Easy-open Can Material Testing

Beverage can material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, coating property testing, base material thickness and uniformity testing, surface quality and cleanliness inspection, and pressure resistance and burst strength 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

Chemical composition analysis: The alloy compositions of the can bodies (usually the 3004/3104 aluminium alloys) and can ends (usually the 5182 aluminium alloys) are precisely determined. The main tested elements include the content percentages of the manganese, magnesium, copper, iron and silicon, etc., ensuring that they meet the specific designation alloy standards. It is the basis of guaranteeing the material mechanical properties and subsequent processing and forming.

Mechanical property testing: Evaluates the behavior of the material under the stressed states, which is crucial for the formability of the can body materials. The core testing items include the yield strength, tensile strength, elongation (especially paying attention to the uniform elongation and total elongation) and hardness (such as the Webster hardness). These indicators directly affect the success rates of the deep drawing and draw and ironing (D&I) processes of the can bodies and the pressure resistance strengths of the finished cans.

Coating property testing: Evaluates the qualities of the inner and outer coatings of the beverage cans (such as the inner epoxy-phenolic resin coatings and outer colour printing inks). The main testing items include the coating thickness, adhesion (cross-cut method or pull-off method), flexibility (such as the T-bend testing of the tinplate substrates), impact resistance and chemical resistance (acid resistance and solvent resistance), to ensure that the coatings do not peel or corrode during the filling, sterilization and storage.

Base material thickness and uniformity testing: The thicknesses of the aluminium alloy sheets or tinplate sheets used for the can bodies and can ends are precisely measured. The thicknesses are required to meet the design tolerances, and the thicknesses at the various points of the plate surfaces are uniform and consistent. The uneven thicknesses will cause the stress concentrations and cracking during the stamping and forming, or affect the pressure resistance and sealing properties of the can bodies. Usually, the high-precision thickness gauges are used for the multiple-point measurement.

Surface quality and cleanliness inspection: Checks the defects on the metal substrate or coating surfaces. Including observing whether there are the scratches, indentations, roll marks, oxidation spots and oil residue residues, etc. The cleanliness testing can be evaluated through the contact angle measurement or surface tension test liquids, ensuring the adhesion of the subsequent printing and coating processes. For the can end tab areas, the surface qualities directly affect the opening performance.

Pressure resistance and burst strength testing: Simulates the internal pressure environments of the beverage cans after filling the carbonated drinks or beer. After sealing the can bodies, the compressed air or water is injected to test their axial pressure resistance strengths and the pressure values until the burst. This item is directly related to the safety of the products during the transportation, stacking and cold chain processes, and is the key indicator of measuring the structural integrity of the can bodies.

Testing Scope

Raw material incoming inspection: The incoming verification is carried out for the purchased aluminium alloy coils or tinplate sheets. According to the purchase technical agreements, the sampling testing of the chemical compositions, mechanical properties, thickness tolerances, surface states and coatings (such as the pre-coated coils) of the material is carried out, controlling the quality from the sources and preventing the unqualified materials from entering the production lines.

Can-making process online monitoring: The real-time or scheduled sampling inspections are carried out in the key processes such as the can body stamping and forming, cleaning, coating, printing, drying and necking and flanging. The emphases are on the wall thickness distributions after the forming, coating curing degrees, printing colour differences and adhesion, and dimension precision (such as the can heights and diameters), realizing the process quality control.

Finished can ex-factory inspection: The comprehensive final inspection is carried out on the complete beverage cans before the packaging. Including the full appearance inspection (deformation, scratches and printing defects) and property sampling inspection (pressure resistance, sealing, opening forces and coating integrity). This stage of testing is the last quality gate before the product delivery, ensuring that the products arriving at the filling plants are 100% compliant.

Specific application scenario verification: The targeted testing is carried out according to the characteristics of the contents. For example, the acid resistance and sulfur resistance testing of the inner coatings need to be strengthened for filling the acidic drinks; the can body pressure resistance strengths and coating anti-peeling properties need to be emphatically focused on for filling the beer; for the food cans used for the high-temperature sterilization, the stability of the coatings under the high-temperature steam needs to be verified.

Failure analysis and quality traceability: When the problems such as the leakage, burst and coating peeling of the beverage cans occur during the filling, transportation or consumer use, the reverse engineering analysis is carried out on the failed samples. Through the means such as the metallographic observation, fracture analysis and composition re-testing, the failure roots, whether the material, process or design problems, are traced, providing the basis for the quality improvement.

Testing Methods

Spectroscopic analysis method: Mainly used for the fast and precise determination of the chemical compositions. Usually using the spark direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF). The OES has the high precision and is suitable for the quantitative element analysis of the metal substrates; the XRF can non-destructively detect the heavy metal contents (such as the lead, cadmium, chromium and mercury) in the coatings, meeting the food contact material regulation requirements.

