3K Plain Weave

1. Metal mandrel The roll wrapping process involves wrapping prepreg carbon fibre around a metal mandrel and so the starting point is to have a metal mandrel of the correct diameter for the size tube that you want to create. Because the carbon fibre will be wrapped around the outside of the mandrel, the mandrel itself needs to have an outside diameter that matches the inside diameter of the carbon fibre tube you will use it to make. The outside diameter of your carbon fibre tube will be determined by the amount of reinforcement (the number of layers) you wrap around the mandrel. In the video we use an aluminium mandrel because the high coefficient of thermal expansion (CTE) of aluminium makes it well-suited to the roll wrapping process. Steel or other metals can be used but metals with higher CTE will make the process easier. If you want to produce a tapered carbon fibre tube then you will most likely need to use a lathe to turn down a solid aluminium rod to create a tapered mandrel. Once you have a correctly sized mandrel, ensure it is completely clean and as smooth as possible to aid with extraction. The mandrel should then be thoroughly prepared with a high temperature chemical release agent, such as Easy-Lease. 2. Prepreg carbon fibre Although in theory it may be possible to use alternative types of reinforcements, such as a dry fabric wetted out with an epoxy resin, in practice only prepreg carbon fibre offers the precision and ease of handling required for the roll wrapping process. In the tutorial we use a layer of XPREG® XC110 210g woven prepreg on the inside of the tube, followed by several layers of XC130 300g unidirectional prepreg, finished with a final layer of the XC110 210g woven prepreg again on the outside of the tube. Because the woven prepreg has fibres oriented in the 0° axis (down the length of the tube) and in the 90° axis (around the circumference of the tube) these layers add what is known as hoop strength to the tube, making the tube less vulnerable to crush or burst forces and splintering. The bulk of the reinforcement, in the 0° axis, gives the tube its longitudinal stiffness. By altering the layup to include more or less woven layers or by changing or alternating the orientation of the unidirectional plies allows the performance of the tube to be precisely optimised for its specific use. For example, a tube for a prop-shaft will encounter primarily torsional forces and so the unidirectional fibre can be aligned off-axis, at 45° for example, specifically to handle these forces. 3. Composites shrink tape Once the prepreg carbon fibre reinforcement has been wrapped around the mandrel as tightly as possible, the reinforcement is then tightly wrapped with a composites shrink tape to provide further consolidation. When applying the shrink tape it is important to ensure there is lots of overlap. Each wrap of the tape advances only a few millimetres down the tube. Although time consuming to do by hand, having lots of overlap in this way will provide much more consolidation pressure when the tape contracts during the cure. 4. Oven cure Besides the mandrel, having an oven to cure the tube in is likely to be the main limiting factor when considering the roll wrapping process outside of a full production environment. However, unlike other prepreg processes, the roll wrapping process does not require precise temperature control or the multi-step 'ramp and soak' cure cycles often needed. Nor does the process require an active vacuum line into the oven. Therefore, the only requirements for an oven to cure a roll wrapped tubes are basic temperature control and sufficient size to fit the tube in. Depending on the size of the tube you want to make, anything from a domestic oven to a powder-coating oven could be used. In the video tutorial, we're using our OV301 precision composites curing oven. The XPREG XC110 prepreg is loaded into the oven at 120°C, flat temperature, and allowed to cure for 1hr 30mins.