1.What is the authoritative basis for color difference determination?
Currently, the most authoritative national standard in my country for color difference testing of galvanized color-coated steel coils is GB/T 12754-2019 "Color Coated Steel Sheets and Strips". The corresponding method for color difference testing is GB/T 13448-2019 "Test Methods for Color Coated Steel Sheets and Strips".
2. What is the permissible range of color difference specified by national standards?
According to GB/T 12754-2019, the color difference ΔE value between the coating and the standard color sample should not exceed 1.5. This is also a key indicator during finished product warehousing or factory inspection. For some applications with higher requirements, such as color-coated steel strips for cleanroom applications, the group standard stipulates that the ΔE value should not exceed 0.5, which is stricter than the national standard.
3.How to scientifically measure and calculate color difference values?
There are two main methods: instrument measurement and auxiliary observation. Instrument measurement provides a more objective basis for judgment:
Instrument Measurement: Measurement is performed using a colorimeter or spectrophotometer.
Calibration: Under a standard light source, the instrument is first calibrated using a calibration plate.
Measurement: The color data of the standard color chart confirmed by both the supplier and the buyer is obtained as a reference; then the sample to be tested is measured to obtain color data.
Calculation: The instrument automatically calculates the color difference value ΔE. Its core is based on the CIE Lab color space model. The ΔE value represents the total color difference; the smaller the value, the smaller the color difference.
4.What is the relationship between color difference and ΔE value? What are the different requirements for different scenarios?
Changes in ΔE value directly reflect the degree of color difference. The acceptable range of color difference varies depending on the application:
ΔE Range | Degree of Color Difference | Typical Application Scenarios and Requirements for Color-Coated Coil
ΔE ≤ 0.5 | Almost indistinguishable to the naked eye | Clean rooms, laboratories, and other high-requirement environments, or high-quality home appliances and automotive parts
ΔE ≤ 1.0 | Visible with professional testing; generally minimal visual difference | Ordinary indoor home appliances, high-end building facades
ΔE ≤ 1.5 | Slight differences may be perceptible to the naked eye | Ordinary building facades, industrial plants; also a general requirement of the national standard GB/T 12754-2019
5.What causes color difference? How to avoid and control it?
The causes of color difference are present throughout the entire production process. Key control points are as follows:
Coating itself: Different batches of coatings may have slight differences in color formulation. Even with the same formulation, insufficient mixing can lead to uneven color.
Substrate material: The surface condition and color differences of the substrate will affect the final color of the coating.
Production process: The rotation speed and pressure of the coating rollers, as well as the temperature and speed of the baking oven, all affect the final color.