Assessing the Effects of Inline Steam Treatment on the Geometric Characteristics of the Thermoformed Plastic-Coated Fibre-Based Products

Abstract

Thermoforming of fibre-based materials faces challenges due to their restrictive three-dimensional formability and lack of compatibility between material properties and traditional thermoforming methods. Treatment techniques present an intriguing area of research to address the gap between material characteristics and process requirements. Therefore, this study examined the influence of inline steam treatment on the thermoforming of plastic-coated fibre-based materials with different product geometries. Three plastic-coated paperboards were subjected to thermoforming, with and without steaming, and their thermoformability was assessed based on the maximum depth and draft angle of the samples. Following 7 s of steaming, the materials exhibited a moisture content 2–4 percentage points higher than that of the original setup, yielding a greater curling rate in the produced samples across different geometries. The curling rate was influenced by the tensile stiffness of the materials and was correlated with the size of the flat-edge area in the studied geometries. Furthermore, findings revealed varying rates at which steaming affected different geometries. Steaming substantially enhanced the achieved depth for geometries where the material was incapable of reaching the mould without moisturization, particularly observed for smaller geometrical shapes. This improvement was linked to steaming, which enhanced pliability, facilitated material penetration, and increased the final depth. However, in geometries where the material could reach the mould without moisturization, steaming expanded the contact area but could also result in a greater spring-back rate and reduced depth, potentially owing to the loss of elastic modulus by moisturization. Nevertheless, the rate of spring-back did not consistently increase with steaming and could be influenced by various material-related factors, including the properties, composition, preparation methods, and postforming drying. Despite depth changes, steaming consistently improved the obtained draft angles, indicating that the materials were drawn more efficiently to the sidewalls across all instances studied. Thus, this study underscored the importance of choosing appropriate materials and moulds for hydrothermal moisturization treatments in the thermoforming process.