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The selection of mold steel is one of the most critical decisions determining a mold’s service life, production quality, and total cost. For Plastic Injection and Hot Forging Molds, the required performance criteria can differ significantly. While surface quality, dimensional stability, and corrosion resistance are paramount in plastic injection molds, high-temperature resistance, thermal fatigue resistance, and toughness are the key factors in hot forging molds.
Incorrect steel selection leads to premature mold failure, high maintenance costs, reduced product quality, and production downtime. The right choice, however, extends mold life, reduces maintenance frequency, and lowers the cost per part.
In this guide, we provide a detailed overview of the steel grades used in Plastic Injection and Hot Forging Molds, along with selection criteria, heat treatment requirements, and application recommendations.
1. What Is Die Steel and Why Is It Important?
Die steels are a specialized subcategory of tool steels used in the forming processes of metal or plastic materials. Possessing properties such as high wear resistance, thermal fatigue resistance, hardness, and toughness, these steels differ fundamentally from general-purpose structural steels.
Die steels are classified into two main categories: cold-work tool steels (typically used in dies operating below 200°C) and hot-work tool steels (used in dies operating above 200°C, particularly in the 300–1200°C range). While plastic injection molds are generally classified under the cold work category, hot forging dies fall under the hot work category. In both categories, the raw steel material—whether round, square, or flat steel bars—is machined using CNC in mold manufacturing facilities to achieve the final mold shape.
2. Selection of Steel for Plastic Injection Molds
A plastic injection mold is a precision tool into which molten plastic is injected under high pressure, cooled to solidify, and shaped into the desired form. The key properties expected from mold steel are as follows:
Good machinability: It must be easily machinable using CNC milling and electrical discharge machining (EDM).
Polishability: It must be capable of achieving a mirror finish (SPI A-1) for products requiring transparent or glossy surfaces.
Dimensional stability: It must exhibit minimal distortion after heat treatment.
Corrosion resistance: Stainless steel grades are required for plastics that produce corrosive gases, such as PVC and ABS.
Wear resistance: High hardness is essential to resist surface wear in glass-fiber-reinforced plastics.
| Steel Grade | DIN Equivalent | Hardness (HRC) | Type | Polishability | Typical Application |
|---|---|---|---|---|---|
| 1.2738 | 40CrMnNiMo8-6-1 | 28–32 | Pre-hardened | Good | Large injection molds, automotive parts |
| 1.2312 | 40CrMnMoS8-6 | 28–32 | Pre-hardened | Medium | Mold bases, holder plates, high machinability |
| 1.2344 | X40CrMoV5-1 | 48–52 | Requires hardening | Good | Hot runner components, high-temperature applications |
| 1.2083 | X42Cr13 | 48–52 | Requires hardening | Excellent | Corrosive plastics (PVC), transparent products, medical applications |
| 1.2316 | X38CrMo16 | 28–32 | Pre-hardened | Excellent | Pre-hardened applications requiring corrosion resistance |
| 1.1730 | C45U | Normalized | Basic | Low | Mold frames, support plates, prototype molds |
2.1. Pre-hardened Die Steels (1.2738, 1.2312)
Pre-hardened die steels are supplied by the manufacturer with a hardness range of 28–32 HRC and can be machined directly on CNC machines without requiring additional heat treatment. This significantly reduces die manufacturing time and eliminates the risk of heat treatment distortion.
1.2738 is the most widely preferred grade worldwide for large-scale injection molds. Thanks to its nickel alloy composition, it ensures a homogeneous hardness distribution even in large cross-sections. 1.2312, on the other hand, features enhanced machinability due to its sulfur content and offers a cost advantage in applications where polishability is not critical, such as mold bases and retaining plates.
2.2. Sertleştirilebilir Kalıp Çelikleri (1.2344, 1.2083)
Yüksek aşınma direnci veya korozyon direnci gerektiren uygulamalarda sertleştirilmiş kalıp çelikleri tercih edilir. Bu çelikler CNC ile yarı finisaj işlemesinden sonra ısıl işleme tabi tutulur ve 48–52 HRC sertliğe ulaşır.
1.2083 (X42Cr13), yüksek krom içeriği (çoğunlukla yüzde 13 Cr) sayesinde paslanmaz özellik gösterir. PVC, asetal ve alev geciktirici katkılı plastiklerin kalıplanmasında korozyon direnci sağlar. Aynı zamanda mükemmel cilalanabilirliği ile şeffaf optik parçalar ve medikal bileşenler için idealdir.
