Vacuum Bagging Process

1. Application of compaction pressure to consolidate plies and minimize voids
2. Extraction of moisture, solvents, and volatiles
3. Allow resin to flow and be absorbed

 

Autoclave Process

A process using a two-sided mold set that forms both surfaces of the panel. On the lower side is a rigid mold and on the upper side is a flexible membrane made from silicone or an extruded polymer film such as nylon. Reinforcement materials can be placed manually or robotically. They include continuous fiber forms fashioned into textile constructions. Most often, they are pre-impregnated with the resin in the form of prepreg fabrics or unidirectional tapes. In some instances, a resin film is placed upon the lower mold and dry reinforcement is placed above. The upper mold is installed and vacuum is applied to the mold cavity. The assembly is placed into an autoclave. This process is generally performed at both elevated pressure and elevated temperature. The use of elevated pressure facilitates a high fiber volume fraction and low void content for maximum structural efficiency.

 

Resin Transfer Molding (RTM)

RTM is a process using a rigid two-sided mold set that forms both surfaces of the panel. The mold is typically constructed from aluminum or steel, but composite molds are sometimes used. The two sides fit together to produce a mold cavity. The distinguishing feature of resin transfer molding is that the reinforcement materials are placed into this cavity and the mold set is closed prior to the introduction of matrix material. Resin transfer molding includes numerous varieties which differ in the mechanics of how the resin is introduced to the reinforcement in the mold cavity. These variations include everything from the RTM methods used in out of autoclave composite manufacturing for high-tech aerospace components to vacuum infusion (for resin infusion see also boat building) to vacuum assisted resin transfer molding (VARTM). This process can be performed at either ambient or elevated temperature.

 

 

Typical Consumable Products Used in Composite Lay-up Processes

 

Peel Plys

Woven fabrics that are generally applied as the first material in the composite laminate sequence. They are designed to be peeled from the surface following cure to leave a textured surface, which is clean and contaminant free. This surface is then ideally prepared for secondary bonding, filling or painting and does not usually require any further mechanical abrasion. The textured surface left on the laminate is an imprint of the weave of the peel ply and therefore fine weaves will impart a finely textured surface. Peel plies are usually made from polyamide (nylon) or polyester fibers. Polyamide fabrics are more commonly used, though polyester fabrics must be used with phenol-based resin systems.   It is important to store peel plies in a cool and dry environment, ensuring the fabric is kept clean and free from possible contamination.

 

Release Films

Peel ply can be either a solid sheet, or it can have perforations which can vary in their spacing.  Choose spacing based on the amount of resin that needs to be bled out; prepreg manufacturers can recommend spacing for their particular products; and net-resin systems of course use unperforated release films.

 

Breather Cloth

Breather Cloth is a material, which will not compress under pressure to the extent that air cannot be drawn through it. Often, an otherwise perfectly sealed bag will not develop full vacuum pressure because the bag under the hose fitting is pinched off preventing evacuation of air from the rest of the part.

 

Bagging Film

Vacuum Bagging Films are used to seal and provide even pressure to the laminate and tooling surface.  Vacuum is applied to ensure compaction, minimize voids and the removal of volatiles. Vacuum Bagging Film should be of a high quality, and possess characteristics such as, resistance to tearing, high modulus, high elongation, resistance to “pin holing” and high temperature resistance. Components, particularly with complex geometries, must be vacuum bagged with plenty of tucks to give ‘depth’ to the film.

 

Sealant Tape

Sealant tapes are used to seal the bagging film to the tool or part. They are very pliable and similar in texture to “chewing gum”. Most sealant tapes are butyl-based material which will stick to most bagging films (except silicone which is self-sealing) and to most fiberglass, aluminum, and plate glass tools. Sealant tapes should be tacky enough to allow for ease of sealing while maintain the vacuum bag able to remove cleanly from the part or tool.