Heat Treatment Standard for Cavities of High-temperature PPS Molds
Polyphenylene sulfide (PPS) belongs to high-performance semi-crystalline engineering plastic, widely used in automotive thermal structural parts, new energy battery modules and electronic high-temperature resistant shells. PPS molding requires high mold temperature ranging from 120℃ to 180℃, and glass-fiber reinforced PPS material contains hard fiber fillers that cause strong abrasive wear on mold cavities during long-term injection molding. Conventional mold steel without standardized heat treatment will suffer rapid surface wear, corrosion and deformation under long-term high-temperature and abrasive working conditions, resulting in dimensional deviation of molded PPS parts, rough surfaces and shortened mold service life. This heat treatment standard formulates targeted heating, quenching and tempering processes for PPS mold cavities, specifies hardness range, tempering temperature and post-treatment requirements, and realizes stable high-temperature wear resistance and dimensional stability of mold cavities.
1. Selection Benchmark of Mold Steel Base Material
Select different mold steel grades according to production batches of PPS products. For molds with production volume below 50,000 shots for ordinary unfilled PPS parts, adopt 1.2344 (H13) hot work mold steel as cavity base material; for mass-production molds over 80,000 shots molding glass-fiber reinforced PPS, choose S136H, STAVAX stainless mold steel or 1.2367 high-temperature wear-resistant steel. Avoid using P20 pre-hardened steel for PPS mold cavities, as P20 steel has poor high-temperature tempering resistance and will soften and wear rapidly under long-term 120℃~180℃ mold temperature environment. All mold steel blanks must undergo stress relief annealing before rough machining to eliminate internal forging stress of steel materials and prevent cavity deformation during subsequent heat treatment and long-term high-temperature production. The annealing temperature is controlled at 650℃~680℃, keep constant temperature for 4 hours and cool down to room temperature inside the furnace slowly, which can effectively reduce deformation rate of finished cavities after heat treatment below 0.02%.
2. Standard Heating and Quenching Process for Cavity Heat Treatment
Adopt segmented heating mode for cavity heat treatment to avoid excessive thermal stress inside steel caused by rapid temperature rise and cavity cracking. First heat the cavity to 600℃ at heating rate 80℃ per hour and keep constant temperature for 120 minutes for preheating; then raise temperature to quenching temperature at 60℃ per hour. The quenching temperature of H13 steel cavities is set at 1020℃~1040℃, while quenching temperature of S136 stainless steel cavities is controlled at 1000℃~1020℃. Adopt vacuum quenching in nitrogen protective atmosphere throughout the whole process to prevent oxidation decarburization on cavity surfaces; decarburized layers on steel surfaces will lead to local hardness reduction and rapid abrasive wear during PPS molding. After constant temperature quenching, conduct gas cooling at cooling rate 35℃ per minute until cavity temperature drops to 220℃, then transfer the mold to a tempering furnace immediately without natural air cooling to avoid martensite transformation stress causing cavity cracking. After quenching, the initial hardness of H13 steel cavities reaches HRC 52~55, and initial hardness of S136 cavities is HRC 51~54, laying a foundation for subsequent tempering treatment.
3. Double Tempering Process Requirements for High-temperature Working Conditions
PPS molds work continuously under 120℃~180℃ high temperature, so single tempering cannot guarantee long-term hardness stability, and double high-temperature tempering is mandatory. The first tempering temperature is 560℃, keep constant temperature for 3 hours and cool to room temperature inside the furnace; then carry out the second tempering at 580℃ with 3 hours of constant temperature heat preservation. After double tempering, the stable hardness of H13 mold cavities is controlled at HRC 48~52, and hardness of S136 stainless steel cavities stabilizes at HRC 47~51. This hardness range balances high-temperature tempering resistance and toughness of mold steel effectively: the cavities will not soften after long-term high-temperature baking of PPS molding, and possess sufficient toughness to resist thermal cycle fatigue cracking caused by repeated cold and hot alternation during injection molding. For precision PPS parts requiring dimensional tolerance within ±0.01mm, add the third low-temperature tempering at 220℃ for 2 hours after double tempering to eliminate residual stress completely and realize ultra-low deformation of cavities in high-temperature production environment. After each tempering process, cool the cavity slowly inside the furnace instead of rapid air cooling to avoid new internal stress generated by temperature difference.
4. Post Heat Treatment Surface Strengthening Treatment
Complete finish machining and mirror polishing of cavities after heat treatment tempering, then carry out surface strengthening treatment aiming at glass fiber abrasion of PPS materials. The mainstream treatment scheme adopts DLC diamond-like carbon coating or CrN nitride coating with coating thickness controlled at 3μm~6μm. The coating can increase surface hardness of cavities to HV 1800~2200, greatly improve anti-abrasion performance against glass fiber fillers in PPS, and prevent scratch wear on cavity surfaces during long-term injection molding. Conduct passivation anti-rust treatment on S136 stainless steel cavities after polishing to enhance high-temperature oxidation resistance of surfaces under PPS mold temperature conditions. After surface strengthening, inspect cavity dimensional accuracy with a three-coordinate measuring instrument; the overall deformation of qualified cavities after heat treatment must be controlled within 0.02mm to meet the precision molding requirement of PPS high-precision structural parts. All flow surfaces of cavities must be polished along melt flow direction to reduce friction between PPS melt and mold steel and lower generation of flow marks on molded products.
5. Heat Treatment Quality Inspection and Acceptance Standards
Carry out hardness inspection at 5 random positions of each cavity after heat treatment; the hardness deviation of all detection points shall not exceed HRC ±1.5, and unqualified cavities with uneven hardness must be reworked for tempering treatment. Use a metallographic microscope to inspect the metallographic structure of cavity sampling sections, require uniform tempered martensite structure without coarse carbide segregation, otherwise the cavity is prone to local hardness deficiency and wear failure. Conduct high-temperature aging simulation test for 24 hours at 180℃ on mold steel samples after heat treatment, detect hardness change before and after aging; the hardness drop value shall not exceed HRC 1 to confirm stable performance under long-term PPS high-temperature working conditions. Before assembling molds, conduct high-temperature mold test at 160℃ for 8 hours to observe whether cavities have micro-deformation, and repair deformed positions precisely to ensure dimensional stability during formal PPS mass production.
Conclusion
The core purpose of heat treatment for PPS high-temperature mold cavities is to endow molds with high-temperature tempering resistance, glass fiber abrasion resistance and low deformation performance. Select matched mold steel according to production batches, adopt vacuum segmented heating quenching plus double high-temperature tempering processes to stabilize cavity hardness within HRC 47~52, which avoids softening failure of molds under long-term high-temperature molding environment of PPS. Match DLC or CrN surface strengthening coating after heat treatment to solve abrasive wear problems brought by glass-fiber reinforced PPS materials. Complete hardness detection, metallographic inspection and high-temperature aging simulation verification after heat treatment to guarantee qualified heat treatment quality of cavities. Implement this heat treatment standard for PPS molds, which can extend mold service life by more than 60%, keep dimensional accuracy of PPS injection parts stable for a long time, reduce defective products caused by cavity wear and deformation, and lower mold maintenance frequency and production cost for injection molding workshops.