3. Selection of Steel for Hot Forging Dies
Hot forging dies are used to shape metal parts heated to temperatures between 900–1250°C under high pressure. These extreme conditions demand very different properties from the die steel:
High-temperature hardness retention: The die surface is continuously exposed to temperatures of 500–600°C and must retain its hardness.
Resistance to thermal fatigue: The rapid heating-cooling cycle during each forging cycle can cause thermal cracking (heat checking) on the surface. The steel must be resistant to this.
Toughness: The ability to deform without fracturing under forging impacts is of critical importance.
Wear resistance: Metal flow at high temperatures wears down the die surface; the steel must be resistant to this.
| Steel Grade | DIN Equivalent | Hardness (HRC) | Max Operating Temp. | Typical Application |
|---|---|---|---|---|
| 1.2344 | X40CrMoV5-1 (H13) | 44–52 | ~600°C | General forging dies, Al/Cu extrusion, die casting |
| 1.2343 | X38CrMoV5-1 | 44–50 | ~550°C | Forging and drawing dies requiring higher toughness |
| 1.2714 | 56NiCrMoV7 | 40–48 | ~500°C | Heavy forging dies, hammer dies, high-impact applications |
| 1.2367 | X38CrMoV5-3 | 44–52 | ~650°C | Superior thermal fatigue resistance, high-performance applications |
4. The Relationship Between Heat Treatment and Steel Quality
The performance of die steels depends on the application of proper heat treatment. Pre-hardened steels (1.2738, 1.2312) are delivered with a pre-set hardness value and therefore do not require additional heat treatment. However, hardenable steels (1.2344, 1.2083, 1.2714) must undergo controlled quenching and tempering.
Heat treatment is particularly critical for hot-forging die steels. For example, to achieve optimal performance with 1.2344, austenitization at 1020–1040°C, followed by oil or air cooling and then two tempering cycles, is typically applied. The tempering temperature is selected between 520–620°C depending on the desired balance of hardness and toughness.
Important note: Dimensional changes occurring during heat treatment are directly related to CNC machining. The general approach for heat-treating die steels is: rough machining → heat treatment → finish machining.
5. Die Steel Raw Material Forms: Bar, Block, and Plate
Die steels are available in various raw material forms. Selecting the correct form directly impacts die manufacturing efficiency:
Round steel bar: Ideal for cylindrical die components (rollers, nozzles, injector bodies). It offers tight tolerances and a smooth surface when supplied in cold-drawn form.
Square steel bar: Used as raw material for small and medium-sized mold inserts, cores, and rollers. Provides a holding advantage for CNC milling.
Steel blocks and plates: Used for large mold bases and cavity blocks. Typically supplied in hot-rolled or forged form.
Steel plates: Suitable for mold guide elements, scraper plates, and support components.
6. Sectoral Differences in Selection Criteria
6.1. Automotive Industry
Automotive plastic parts are typically produced in high volumes (hundreds of thousands or even millions of units). For this reason, 1.2738 is preferred as mold steel, and 1.2344 is preferred for glass-fiber-reinforced plastics. In hot forging, 1.2714 and 1.2344 are commonly used for forging critical automotive components such as crankshafts, connecting rods, and gears.
6.2. Medical and Food Industry
Due to hygiene requirements, corrosion resistance is a top priority. Stainless steel grades such as 1.2083 or 1.2316 are preferred. High polishability reduces bacterial adhesion and facilitates cleaning.
6.3. Home Appliances and Consumer Products
Surface finish quality is critical. While 1.2738 pre-hardened die steel is the standard choice, 1.2083 stands out for its mirror-finish capability in transparent parts.
7. Common Mistakes and Recommendations
- Over-engineering: Using high-alloy steel for prototypes or low-volume dies creates unnecessary costs. 1.1730 (C45U) may be sufficient in this case.
Incorrect heat treatment: Even high-quality steel loses its performance with improper heat treatment. The correct tempering temperature and duration are particularly critical for hot forging dies. - Ignoring corrosion risk: Using standard steel when processing PVC and flame-retardant plastics leads to premature corrosion on the die surface.
Compromising on raw material quality: The distribution of inclusions and homogeneity in die steel directly affect polishability and die life. Low-quality raw material is cheap in the short term but expensive in the long term.
Future Trends in Die Steel and Surface Treatments
The die steel industry is constantly evolving in response to industrial demands and technological innovations. Certain trends that have emerged in recent years, in particular, are directly influencing steel selection and die design.
