The closed mold technique is an efficient way to make products that are of higher quality, can be produced faster, cause less environmental pollution and produce less waste than with open mold technique. Fiber reinforcements are placed in the mould, the mold is closed and sealed and the resin is injected into the mold. After the resin has cured, the mold can be opened to take out the part.
There are numerous techniques that can be considered part of vacuum engineering, including ‘VARTM (Vacuum-Assisted Resin Transfer Molding) or SCRIMP (Seeman’s Composite Resin Infusion Molding Process). These techniques all have in common that the workpiece is soaked using atmospheric pressure. This is only possible if the workpiece is hermetically sealed and air is extracted from one side with a vacuum pump.
Vacuum foil can be used to seal a workpiece airtight. A lower and upper mold is necessary to prevent the workpiece from creasing. This makes it easier to obtain the correct fiber volume content and to obtain a good surface quality on both sides. This is especially advisable when manufacturing test pieces for material testing. Vacuum technology is a simple principle with which high-quality products and series can be made.
Tools for vacuum techniques
A release layer is first applied using a mold in the desired shape of the end product. This allows the product to be removed from the mold after curing (most resins have good bonding power!). The laminate is then built up according to the requirements. With a mold on one side, a tear cloth (‘peel ply’) is often used; this acts as a release layer, but also ensures a certain roughness of the surface (for example, favorable for gluing). In some cases, a piece of cloth or gauze is placed over part of the product to allow the resin to penetrate all over the product before curing begins. Finally, the entire stack is sealed with vacuum foil. The tape (‘tacky tape’) is pasted around the product, which is malleable and very sticky. The release layer is not covered in this case.
Usually the foil is not completely stretched over the mold, but it has some folds in it to compensate for deformations during the process. As soon as everything is airtight, a vacuum is applied at one or more points. As a result, the resin is sucked through the product. To prevent excess resin from entering the pressure gauge or vacuum installation, a resin trap can be installed on the discharge side.
For the infusion of large products, you must carefully consider in advance where the resin supply and air exhaust are connected to the product. These provisions influence the “infusion strategy”. When applied incorrectly, the two most common problems can occur:
- Incomplete infusion due to too long infusion path: If the infusion takes too long, the resin will thicken and flow through the product will be reduced. If too long distances are used between the supply and discharge points, the resin cannot enter the product in all places.
- Incomplete infusion due to incorrect preferred path: The resin seeks the path of least resistance between inlet and outlet. If there is a fiber pack (less permeable to resin) over which a flow-through medium (highly permeable to resin) is laid throughout the infusion path, the resin will tend to flow through the flow-through medium rather than through the fiber pack. This also happens, for example, if the resin supply lines (runners) extend too far into the set-up; then ‘race tracking’ can occur, causing dry spots between the supply pipes.
Software has been developed based on the combination of the finite element method and Darcy’s law that allows you to analyze the consequences of your infusion strategy. By means of the predicted infusion time you can also improve and optimize your production process.
* Nijssen, R. (2015). Composieten Basiskennis. Retrieved from Composites NL: https://compositesnl.nl/wp-content/uploads/2020/03/Composieten-Basiskennis-3e-druk-NL-CNL.pdf