Development of hybrid rotomoulded composite structures

Abstract

Rotational moulding is a manufacturing process that produces huge hollow plastic parts compared to injection or blow moulding. In a typical rotomoulding cycle, a polymer powder is loaded into a metal mould to be bi-axially rotated and heated. Once a molten plastic layer entirely covers the mould, the tool is moved to a cooling chamber where the temperature decreases until the part is solidified to be de-moulded. Rotational moulding is well known in the tank manufacturing industry as it is one of the most extended techniques for manufacturing large plastic tanks. Also, rotomoulding can produce articles such as shipping containers, recycling bins, boats or kayaks. Despite its advantages, the method's full potential is not exploited due to the limited number of materials available for moulding and their mechanical properties.During the classical rotomoulding cycle, specifically during heating, the powder particles undertake a process in which they soften and merge to form a homogeneous layer. This phenomenon is driven by surface tension and is known as sintering, which constitutes the fundamental mechanism by which parts are formed in rotomoulding. The complexity of sintering makes adding and combining different materials to gain part strength incredibly challenging.Combining two or more materials can create compounds of different properties, known as composites, that produce a result structure that offers more desirable properties than the separate sources. However, no method has made standard the incorporation of reinforcements to produce composite structures using rotational moulding. The present work aims to develop effective means to reinforce rotationally moulded parts to enhance strength and mechanical performance. The work is divided into three main blocks.In block one, the work starts with determining the optimum material composition and structure. An experimental study identifies the critical parameters of the composite part's optimum material composition and structure. The study also aids in developing a hybrid manufacturing process by defining the addition method, processing parameters and moulding technique. An experimental study focuses on how polymer particles soften, bond, and densify with reinforcing fibres. Adhesion measurements were based on the angle formed between the liquefied resins and the reinforcing fibre bundle surface. It was discovered that by modifying heating conditions, resin viscosity, and fibre chemical surface, the adhesion of the polymer to the fibre could be wettable enhanced.In block two, the work continues developing a new in-mould visual imaging system, named RotoCam™, capable of obtaining images of the polymer during the process. The high temperatures, bi-axial rotation, and enclosed mould complicate studying the polymer melt from the inside of the mould. An investigation into the system's main characteristics is undertaken, identifying the key features it requires and assessing the image quality and level of magnification. The layouts included developing three versions, achieving the optimum image quality and resolution to acquire measurable images from the process in the last design. The work finishes by applying the optimum material composition, structure, and hybrid manufacturing process in a full-scale set of trials in block three. During the trials, RotoCam™ acquired images to assess fibre-polymer adhesion, validating the material and the process. The trial set comprised mouldings using the resins, fibres and processing parameters established in block one to determine the relation between fibre adhesion with composite fibre content. Finally, conclusions and recommendations for future work are presented in the last chapter to produce the most optimum composite structure in rotational moulding.

Thesis embargoed until 31 July 2024.

Date of Award	Jul 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	European Union’s INTERREG VA Programme, Special EU Programmes Body, Northern Ireland Department for the Economy & Department of Jobs, Enterprise and Innovation
Supervisor	Peter Martin (Supervisor) & Joseph Butterfield (Supervisor)