When bottle manufacturers evaluate a new PET preform mold, cavity count and hot runner systems often dominate the conversation. Yet the steel grade inside the mold base and core inserts directly determines how long that mold produces clear bottles before corrosion, wear, or dimensional drift forces costly rework. At HEYAN Mold, engineers match PET preform mold steel grade S136 2316 and other alloys to each customer’s resin type, water chemistry, and production volume. This guide explains why that choice matters and how the grades compare in real factory conditions.
Why Steel Grade Matters in PET Preform Molds
PET preform molding subjects steel to a combination of heat, pressure, and chemical attack that differs from standard injection molding. The melt temperature reaches 270–290 °C. Resin degradation releases acetaldehyde and acidic by-products. Cooling channels circulate water that may carry chlorides or hardness minerals. Over thousands of cycles, the wrong steel grade shows pitting on core pins, loss of polish on cavity walls, or dimensional changes that make bottle clarity inconsistent. Selecting the correct alloy at the design stage prevents these failures before the first shot.
Corrosion Resistance from PET Resin Degradation
PET resin begins to degrade at temperatures above 280 °C, releasing acidic compounds that attack tool steel. In aggressive environments—high melt temperatures, long residence times, or frequent color changes—corrosion appears first on core pins and near gate areas where flow velocity is highest. Stainless tool steels with chromium content above 12 % resist this attack. S136, with 13.6 % Cr, forms a passive layer that slows chemical penetration. Standard pre-hardened steels without this chromium level show measurable weight loss after extended exposure.
Wear Resistance and Polishability for Bottle Clarity
The inner surface finish of a PET preform cavity determines light transmission in the final bottle. Any scratch, pit, or rust spot on the cavity wall creates optical defects. Steels that hold a mirror polish after thousands of cycles maintain bottle clarity. S136 and 1.2316 both accept polish levels below Ra 0.01 µm, but the difference appears after 500,000 cycles: corrosion-resistant grades retain their finish, while non-stainless grades require repolishing or chrome plating to recover surface quality.
Heat Treatment Hardness and Dimensional Stability
PET mold components see repeated heating and cooling. Core inserts must resist plastic deformation under clamping pressure, while maintaining dimensional tolerances across the temperature swing. Most PET mold steels are through-hardened to 48–52 HRC. S136 reaches 50–52 HRC after quenching and tempering, with minimal distortion when vacuum heat treated. 1.2316 achieves slightly lower peak hardness but offers better dimensional stability during machining, reducing benchwork before final assembly.
PET Preform Mold Steel Grade Comparison: S136 vs 1.2316 vs Others
The table below summarizes the alloys most commonly specified for PET preform mold cores and cavities.
| Steel Grade | Standard | Cr Content (%) | Hardness After HT (HRC) | Corrosion Resistance | Relative Cost |
|---|---|---|---|---|---|
| S136 | ASSAB | 13.6 | 50–52 | High | Higher |
| 1.2316 | DIN | 12.0–13.5 | 46–48 | Moderate–High | Medium |
| 1.2083 / 420 | DIN / AISI | 12.0–14.0 | 48–52 | Moderate | Lower |
| NAK80 | Daido | — | 37–43 (pre-hardened) | Low | Medium |
| PX5 | Nachi | — | 30–36 (pre-hardened) | Low | Lower |
S136 (ASSAB) — High Corrosion Resistance for Aggressive Environments
S136 is a martensitic stainless tool steel developed by ASSAB (Uddeholm). Its 13.6 % chromium content provides strong resistance to the acidic by-products of PET degradation. At HEYAN Mold, S136 is the default choice for molds running aggressive resins, high recycled content, or factories with poor water treatment where cooling channels are prone to chloride attack.
Key data points:
– Through-hardened hardness: 50–52 HRC after vacuum quench and double temper.
– Polishability: Achieves Ra < 0.01 µm; used for optical-grade preforms.
– Typical applications: 32- to 96-cavity preform molds, core inserts, and cavity blocks.
– Limitation: Higher material cost and longer lead time due to heat treatment requirements.
1.2316 (DIN) — European Standard for Cost-Effective Durability
1.2316 is a DIN-standard martensitic stainless tool steel with chromium content close to S136 but with different carbon and molybdenum balance. It offers a practical middle ground: better corrosion resistance than 1.2083, lower cost than S136, and good availability from European and Asian suppliers.
Key data points:
– Through-hardened hardness: 46–48 HRC.
– Corrosion resistance: Adequate for standard PET grades and treated cooling water.
– Machinability: Slightly easier to machine than S136 in the annealed state, reducing electrode wear in EDM work.
– Typical applications: 8- to 48-cavity molds where water quality is controlled and resin is virgin or lightly recycled.
For buyers evaluating total cost of ownership, 1.2316 often wins when the production environment is stable and aggressive chemicals are not present.
