Johnny Xiong
Rapid Tooling Expert
Contents
For custom manufacturing, the mold is arguably your most significant upfront investment. Whether you are launching a consumer electronic device or a medical component, the steel you choose for your mold core and cavity dictates the success of your production run.
At HordRT, we have overseen thousands of tooling projects, and we often see clients struggle with a common dilemma: choosing the wrong steel. Select a steel that is too soft, and you risk early tool failure, flash on parts, and costly repairs. Select a steel that is overkill for your volume, and you waste thousands of dollars and weeks of lead time on durability you don't actually need.
There is no single "best" material in mold making. However, understanding the specific types of steel for molds is crucial for balancing budget, timeline, and part quality. In this guide, we will break down the three most common steel grades we use at HordRT—P20, H13, and S136—to help you make the right engineering decision.
Key Criteria for Choosing Mold Steel
Before diving into specific grades, it is important to understand the variables that our engineers evaluate when a Request for Quote (RFQ) lands on our desks. When we analyze your 3D CAD files, we aren't just looking at geometry; we are looking at the lifecycle of the project.
1. Hardness & Wear Resistance (HRC)
Hardness is measured on the Rockwell C scale (HRC). The logic is simple: harder steel resists abrasion better. If you are molding glass-filled nylon (which is very abrasive), a soft steel will wear down quickly, ruining the dimensional accuracy of your part. However, harder steel takes longer to machine, which increases the initial tooling cost.
2. Corrosion Resistance
Not all plastics are chemically inert. Materials like PVC (Polyvinyl Chloride) release corrosive gases during the molding process that can eat away at standard tool steel, leading to rusted mold surfaces and pitted parts. If you are using corrosive resins, or if the mold will be stored in a humid environment (like certain areas here in Southern China) without constant maintenance, corrosion resistance is non-negotiable.
3. Polishability and Surface Finish
Does your part require a high-gloss, mirror-like finish (SPI A1 standard)? Or is a textured, matte finish acceptable? Some steels have a grain structure that prevents them from achieving a true optical mirror polish. If you need a clear lens or a cosmetic cover, you need a high-purity steel that can be polished without "orange peel" defects.
4. Machinability and Cost
This is often the deciding factor for startups and rapid prototyping. "Machinability" refers to how fast our CNC machines can cut the steel. Harder steels usually require slower cutting speeds and often a post-machining heat treatment process, which adds days or weeks to the lead time.
Detailed Breakdown of Steel Types
At HordRT, while we can source virtually any material, 90% of our projects utilize one of the following three steels. Here is how they stack up.
1. P20 Steel: The Versatile Workhorse
P20 is the industry standard for general-purpose molding. It is classified as a pre-hardened alloy steel. "Pre-hardened" means it arrives at our factory already heat-treated to a hardness of HRC 28-32. We can load it directly into our CNC machines, cut the core and cavity, and move straight to polishing and assembly without sending it out for further heat treatment.
- Characteristics: Moderate hardness, excellent machinability, good tensile strength.
- Best For: Mid-volume production (50k – 100k shots). It is ideal for structural parts, housings, and general enclosures using non-corrosive plastics like ABS, PP, or PE.
- Pros:
- Cost-Efficiency: It is the most affordable option among the three.
- Speed: Because it skips the post-machining heat treatment step, P20 molds have the fastest lead times (often ready in 15–20 days).
- Repairability: If an engineering change is needed, P20 is relatively easy to weld and re-machine.
- Cons: It is not suitable for abrasive glass-filled materials or corrosive plastics. It will wear out over time if pushed beyond 100,000 cycles.
2. H13 Steel: The High-Volume Performer
When a client tells us, "We need to run this mold for years," we point them toward H13. This is a hot-work tool steel. Unlike P20, we typically machine H13 in a softer "annealed" state and then send it out for heat treatment to achieve a hardness of HRC 48-52.
- Characteristics: Through-hardened, extremely tough, and possesses excellent thermal shock resistance. It can withstand the constant heating and cooling cycles of injection molding without "heat checking" (surface cracking).
