What Causes The Coating Peeling And Cracking of Color Aluminum Food Packaging During Subsequent Stamping, Deep Drawing And Edge Folding Processes, And How To Improve It?

1. Introduction

Color-coated aluminum materials are widely applied in food packaging fields such as food cans, sealing covers and aluminum foil containers due to their excellent barrier properties, lightweight characteristics and decorative appearance. In actual production, color aluminum food packaging needs to undergo secondary plastic forming processes including stamping, deep drawing and edge folding to realize final product shaping. However, coating peeling, micro-cracking and macroscopic cracking defects frequently occur during these processes. These defects not only damage the surface aesthetics of packaging products, but also destroy the protective barrier of the aluminum substrate, leading to risks such as food oxidation, moisture contamination and heavy metal precipitation, which seriously affects the food safety and service life of packaging products. Therefore, systematically analyzing the root causes of coating failure in plastic forming processes and formulating targeted improvement measures is of great significance to improve the yield and comprehensive quality of color aluminum food packaging.

2. Main Causes of Coating Peeling and Cracking

The coating failure of color aluminum food packaging during stamping, deep drawing and edge folding is a comprehensive result of substrate performance, coating system characteristics, pretreatment process and forming process parameters. The specific causes are classified and analyzed as follows.

Defects of Substrate and Coating System Matching

The bonding stability between the aluminum substrate and the coating is the core premise to resist forming deformation damage. Most food-grade color aluminum substrates adopt thin aluminum alloy sheets with high ductility, while the matching primer and topcoat formulas are mostly universal products, resulting in poor system compatibility. Firstly, ordinary universal primers lack targeted activity for aluminum substrates. Unlike steel substrates, aluminum surfaces are prone to form dense inert oxide films. Non-specialized phosphating primers cannot form molecular fusion bonding with the aluminum conversion film, resulting in low interface adhesion strength. When the substrate undergoes tensile and compressive deformation during deep drawing and stamping, the coating is easy to separate from the substrate and peel off.

Secondly, the mismatched hardness and ductility of primer and topcoat cause internal stress concentration. The topcoat with excessive hardness and poor flexibility cannot follow the plastic deformation of the aluminum substrate synchronously. During high-strain processes such as deep drawing and edge folding, the topcoat bears excessive tensile stress alone, resulting in micro-cracks. In addition, unreasonable coating thickness distribution also induces defects. Excessively thick coating increases brittleness and internal curing stress, while uneven local thickness leads to inconsistent deformation resistance, causing localized cracking and peeling during forming.

Inadequate Substrate Pretreatment and Curing Process

Surface pretreatment is the key link to ensure coating adhesion, and incomplete pretreatment is a common hidden cause of coating failure. Residual oil stains, dust and oxide residues on the surface of the aluminum substrate will form isolation layers between the substrate and the coating, seriously reducing the interface bonding force. Simple wiping and conventional cleaning processes cannot completely remove microscale contaminants, resulting in partial weak adhesion areas. These weak areas will first peel and crack under the stress of stamping and edge folding processes.

Unreasonable coating curing parameters further aggravate coating defects. Excessively high curing temperature or long curing time leads to excessive cross-linking of the coating resin, making the coating overly brittle and losing ductility. Conversely, insufficient curing temperature or time results in incomplete resin cross-linking, low coating strength and poor wear resistance and deformation resistance. In addition, uneven curing temperature in the production furnace causes inconsistent curing degree of the coating on the same aluminum sheet, forming local fragile areas that are prone to failure during subsequent forming.

Unreasonable Forming Process Parameters

Stamping, deep drawing and edge folding processes involve complex tensile, compressive and shear stresses, and improper process parameters will directly cause coating damage. Excessive blank holder force is the main cause of deep drawing coating failure. Excessive clamping force restricts the free deformation of the aluminum sheet flange, resulting in excessive tensile stress on the side wall and corner of the workpiece, which exceeds the deformation limit of the coating and causes cracking and peeling. Meanwhile, unreasonable forming speed also has a significant impact. High-speed stamping and deep drawing produce instantaneous impact stress and friction heat, leading to uneven substrate deformation and local coating overheating and embrittlement, inducing micro-cracks.

In addition, unreasonable mold structure and precision also trigger defects. Sharp mold fillets cause severe stress concentration at the bending and drawing corners of the workpiece, making the coating unable to bear concentrated strain and crack. Mold surface wear, burrs and insufficient lubrication will produce friction scratches on the coating surface during forming, which expand into peeling and cracking defects under continuous deformation stress. The excessive single-pass drawing depth and unreasonable draw ratio will also cause excessive thinning and tensile deformation of local substrates, exceeding the coating's synchronous deformation capacity.

