Incorrect mold steel selection can ruin a project. It causes premature wear, substandard part quality, and totally blown budgets. Picture your mold breaking in the middle of production, causing huge delays and rework. Getting to know the main differences between P20, H13, and S136 is the beginning of preventing costly errors and making the ideal selection for your particular requirements each time.
The ideal mold steel is completely up to your project. P20 is a general-purpose, pre-hardened steel great for low production runs and general-purpose molds. H13 is a hard, hot-work tool steel great for high-temperature plastics and high production runs because of its better hardness. S136 is a high-end stainless steel selected because it is great at resisting corrosion and can be polished to a high finish, so it is required for optical components or when working with corrosive resins such as PVC.
One of the most vital decisions in the mold design is about making the right decision about steel. My experience in the field has led me to witness its breakdown and success of projects. Cost is not only the initial cost of purchase, it is the overall cost of ownership, which includes of maintaining, having to wait and the quality of the end product parts. Okay, so how do we go about examining these materials and then right down one by one to find out why each one of them rises to the occasion? This will assist you to find suitable steel in your next project with no doubts.
What is the difference between H13 steel and P20 steel?
You will find P20 and H13 on spec sheets everywhere but there is not always a clear distinction between spec and reality. It is a tremendous gamble to make judgments based on price. You may wind up with a mold that cannot hold up to the heated temperatures of an engineering grade plastic, or one that will wear out far too soon and cost you precious time and money to rebuild. To learn how to apply these properties, we will consider the essential properties of them.
The biggest distinction is the purpose of use and the hardness. P20 is a general purpose steel, which is distributed at approximately 30-36 HRC, that is not hardened. This is ideal on prototypes and medium runs using non abrasive plastics such as PP or ABS. A more difficult hot-work tool steel is H13, which must be heat-treated to a maximum hardness of 48- 52 HRC. It performs at high-temperature use and abrasive, high-volume engineering plastics.
I recall a customer early in my career that wanted to save a couple of thousand dollars off a mold of a polycarbonate (PC) part. When I recommended the use of P20 steel they insisted on it. Polycarbonate needs very high melt temps and after only 50,000 shots the mold cavity was showing heat stress, and wear. The part dimensions moved out to their limits and stops in production had to be called. The core and cavity were required to be remade of H13 steel. The rework and the downtime of the company production cost entirely the initial savings of the company. This experience came in handy and gave me a lesson, never to match more steel with the requirements of the plastic resin.
Hardness and Durability
Here the greatest variation is to be found. P20 is pre-hardened meaning it does not require another form of heat treatment before it is machined. This is cost effective. Its hardness however is moderate thus it can be used where the demand on its use is lighter. H13 however is normally available in annealed (soft) condition. Upon machining, it requires heat-treatment so that it can become very hard. This additional measure is not only more expensive and time-consuming, however, the result is a mold that can tolerate significantly hotter temperatures, pressures and abrasive products and materials resulting in a much longer durability.
Cost vs. Performance
It’s crucial to look beyond the initial material cost and consider the overall project value.
| Feature | P20 Steel | H13 Steel |
|---|---|---|
| Hardness (Typical) | ~30-36 HRC (Pre-hardened) | ~48-52 HRC (After Heat Treat) |
| Heat Treatment | Not required | Required |
| Best For | Prototypes, low-medium volume | High volume, high temp plastics |
| Typical Plastics | PP, PE, ABS, PS | PC, PA, PA+GF, POM |
| Mold Life (Est.) | Up to 500,000 shots | 1,000,000+ shots |
| Initial Cost | Lower | Higher |
| Machinability | Good | Good (before heat treat) |
P20 is cheaper upfront and faster to get into production. For a prototype mold or a short run of 100,000 parts using a simple material like polypropylene, P20 is often the smartest economic choice. But for a high-volume program running a million parts with glass-filled nylon, H13 is the only viable option. Its durability ensures consistency over the long run and prevents costly mid-production failures.
What is S136H material?
You need to form clear lenses or parts from a corrosive plastic like PVC, but regular steels just aren’t capable of that. Imagine rust stains on your high-polish mold surface after a few weeks. This fouls up every part and necessitates expensive, time-consuming re-polishing, ruining your productivity and profits. This is exactly the kind of scenario where a high-performance steel like S136H comes to the rescue.
S136H is a premium chromium tool steel which is highly corrosion-resistant and can be polished to a mirror finish. The ‘H’ suffix means that it’s pre-hardened (similar to P20), which is a time-saving in heat treatment. It’s the premium choice for molds handling corrosive plastics, wet environments, or optical clarity and a flawless surface finish, such as lenses, medical devices, or clear covers.
