Investigating the effect of tooling coatings on the spring-back behaviour of paperboard materials in thermoforming applications

Abstract

This thesis investigates how the surface of tools influences the spring-back behavior of thermoformed paperboard to improve the accuracy of shapes made from fibre-based materials. The two types of paperboards reviewed in this study were produced under the same controlled atmosphere with uncoated aluminium molds as well as molds coated using a 10–15 µm layer of nickel-phosphorus. Testing was carried out by heating at 110 °C and a pressure of 2 bar, while changing the depth of the mold. The temperature of the surface was studied with thermal imaging, while the results were checked using optical profilometry. It was found that uncoated molds led to faster and uniform cooling and less spring-back, especially for Paperboard 2, which was both more flexible and can be formed easily. Unlike smooth molds, coated molds remained warmer, and this resulted in temperature change and change in the forming depth during plastic injection. The researchers confirmed that the way mold surfaces function greatly affects thermoforming and that to get precise paperboard packaging, it is necessary to focus on both the design of the tooling and material choice.