Post-Coating Residual Stress and Environmental Factors

Residual stress generated during coating and curing will remain inside the coating for a long time. During subsequent secondary forming, the residual stress superimposes with the forming stress, breaking the stress balance of the coating and causing crack propagation and peeling. Moreover, color aluminum packaging materials are vulnerable to environmental humidity and temperature changes during storage and transportation. Moisture invasion will reduce the interface bonding force between the coating and the substrate, and low-temperature environment will reduce the coating's toughness, making the coating more sensitive to forming stress and prone to brittle fracture.

3. Targeted Improvement Strategies

Aiming at the above defect causes, comprehensive improvement measures are proposed from four dimensions: material system optimization, pretreatment and curing process upgrading, forming process parameter optimization, and production environment control, to solve the coating peeling and cracking problems fundamentally.

Optimize Coating System and Material Matching

Select food-grade special coating systems matching aluminum substrates, abandon universal primers and topcoats. Adopt acid-resistant phosphating primers with high activity for aluminum materials to form stable chemical bonding with the aluminum conversion film, significantly improving interface adhesion strength. Match flexible topcoat resins with good ductility and food safety certification, balance the hardness and toughness of the coating, and ensure the coating can synchronously follow the plastic deformation of the aluminum substrate without cracking or peeling.

Strictly control the coating thickness uniformity, set the optimal thickness range according to the forming process requirements, and avoid excessive coating brittleness caused by overly thick coating and insufficient protection caused by overly thin coating. Optimize the interlayer matching of primer and topcoat to ensure consistent deformation coordination, eliminate internal stress between coating layers, and improve the overall deformation resistance of the coating system.

Upgrade Substrate Pretreatment and Curing Process

Improve the substrate pretreatment process to build a high-adhesion interface. Adopt degreasing, pickling, phosphating and pure water cleaning multi-stage pretreatment processes to completely remove surface oil, oxide and dust contaminants. Strictly detect the surface cleanliness and roughness of the aluminum substrate before coating to ensure that the substrate surface has high surface energy and uniform roughness, providing a good bonding foundation for the coating.

Optimize coating curing parameters and formulate segmented temperature control procedures. According to the coating resin characteristics, accurately control the curing temperature, time and furnace temperature uniformity to ensure complete and uniform cross-linking of the coating resin. Avoid excessive curing brittleness and insufficient curing strength defects. Regularly calibrate the curing furnace temperature and maintain production equipment to eliminate uneven curing caused by equipment failure.

Optimize Forming Process Parameters and Mold Conditions

Adjust stamping, deep drawing and edge folding process parameters scientifically. Adopt moderate and low blank holder force, and equip with polyurethane blank holder rings to reduce clamping friction and avoid coating scratch damage. Optimize the forming speed, adopt hydraulic press low-speed forming (50–100mm/min) to eliminate instantaneous impact stress and friction heat accumulation, and ensure stable and uniform substrate deformation. For deep drawing processes with large deformation, adopt multi-stage drawing instead of single-pass deep drawing to reduce single deformation strain and avoid excessive tensile stress on the coating.

Optimize mold structure and surface precision. Polish mold fillets to smooth transition, eliminate stress concentration points at workpiece corners. Regularly repair and polish mold surfaces to remove burrs and wear marks. Adopt special aluminum stretching oil or dry film lubricants with food safety grade to reduce mold-workpiece friction, prevent friction-induced coating damage, and improve forming uniformity.

Control Residual Stress and Production Environment

Release coating residual stress effectively. Set a proper aging time after coating curing to release internal residual stress of the coating before secondary forming processing, avoiding stress superposition induced failure. Standardize the storage and transportation environment of color aluminum materials, maintain constant temperature and humidity, avoid moisture erosion and low-temperature embrittlement of the coating. Strengthen batch quality inspection, detect coating adhesion and toughness before forming, and eliminate unqualified raw materials in advance.

4. Conclusion

The coating peeling and cracking of color aluminum food packaging during stamping, deep drawing and edge folding is mainly attributed to mismatched coating system, inadequate substrate pretreatment and curing, unreasonable forming process parameters and residual stress superposition. The defect mechanism involves interface bonding failure, coating brittle fracture and stress concentration damage. By optimizing aluminum-specific food-grade coating systems, upgrading pretreatment and curing processes, refining forming process parameters and mold conditions, and standardizing environmental control, the synchronous deformation coordination ability between the coating and aluminum substrate can be effectively improved. These improvement measures can fundamentally reduce coating failure defects, ensure the surface quality and food safety performance of color aluminum packaging products, and provide reliable technical support for stable mass production of high-quality food packaging aluminum materials.