Developing a smart mould to improve the vacuum infusion process
This research is carried out by Marco Withag, the goal of the research is to investigate if the vacuum infusion process can be improved by actively
controlling the resin front flow using a closed loop control system, without the need of human labour
involved. This is done by creating a smart mould using sensors or a vision system to gather continuous location data of the resin front flow and simultaneously adjusting the flow properties when needed.
Various sensors and methods to adjust the resin front flow speed have been investigated,
One can for instance think about controlling the pressure difference between the inlet and outlet of the mould cavity, or variation of the temperature locally on the mould.
From the research emerged that controlling the total discharge rate is the best way to control the resin front flow.
The total discharge is the volumetric flow of the resin through a cross-sectional area, and is described by Darcy’s law.
Darcy’s law describes the variables that influence the volumetric flow rate, namely the pressure difference between the inlet and outlet (pb − pa), the permeability of the fiber preform (κ), The cross-sectional area(A), the viscosity of the resin (μ) and the distance over which the pressure drop takes place (L).
To control the filling speed, the total discharge through the fiber preform must be adjusted. This is done by controlling the total amount of resin that is able to enter the fiber preform. The volumetric resin flow is limited by servo controlled valves that are connected to a micro controller. The servo motors are controlled based on the resin front flow speed. The resin front flow speed is measured with the use of a camera and a computer vision script programmed in Python.
Besides the control of the volumetric resin flow, also the distance over which the pressure drop takes place is controlled. This is done by opening new inlet ports automatically when the resin flow arrives.
To lead the resin flow to a specific area on the mold a method called vacuum preform relaxation can be used. Vacuum preform relaxation is the method of applying an external vacuum on the vacuum bag. The external vacuum reduces the pressure on the fibres, and therefore increases the permeability of that area, while maintaining the pressure difference between the resin inlet and outlet. A higher permeability means that there is less flow resistance. Liquids flowing through a porous medium follow the path of least resistance, and therefore fill up the area under the external vacuum.
This method is used to create resin runners. To test the method circular resin runners are embedded in the silicon vacuum bag. This is done to ensure that the infusion pattern keeps a circular shape and to increase the cross-sectional area to increase the total discharge. In Figure 4 is shown how the runners are created by applying vacuum. When the part is filled the pressure inside the runners goes back to atmospheric pressure, to remove the excess resin from the part.
For the automated infusion test-set up a silicon vacuum bag is designed and build to increase the knowledge on this type of vacuum bags. The silicon is brushed or sprayed on the mould cavity including the thickness of the product. This results into a vacuum bag that fits tight into all the sharp corners and edges of the product. The silicon bags are sealed to the mould flange by a built-in seal profile. Those features reduce the bag up time to a few seconds till a few minutes for larger products. Besides the significant reduction in bag up time, there is no consumable plastic film and tacky tape needed. Furthermore, the flow mesh profile can be embedded in the silicon vacuum bag, which removes the waste of consumable flow mesh from the vacuum infusion process. By using a silicon vacuum bag it can be guaranteed that the runners are always on the same position, and therefore repeatability of the process can be ensured. The use of silicon vacuum bags can improve the product quality consistency and reduces the unnecessary waste of plastics.
The final goal of the smart mould is to test the influence that different infusion speeds have on the quality of the composite product. One can think about reducing the amount of voids and eliminating the formation of dry spots. This is done by controlling the resin front flow speed during the infusion of flat glass fiber laminates. The laminates will be compared to each other to see which infusion speeds give the best results. The tests will be executed with the demonstrator shown in Figure 5.
In the animations below, various infusions are shown that are discussed in the report.