Surface treatments and coatings are critical technologies that extend die life. The nitriding process significantly increases wear resistance by forming a thin nitride layer on the die surface with a hardness of 800–1200 HV. PVD (Physical Vapor Deposition) coatings, particularly TiN, TiAlN, and CrN coatings, reduce sticking in plastic injection molds and protect the die surface. In hot forging dies, nitriding is the most common surface treatment and can extend die life by 30–50 percent.
Conformal cooling channels are cooling channels produced using 3D metal printing technology that follow the part’s geometry within the die, unlike traditional straight holes. This technology can reduce cycle time by 20–40% and minimize warping and distortion defects. Conformal cooling inserts are typically 3D-printed from maraging steel or special alloys like 1.2709 and integrated into the traditional die steel body.
Powder metallurgy (PM) mold steels are another emerging technology. Steels produced via the PM method offer a much more homogeneous carbide distribution compared to traditional cast steels. This provides significant advantages, particularly in optical and medical applications that require high polishability and long mold life.
From a sustainability perspective, die steels have a high recyclability rate. Dies that have reached the end of their service life are classified as scrap and returned to the steel production cycle. With the introduction of European CBAM regulations, the supply of steel with a low carbon footprint is becoming an increasingly important criterion for supplier selection in the die industry as well.
On the supply side, the trend toward digitalization is gaining momentum. Die manufacturers have begun expecting digital material certificates, real-time inventory information, and rapid cutting services from steel suppliers. Suppliers capable of delivering quickly from multiple locations and maintaining inventory across a wide range of diameters and profiles are gaining a competitive advantage.
Frequently Asked Questions (FAQ)
What type of steel is most commonly used for plastic injection molds?
1.2738 (pre-hardened, 28–32 HRC) is the most widely used plastic injection mold steel worldwide. It can be machined directly without requiring additional heat treatment, provides uniform hardness across large cross-sections, and offers good polishability. For corrosive plastics, 1.2083 or 1.2316 is preferred.
What type of steel is used to make hot-forging dies?
1.2344 (H13) is the most commonly used grade for hot-forging dies. It offers hardness retention at high temperatures and resistance to thermal fatigue. For hammer forging, where heavy impact loads are present, 1.2714 is preferred due to its higher toughness.
What is the difference between pre-hardened and hardened die steel?
Pre-hardened steels (1.2738, 1.2312) are supplied with a ready-to-use hardness value (28–32 HRC) and require no additional heat treatment; the mold manufacturing time is short. Hardened steels (1.2344, 1.2083), on the other hand, are heat-treated after machining to achieve a hardness of 48–52 HRC; they offer higher wear resistance but have a longer production cycle.
In what form should I purchase the raw material for die steel?
Cold-drawn round bars are preferred for cylindrical mold components, square bars for small-to-medium inserts, and blocks or plates for large cavity blocks. Cold-drawn bars reduce CNC machining costs thanks to their tight tolerances and good surface quality.
The delivery condition of the steel bar is also important when selecting the raw material form. Cold-drawn bars can be delivered in annealed or normalized condition. In pre-hardened mold steels (1.2738, 1.2312), the bar is already delivered at a hardness of 28–32 HRC, making it ready for direct CNC machining. For grades that require hardening, the bar is received in a normalized or annealed condition, undergoes rough machining first, and then undergoes heat treatment. This sequence is critical for efficiency and dimensional accuracy in die manufacturing. Having your supplier stock different delivery conditions provides flexibility for your die projects.
Conclusion: Die Steel Guide: The Right Material for Plastic Injection and Hot Forging Dies
The selection of mold steel should be based on the type of mold (plastic injection or hot forging), production volume, the properties of the material to be processed, and the expected mold life. In plastic injection molding, 1.2738 and 1.2083 are prominent industry standards, while in hot forging, 1.2344 and 1.2714 are the leading choices.
When the right steel material is combined with the correct heat treatment and the appropriate die design, die life is extended, maintenance costs are reduced, and the production cost per part is optimized.
As Uyar Çelik, we supply cold-drawn round, square, and flat steel bars in grades 1.2344, 1.2738, 1.2714, C45, and other high-quality steel grades to die-casting foundries.
For the supply of Type 3.1 certified, tight-tolerance bars from our locations in Istanbul, Karabük, Kocaeli, and Düsseldorf:
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April 2, 2026