What is the heat treatment process for electronic scale molds
The heat treatment process of electronic scale molds needs to be combined with their core requirements - high dimensional stability (to avoid deformation affecting weighing accuracy), moderate hardness (to balance processability and wear resistance), low internal stress (to prevent long-term use cracking), and also need to be adapted to commonly used mold materials (such as pre hardened plastic mo
The heat treatment process of electronic scale molds needs to be combined with their core requirements - high dimensional stability (to avoid deformation affecting weighing accuracy), moderate hardness (to balance processability and wear resistance), low internal stress (to prevent long-term use cracking), and also need to be adapted to commonly used mold materials (such as pre hardened plastic mold steel 718H, ordinary plastic mold steel P20, or 45 # steel for metal structural component molds). The following is a typical process flow and key link description:
1、 Core premise: Material selection and pre-processing
Different materials correspond to different heat treatment schemes. Electronic scale molds are mainly "plastic molds" (such as shell and tray molds), with a few being metal structural component molds. Pre treatment is the foundation for ensuring stable subsequent processes
1. Material matching:
Mainstream choice: 718H (pre hardened plastic mold steel) - has reached 28-32HRC at the factory, does not require overall quenching, only local aging strengthening is needed;
Economic choice: P20/45 # steel - requires "quenching and tempering+surface hardening" to achieve performance, suitable for low precision, small batch molds.
2. Pre treatment (stress relief annealing):
Purpose: To eliminate internal stress caused by forging/rough machining of mold blanks and avoid deformation during subsequent heat treatment;
Process: Heat to 600-650 ℃, hold for 2-4 hours (depending on the mold size), slowly cool in the furnace to below 200 ℃, and remove from the furnace;
Key: For complex cavities (such as anti slip patterns on electronic scale trays), an additional stress relief annealing is required after rough machining and before precision machining.
2、 Mainstream Process: Typical Heat Treatment Process by Material
1. Pre hardened steel (718H): local strengthening+stabilization treatment (most commonly used)
The 718H has been pre hardened to 28-32HRC before leaving the factory, and does not require overall quenching (to avoid deformation of the mold cavity). Only the parts that are prone to wear (such as parting surfaces and gates) need to be strengthened and the processing stress eliminated
Step 1: Stress relief after rough machining
Heat to 550-580 ℃, hold for 3 hours, then cool in the furnace to 300 ℃ and air cool out of the furnace (temperature below the critical point of Ac1 to avoid phase transition) to eliminate the stress generated by rough milling.
Step 2: Local surface hardening (optional)
If the mold parting surface and gate require higher wear resistance (to prevent long-term opening and closing wear), low-temperature nitriding should be used:
Temperature: 450-500 ℃, keep warm for 15-20 hours;
Effect: A 5-15 μ m nitride layer is formed on the surface, with a hardness of 60-70HRC, and the core still maintains 28-32HRC (balancing toughness and preventing cracking).
Step 3: Stable aging after precision machining
Precision machining of molds (after the completion of key dimensions such as cavities and positioning holes), heating to 500-520 ℃, holding for 4-6 hours, and slow cooling with the furnace;
Purpose: To further stabilize the organization (reduce dimensional drift during subsequent use), while releasing precision machining stress, ensuring that the cavity size error of the electronic scale mold is ≤ 0.02mm after long-term use (meeting the assembly accuracy of weighing components).
2. Ordinary mold steel (P20/45 # steel): quenched and tempered+surface hardened (economical type)
P20/45 # steel is not pre hardened and needs to be quenched and tempered to obtain basic strength, followed by surface strengthening to improve wear resistance. It is suitable for electronic scale shell molds with low precision requirements:
Step 1: Quenching and tempering treatment (overall performance improvement)
Quenching: Heat to 830-860 ℃ (upper limit for P20, lower limit for 45 # steel), hold for 1-2 hours (depending on thickness), and cool with oil to room temperature;
High temperature tempering: Heat to 580-620 ℃, hold for 2-3 hours, and air cool;
Effect: The hardness reaches 22-26HRC, the microstructure is composed of martensite, and it combines strength and machinability, laying the foundation for subsequent precision machining.
Step 2: Surface quenching (local wear-resistant strengthening)
Induction heating surface quenching is used for vulnerable parts such as parting surfaces and gates
Temperature: 900-950 ℃, rapid heating (to avoid phase transition in the core), water spray cooling;
Effect: The surface hardness reaches 50-55HRC, the depth of the hardened layer is 0.5-1mm, and the core still maintains 22-26HRC (anti brittle fracture).
Step 3: Low temperature stress relief tempering
After surface quenching, heat to 180-200 ℃, hold for 2 hours, and air cool;
Purpose: To eliminate internal stress caused by surface quenching and prevent cracking during mold use.
3、 Key control steps: avoiding deformation and ensuring accuracy
The electronic scale mold has high requirements for dimensional accuracy (such as the coaxiality between the mold cavity and the installation position of the weighing sensor), and it is necessary to focus on controlling three aspects:
Heating rate:
When heating large molds (such as electronic scale platform panel molds), the rate should be ≤ 100 ℃/hour to avoid thermal stress deformation caused by excessive temperature difference;
Cooling method:
Pre hardened steel (718H): slow cooling or air cooling throughout the furnace, water cooling is prohibited (to prevent cracking);
Ordinary steel quenching and tempering: oil cooling (45 # steel) or water-soluble quenching agent cooling (P20) after quenching to avoid deformation caused by too fast cooling rate;
Time stability:
After precision machining, 1-2 aging treatments (500-520 ℃) must be carried out, and key dimensions must be measured after each insulation until the dimensions are stable (change ≤ 0.005mm).
4、 Process validation: performance testing
After heat treatment, two core tests are required to ensure that the usage requirements are met:
Hardness testing:
Core hardness: 28-32HRC required for 718H, 22-26HRC required for P20/45 # steel;
Surface hardening layer: After nitriding/quenching, it needs to reach 50-70HRC (depending on the location requirements);
Dimensional inspection:
Use a coordinate measuring instrument to detect the size of the mold cavity and the position of the positioning hole, with an error of ≤ 0.02mm (to avoid affecting the assembly and weighing accuracy of the electronic scale).
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