- Best For: High-volume production (500k – 1 Million+ shots). It is the standard for high-precision parts and materials containing abrasive fillers (like Glass-Filled Nylon).
- Pros:
- Durability: It holds tight tolerances over millions of cycles.
- Wear Resistance: It resists the abrasive nature of fiber-reinforced plastics.
- Cons:
- Cost & Time: h13 mold steel is harder to cut and requires the extra step of heat treatment, which adds cost and extends the lead time.
- Modifications: Once hardened, engineering changes are difficult and expensive to implement.
3. S136 Steel: The High-Precision Specialist
S136 is a high-grade stainless tool steel. Its defining feature is its purity and high chromium content, which gives it two superpowers: corrosion resistance and polishability.
- Characteristics: Stainless steel grade, high purity structure. It can be heat-treated to HRC 48-52.
- Best For: Medical parts, food-grade products, and transparent parts (Polycarbonate, PMMA/Acrylic) that require a mirror finish. It is also essential for corrosive resins like PVC.
- Pros:
- Surface Finish: Because the steel is so pure, mold core s136 can be polished to an SPI A1 (lens quality) finish without pitting.
- Rust-Proof: It resists oxidation from water channels and corrosive off-gassing from plastics.
- Maintenance: These molds require less maintenance in humid environments.
- Cons: It is the most expensive material option and requires specialized machining techniques.
Quick Comparison Table
|
Feature |
P20 Steel |
H13 Steel |
S136 Steel |
|
Material Cost |
Low ($) |
Medium ($$) |
High ($$$) |
|
Machining Cost |
Low |
High |
High |
|
Hardness (HRC) |
28 - 32 (Pre-hardened) |
48 - 52 (Hardened) |
48 - 52 (Hardened) |
|
Typical Cycle Life |
50k - 100k shots |
500k - 1 Million+ shots |
500k - 1 Million+ shots |
|
Corrosion Resistance |
Poor |
Moderate |
Excellent (Stainless) |
|
Polishability |
Good (Standard) |
Good |
Excellent (Mirror/Optical) |
|
Ideal Application |
Prototypes, Consumer Electronics, ABS/PP |
Automotive, Glass-Filled Nylon, High Volume |
Medical, Optical Lenses, PVC |
How to Decide: A Checklist for HordRT Clients
Navigating these types of steel for molds can be complex. To simplify the process, we use this internal checklist when advising our clients. Ask yourself these three questions:
Check 1: What is your production volume?
- Prototype to Low Volume (Under 100k): Go with P20. There is no need to pay for a mold that will outlive your product's market lifecycle.
- Mass Production (Over 500k): Go with H13. The initial investment pays off by avoiding mold repairs and downtime later.
Check 2: What material are you injecting?
- Standard Plastics (ABS, PP, PE): P20 is perfectly adequate.
- Abrasive Plastics (Glass-Filled): You need the hardness of H13 to prevent the gate and cavity from wearing away.
- Corrosive Plastics (PVC, POM): You must use S136 to prevent the mold from rusting chemically.
Check 3: What is the appearance requirement?
- Tech/Industrial Parts: P20 or H13 with a standard EDM or bead-blast texture.
- High Gloss / Optical Clarity: If you need a "piano black" finish or a clear lens, S136 is the only safe choice. P20 cannot sustain a high polish without inconsistencies appearing in the steel grain.
Conclusion
At HordRT, we believe that informed clients make better products. The difference between p20 mold steel, h13 mold steel, and mold core s136 isn't just about metallurgy—it's about aligning your manufacturing strategy with your business goals.
There is no "best" steel, only the right steel for your specific project. P20 offers speed and economy; H13 offers endurance and toughness; S136 offers perfection in surface finish and resistance.
Not sure which steel fits your budget and timeline?
Don't guess with your investment. Contact HordRT today. Our engineering team will review your CAD designs and production requirements to recommend the optimal mold configuration for your specific needs, ensuring you get high-quality parts on time and on budget.
