Key Takeaways
- The Golden Rule: There is no "best" steel, only the "fittest." Selection depends on Production Volume, Plastic Material, and Surface Finish.
- For General Use: P20 or 718H are the industry standards for medium volume.
- For High Wear (Glass Fiber): Choose high-hardness steels like H13 or D2.
- For Corrosive Plastics (PVC): Stainless steels like S136 (420) are mandatory to prevent rusting.
- For High Polish: Use Vacuum Arc Remelted (VAR) steels like S136 or NAK80 for mirror finishes.
1. Introduction: Choosing the right steel for plastic injection molds is one of the most critical decisions in plastic injection mold manufacturing.
- Incorrect materials not only lead to high maintenance costs but can also result in inconsistent product quality and even a significantly shortened mold life.
- This guide summarizes the most important steel selection factors in the industry to help you make a systematic evaluation and decision.
2. Understand Why Mold Steel Matters
Mold steel directly affects the following key aspects:
- Mold life and durability.
- Injection molded parts precision and surface quality.
- Maintenance costs and cycles.
- Processing difficulty and manufacturing efficiency.
Incorrect selection can lead to premature wear, dimensional deviations, chemical corrosion problems, etc., all of which are common pain points.
3. Core Selection Factors
3.1 Hardness
Hardness is a fundamental property of steel that resists the high-pressure impact and abrasion of molten plastic. The higher the hardness of the steel, the less likely it is to deform under long-term high-temperature and high-pressure cycling.
However, high hardness is usually accompanied by decreased machinability and longer processing and polishing times. Therefore, a trade-off must be made between durability and processing costs.
Recommended for high-wear applications: high-hardness mold steels.
3.2 Toughness
Toughness determines the resistance of steel to fracture under impact or high stress. This is especially important in high-cycle production and when encountering sudden impacts on molds.
Low-toughness steels are prone to cracking or stress concentration, thus shortening mold life.
3.3 Wear Resistance
Abrasion resistance is a key indicator of a mold's long-term resistance to wear, especially when using reinforced glass fiber or mineral-filled plastics.
Good abrasion resistance can significantly reduce mold repair frequency and increase production capacity.
3.4 Corrosion Resistance
Some plastics (such as PVC) release corrosive gases at high temperatures, causing chemical corrosion to the mold.
In such cases, it is necessary to select steel grades with high corrosion resistance (such as stainless steel) to extend mold life and ensure product quality.
3.5 Thermal Properties
High thermal conductivity facilitates rapid cooling and reduces cycle time; high thermal stability resists dimensional deviations or thermal fatigue cracks caused by repeated thermal cycling.
Therefore, thermal properties are a crucial factor to consider in high-temperature plastic injection molded parts.
3.6 Machinability & Workability
The machinability of steel directly affects the mold manufacturing cycle and cost.
Easy-to-machinable steel can shorten processing time and reduce tool wear, making it an ideal choice for low- to medium-volume mold production.
3.7 Surface Finish
The surface quality of the mold directly determines the appearance of the plastic part. For high-gloss or textured surfaces, a highly polishable steel grade should be selected.
This is especially important for transparent parts and precision vision components.
3.8 Cost & Lifecycle Value
Initial material costs are only part of the overall cost. Other factors to consider include:
- Processing and manufacturing costs
- Mold repair and maintenance cycles
- ROI resulting from service life
High-performance steel typically has higher initial costs but lower long-term maintenance costs.
4. Consider Production Volume
Production volume is the core factor determining material selection:
- Short production/sample molds: Cost-effective and easy-to-process pre-hardened steel can be selected.
- Medium production: Medium-hardness mold steel.
- High production: Requires high wear resistance and high thermal stability steel. High production volumes place even higher demands on mold life, wear resistance, and thermal stability.
5. Material Type of Plastic
The type of plastic being molded has a direct impact on steel selection:
| Plastic Type: | Recommended Steel Characteristics: |
| General-purpose resins | Medium hardness, good machinability |
| Highly abrasive materials | High hardness + high wear resistance |
| Corrosive plastics | High corrosion-resistant steels |
| High-polish requirements | Steels with excellent polishability |
6. Common Mold Steel Types
Below are some commonly used plastic mold steels and their typical applications:
- P20 (1.2311) Steel: General-purpose mold steel, good cost performance, suitable for medium production volumes.
- H13 (1.2344) Steel: Hot-work mold steel with excellent heat resistance and wear resistance.
- S136 / 420 (1.2038) Stainless Steel: High corrosion resistance, ideal for transparent and optical parts.
- Pre-hardened Steels: Easy to machine, suitable for prototypes and short production runs.
Steel properties vary significantly. Final selection should always be based on mold design, plastic material, and production volume.
7. Practical Tips
✅ Do not choose mold steel based on low price alone for mass production—this often leads to higher long-term maintenance costs.
✅ For products with extremely high surface quality requirements, prioritize highly polishable steels.
✅ Work closely with mold makers or steel suppliers when selecting materials, taking machining capability and material availability into account.
Conclusion
Selecting the right mold steel is about balancing Volume, Plastic Type, and Surface Finish. However, don't let "hard-to-machine" fears limit your material choice.
By pairing the right steel with high-performance machining solutions from ASIATOOLS, you ensure that your molds are not only durable but also manufactured with maximum efficiency and precision.
Ready to machine your next mold? Explore how ASIATOOLS High-Rigidity Machining Centers handle the toughest mold steels with ease.