Vacuum-Bag Molding

Pressure bag molding This is a take-of to vacuum bag molding where the bag and mold is placed in a closed system and is subjected to pressure during the curing cycle. 

Vacuum-Bag Molding. After hand lay-up, the assembly is covered with an air-tight film, typically polyvinyl alcohol, occasionally nylon or other material. Then a vacuum is pulled on the underside of the film, to let atmospheric pressure squeeze out air and excess resin. Use of a hand-held paddle may help. The assembly is allowed to stand until cured. Products are void-free, and quality is better than simple hand lay-up. 

Pressure-Bag Molding. This is similar to vacuum-bag molding, but 345 kPa (50 psi) air... 

Compression Molding Flexible Plunger Flexible Bag Molding Laminate Hand Lay-Up Vacuum Bag Molding Vacuum Bag Molding and Pressure Pressure Bag Molding Autoclave Molding Autoclave Press Clave Wet Lay-Up... 

In vacuum-bag molding the reinforcement and the resin mixed with catalyst are placed in a mold, as in the hand layup method, and an airtight flexible bag (frequently rubber) is place over it. As air is exhausted from the bag, atmospheric air forces the bag against the mold (Figure 2.46). The resin and reinforcement mix now takes the contours of the mold. If the bag is placed in an autoclave or pressure chamber, higher pressure can be obtained on the surface. After the resin hardens, the vacuum is destroyed, the bag opened and removed, and the molded part obtained. The technique has been used to make automobile body, aircraft component, and prototype molds. 

FIGURE 2.46 Vacuum-bag molding (a) before vacuum applied (b) after vacuum applied. 

Vacuum bag molding is a modification of hand lay-up, in which the layup is completed and placed inside a bag made of flexible film, and all edges are sealed. The bag is then evacuated, so that the pressure eliminates voids in the laminate, forcing excess air and resin from the mold. By increasing external pressure, a higher glass concentration can be obtained, as well as better adhesion between the layers/plies of laminate. Some items for the process can be disposable. 

The U.S. Navy pioneered in glass-TS polyester RP (hand lay-up) large boat construction with the production of an 8.5 m (28 ft) hull in 1947. RP Navy boats (in the USA and other countries) range from 3.7 m (12 ft) to over 30.5 m (100 ft). Small boat construction was initiated during 1944 at the Philadelphia Navy Yard, Materials Laboratory (D. V. Rosado involved unfortunately the first vacuum bag molded 15 ft boat gradually sank due to unsatisfactory cure cycle but thereafter none sunk). 

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