Determining Polyethylene Specific Gravity Thresholds for Tariff Entry
Polyethylene tariff entry requires density testing per ISO 1183-1 Method A on annealed plaques, where crossing 0.940 g/cm³ shifts classification from 3901.10 to 3901.20.
Float Calibration
Meaning
Laboratory maintenance tasks establish the accuracy of measurement instruments by comparing them against known reference standards. The float calibration process involves measuring the density of glass floats used in a density gradient column against a certified liquid or standard. This process ensures that the column remains accurate and reliable for resin testing.
Density Standard
Reference liquids with known density profiles are used to verify the floats. During float calibration, the positions of the floats are tracked as they settle in the test liquid. These floats must be calibrated annually to account for any changes in the glass structure over time.
This calibration is necessary for maintaining the accuracy of the gradient column.
Temperature Control
Precise thermal control is required during the calibration process to prevent errors. Because the density of the reference liquids changes with temperature, float calibration must be performed in a temperature controlled water bath. This bath keeps the temperature within 0.1 degrees Celsius of the target.
A small deviation in temperature creates a large change in the measured density.
Measurement Integrity
Quality control depends on the accuracy of the density measurements made during production. If a moulder uses floats that have not undergone float calibration, the density measurements of the molded parts will be incorrect. This error can lead to the acceptance of parts with incorrect crystallinity and mechanical properties.
Correct calibration prevents these defects from reaching the customer and ensures that the resin meets the required specifications.