Hardness testing of rubber O-rings is an important step in evaluating their quality and performance. The following are the standardized testing procedures and key points:
I. Preparations before Testing
Preparation of equipment and tools
Hardness tester selection: Choose either Shore A type (soft rubber, 0-100HA) or Shore D type (hard rubber, 0-100HD) based on the hardness range of the O-ring.
Calibration verification: Use standard hardness blocks to calibrate the hardness tester to ensure that the measurement error is within ±1HA.
Auxiliary tools: Prepare a cleaning cloth, a marker pen, a data record sheet, and a constant temperature box (if temperature control is required).
Sample preparation
Condition check: Remove O-rings with surface cracks, bubbles, oil stains or deformations.
Thickness requirement: The thickness of a single layer should be ≥6mm; If insufficient, O-rings of the same batch can be stacked (the number of layers needs to be recorded).
Temperature balance: Place the sample in an environment of 23℃±2℃ for at least 1 hour to eliminate the influence of temperature.
Ii. Inspection Operation Process
Selection of measurement position
Multi-point distribution: Mark measurement points at the top, bottom, left, right and center of the O-ring, avoiding proximity to the edge (≥12mm from the edge).
Avoid defects: Skip areas with bubbles, impurities or scratches.
Hardness measurement steps
Vertical pressure application: Press the hardness tester's indenter foot vertically into the surface of the O-ring to ensure that the indenter pin is in full contact with the sample.
Stable reading: Hold the pressure for 3 seconds and then read the value. If the pointer swings, take the middle value.
Repeated measurement: Measure each measurement point continuously for three times, record the data and calculate the average value.
Environmental control
Temperature recording: If the detection environment has a non-standard temperature (23℃±2℃), the actual temperature should be recorded and the correction factor should be stated in the report.
Humidity requirement: The relative humidity should be controlled at 50%±10% to prevent high humidity from causing the rubber to absorb moisture and affecting its hardness.
Iii. Data Processing and Judgment
Data collation
Outlier elimination: If the data of a certain measurement point deviates from that of other points by more than 5HA, a remeasurement is required and the reasons analyzed.
Average value calculation: Take the arithmetic mean of valid data and retain it to one decimal place.
Result determination
Standard comparison: Refer to standards such as ASTM D2000 and ISO 3601 to determine whether the hardness is within the allowable range (such as Shore A 70±5HA).
Batch consistency: If multiple O-rings are tested, calculate the hardness range (maximum value - minimum value). A range of ≤5HA is considered qualified.
Iv. Precautions and Common Issues
Key precautions
Indenter protection: Prevent the indenter of the hardness tester from colliding with hard objects to avoid damage.
Surface cleaning: Before each measurement, wipe the surface of the O-ring with a lint-free cloth to prevent oil stains from interfering.
Equipment maintenance: After use, the hardness tester should be cleaned and stored in a dry environment, and calibrated regularly.
Common Problem Handling
Uneven hardness: If the hardness difference of the same O-ring is greater than 10HA, it is necessary to check the material uniformity or the vulcanization process.
Temperature correction: When testing at non-standard temperatures, correct the hardness value according to the formula (for example, for every 1℃ increase, the Shore A hardness approximately decreases by 0.5HA).
Data deviation: If the data fluctuates greatly after multiple measurements, check the status of the hardness tester or replace the sample for retesting.
V. Contents of the Test Report
Basic information: Sample number, material, specification, testing date.
Testing conditions: temperature, humidity, hardness tester model and calibration status.
Measurement data: Hardness values at each measurement point, average value, and range.
Determination result: Whether it meets the standard requirements, the conclusion is clearly "qualified" or "unqualified".
Note: Abnormal situation description (such as temperature correction, number of superimposed layers, etc.).