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Composite Material Processing

composite patent© Murgitroyd & Company

The production of carbon-fibre materials can be expensive due to both material and processing costs. Processing typically involves manual processes or ones that require considerably long cycle times, constraining product quality.

In February 2018, Creative Composites Ltd was granted a UK patent, GB 2515316, titled Methods for composite material processing. The disclosed process enables the production of lower cost products utilising high performance carbon-fibre material systems. It comprises of a pre-moulding stage and a compression moulding stage. The premoulding stage may be adapted for use with pre-impregnated carbonfibre materials, or prepregs, for use in the production of carbon-fibrebased composite components via moulding processes.

In the pre-moulding stage, shown as steps (a)-(d), a prepreg carbon-fibre material is received and/or stored in a refrigerated and ‘frozen’ state. The carbon-fibre material may be substantially rigid or semi-rigid such that the matrix – resin such as epoxy – is cooled and not freely malleable or flexible.

Prepreg materials, for example CYCOM 934, can have a timelimited shelf life and may be received in either a refrigerated state or at ambient temperature. In step (a), however, the prepregs may be refrigerated and stored preferably below -ITC.

From their refrigerated storage, the prepreg carbon-fibre materials may then be transferred to a preforming tool. The prepregs are preformed to have a near net shape of the final product. Optionally, preforming takes place at ambient temperature whereby the prepreg carbon-fibre material is in a malleable state – this may reduce part anomalies and aid prediction of material flow characteristics during the subsequent compression moulding stage.

The preform is then debulked on the preforming tool, preferably under vacuum – this may reduce the number and/or size of voids in the preform and increase ply consolidation enabling the subsequent formation of a high-quality component.

The preform is stored at a reduced temperature so that it is frozen prior to the compression moulding stage. In this way, the preformed shape may be reinforced and the likelihood of deformation during transfer between further processing steps is reduced. Further, preformed parts may be stacked in a stockpile prior to the moulding stage.

In the compression moulding stage, shown as steps (i)-(vi), the frozen preform is inserted into a heated open mould cavity – and its placement may be more easily achieved due to its frozen state.

The mould is moved towards a closed position whereby at least one half of the mould cavity is maintained in a dwell position that is spaced apart from a surface of the preform – the mould may be closed on stops that prevent the preform being compressed below a desired or optimal thickness.

The dwell position is maintained for a period of time and the mould is then closed at a desired pressure and temperature to form a moulded component. A vacuum is applied to the preform under compression – this may reduce porosity in the moulded component. The moulded component is cured for a period of time and the mould is opened to retrieve the moulded component.

Carbon-fibre composite products manufactured using compression moulding techniques may exhibit higher quality surfaces as compared with those formed using conventional techniques such as autoclaving. Further, the patented process reduces cycle times as compared with autoclaving, or resin transfer moulding, by eliminating the need to ramp up or cool down the temperature of the moulding tools. With pressure and heat being sufficient to produce components with comparable material properties, the lower cycle times achieved therefore improve manufacturing throughput thereby reducing production costs.

Read the full patent here

This article first appeared in the July 2019 issue of Materials World, the member magazine of the Institute of Materials, Minerals and Mining.