Effect of temperature on the shaping process of an extruded wood-plastic composite (WPC) profile in a novel post-production process

Abstract

Wood-plastic composites (WPC) as a result of their ability to be produced from a wide range of materials and mixtures of different materials, including recycled materials and materials hithertosent to waste disposal, have become an area of increasing interest worldwide. Recent changes to environmental legislation mandating higher recycling targets have created additional interest. WPC material is currently used in the manufacture of a number of indoor and outdoor products, but these products are limited to profiles that have uniform cross section due to a lack of suitable post-production processes.

The feasibility of producing three-dimensional wood thermoplastic composite products in a novel post-production process using extruded WPC profiles in a press forming method and the effect of temperature on this process are studied in this thesis. The aim of this work was to investigate and characterize the effect of temperature as the key parameter in the shaping process and to improve forming quality by regulating the process temperature based on the material characteristics.

Experiments undertaken included primary tests to investigate the feasibility of the post-extrusion shaping process using a press forming method and tests to establish the key factors determining the forming process of the thermoplastic WPC sheet. In these tests, the forming quality of the samples was investigated based on material characteristics such as variation in thickness, surface roughness and fiber direction. Preliminary forming tests revealed the importance of temperature as the key process parameter. A diverse set of related tests including cooling rate measurements, flexural tests, tensile tests, conveyor surface energy measurements and numerical simulation were conducted to ascertain material behavior in the shaping process with the aim of improving the forming process.

The results showed that the proposed shaping process is feasible and forming can be done in an uninsulated WPC post-production line. The cooling rate is a key factor in the process and needs to be considered carefully in order to find the right time for the pressing process. Additionally, the cooling rate sets limits on the slowest web speed attainable. Predominant fiber orientation also affects the forming result. The samples produced were accurate with respect to geometrical shape and deformation of the product after cooling. Material variation of the primary extruded WPC sheet affected the quality of the final product; however, the shaping process improved the quality of the material, i.e., resulted in reduced surface roughness and thickness variation. Material behavior under pressing forces needs to be considered during design of the forming tools. Numerical simulation was performed using an elasto-visco-plastic material model and the simulation results were in good agreement with experimental values, indicating the feasibility of using a simulation approach for preliminary evaluation of the formability of different WPCs based on their material characteristics. At elevated temperatures, the studied WPC material showed more ductile behavior and a change in ultimate tensile strain. At higher strain rate, increasing the temperature increased the ultimate strain capacity significantly, whereas at lower strain rate, increasing the temperature decreased the ultimate strain capacity of the material.