Mechanical testing machine method: The standard method of carrying out the mechanical property testing. Using the universal material testing machines, according to the standards such as ASTM E8/E8M or GB/T 228.1, the standard tensile specimens are tested to obtain the stress-strain curves, thereby calculating the yield strengths, tensile strengths and elongation. For the hardness testing, the Webster hardness testers or Rockwell hardness testers are commonly used.

Coating thickness measurement method: Including the magnetic thickness measurement method (suitable for the measurement of the non-magnetic coatings such as the paints on the steel substrates) and eddy current thickness measurement method (suitable for the measurement of the non-conductive coatings on the non-ferrous metal substrates such as the aluminium). The portable or bench-type thickness gauges are used, and the operation is carried out according to the standards such as GB/T 4956/4957 or ISO 2178/2360.

Electrochemical testing method: The important method of evaluating the coating corrosion resistance and substrate protection effects. The common methods include the electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve measurement. Through the analysis of the impedance and corrosion current densities of the coatings in the electrolyte solutions, the shielding properties and durability of the coatings can be quantitatively evaluated, predicting their service lives.

Chromatography-mass spectrometry combined method: Used to detect the substances that may migrate from the inner coatings of the beverage cans into the food, such as the residual monomers (bisphenol A, BADGE, etc.), solvent residues or degradation products. Usually adopting the gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS), the methods are sensitive and accurate, and are the key technologies of guaranteeing the food safety.

Simulated filling and accelerated aging testing: The comprehensive properties are evaluated by simulating the real filling and storage conditions in the laboratories. For example, the test cans are filled with the specific contents (or simulation liquids), and after the pasteurization or high-temperature treatment, stored under the specific temperature and humidity conditions, and the conditions such as the can body corrosion, coating peeling and contents flavour changes are regularly checked.

Testing Standards

Domestic Standards:

1. GB/T 9106.1, Packaging containers — Aluminium easy open end and two-piece can — Part 1: Aluminium easy open end and two-piece can

2. GB/T 3880.1, Wrought aluminium and aluminium alloy plates, sheets and strips for general engineering — Part 1: Technical conditions of inspection and delivery

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

4. GB/T 4956, Magnetic method for measuring the thickness of non-magnetic coatings on magnetic substrates

5. GB/T 4957, Non-conductive coatings on non-magnetic basis metals — Measurement of coating thickness — Eddy current method

International Standards:

1. ASTM E8/E8M, Standard test methods for tension testing of metallic materials

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 basis materials — Measurement of coating thickness — Amplitude-sensitive eddy-current method

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

Direct-reading spectrometer: The core equipment of the quantitative analysis of the chemical compositions of the metal materials. Its working principle is to excite the samples through the high-voltage sparks and measure the intensities of the characteristic spectral lines of the various elements to determine the contents. It has the advantages of the fast analysis speed (results within one minute), high precision and simultaneous determination of the multiple elements, and is the necessary instrument of the raw material acceptance.

Universal material testing machine: Used to carry out the mechanical testing such as the tensile, compression and bending. The equipment is composed of the loading frames, servo control systems, force transducers and extensometers, which can precisely control the loading rates through the computer software and record the data in real time, generating the complete test reports. It is the basic equipment of evaluating the material mechanical properties.

Coating thickness gauge and adhesion tester: The coating thickness gauges are divided into the magnetic induction type and eddy current type, which are portable and light, suitable for the fast on-site measurement. The adhesion testers are usually the cross-cut testers (combined with the tapes) or pull-off adhesion testers. The latter can quantitatively measure the bonding strengths between the coatings and the substrates, providing the more precise data.

Sealing tester and pressure resistance burst testing machine: The sealing testers inject the compressed air into the cans and immerse them in the water, observing whether the bubbles escape, or judging the micro leakages by monitoring the pressure decays. The pressure resistance burst testing machines continuously pressurize until the can body rupture, recording the maximum pressure values, used to evaluate the ultimate load-bearing capacities of the can bodies.

Metallographic microscope and electron microscope: The metallographic microscopes are used to observe the micro-structures of the material (grain sizes and second phase distributions), analysing the rationality of the forming processes. The scanning electron microscopes (SEM) combined with the energy dispersive spectrometers (EDS) can be used for the high-magnification morphology observation and micro-area composition analysis of the failed fractures, and are the powerful tools of carrying out the in-depth failure analysis.

Gas chromatography-mass spectrometry instrument: The high-end analysis instrument for the trace volatile organic compound detection. The GC is responsible for separating the mixed components, and the MS is responsible for the qualitative and quantitative analysis of the separated components. In the beverage can testing, it is specially used to analyse the extremely tiny amounts of the harmful substances such as the inner coating migrants and ink solvent residues, ensuring the food safety compliance.