I was working for a medical firm making a new diagnostic device with a clear plastic window. The material was PMMA (acrylic), and the part has to be optically perfect. Furthermore, the mold would be warehoused in a space where humidity could be a problem. P20 or H13 was out of the question; any tiny rust spot on the mold surface would be transferred to the part, rendering it useless. We employed S136H. That it could be polished to an SPI A-1 (mirror) finish was critical, and its stainless composition meant we never had to worry about corrosion during production or warehousing. It was a little more expensive, but in this use, it was our only choice.
The Power of Corrosion Resistance
Chromium is the most important ingredient in S136H which is normally approximately 13-14%. This abundance of chromium forms a passive covering layer of oxide on the steel surface that saves it against rust and corrosion. This is particularly important in case of molding materials that excrete corrosive byproducts e.g. PVC or a molding environment that is humid. The property also makes maintaining the equipment less costly since you no longer have to apply corrosion-resistant coatings so regularly.
Achieving a Mirror Finish
Beyond corrosion resistance, S136H is manufactured with very high purity and a uniform microstructure. This allows it to be polished to an extremely high level, often referred to as a "mirror finish." This is impossible to achieve with standard steels like P20, which have more impurities and a less uniform structure.
| Feature | P20 | H13 | S136H |
|---|---|---|---|
| Corrosion Resistance | Poor | Poor | Excellent |
| Polishability | Good (SPI B-2) | Very Good (SPI A-2) | Excellent (SPI A-1) |
| Primary Advantage | Low Cost, General Purpose | Hardness, Wear Resistance | Corrosion Resistance, Polish |
| Typical Use Case | Opaque Housings | Gears, Structural Parts | Lenses, Medical Parts, PVC |
Choosing S136H is a decision driven by the part’s requirements. If your part needs to be transparent, requires a perfect cosmetic surface, or is made from a corrosive resin, the extra investment in S136H is not just recommended; it’s necessary for success.
What is P20 mold steel used for?
P20 mold steel is generally utilized in the production of molds for plastic injection molding, die casting, stamping, and rubber molding. It is commonly used in various applications because of its outstanding machinability, homogeneous hardness, wear resistance, and ability to endure high pressure and temperature during the manufacturing process. The steel’s characteristics make it ideal for making mold bases, die holders, backing plates, and other tooling components.
P20 steel is utilized in a variety of engineering applications, including automobile parts such as engine components, frames, and brake system parts, as well as mechanical engineering for agriculture and construction machinery components. Its great strength, durability, and fatigue resistance enable precision manufacture and long service life in various industries.
Which steel is best for molding?
And with all these choices you may still ask one more time: is there one single best steel? This fear of erring in choosing may cause analysis paralysis as you take time explaining the risks plus the advantages of a project. You require an evident system of making a decision. The reality is; the best steel is always whatever works in the unique needs of your project.
No one kind of steel is best in all molding. An optimum combination of three main issues- the plastic that is to be molded, the target production volume (mold life), and your finances- should be sought. P20 is the most suitable with general purpose, low to medium volume projects. High-volume and high-temperature engineering plastics are made best with H13. S136 is the hero of high-precision, high-polish or corrosive uses where one has to concern less about the quality of the parts.
Imagine, it is a question of selecting a car. No one would ride a sports car to deliver lumber and a heavy-duty truck to commute in the city. And each is the best in the sense that it is made to do just what it is meant to do. Exactly the same sense can be applied to mold steels. When I advise my clients, I approach the decision-making process with regards to the needs of the particular project and not seek a single solution. How to think through this. Lets break it down.
Factor 1: The Plastic You’re Molding
The resin you choose is the first and most important factor. Some plastics are gentle on a mold, while others are incredibly aggressive.
- General Purpose Plastics (PP, PE, ABS): These have lower melting temperatures and are not abrasive. P20 is perfectly suitable and cost-effective for these materials.
- Engineering Plastics (PC, PA, POM): These require higher molding temperatures, which can cause heat stress in lesser steels. H13 is the standard choice here due to its excellent heat resistance.
- Abrasive Plastics (e.g., PA+GF): Plastics with glass fiber (GF) or other fillers act like sandpaper on the mold surface. The high hardness of heat-treated H13 is essential to resist this wear.
- Corrosive Plastics (PVC) or Optically Clear Plastics (PMMA, PC): For these, S136 is the clear winner due to its corrosion resistance and superior polishability.
Factor 2: Your Production Volume Needs
How many parts do you need to make? This directly relates to the required lifespan of the mold.
- Low to Medium Volume (< 500,000 shots): For prototypes or products with a shorter market life, P20 offers a great balance of performance and cost.
- High Volume (> 1,000,000 shots): For long-running, high-volume production, the durability of H13 is a necessary investment to ensure the mold lasts the entire product lifecycle without major failure.
- Specialty High Volume: S136 also offers excellent wear resistance and can handle high volumes, especially when its other properties (corrosion resistance, polish) are also required.
Factor 3: Surface Finish and Special Requirements
Finally, what does the final part need to look like and do?
