Stage 4 Bagging Up

A very critical stage as the work done so far could all be lost if the product fails in the autoclave cycle due to insufficient pressure caused by faulty bagging up.

The whole area of the mould is first covered with a release film to do much the same job as the release polish on the mould face. It also ensures that the surfaces of the pre-preg do not come into contact with the cloth wrap. The surface of the release film should be free from creases if possible, as a line will be left on the surface of the moulding where any folds are left.

The cloth wrap covers all the mould and does two jobs

1-       It protects the vacuum bag from any sharp edges that may puncture it

2-       It provides a passage over the whole mould for the air to be sucked out

With great care the vacuum bag is prepared. The bag is usually made from an envelope of a tube on a roll of different widths. The complete mould is placed into the bag and the ends sealed with ‘tacky tape’ which is a double sided soft plastic strip used to stick the open ends of the bag together such that they are air tight.

When one end has been sealed the valves are placed in to the bag. They have a sealing ring to clamp the area where the connection pierces the bag. They are placed in positions where they will not leave a mark on the surface of the mould and they are accessible for the air lines.

After ensuring there are no obvious points that will cause the bag to puncture partial evacuation is done. The bag is then pushed into recesses and edges that may not have enough material when vacuumed down. A vacuum gauge is placed onto the one valve whilst the evacuation is carried out. After this is complete it is a good policy to see that the bag retained its vacuum for 5 minutes, without dropping the vacuum.