In mold manufacturing, material selection often determines success or failure. Why can some molds only last tens of thousands of cycles before wearing out, while others reliably produce over a million parts? The answer lies in choosing the right injection mold materials. This choice not only affects mold life but also directly impacts product quality, production efficiency, and overall cost.
This article will break down mold material selection from three critical perspectives: plastic part characteristics, production volume and efficiency, and mold cost & economics. Mastering these three points will help you quickly determine which material is most suitable for your project.
Mold material isn’t always better when harder; it must match the product volume, material characteristics, and budget.
1. Plastic Part Characteristics: The Primary Determinant of Mold Material
The relationship between a mold and the plastic it shapes is like shoes and feet. Without understanding the properties of the plastic, it’s difficult to select the right mold material. Key considerations include:
Corrosiveness
Plastics such as PVC or those with halogen flame retardants release corrosive gases at high temperatures, which can rust mold surfaces and cause pitting over time.
A common solution is to use stainless steel (e.g., S136, 440C) or apply surface plating like chrome or nickel on standard steel.
Wear Resistance
Glass fiber reinforced plastics (e.g., PA+GF) flow like “sandpaper” during injection, causing severe abrasion to cavities and runners.
High-hardness steel such as H13, or inserting carbide in critical areas, is typically required.
Flow Characteristics and Strength Requirements
Plastics like PC and PMMA have poor flowability and require higher injection pressure. Molds must be rigid enough to prevent deformation.
Transparency and Surface Finish
Transparent or mirror-finish parts demand extremely high mold surface quality. Materials like S136 and NAK80 are commonly used.
Thermal Performance
High-temperature plastics (e.g., PEEK, PPS) require mold materials that maintain strength at elevated temperatures. Hot-work steels (H13, 2344) are standard choices.
Mold failures are often not caused by the material itself, but by a mismatch between material and application scenario.
Understanding the characteristics of the plastic part is the first step in selecting a mold material.
2. Production Volume and Efficiency: Determining Material Grade
How long a mold can produce and how fast it operates depends on its “workload” and operating environment.
Typical Material Choices Based on Production Volume
| Production Scale | Common Materials | Characteristics | Suitable Applications |
|---|---|---|---|
| Small Batch (thousands–tens of thousands of cycles) | P20, 718, Aluminum | Low cost, fast manufacturing, short life | Prototype molds, small batch orders |
| Medium Batch (hundreds of thousands of cycles) | 718H, 738H, NAK80, H13 | Balanced performance, stable lifespan | Consumer electronics parts, automotive interior components |
| Large Batch (over 1 million cycles) | Premium H13, powder steel, carbide inserts | High initial cost, long life, low per-piece cost | Automotive exterior parts, large structural components |
Efficiency Considerations
Cooling Rate: High thermal conductivity materials like beryllium-copper are often used in local mold areas to speed up cooling.
Automation: Automated production lines require molds to be durable, wear-resistant, and reliable over long periods.
Demolding Performance: Good polishing or surface treatments reduce ejection issues and improve production smoothness.
The value of a mold isn’t just how many cycles it can produce, but how quickly it can be delivered and put into production.
In other words, the larger the batch and the higher the efficiency requirement, the higher the performance grade of steel required for stable long-term operation.
Common Injection Mold Materials Comparison
| Material | Key Features | Machining | Wear/Corrosion Resistance | Approx. Lifespan | Typical Applications |
|---|---|---|---|---|---|
| P20 | Pre-hardened, low cost | Easy to machine, fast | Moderate | ~50k–100k cycles | Small batch, prototype molds |
| 718 / 718H | Imported pre-hardened steel, balanced performance | Good | Good | ~300k–500k cycles | Medium batch, consumer parts |
| NAK80 | High mirror finish, excellent polishability | Easy | Moderate | ~300k–500k cycles | Transparent parts, mirror-finish products |
| H13 / 2344 | Hot-work steel, high strength & heat resistance | Harder to machine | High | ~500k–1M+ cycles | High-temperature plastics, large-volume production |
| S136 | Stainless steel, corrosion & wear-resistant | Excellent polish | Excellent | 500k–1M+ cycles | Transparent, corrosive plastics (PVC) |
| Powder Metallurgy / High-Speed Steel | Extremely hard and tough, top performance | Difficult, can use EDM | Very high | Over 1M cycles | High-wear large batch, automotive molds |
| Aluminum Alloy (7075, QC-10) | Low cost, good thermal conductivity, fast manufacturing | Very good | Poor | ~10k–30k cycles | Prototype molds, small batch, high-heat-dissipation parts |
| Beryllium-Copper Alloy | Excellent thermal conductivity | Difficult to machine | Moderate | Usually used as inserts | Local mold areas (hot spots, deep cavities) |
3. Mold Cost and Economics: Understanding “Per-Part Cost” Is the Real Savings
When selecting mold materials, many focus on initial cost. However, from a production perspective:
“The key measure of mold economics isn’t the initial cost, but the per-part production cost.”
For example, a low-cost mold may only last 20,000 cycles, while a premium mold can reliably run 500,000 cycles. The former seems cheaper initially, but the latter often achieves a much lower per-part cost in high-volume production and reduces downtime and maintenance.
Material Cost Differences：
P20 is cheapest, 718H and NAK80 are mid-range, H13 requires heat treatment, and powder steel or carbide is most expensive.
Machining and Maintenance Costs：
Harder steels are more difficult to machine, consuming more tool life, but last longer and require less maintenance. Cheaper steels are easier to process but may need frequent repairs, increasing total cost.
Per-Part Cost Formula：
Per-Part Cost = (Total Mold Cost + Maintenance Cost) / Total Number of Parts
A truly smart choice isn’t the lowest mold price, but the lowest Total Cost of Ownership (TCO).
Conclusion
Mold material selection has no absolute standard. It requires a comprehensive assessment of plastic part characteristics, production volume & efficiency, and overall economics.
If you’re struggling to decide, ask yourself:
- What type of plastic am I using?
- How many parts will I produce?
- Do I prioritize initial cost or long-term stability?
Answering these questions, combined with supplier expertise, greatly improves material selection accuracy.
At RJC Mold, we frequently help clients balance these factors, avoiding both “over-design” and “under-design.” If you’re planning a plastic injection mold or injection molding project, feel free to consult our engineering team for the most suitable solution.