Today’s preforming technologies are largely manual, thus increasing the costs of Liquid Composite Moulding (LCM) technologies such as Resin Transfer Moulding (RTM). Systems that can drape 3D profiles automatically and continuously, such as the one developed by FIBRE, have just left the development stage but are in need of further development to in-crease productivity and quality.
The chemical stitching (CS) technology offers a way to reduce the lead time by replacing the time-consuming binder application with localised adhesives application. For the application, needles will be used which inject either a thermoplastic (hot melt) or thermoset (microwave curing) adhesive between the layers. This will also improve permeability and thus the quality of the finished part. Furthermore, the chemical stitching technology is more energy efficient because melting or curing of the adhesive is restricted to minimal areas and volumes.
The aim of this proposal was to develop an efficient process chain for the continuous pro-duction of profiles such as stingers and spars for the CS EDA WP2 torsion box generator.
From the singular requirements concerning the component geometry as well as the equipment footprint it became clear, that a new machine needed to be designed and constructed. Extensive knowledge gathered during the development of 3D preforming technology by the applicant reduced the development time of the core preforming equipment, so that the majority of the work can be focused on developing the integrated preforming/chemical stitching process. Nevertheless, the requirements for using different sorts of fabric (woven, nonwoven, NCF, UD) required testing of the new equipment to find the correct set of process parameters for each fabric. The new equipment was designed in modules, so as to ease later expansion of its capabilities. Thus, the equipment could be enhanced in the future with the ability to produced curved profiles or with additional CS substructures.
The Chemical Stitching und Continuous Preforming technologies had been developed independently and needed to be combined into an integrated process. This necessitated extensive design work and the manufacture and assembly of a new preform production line.
The forming, stitching and curing units though had to be designed specifically for the final product(s) to be produced on the device. The first step in the production was the cross section forming. To learn more about the possibilities of the cross sectional several concepts for draping mechanisms were considered and tested in the form of lab scale prototype devices (D2-2) to help choose the best technology for the final design. The two possibilities that existed were preforming with a solid forming tool and preforming using a set of rollers. For the preform geometry with L sections and flat doublers, the forming method of the roller-based technique was chosen for best quality.
The rollers gradually change the flat textile into the L shape. After the forming and before the stabilisation of the stitching the shape is kept by a snug fitting solid tool, that was able to guide the preform but is too narrow for the flat doubler. The doubler preform was transported up and over the roller section of the preforming unit and passes the section thus without any clearance issued.
The chemical stitching was also advanced. First, concepts were developed how multiple stitching points can be set simultaneously. Also, it was considered what kind of injectors could be used in the final design. The simple injector used before was replaced with one that can deliver the controlled resin amounts necessary for the industrial application of this technology, this new injector was tested in a lab scale prototype of the stitching unit to determine the best process parameters concerning adhesive amount, curing time, curing temperature etc. In the detailed design of the production line the stitching unit was designed to reflect the production of the two different profile geometries.
The manufacturing and assembly of the new preforming line was completed on time. The integration of the resin injectors was the more challenging task. The injectors must move at the same speed than the textile layers through the preforming device. At the same time, they must also move up and down. These individual movements must be synchronised for the process to function. Other parameters like the resin viscosity, which depends on the amount of curing agent, also influence the setting of these movements. The viscosity influences the injection time, which was limited by the maximum pressure the injectors can achieve. The amount of curing agent curing agent must be set according to the speed of the textile, because this determined the amount of time the resin points will remain in the hot curing zone.
After lengthy optimisation of these parameters and the chemical stitching device, a functioning, stable and high quality process was achieved. Due to the automated and continuous manner of the preform production line, even a 24-hour usage of the device is viable with minimal downtime. Under these conditions, a production rate of 1000m of stacked, shaped, stabilised, cut and infusion ready preforms was not unrealistic.