Laminated Steel Plate Testing

Laminated steel plate inspection mainly conducts scientific testing and analysis on the peel strength testing, coating thickness testing, hardness testing, corrosion resistance testing, colour and appearance testing and mechanical property testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Peel strength testing: Evaluates the interfacial bonding performance between the base plate and the polymer coating of the laminated steel plate. Usually through the tensile or peel testing, the force per unit width required to separate the coating from the base plate is measured. It is the core indicator for measuring the durability and reliability of the composite material, directly affecting the delamination and blister resistance of the product.

Coating thickness testing: Precisely measures the total thickness of the polymer coating and the thicknesses of the various layers. The eddy current thickness gauges or metallographic microscope method is used to ensure that the coating thickness meets the design specifications. The thickness uniformity is the key to guaranteeing the corrosion resistance, insulation and appearance quality of the product, and the excessive thinness or thickness will affect the final performances.

Hardness testing: Evaluates the ability of the surface coating of the laminated steel plate to resist the indentation or scratches. The common methods include the pencil hardness testing, Shore hardness testers or micro-hardness testers. The hardness indicator reflects the wear resistance, scratch resistance of the coating and the surface protection ability during the processing and forming.

Corrosion resistance testing: Inspects the protection ability of the laminated steel plate under the specific corrosive environments. The main tests include the salt spray testing, damp heat resistance testing and chemical reagent resistance testing. By observing whether the phenomena such as the rust, blistering and peeling appear on the specimen surface, the reliability of its long-term service is evaluated.

Colour and appearance testing: The colour consistency, gloss, flatness and whether there are the defects (such as the spots, scratches and pits) on the laminated steel plate surface are evaluated through the visual inspection or instruments. The equipment such as the colour difference meters and gloss meters is used to quantify the colour deviations, ensuring the stable appearance quality between the product batches.

Mechanical property testing: Tests the tensile strength, yield strength, elongation and cupping values of the laminated steel plate. These properties determine the forming ability of the material in the subsequent processes such as the stamping and bending, as well as the load-bearing capacity of the final components, and are the important basis for the material selection.

Weather resistance and aging testing: Simulates the effects of the natural environments on the material properties. The common methods include the xenon lamp aging testing and UV aging testing. By detecting the changes of the colour, gloss and mechanical properties of the samples before and after the aging, its long-term outdoor service life and colour retention are evaluated.

Testing Scope

Building materials field: Mainly used for the building exterior wall panels, roof panels and interior decoration panels. The testing emphasis is on the weather resistance, colour durability, wind pressure deformation resistance and fire rating assessment of the coatings, ensuring that they can meet the requirements of the building safety and long-term aesthetics.

Household appliance housings field: Applied to the housings and internal components of the household appliances such as the refrigerators, washing machines, air conditioners and microwave ovens. The testing focuses on the surface hardness, scratch resistance, fingerprint resistance, detergent corrosion resistance and compliance with the food contact material safety regulations.

Transportation field: Used for the automobile interior panels, bus bodies and container plates. The testing items include the high and low temperature cycles, vibration fatigue resistance, oil stain resistance, flame retardancy and the VOC (volatile organic compounds) release testing of the interior materials.

Office furniture field: Used for the filing cabinets, office partitions, desks and chairs. The testing core is the surface wear resistance, impact resistance, stain resistance (such as the ink and coffee) and antibacterial performance, to meet the requirements of the high-frequency use and cleaning maintenance.

Electronic products field: Applied to the computer cases, server cabinets and electronic equipment housings. The testing needs to focus on the electromagnetic shielding effectiveness (if required), insulation resistance, arc resistance and the protection ability of the coatings for the internal precision components.

Industrial equipment field: Used for the control cabinets, instrument housings and clean room panels. The testing emphasis is on the chemical corrosion resistance (such as the acid and alkali atmospheres), antistatic performance, wear resistance of the coatings and the dimensional stability under the high-humidity environments.

Testing Methods

Peel strength testing method: In accordance with the standards such as ASTM D903 and GB/T 2792, the 180° or 90° peel mode is adopted. The peeling layer at one end of the specimen is pre-separated and fixed on the tensile testing machine, and the peeling is carried out at a constant speed, recording the force value curves during the peeling process and calculating the average peel strength.

Coating thickness measurement method: The magnetic thickness measurement method (ISO 2178) is used to measure the thicknesses of the non-magnetic coatings on the steel substrates; the eddy current thickness measurement method (ISO 2360) is used to measure the thicknesses of the non-conductive coatings on the non-ferrous metal substrates. The metallographic microscope method is the arbitration method, and the cross-sections are directly observed after the cutting, mounting, polishing and etching.

