In injection mold manufacturing, steel selection often comes down to one core conflict: how to control cost while still ensuring the mold performance meets production requirements. In real workshops, customers commonly show a plastic part drawing and ask: “I’m only making 5,000 parts—what steel should I choose so I’m not wasting money?” Many mold makers worry that “cheap means poor quality” and the mold life won’t be long enough, or blindly pursue high-grade steels and end up with unnecessary cost.
The real key is precision matching—making steel performance equal to actual demand, so you don’t pay for “unused performance.” This content is compiled from Moldsteells’ practical experience in mold steel selection projects, for your reference.
Core Budget-Friendly Steel Options: How to Choose 45#, S50C, P20, and 718H?
To choose the right steel, you must first understand each grade’s “cost-performance boundary.” The following four budget-friendly steels cover most low- to mid-range injection molds, but their prices, performance, and applications differ significantly (data compiled based on current market conditions).
| Steel Grade | Unit Price (USD/kg) | Hardness (HRC) | Typical Mold Life (Shots) | Key Characteristics |
|---|---|---|---|---|
| 45# | 0.57–1.14 | ~20 as-supplied; 30–35 after heat treatment | 5,000–10,000 | Lowest cost; requires heat treatment; good machinability |
| S50C | 0.71–1.29 | ~25 as-supplied; 35–40 after heat treatment | 10,000–50,000 | Upgraded 45#; slightly higher hardness and wear resistance |
| P20 | 1.43–2.14 | Pre-hardened 28–32 | 50,000–100,000 | Entry-level alloy pre-hardened steel; no additional heat treatment needed |
| 718H | 2.14–3.14 | Pre-hardened 29–33 | >100,000 | Pre-hardened; uniform hardness; good polishability |
Current mold steel pricing is influenced by raw materials. Plain carbon steels like 45# and S50C usually fluctuate less, while pre-hardened alloy steels like P20 and 718H can be more affected by alloying elements (e.g., chromium and nickel). When selecting, it can be helpful to watch short-term price trends.
45# steel is the absolute cost-floor option, suitable for simple plastic parts with up to 10,000 shots. For example, a small PP bottle cap mold: if the steel usage is about 50 kg, the steel cost is only about USD 28.57–57.14. However, note: its as-supplied hardness is low, and it can deform easily, so it must be paired with quenching and tempering.
S50C is a “budget upgrade” over 45#, and is often more cost-effective in mid-volume scenarios (10,000–50,000 shots). For example, a standard ABS housing mold: using S50C can extend mold life by about 3× compared with 45#, while the steel cost only increases by around 20%.
P20’s main advantage is saving process steps. With a pre-hardened hardness of HRC 28–32, it can be milled directly, eliminating the need for heat treatment. It’s suitable for 50,000–100,000 shots, especially when lead time matters—such as fast tooling for e-commerce “hot-selling” plastic parts.
718H is often the “best value” choice for mid-to-high volume, and is recommended when production exceeds 100,000 shots. Its hardness uniformity is better than P20, and polishing can reach Ra 0.4, making it suitable for glossy surface requirements (e.g., home appliance housings). Its price is only about 15%–20% higher than P20, but mold life can increase by more than 50%.
Key Selection Logic: Output Volume and Part Requirements Decide “Where the Money Should Go”
The essence of steel selection is: use the least money to cover the maximum requirement. Focus on two things:
1) Cost by volume tier
Small batch (< 10,000 shots): 45# usually offers the best cost-performance. For example, a 10,000-shot PC toy mold: using 45# might cost about USD 28.57–57.14 for steel; choosing S50C would be about USD 35.71–64.29. The extra cost can be pure “performance redundancy.”
Medium batch (10,000–100,000 shots): choose between S50C (requires heat treatment) and P20 (no extra heat treatment). If the factory has in-house heat treatment, S50C can be cheaper; if not, P20 avoids outsourced heat treatment, which commonly costs about USD 142.86–428.57 per ton (depending on process complexity and region) and may reduce total cost.
Large batch (> 100,000 shots): go directly to 718H. In some cases, P20 molds may require mid-life repair (e.g., welding) due to insufficient life, resulting in a higher total cost than starting with 718H (exact differences must be calculated based on real operating conditions).
2) Performance baseline by plastic material
Common materials (PP, ABS, PE): all four steels can work; volume becomes the main deciding factor.
Special materials: for plastics containing PVC (corrosive) or glass fiber (high wear requirement), you should exclude 45# and S50C. For example, PVC pipe fitting molds: 718H can help prevent rust-related surface pitting, while a 45# mold may be scrapped after around 30,000 shots due to corrosion.
3 Practical Cost-Saving Strategies: Don’t Buy Expensive—Buy “Correct”
To control mold steel cost, the key is cutting redundancy precisely. These three methods have been validated in real workshops:
Strategy 1: Choose steel by surface roughness threshold—avoid “blind upgrading”
Plastic part surface roughness directly sets the upper limit of steel selection:
- Ra ≥ 3.2 (visible texture): 45# is often enough; even without heat treatment it can be machined
- Ra 1.6–Ra 3.2 (slight gloss): choose S50C or P20
- Ra 0.8–Ra 1.6 (mirror-like effect): choose 718H
Example: if a children’s toy part requires Ra 3.2, using P20 can be wasteful—45# plus sandblasting after machining can reduce cost by about 40%.
Strategy 2: Prioritize pre-hardened steels—what you save is more than heat treatment cost
Pre-hardened steels like P20 and 718H can shorten lead time. One mold shop’s internal statistics suggested that for similar mold complexity, using P20 versus 45# (which requires heat treatment) shortened tooling lead time by about 5 days (case data for reference only; actual results vary by mold complexity and process capability), indirectly reducing WIP and shop-floor inventory cost.
Strategy 3: For low volume but high appearance, use “process” to compensate for “material”
If the part requires better appearance but the volume is low (e.g., 5,000-shot mirror-like parts), you can try 45# + fine polishing instead of 718H. In practice, after fine polishing above 1200 grit, 45# surface roughness can approach Ra 1.6 (can be tried for low-volume and lower mirror requirements; for high-volume or true mirror finishing, 718H is still recommended). This approach can reduce costs by about 35% compared to 718H while meeting low-volume needs.
Conclusion
The core rule for injection mold steel selection is: match volume to mold life, and match part requirements to steel performance. Use 45# for small batches; choose S50C/P20 for medium batches; go with 718H for large batches—avoid paying for “performance you won’t use.”
- For small-batch production, calculate steel weight first to avoid wasting “large stock cut into small sizes.”
- For glass-fiber-filled or PVC parts, exclude 45#/S50C and prioritize 718H.
- For pre-hardened steel machining, choose cut-to-size to reduce offcut scrap (can save about 10%–15% material).
