Vacuum bagging system

To maximize properties in the product, higher pressure is needed in the conventional vacuum bag system. A second envelope can be placed around the whole assemblage. Air under pressure is admitted between the inner bag and the outer envelope after the initial vacuum cycle is completed. Still higher uniform pressures can be obtained by placing the vacuum assemblage in an autoclave. By this technique, an initial vacuum may or may not be employed. Using an autoclave assures good results. 

Due to the flexible vacuum bagging system used in VARTM, a composite part manufactured by VARTM could have a thickness non-uniformity issue. Generally, better thickness control (i.e., preform compaction relaxation) can be achieved during the post-mold filling process if one allows enough time for the excess resin to be completely vented out from the resin saturated fiber preform with all resin inflow gates closed. Under such a scenario, the whole fiber preform could be uniformly compacted to its compaction limit (or fiber volume fraction limit) under the pressure difference between the environmental pressure and the vacuum pressure. The preform compaction relaxation can either be measured from experiments or predicted by numerical simulation tools. 

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. 

The transfer of autoclave pressure to the resin in the laminate does not occur hydrostatically because the resin is not enclosed in a constant-volume system. Flow can occur initially both vertically (thickness direction) and horizontally. Furthermore, the network of fibers can also eventually act as a network of springs to which the vacuum bag and bleeder assembly transfer the stress from the autoclave pressure. This stress can then be transferred... 

The crude PA is thermally pretreated (7) and then fed to the vacuum distillation system. Low boiling (LB) impurities are removed in the lights column (8) as LB residues. The high-boiling (HB) residue from the pure PA column (9) is sent to the residue boil-out vessel for PA recovery. Pure PA obtained as a distillate can be stored either in the molten state or flaked and bagged. 

Vacuum bag film is usually a polypropylene film, modified to resist elevated temperatures. The elastic nature of the film offers high formability and resistance to puncture, also permitting the film to be stretched over complex molds without need for a large number of tucks and folds, so improving the efficiency of the vacuum. The film is suitable for use with prepreg and wet lay-up laminating systems in an autoclave. 

Tacky tape is a butyl-based vacuum bag sealant giving high elasticity and tenacity. It has exceptional sealant properties, eliminating the risk of imperfect seals often fi>und in the initial phases of vacuum bag application, so improving the vacuum efficiency and reducing labor. It is suitable for polyester, vinyl ester, and epoxy laminating systems. 

Boat hulls were among the earliest applications of modern reinforced plastics. The United States Navy introduced their first 8.5 m (28 foot) fibreglass personnel boat in 1947, and was contracted to pass 3000 vessels by 1966. The orders for the first two vessels specified construction by the pressure bag method and by the vacuum injection system in order to achieve higher quality than was obtained by the then current hand-lay-up technique [1]. One of the earliest large GRP structures for submerged... 

A simple definition for a composite is two or more materials, that when combined, will retain their original identities but do interface chemically and physically when subjected to specific conditions of either time, temperature, pressure, or chemical catalyzation. Composites are widely available with different resin systems and reinforcement combinations. They are produced in several physical forms and can be shaped with conventional molding methods as well as a variety of other less conventional methods such as resin transfer molding (RTM), resin injection molding (RIM), structural resin injection molding (SRIM), hand-layup, and autoclave molding(i.e., vacuum bag technique). 

A recent introduction to prepreg technology is the use of a resin system such as 3M s Scotchply SP 381, which can be used in a one side tacky (OST) form. Processing using a vacuum bag enables an easy path for air to exit between the plies prior to resin saturation and achieve a composite with less than 0.4% voids using only a vacuum cure. 

Figure 21.77 A vacuum bag lay-up. Source Reprinted from SP Systems, Newport, lOW, technical data, with kind permission.

Thermoset based composite laminates are generally produced bj Autoclave/Vacuum Degassing Lamination Process (38, 39). The characteristics of this inocess are shown in Fig. IS. In this process, pr eg plies of desired shape are laid up in a prescribed orientation to form a laminate. The laminate is covered with successive layers of an absorbent material (glass bleeder fabric), a fluminated film to prevent sticking, and, finally, with a vacuum bag. The mitire system is placed upon a smooth metal tool surface into an autoclave, vacuum is a Ued to the bag and the temperature is increased at a constant rate in order to promote the resin flow and polymerization. The autoclave process will be used along this section as a case study to describe the influence of the matrix characteristics on the processing behavior of hi performance conqmsites. 

---