Development of a Low-Cost Automated In-Situ Consolidation Process for Self-Reinforced Polypropylene Composites

Self-reinforced composites are a new trend of “composite” materials in which both the reinforcement and the matrix are the same polymer but present different morphologies. Other benefits of self-reinforced composites are recyclability and lightweight. There are several technologies to produce self-reinforced composites, but the most common procedure is based on Hot compaction of high modulus fibres/tapes or bicomponent tapes by selectively melting a thin surface of each fibre/tape, so when cooling, this region recrystallises as the matrix, making a self-reinforced composite.
This thesis presents a flexible approach to the hot compaction procedure by adopting a low-cost automated in-situ consolidation process for self-reinforced polypropylene composites. Here a closed mould is replaced by only one side mould, while the other side mould is replaced by a small tool. The tool can in-situ consolidate a stack of self-reinforced composite fabrics using automated forming paths while ensuring a precise temperature control that prevents the reinforcement phase from molecular relaxation and, therefore, a loss in mechanical properties. The physics behind the automated process is explained, including the bonding mechanism of two polymer interfaces and the heat transfer mechanism.
The machine consists of a gantry structure integrating open-source hardware, a piezoelectric force sensor, temperature sensors with PID temperature controllers, and control software. The control software provides several functions, including hardware communication, machine motion control with a g-code programming language, graphical user interface, monitoring, recording, online processing of force signals, and a software-PID force closed-loop actuation.
To prove the feasibility of the automated in-situ consolidation process, two-dimensional and three-dimensional laminates were formed under different processing parameters such as temperature and pressure. The laminates were characterised and compared against a benchmark manufacturing process: hot press. The results of this project demonstrate it is possible to achieve in-situ consolidation of self-reinforced polypropylene composites which also opens a window towards mould-less fabrication.