High pressure liquid moulding of composites

Martin, Ffion A. (2018) High pressure liquid moulding of composites. EngD thesis, University of Nottingham.

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Abstract

Automotive manufacturers are responding to increasingly stringent demands on fuel consumption by developing novel lightweight designs with new materials. Carbon fibre composites are seen to offer high specific properties – leading to low weight solutions, but their cost and manufacturing rate are currently unsuited to automotive high volume cost targets.

This thesis presents a novel composites manufacturing process offering high production rates and low process costs. The process is termed ALCM, Advanced Liquid Compression Moulding and is a novel derivative of High Pressure Resin Transfer Moulding. In the two-stage ALCM process, features of both liquid moulding and compression moulding techniques are exploited. In the first stage the mould tool is held open by a small amount during the injection phase. This allows the fibre preform to expand into the mould cavity, increasing its permeability, thus reducing overall injection pressure requirements. In the second stage the tool is closed to finalise the resin infusion and form the finished component to the required geometry and fibre volume fraction.

Two work packages are presented in this thesis: Objective 1 is to study a range of experimental parameters to characterise the ALCM process; Objective 2 is to simulate the ALCM manufacturing process using the Finite Element method.

In Objective 1 a range of experiments explore the feasibility of the ACLM process to manufacture large structural components using carbon fibre NCF (non-crimp fabrics) with fast-curing epoxy resin. The experiments were performed on the newly-installed high pressure RTM injection equipment and large bed press at the National Composites Centre, UK. Characterisation of the ALCM process was undertaken using two mould tool geometries – a 1240x790mm rectangular flat plate tool and a top hat shape tool with the same planform area and 180mm depth. Generally, high quality mouldings were produced, with void content seen to be typically lower than 0.5%, and injection flow rate across the experimental range did not affect the void content.

Flat plaque experiments were performed at resin injection flow rates between 35 and 85g/s and three different levels of tool opening during the first resin injection stage, leading to in-mould pressures of 100bar. Results from mouldings filled with between 18% and 100% of total resin volume demonstrated that, in the liquid moulding stage of ALCM whilst the mould tool is held open, the resin flow in the preform is in-plane.

The in-plane flow through the preform follows Darcy’s law for flow through porous media, however it was seen that spaces around the preform inside the rigid mould tool were conduits for resin flow governed by Navier-Stokes flow. It was found that in the resin injection stage, after a minimum tool opening, the resin channels in the mould alleviate resin injection pressure to a greater affect than increasing permeability (through lower fibre volume fraction).

In the flat panel mouldings, resin channels around the component perimeter determine the fill pattern. In the top hat shape mouldings, resin channels were present at corner bend radii and were seen to be advancing the flow front. Resin channels were found to be problematic through reducing the proportion of injected resin volume that remained in the preform. Injected resin volume determined by feedback from an in-mould pressure sensor, rather than prescribed volume of resin, demonstrated the process consistency across sixteen repeated flat panels.

Preform/fibre wash was found to be a critical problem as a result of the high injection pressures. The effect varied from local wrinkle defects to complete wash out and was shown to be linked to the degree of preform stabilisation by binder or edge clamping and fibre tow wash to the fabric stitch and inlet configuration. Critically the extent of fibre wash was sensitive to small variations in preform geometry at the higher injection flow rates experimented.

In Objective 2, the simulation methodology developed using the well-known University of Delaware, LIMS Liquid Injection Moulding Simulation software. The modelling methodology was successfully adapted to the ALCM process, employing a low FVF injection using a Finite Element Control Volume approach, followed by compression as each node becomes a resin injection gate. The simulation was used to predict fill pattern, mould pressure and press clamping force.

Results from simulation models representing the flat and top hat shape panels correlated to experiments and found that the Darcy flow assumption is valid for predicting fill pattern provided that the preform remains fixed and resin channels are represented. Correct representation of changing resin viscosity is necessary to reliably calculate pressure and therefore press clamping force.

A full-size vehicle floor was the technical demonstrator component. This large component, with surface area 3m2 was ultimately manufactured within a 5 minutes cure cycle. At the time of writing, this is believed to be the largest structural composite component manufactured at this production rate. It was found that the methodology developed in Objective 2 could be applied to give good correlation between simulation and experimental results for resin fill pattern and process pressure, with the additional requirement that resin outflow from the preform into the tool be characterised.

The floor component is produced from three preforms. Fibre wash at the preform joint was observed at resin injection flow rates of 85g/s and above. An innovative design of Anchored Through-Thickness Reinforcement at the preform joint was incorporated and was seen to stabilise the preform joint, eliminating fibre wash

The two part resin used in the process is a new formulation developed to have short cure time. At moulding temperatures between 130⁰C and 110⁰C the resin cure time was between 3 and 8 minutes. It was found that conventional rheometry characterisation techniques were not satisfactory for defining the resin viscosity in the initial reaction stages. The dielectric analysis technique was investigated for the first time in this application. Results found that this method is suitable for fast cure resin rheo-kinetics characterisation.

Item Type: Thesis (University of Nottingham only) (EngD)
Supervisors: Warrior, N. A.
Potter, K.
Keywords: Plastics, Molding; Composite materials, Mechanical properties; Automobiles, Bodies, Design and construction; High pressure (Technology)
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 51888
Depositing User: MARTIN, Ffion
Date Deposited: 22 May 2018 08:53
Last Modified: 08 May 2020 08:04
URI: http://eprints.nottingham.ac.uk/id/eprint/51888

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