1.2083 / 420 Stainless — Budget Option with Trade-offs
1.2083 (DIN) and its AISI equivalent 420 are widely available martensitic stainless steels used in mold making. They provide baseline corrosion resistance at a lower alloy cost.
Key data points:
– Through-hardened hardness: 48–52 HRC.
– Corrosion resistance: Lower than S136 and 1.2316; requires chrome plating or nitriding in aggressive environments.
– Polishability: Good initial finish, but surface degradation appears earlier without protective coatings.
– Typical applications: Low-cavity molds, prototype tooling, or markets where upfront cost is the primary constraint.
The trade-off is maintenance frequency. A 1.2083 mold running 24/7 in warm, untreated cooling water may need cavity rework after 300,000–400,000 cycles, compared to 800,000+ cycles for S136 under the same conditions.
NAK80 / PX5 — Pre-Hardened for Shorter Lead Times
NAK80 and PX5 are precipitation-hardened (pre-hardened) steels delivered at working hardness without additional heat treatment. They offer the shortest possible delivery time for mold components.
Key data points:
– Hardness: 37–43 HRC (NAK80), 30–36 HRC (PX5) — no heat treatment required.
– Corrosion resistance: Low; NAK80 is not stainless. Requires chrome plating or coating for PET use.
– Wear resistance: Adequate for short-run production or prototype validation.
– Typical applications: Tight-delivery prototype molds, sampling tools, or markets where fast validation is more important than multi-year life.
HEYAN Mold occasionally uses NAK80 for prototype cavities when a customer needs samples in two weeks rather than six, with the understanding that production molds will switch to through-hardened stainless grades.
How to Choose the Right Steel Grade for Your Application
Water Quality, Resin Type, and Production Volume Factors
The operating environment is the first filter:
- Cooling water chemistry: High chloride or sulfate levels accelerate corrosion in non-stainless grades. If your factory cannot maintain deionized or treated water, S136 provides a safety margin.
- Resin type: Virgin PET is chemically stable. High recycled content, barrier resins, or specialty grades with additives increase degradation by-products. S136 or 1.2316 is recommended.
- Production volume: A mold running 3 million cycles per year justifies S136 or 1.2316. A mold running 200,000 cycles per year may not recover the premium in avoided maintenance.
Matching Steel Grade to Cavity Count and Mold Complexity
Higher cavity counts multiply the cost of downtime. A 96-cavity mold stopping for cavity rework loses more output per hour than an 8-cavity mold. HEYAN Mold’s steel selection policy reflects this:
- 8–16 cavities: 1.2316 or 1.2083 with chrome plating is often sufficient, especially with treated water.
- 24–48 cavities: 1.2316 is the standard; S136 specified for aggressive environments.
- 64–96 cavities: S136 for core inserts and critical cavity surfaces; 1.2316 accepted for secondary components.
This tiered approach balances durability with material cost. HEYAN Mold is capable of producing up to 96-cavity molds using this framework.
HEYAN Mold’s Steel Selection Policy
HEYAN Mold (合研科技) was established in 2018 and holds 27 patents in PET preform mold and blow molding machine design. Core team members have worked in PET mold engineering since the early 2010s. The company’s production capacity scales from 8 to 96 cavities across preform mold lines.
Steel selection follows a documented process:
- Customer questionnaire: Water source, resin specification, target cycle count, and maintenance capability.
- Environmental scoring: Chloride level, pH, and temperature of cooling water are rated from 1 (mild) to 5 (severe).
- Steel assignment: S136 for scores ≥ 4; 1.2316 for scores 2–3; 1.2083 or coated NAK80 only for scores ≤ 2 with customer acknowledgment of shorter service intervals.
- Documentation: Heat treatment certificates, hardness test reports, and steel mill certificates are provided with each mold.
This policy eliminates guesswork. A customer in West Africa with borehole water and high recycled PET receives S136. A customer in Northern Europe with municipal water and virgin resin receives 1.2316. Both get the correct alloy for their actual conditions.
Conclusion
The choice between PET preform mold steel grade S136 2316 and other alloys is not about buying the most expensive material. It is about matching the steel’s corrosion resistance, hardness, and polishability to the resin chemistry, water quality, and production volume of the specific line. S136 provides maximum protection in aggressive environments. 1.2316 delivers solid performance at moderate cost. 1.2083 and pre-hardened grades fill niches where upfront cost or lead time is the overriding constraint.
For a broader view of how mold design choices interact with production planning, see our article on PET Preform Mold Cavity Count Selection. If you are evaluating suppliers and want to know what documentation to request, our PET Preform Mold Quotation Checklist covers steel certificates, heat treatment records, and hardness verification steps.
— HEYAN Mold Operations Team
Need a steel recommendation for your next PET preform mold? Contact HEYAN Mold with your resin specification, water analysis, and target production volume. The engineering team will specify the grade that delivers the lowest cost per thousand bottles over the mold’s lifetime.
