision. Talk to an expert!
