For previous reading, please refer to A Guide to Injection Mold Steel Performance and Selection.

 

H13 is a hot work mold steel specified by US AISI standard. Although often categorized as a hot work steel, its excellent high-temperature resistance and wear resistance have earned it a prominent position in plastic mold industry (particularly engineering plastics molding). It is a core material for high-end hot runner molds and large, thick-walled plastic parts. Its composition is designed around a balance of high-temperature stability and toughness: carbon content is precisely controlled between 0.32% and 0.45%, ensuring matrix strength while avoiding brittleness. Chromium content, at 4.75% to 5.50%, is crucial for forming an oxidation- and corrosion-resistant surface layer. Molybdenum (1.00% to 1.50%) and vanadium (1.00% to 1.50%) enhance high-temperature hardness stability, while vanadium improves wear resistance by forming carbides. Silicon (0.80% to 1.20%) further optimizes toughness. Unlike common plastic mold steels (such as S136 and NAK80), H13 is typically shipped in an annealed state, with a hardness of approximately HB220-250. It requires a "quenching and tempering" process (quenching temperature 1020-1050℃, tempering temperature 520-620℃) to achieve a stable hardness of HRC42-48, making it suitable for plastic molding applications with varying strength requirements.

High-Temperature Stability: H13 maintains a hardness above HRC38 even at temperatures of 600℃, far exceeding that of common plastic mold steels (for example, S136 drops below HRC30 at 600℃). This characteristic enables it to meet high-temperature injection molding requirements of engineering plastics (such as PC, PA66, and PBT). Molding temperatures for these plastics often reach 250-320℃, and thick-walled parts require prolonged holding pressure. H13 molds prevent cavity deformation caused by softening at high temperatures, ensuring consistent product dimensions. Furthermore, its coefficient of thermal expansion is only 12.5×10^-6/℃, significantly reducing risk of mold cracking during repeated hot and cold cycles.
High Toughness: After optimized heat treatment, H13 achieves an impact toughness of 15-20 J/cm² (at room temperature). Even at 300℃, it maintains over 80% of its room-temperature level. For complex plastic molds with deep cavities and thin walls (such as automotive engine hood molds and large appliance housing molds), where uneven forces are applied to various parts of mold during molding, H13's high toughness effectively prevents stress concentration, reduces mold failures such as cracking and chipping, and extends mold life.
Strong Wear Resistance: Engineering plastics often contain fillers such as glass fiber and mineral powder (for example, glass fiber-reinforced PA66 can contain up to 30%-50%). These materials can cause severe erosion and wear on mold cavities and gates during injection molding process. Vanadium in H13 combines with carbon to form a VC hard phase, achieving a hardness of HV1800-2000. This effectively resists wear of fillers and, compared to ordinary mold steels (such as 718H), offers 2-3 times greater wear resistance. This allows mold cavity accuracy to be maintained for over 50% longer, reducing maintenance frequency.
Easy Machining and Repairability: H13 exhibits excellent cutting performance in annealed state. Using carbide tools, efficient milling and drilling can be achieved, with comparable efficiency to annealed S136. Although hardness increases after quenching, precision machining can still be achieved by adjusting cutting parameters (such as reducing cutting speeds and using cubic boron nitride tools). In addition, it offers excellent welding performance. If a mold experiences localized wear or minor cracks, repair with argon arc welding and tempering can restore its hardness to over 85% of parent material, eliminating need for complete mold replacement and significantly reducing costs.

H13's applications focus on engineering plastics and high-demand thermoforming molds, primarily including:
Automotive: Molds for high-temperature-resistant plastic parts surrounding engines (such as intake manifolds, sensor housings, and transmission seals);
Home Appliances: Molds for high-temperature operating components (such as microwave oven liner brackets, air conditioning compressor plastic parts, and electric water heater heater tube mounts);
Industrial: Molds for highly filled engineering plastics (such as glass fiber-reinforced PA66 gears, mineral-filled PBT structural parts, and PC+ABS alloy thick-walled housings);
Special Applications: Core components for hot runner systems (such as hot nozzles and manifolds), and small hot work molds (such as zinc alloy die-casting molds). Compared to similar steels, H13 forms a clear division of labor with S136 and 718H. S136 focuses on corrosion resistance and a super-mirror finish, making it suitable for unfilled, high-appearance plastic parts (such as cosmetic casings and optical lenses). 718H, in its pre-hardened state, offers a high cost-effectiveness and is suitable for general unfilled plastic parts (such as automotive interior and exterior trim, common appliance panels). H13 excels in high-temperature stability and wear resistance, making it suitable for highly filled, high-temperature molding engineering plastic parts. It also offers a reasonable cost (imported H13 costs approximately 80-100 yuan/kg, lower than S136; domestic H13 mold steel, such as from Shanghai No. 5 Steel Plant and Fushun Special Steel, costs around 40 yuan/kg). If molds will come into contact with high-temperature, high-wear plastic materials, H13 is optimal choice.

(I) Daily Maintenance
Post-molding Cleaning: After each production batch, immediately wipe mold cavity, gate, and runner with a dedicated mold cleaner (such as a neutral solvent-based cleaner) to remove any residual plastic and filler (especially glass fiber and mineral powder). This prevents residual material from carbonizing at high temperatures and adhering to mold surface, potentially causing missing parts or scratches in next molded product. After cleaning, dry mold with compressed air to prevent residual cleaning agent from corroding mold.
Regular Lubrication and Inspection: After every 10,000-20,000 molds, apply high-temperature grease (molybdenum disulfide-based grease is recommended) to sliding parts such as mold guide pins, guide bushings, and ejector pins to ensure smooth movement. Also, inspect mold cavity surface for accuracy. Any damage should be polished and repaired promptly.
Idle Protection: When a mold is idle for an extended period (over 1 month), apply a hard-coated anti-rust oil to metal surfaces of mold cavity, guide pins, and other areas. Wrap mold with moisture-proof paper and store in a dry, ventilated environment (relative humidity ≤60%, temperature 5-30℃) to prevent rust. Before reactivation, thoroughly clean anti-rust oil to avoid contamination of plastic product.
(II) Troubleshooting
Minor Wear Repair: If minor scratches or wear (depth ≤0.5mm) appear on mold cavity surface, manually polish with an 800-1200# fine-grit oilstone. Polish along mold's forming surface flow lines, avoiding cross-cutting scratches. After polishing, wipe clean with alcohol and check to see if surface roughness has returned to Ra 0.8μm or higher.
Local Crack Repair: When minor cracks (length ≤5mm, depth ≤2mm) are discovered, penetrant testing is performed to determine crack extent. TIG welding is then used for repair. H13 welding wire (such as ER80S-G, diameter 1.2-1.6mm) is used. Preheat to 300-350℃ before welding. Immediately after welding, maintain weld at 250-300℃ for 2 hours for stress relief. Finally, temper weld at 520-550℃ twice (2 hours each). Hardness can be restored to HRC 40-45.
Replacement for Severe Wear: If wear depth exceeds 2mm in key areas of cavity, or if through-hole cracks appear, mold core must be replaced. Before replacement, mold base must be dimensional inspected to ensure accurate positioning. After assembly, new mold core must be tested to verify that product's dimensions and appearance meet standards.