- Standard Finish: For many internal or non-cosmetic parts, a standard machine finish or a light polish (SPI C or B grade) is sufficient. P20 handles this well.
- High Gloss/Textured Finish: H13 can be polished to a high gloss (SPI A-2) and holds up well to texturing processes.
- Mirror/Optical Finish: For lenses, light pipes, or premium cosmetic parts, nothing beats the SPI A-1 mirror finish achievable with S136. Its purity is essential for a flawless surface.
Basic Requirements for Mold Steel Materials
Hardness and wear resistance of steel are significant in mold manufacture. This is because molds wear continuously when molten plastic is being injected into them, mainly when the plastic contains hard fillers like glass fibers, which tend to be damaging. Molds also encounter impacts during operation, for which a certain toughness is needed in order not to crack. During high-temperature plastic processing, the need for wear-resistant steel is greater, and hence, there is a need to use high-performance mold steel.
Minimal Heat Treatment Deformation
Mold steel hardens during heat treatment, making machining more challenging and hence more expensive. Steel materials with consistent internal structures that show minor deformation under heat treatment are required to increase manufacturing efficiency and lower costs. These materials guarantee the accuracy and durability of the mold by not developing excessive internal tensions or deformities.
Polishing Performance
The high-quality mold steel should have a very high purity with fine and uniform structure in order to impart a perfectly polished mold surface. Any pores, impurities, or other surface defects on the mold surface will adversely affect the appearance of the product. For instance, S136 steel is renowned for its outstanding polishing characteristics.
Weldability Molds
It will also experience wear and accidental damage during use, and sometimes the design changes need modifications. The weldability of mold steel is thus very important as it will determine the ease and feasibility of mold repairs and alterations.
How to calculate mold life?
You’ve chosen a steel, but your supervisor or client wants expected life in hard figures. Guessing is dangerous. If you promise an impossible mold life, you lose your reputation and can create unwanted, costly production downtime in the future. There is no formula you can apply as a rule, but you can make an excellent professional guess if you look at key factors.
It is estimated that mold life is a function of abrasiveness of the steel against the plastic resin, the wear resistance and hardness of the steel, cycle times in the production process, and injection pressure. P20 mold making non-abrasive polypropylene might have a mold life of up to 500,000 cycles, for instance. H13 mold making abrasive glass-filled nylon might have up to 1,000,000 cycles. S136 has an incredibly long life, particularly with corrosive materials. These are estimates; the trick to maintaining them is consistent and regular maintenance.
I always stress to my clients that a mold life number is an estimate, not a guarantee carved in stone. The single biggest variable that they control is maintenance. I once had two clients using nearly identical H13 molds to produce the same PA+GF part. One client followed a strict weekly cleaning and inspection schedule. The other only performed maintenance when a problem occurred. The first client’s mold easily surpassed 1.2 million shots. The second client’s mold started having flashing issues and broken core pins before it even hit 700,000 shots. Same steel, same part, different outcomes—all because of maintenance.
Key Variables in Mold Life Estimation
Estimating mold life is about weighing several interconnected factors. It’s a skill that combines material science with practical experience.
| Influencing Factor | Impact on Mold Life | Explanation |
|---|---|---|
| Steel Type & Hardness | High | Harder steels like heat-treated H13 resist wear far better than softer steels like P20. |
| Plastic Resin Abrasiveness | High | Resins with fillers like glass fiber (GF) or mineral fillers dramatically accelerate wear on the mold cavity and core. |
| Part Complexity | Medium | Parts with thin walls, sharp corners, or delicate core pins create high-stress areas that are more prone to wear or breakage. |
| Cycle Time & Pressure | Medium | Aggressive, fast cycle times with high injection pressures increase the rate of wear and thermal stress on the steel over time. |
| Maintenance Protocol | High | Regular cleaning, lubrication, and inspection can double a mold’s effective life by preventing premature wear and damage. |
The Critical Role of Maintenance
You cannot overstate the importance of a good maintenance plan. It is the single most important factor in maximizing the life you get from your steel choice. A proper plan includes:
- Regular Cleaning: Removing resin buildup and gas deposits prevents contamination and corrosion.
- Lubrication: Ensuring all moving parts like ejector pins and slides move freely prevents galling and breakage.
- Inspection: Regularly checking for wear on the parting line, cavity surfaces, and core pins allows you to catch small problems before they become catastrophic failures.
When you invest in a high-quality steel like H13 or S136, you must also invest in the procedures to protect it. Doing so ensures you get every bit of value and life out of your tool.
Conclusion
The decision between P20, H13 and S136 is not between what is the single best steel but which is the right steel to use. It is a strategic choice in which you have to set off cost versus performance and your project needs. With a clear knowledge of the key strengths of the two materials at hand, you can choose the one successfully and ensure that your mold achieves success, durability, and profitability. Perfect this and you are halfway on the path towards perfecting molding.