Layer by layer curing

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Layer by layer curing

Host Institution: Cranfield University

Lead Investigator: Alex Skordos

Aims

This project offers the opportunity to open up a new approach to composites manufacturing, delivering a step-change improvement in the speed of processing of large high value structures by rewriting the basic assumptions of current manufacturing processes. Thermosetting composite manufacturing currently presents costly challenges related to inherently low processing rates, a high rate of defect generation and limited flexibility of process conditions. The major stages of processing – placement, consolidation and cure - are separated, with ply placement and consolidation taking place at temperatures at which cure is very slow. This is driven by the need to achieve acceptable quality prior to cure, while the material is still highly deformable.
The execution of placement, consolidation and cure in series is naturally costly whilst opportunities for accelerating consolidation and cure by increasing the temperature are missed. The inefficiencies of the current processing philosophy are intensified as component size - especially thickness - increases. Placement/forming and consolidation often need to be carried out in stages, while thermal lags increase non-linearly with thickness. For the largest structures, there is a significant challenge to lay up the part inside the prepreg’s out-life. This is currently driving part of the trend to dry fibre processing. Furthermore, in the case of thick and ultra-thick components, thermal profiles need to be conservative and long to eliminate detrimental temperature overshoots.

The concept proposed in this project addresses the limitations and inefficiencies of current strategies through layer by layer cure of prepreg during placement/consolidation. This new strategy is applicable in process setups where whole layers are placed sequentially as well as in variants such as AFP/ATP in which local placement is carried out. The process takes place on a heated tool, with the current upper layer heated through contact in a positive pressure bag press in the whole layer version of the process or, using the heating head in the AFP version.

The overall aim of the project is to establish the capability of producing composites by processing in a single layer by layer (LbL) step. The following objectives will be addressed:
1. Simulation of the layer by layer process - including consolidation, thermal and curing effects - to enable investigation of process scenarios to be carried out.
2. Evaluation of interlaminar properties at interfaces produced using partially cured sub-laminates to establish material state limits appropriate for adhesion of successive layers.
3. Process optimisation to identify conditions combining high speed and sufficient layer adhesion.
4. Implementation and demonstration of the whole layer variant of the process.
5. Assessment of product quality to validate the development.

Successful implementation of the LbL process concept will result in the following benefits:
1. Acceleration of consolidation due to the increased processing temperature.
2. Speeding up of curing due to immediate heating of the current upper layer.
3. Ability to process thick/ultra-thick components with cost scaling linearly as a function of size and elimination of thickness processing limitations due to the additive nature of the process.
4. Ability to process large prepreg structures due to the elimination of prepreg life limitations.
5. Opportunity to adapt process conditions layer by layer to address potential defect formation.
6. Opportunity to vary the material and conditions as the process evolves, e.g. using thermoplastic interlayers or secondary materials for multifunctional composites that cannot tolerate the protracted high temperature exposure of conventional cure.
7. Opportunity to inspect the nearly finished product layer by layer through the thickness.

The aim and objectives of the study support directly the High rate deposition and rapid processing technologies research priority area of the Hub through the acceleration of processing as a result of merging currently separate stages of manufacturing and accelerating each stage due to the ability to carry out the process at higher temperatures. Furthermore, the opportunities generated by the layer by layer completion of the process enable new strategies of inspection and integration of multi- material solutions contributing to the areas of Inspection and in-process evaluation and Manufacturing for multifunctional composites and integrated structures.

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