Answer to Question #208250 in Material Science Engineering for Kels

Question

Sketch and describe vaccum bag moulding process

Answer 1

External air pressure exerts stress on the bag by lowering the pressure within the vacuum bag. The pressure applied to the laminate eliminates entrapped air and surplus resin while also compacting the laminate, resulting in a larger percentage of fibre reinforcement.

Wet-lay laminates and prepreg advanced composites can both benefit from vacuum bagging. The reinforcement is soaked using hand lay-up in wet lay-up bagging; then, the vacuum bag is put on the mould and utilized to compact the laminate and eliminate air spaces. In pre-impregnated advanced composites moulding, the prepreg material is put up on the mould, the vacuum bag is attached, the mould is heated, or the mould is placed in an autoclave that provides both heat and external pressure, therefore increasing the force of air pressure. The prepreg-vacuum bag-autoclave technique is most commonly employed in the production of sophisticated composite aircraft and military goods.

Traditional hand layup procedures can result in resin-rich structures, which can be remedied by vacuum bagging. Furthermore, if the technique is followed correctly, a full-fibre wet-out can be achieved. Vacuum bag processing also has the potential to improve core bonding.

Process

The simplest type of vacuum bagging involves placing a flexible film (PVA, nylon, mylar, or polyethene) over the wet layup, sealing the edges, and drawing a vacuum. A release film is applied to the laminate in a more sophisticated type of vacuum bagging, followed by a bleeder ply of fibreglass cloth, non-woven nylon, polyester cloth, or other material that absorbs excess resin from the laminate. A non-woven fabric breather ply is put over the bleeder ply, and the vacuum bag is mounted over the complete assembly. A vacuum is drawn from within the bag using air pressure to remove cavities and push extra resin from the laminate. The increase of pressure leads to even more fibre concentration and improved adhesion between sandwich construction layers. Vacuum bagging is the technique of choice for placing non-contoured PVC foam sheets or balsa into a female mould to guarantee good secondary bonding of the core to the outside laminate.