Salt spray testing method: The neutral salt spray testing is carried out in accordance with GB/T 10125 (equivalent to ISO 9227). The specimen is placed in an enclosed salt spray chamber, and the 5% sodium chloride solution is continuously sprayed to form the corrosive environment. The times when the red rust, white rust or coating blistering appear are regularly observed and recorded, evaluating its corrosion resistance grade.

Pencil hardness testing method: In accordance with GB/T 6739 (equivalent to ISO 15184). The drawing pencils of the different hardnesses from 9B to 9H are used to move across the coating surface at a 45° angle, and the grade of the hardest pencil that does not scratch the coating is taken as the pencil hardness of the coating.

Cupping testing method: In accordance with GB/T 9753 (equivalent to ISO 1520). The specified steel balls or punches are used to apply the pressure to the clamped specimen until the coating first appears the cracking or peeling. The indentation depth (cupping value) at this time is measured, thereby evaluating the extensibility and adhesion of the coating-substrate composite during the forming process.

Colour difference and gloss measurement method: The colour difference meter (in accordance with GB/T 11186) is used to measure the ΔE values of the specimen and standard plate in the CIEL*a*b* colour space. The gloss meter (in accordance with GB/T 9754) is used to measure the specular gloss of the coating surface at the 20°, 60° or 85° incidence angles, evaluating its appearance quality.

Testing Standards

Domestic Standards:

1. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

2. GB/T 2792, Adhesive tapes — Measurement of peel adhesion strength

3. GB/T 6739, Paints and varnishes — Determination of film hardness by pencil test

4. GB/T 9753, Paints and varnishes — Cupping test

5. GB/T 9754, Paints and varnishes — Determination of specular gloss of non-metallic paint films at 20°, 60° and 85°

6. GB/T 11186, Methods of measurement of colour co-ordinates of paint films

7. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

International Standards:

1. ASTM D903, Standard test method for peel or stripping strength of adhesive bonds

2. ISO 2178, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method

3. ISO 2360, Non-conductive coatings on non-magnetic electrically conductive base metals — Measurement of coating thickness — Amplitude-sensitive eddy-current method

4. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

5. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electronic universal material testing machine: The core equipment for carrying out the mechanical property tests such as the peel strength and tensile strength. Equipped with the high-precision load cells and displacement encoders, it can precisely control the test speeds, automatically record the force-displacement curves and output the various mechanical parameters.

Salt spray corrosion test chamber: The accelerated corrosion equipment simulating the marine or salt-containing damp atmospheric environments. With the functions such as the automatic temperature control, saturated air barrel heating and spray volume adjustment, it can carry out the various tests such as the neutral salt spray, acetic acid salt spray and copper-accelerated acetic acid salt spray.

Coating thickness gauge: Including the two main types of the magnetic thickness gauges and eddy current thickness gauges. The portable equipment is used for the fast non-destructive on-site testing, and needs to be calibrated on the standard calibration foils; the laboratory-grade equipment has the higher precision, usually equipped with the multiple probes to adapt to the different substrates and coatings.

Xenon lamp aging test chamber: Simulates the effects of the full-spectrum sunlight, rain and dew on the material aging. By controlling the spectral distribution through the optical filters, and precisely controlling the light intensity, black panel temperature, chamber humidity and spraying cycles, it is the key equipment for evaluating the weather resistance.

Precision colour difference meter and gloss meter: The colour difference meter adopts the integrating sphere type or 0°/45° optical structures, and can accurately measure the absolute colour values and differences. The gloss meter quantifies the surface gloss by measuring the reflected luminous flux at the specified angles, and both are the standardized tools of the appearance inspection in the quality control.

Cupping testing machine: The equipment specially used for evaluating the forming properties of the metal plates and their coatings. Composed of a rigid frame, hydraulic or mechanical driving system, standard dies and depth measuring device, the testing process is stable, and can detect and define the critical points of the coating failures.

Micro-hardness tester and metallographic microscope: The micro-hardness testers (such as the Vickers and Knoop) are used to measure the hardness of the micro-areas of the coatings. The metallographic microscope, combined with the sample preparation equipment such as the cutting and mounting machines, polishing machines and grinding machines, is used to observe the coating cross-section morphologies, measure the thicknesses of the various layers and analyse the interfacial bonding conditions.