7th March 2016
Many manufacturing methods are available for composites. The design of composite structures has to consider manufacturing methods since this can have a large impact on the performance and cost of the structure. Some of the common manufacturing methods are as follows:
Wet Layup is the most common composite production method and can be performed with or without the use of vacuum bagging. By using Wet Layup the operator will place the dry fibres in a mould which represents the negative part to be created. Each layer of fibre will then need to be wetted with a resin (epoxy, polyester, etc) using a roller or a squeegee. This process will be repeated for each layer.
For this process the curing of the resin can be done at ambient temperature or, if a faster process is desired, inside an oven.
The same technique can be improved by using a vacuum bag allowing to better consolidate the various layers and soak out any excess of resin. As a consequence, extra consumables will be required, for instance, release film, breather and vacuum foil resulting in slightly higher prices of processing compared to the standard wet layup.
Wet layup is very simple and affordable. Nonetheless, Wet Layup is very labour intensive, not repeatable, and the quality of the components depend heavily on the operator’s skills and experience.
Wet layup is cost effective for the production of small components; however, very expensive for big parts (i.e. boats or wind turbine blades). If planning to produce larger parts, other process methods should be taken into consideration.
Vacuum Infusion can be considered as an improvement of the Wet Layup with vacuum bagging. This technique it is better suited for the manufacturing of larger components. Therefore, the operator needs the same ancillaries used for Vacuum bagging with the addition of flow medias for a better distribution of the resin inside the fabric.
The operator positions the dry fibres inside the mould and after covering the complete layup with plastic foil starts a vacuum pump pressing the resin inside the laminate. At the same time the vacuum created will push out any air trapped inside the fabric.
This composite production method is more precise process and cleaner as Wet Layup Vacuum bagging. Furthermore, the technique is more precise and gives more control of material placement. Additionally, a higher fibre to resin ratio is achievable resulting in better mechanical properties. Moreover, the control of temperature during cure can be used to reduce the production time.
As mentioned before this process is very cost effective for producing large components, for instance, wind turbine blades and boats, as the operator is only using dry fibre during the layup stage.
The Hand Layup-Prepreg is the most widely used method; therefore often used for aerospace composites parts and structures or within motorsport. For these components quality and repeatability is mandatory. When using a pre-impregnated fabric the material is already provided with the correct amount of resin to ensure the desired mechanical properties.
RESIN TRANSFER MOLDING
By Resin Transfer Moulding the dry fibre is located inside the mould and the mixed resin is injected into a closed mould through one or more ports using high pressure (in general provided from an injector). As a consequence, the resin has to be characterised by low viscosity due to the need of wetting the dry laminate inside a closed mould. Therefore, the operator can easily control the thickness and the surface quality of the part. In fact the tools used for the RTM process (contrary to the resin infusion were a single side tool is used) are more complex. One reason for this is to counteract the high pressure created during the injection process.
The process allows achieving high fibre volume content and therefore high mechanical properties with a high degree of repeatability.
As RTM does not require Prepreg the costs of the material are lower. Therefore, it is possible to combine any type of fibre with the desired resin system allowing a higher degree of flexibility compared to Prepreg. A lot of development is ongoing in the production of fast curing resin systems. As a result the curing time is very short and easily achievable.
Furthermore, the process allows to produce products with high quality. Nonetheless, it is very expensive; therefore, it is better suited for small to medium mass production runs.
As the process allows for more intricate parts to be reliably produced, RTM may allow manufacturers to substitute more metallic components in traditional designs to drive down product weight.
This video from the Discovery Channel shows a vacuum assisted RTM process
Filament Winding is one of the oldest composite manufacturing methods and is less costly compared to other methods. In addition, it is well suited for automation and moreover, effective for mass production.
By using this method the operator can create a wide variety of structures and very high fibre volumes are achievable.
At the beginning of the process the filaments (fibres or rovings) are undergoing a resin bath to be impregnated. Afterwards they are winded around a rotating mandrel.
While the fibres move diagonally, the mandrel rotates and a fibre angle is created that can vary between 0 and 90 degree.
Additionally, the composite structure can be produced at room temperature or with higher temperatures in an oven – if special curing cycles are desired.
On one hand the Filament Winding is a low labour process and the operator can control the resin to fibre ratio relatively easy; on the other hand special equipment is necessary that requires high investment cost. Resulting companies considering using either one of the techniques described above should always consider the benefits and drawbacks to balance costs and results.