Why is it said that the dimensional accuracy of electronic scale molds is high
The high dimensional accuracy of electronic scale molds is determined by the strictness of their application scenarios, the complexity of manufacturing processes, the high requirements for material properties, and the standardization of industry standards. The reasons for its high precision are analyzed from the dimensions of technical principles, production requirements, quality control, etc. as
The high dimensional accuracy of electronic scale molds is determined by the strictness of their application scenarios, the complexity of manufacturing processes, the high requirements for material properties, and the standardization of industry standards. The reasons for its high precision are analyzed from the dimensions of technical principles, production requirements, quality control, etc. as follows:
1、 Mandatory requirement for precision in core functions
1. Sensor installation and signal accuracy
Matching of strain gauges:
The core component of an electronic scale is a strain gauge weighing sensor, which transmits weight signals through small deformations of elastic materials such as aluminum alloy and stainless steel. The mold needs to reserve installation slots, holes and other structures for sensors, and the size error should be controlled within ± 0.01~0.03mm (equivalent to 1/5~1/3 of the diameter of a hair thread), otherwise it will cause:
Uneven installation stress of sensors can cause measurement deviation (such as a weighing error of 5-10g due to a 0.1mm error in an electronic scale with a 5kg range);
The friction between the sensor and the housing accelerates the accuracy degradation after long-term use.
Case: If the diameter deviation of the sensor installation hole in the commercial platform scale mold exceeds 0.05mm, it may cause the sensor pins to be unable to be inserted into the circuit board, resulting in the failure of the entire machine.
2. Mechanical conduction requirements for the structure of the scale body
Stability of force transmission path:
The weight of the object needs to be linearly transmitted through the scale, support structure, and sensor. The dimensional accuracy of the support columns, load-bearing beams, and other structures formed by the mold (such as length error ≤ ± 0.02mm, verticality error ≤ 0.01 °) directly affects the transmission efficiency. For example:
Inconsistent height of support columns (error>0.03mm) can cause force eccentricity on the scale body, resulting in "four corner error" (different results of weighing the same weight at different positions on the scale);
The thickness deviation of the load-bearing beam>0.05mm may change its elastic modulus, causing the sensor signal to be nonlinear and beyond the calibration range.
3. Assembly accuracy of display and operation components
Coordination of display screen, buttons, and casing:
The mold needs to reserve installation positions for LCD displays, buttons, etc., with a gap error of ≤± 0.1mm, otherwise:
The display screen is embedded too loosely, causing shaking and affecting the stability of readings;
The buttons are stuck or the gap is too large, which affects the operation feel (such as a home electronic scale button stroke error>0.2mm, which will cause insensitive pressing feedback).
2、 High precision guarantee of manufacturing process
1. Digitization and simulation of mold design
Application of CAD/CAM/CAE technology:
Using 3D design software such as UG and Pro/E, modeling is done in 0.001mm units, and finite element analysis (FEA) is used to simulate the stress and temperature field distribution of the mold, optimizing the structure in advance (such as avoiding dimensional deviations caused by stress concentration). For example:
The flow channel design of injection molds requires correct calculation of the molten material flow rate. If the diameter error of the flow channel is greater than 0.1mm, it may cause local overheating or uneven cooling, resulting in inconsistent shrinkage rate of the scale body shell (error>0.3%) and affecting assembly accuracy.
Tolerance chain optimization:
From mold parts (such as cores and cavities) to products, a full process tolerance allocation system needs to be established (such as mold part tolerances taking 1/3 to 1/5 of product tolerances). For example, if the product requires a dimensional tolerance of ± 0.1mm, the tolerance of the mold parts should be controlled within ± 0.02~± 0.03mm.
2. Application of precision machining equipment
Numerical Control Machining Technology:
CNC machining center: with a positioning accuracy of ± 0.005mm and a repeatability accuracy of ± 0.003mm, used for machining high-precision cavities of molds (such as sensor mounting slots);
Electric Discharge Machining (EDM): Complex curved surfaces (such as the curved edges of scales) are machined by electrode discharge, with an accuracy of ± 0.002mm and a surface roughness Ra ≤ 0.8 μ m;
Slow wire cutting (WEDM): When cutting mold inserts, the dimensional error is ≤± 0.001mm, suitable for narrow seam processing below 0.1mm (such as button limit slots).
Case: The sensor mounting hole of a high-precision electronic scale mold is machined using five axis CNC linkage, and calibrated in real-time with a laser tool setter. The hole diameter tolerance is controlled within ± 0.008mm, and the cylindricity error is less than 0.005mm.
3. Stability of materials and heat treatment
Selection of mold steel:
Commonly used mold steels such as S136 and H13, after vacuum quenching and cryogenic treatment, have a hardness of HRC52-58 and a deformation of ≤ 0.002mm/100mm, ensuring stable mold dimensions during long-term production (such as a cavity wear of<0.005mm after continuous production of 100000 products).
Temperature control technology:
The injection mold is equipped with a conformal cooling water circuit, which controls the mold temperature fluctuation within ± 1 ℃ through a temperature controller, reducing plastic cooling shrinkage errors (such as the shrinkage rate difference of 0.2%~0.5% for ABS material at mold temperature of 40 ℃ vs 50 ℃).
3、 The strictness of the quality control system
1. High precision detection equipment
Coordinate Measuring Machine (CMM):
Using equipment from Germany's Zeiss and Japan's Sanfeng, conduct full area scanning of mold parts with a measurement accuracy of ± 0.002mm (if detecting the surface profile of the core, the error should be less than 0.005mm);
Image measuring instrument:
Non contact detection of microstructures (such as 0.3mm button identification fonts), with a size error of ≤± 0.001mm;
Laser interferometer:
Calibrate the straightness of the mold guide rail and screw (error<0.001mm/m) to ensure motion accuracy during the machining process.
2. Multi stage verification process
First Article Inspection (FAI):
After the mold trial, the first product is subjected to full-scale inspection (such as the length, width, height, wall thickness, installation hole position, and more than 50 other dimensions of the electronic scale shell), and the pass rate must be ≥ 99.5% before mass production can be carried out;
Long term stability testing:
After continuous production of the mold for 30 days (about 50000 products), the mold size should be rechecked. The wear of key parts should be less than 0.003mm, otherwise surface coating repair (such as PVD coating thickening by 0.002mm) is required.
4、 Industry standards and application scenario driven
1. Mandatory requirements of international standards
OIML (International Organization for Legal Metrology) standards:
Commercial electronic scales require OIML Level III certification, with a weighing error of ≤ 0.1%~0.5%, which directly forces the improvement of mold accuracy (for example, the structural size error of the scale body needs to contribute<10% of the total error, i.e. ≤ 0.01%~0.05%);
GB/T 7724-2020 "Electronic Weighing Instruments":
After the assembly of the scale body, the flatness error of the sensor installation surface must be ≤ 0.02mm, and this requirement needs to be directly guaranteed through mold forming accuracy.
2. Special requirements of the scene
Laboratory precision balance (accuracy of 0.1mg level):
The mold needs to ensure that the resonance frequency error of the scale support structure is less than 0.1Hz (controlled by dimensional accuracy to ensure mass distribution), otherwise small vibrations will interfere with weighing stability;
Medical dialysis scale (requires anti electromagnetic interference):
The size error of the metal shielding cover formed by the mold should be less than 0.01mm, ensuring that the electromagnetic leakage at the joint is ≤ -80dB (if the gap is greater than 0.02mm, it will cause a decrease in shielding effectiveness of more than 10dB).
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