The material selection and heat treatment process for custom-made plastic molds for home appliances directly determine the mold's molding precision, service life, and production stability. Currently, the home appliance industry is transforming towards lightweight, precision, and intelligent manufacturing. Molds need to be compatible with various plastic materials such as ABS, PC, PP, and PA + glass fiber, while simultaneously meeting the requirements of wear resistance, deformation resistance, and ease of processing in mass production. Therefore, a scientific selection and heat treatment plan must be developed based on the type of home appliance, the function of the plastic part, and the scale of mass production.
I. Core Material Selection
1. Classification by Appliance Type and Plastic Part Function
Large Appliance Molds (Refrigerator Side Panels, Air Conditioner Housings): P20 and 718H pre-hardened steel, with a pre-hardening hardness of HRC28-35, are used. They can be directly machined and polished without secondary heat treatment, shortening the mold opening cycle.
Small Appliance Molds (Soymilk Maker Cups, Rice Cooker Lids): S136 and 2316 corrosion-resistant steel are used, suitable for thin-walled plastic parts, avoiding cavity corrosion under high-temperature injection molding.
Functional Component Molds (Appliance Gears, Motor Brackets): H13 and SKD61 hot-work steel are used, resistant to wear from PA + glass fiber reinforcement materials, with a service life of over 500,000 molding cycles.
Transparent Plastic Part Molds (Refrigerator Drawers, Air Conditioner Air Guide Plates): High-purity mold steel is used to ensure a mirror-polished effect, with a surface roughness Ra≤0.01μm.
2. Key Performance Adaptation Requirements
High-Temperature Components (Oven Inner Liner, Water Heater Accessories): Mold materials must possess good heat resistance, able to withstand temperatures around 600℃, and resist thermal fatigue cracking;
High-Frequency Mass Production Molds: Materials must balance hardness and toughness to prevent breakage due to prolonged mold opening and closing;
Export Appliance Molds: Lead-free, low-sulfur environmentally friendly steel must be used, complying with EU RoHS and Chinese GB/T 26572 environmental standards;
Ultra-thin Plastic Part Molds (Wall Thickness ≤ 1.2mm): High-strength, high-toughness new steel must be used, combined with surface coating technology to reduce melt flow resistance.
3. Material Innovation Application Trends
Composite Structure Design: High-strength mold steel is used for the cavity to ensure precision, while aluminum alloy is used for the mold base to reduce weight by more than 30%, improving the mold opening and closing efficiency of the injection molding machine;
Cost-Adaptive Selection: For small-batch production, 45# steel with surface strengthening treatment can be selected to reduce costs while meeting basic precision requirements;
Upgraded Corrosion Resistance: For plastic parts containing special additives, mold steel with stronger corrosion resistance is selected to avoid oxidation and peeling on the cavity surface.
II. Heat Treatment Standards and Practical Application
1. Pretreatment Process
Pre-hardened steel (P20, 718H): Annealing at 720-760℃, holding for 2-3 hours, then furnace cooling to ensure uniform hardness and eliminate forging stress;
Non-pre-hardened steel (H13, S136): Normalizing at 850-900℃, holding for 1-2 hours, then air cooling to reduce hardness and improve machinability;
High-carbon steel: Stress-relief annealing at 650-700℃, holding for 3 hours to reduce the risk of deformation during subsequent heat treatment.
2. Final Heat Treatment Parameters
H13 Steel (Functional Component Mold): Quenched at 950-1050℃, held at that temperature, then oil-cooled, followed by three tempering treatments. The final surface hardness reaches HRC50-55.
S136 Steel (Transparent Plastic Part Mold): Quenched at 1020-1050℃, then subjected to deep cryogenic treatment at -80℃ to -120℃ to effectively control mold deformation.
2316 Steel (Corrosion-Resistant Mold): Quenched at 980-1020℃, followed by two tempering treatments. The surface hardness is controlled at HRC48-52, enhancing corrosion resistance.
3. Post-processing and Quality Control
Abrasion-resistant treatment: The cavity surface undergoes gas nitriding treatment at 500-550℃ for 4-6 hours. The nitrided layer hardness reaches HV800-1000, extending mold life by 2-3 times.
Mirror finish treatment: Following a "rough polishing - fine polishing - mirror polishing" process, a high gloss finish is achieved to meet the requirements for molding transparent plastic parts.
Quality inspection: Focus is placed on checking hardness uniformity, flatness error, and surface defects. Key dimensional accuracy is controlled at the micron level to ensure the absence of cracks, pitting, and other problems.
III. Key Points for Adapting to Production Scenarios
1. Mold Structure and Material Matching
Complex cavity molds (e.g., appliance control panels): Use 718H steel with good machinability to reduce cutting difficulty and tool wear;
Deep cavity molds (e.g., washing machine inner tubs): Use high-toughness H13 steel to prevent cavity chipping during demolding;
Multi-cavity molds (e.g., appliance button molds): Use S136 steel with uniform hardness to ensure dimensional consistency of plastic parts in each cavity.
2. Injection Molding Process Co-optimization
High-speed injection molding (thin-walled plastic parts): Mold material must have high rigidity to prevent cavity deformation, which can be strengthened through quenching and tempering processes;
High-temperature injection molding (PC, PA + glass fiber): Molds need to undergo deep cryogenic treatment to reduce dimensional deviations caused by thermal expansion;
Injection molding of plastic parts containing glass fiber: Mold cavities undergo nitriding treatment to improve wear resistance and resist glass fiber erosion.
3. Mold Maintenance and Lifespan Extension
Regular Polishing: For worn cavities, professional polishing materials are used to restore surface smoothness and prevent damage to plastic parts.
Rust Prevention: Idle molds are coated with rust-preventive oil and stored in a dry environment to prevent corrosion.
Repair Process: Localized wear in cavities can be repaired using argon arc welding with welding wire of the same material. After repair, local heat treatment is performed to ensure matching hardness.
Custom-made plastic molds for home appliances must adhere to the principles of "material matching with plastic parts, heat treatment matching materials, and process synergy optimization." Mold steel is scientifically selected based on the type of home appliance, production scale, and characteristics of the plastic parts, and standardized heat treatment procedures are implemented. Through innovative material applications, precise process control, and adaptation to production scenarios, molds can be ensured to possess excellent molding accuracy, wear resistance, and stability. This meets the precision and mass production needs of the home appliance industry while reasonably controlling costs and shortening production cycles, laying a core foundation for improving the quality of home appliance products.
HUIWEN Professional Injection Plastic Parts,like Household appliances plastic parts, Automotive Plastic parts, agricultural tools plastic parts, medical equipment plastic parts, toys plastic parts, food bottles, boxes, buckets and so on
