Bleeder cloth

A typical cure cycle is shown in Fig. 9.12. Somewhere in the plateau at 135°C the autoclave pressure is applied. This step provokes the elimination of resin, which is absorbed by the bleeder cloth, and the increase in the fiber... 

Vf is the volume fraction of fibers, L is taken as 2 cm to simulate the cure of a thick part, and Tw(t) is the cure cycle described in Fig. 9.12. It is (arbitrarily) assumed that the boundary between the laminate and the bleeder cloth behaves adiabatically. A better simulation would have to consider the heat transfer to the bleeder cloth. 

Figure 9.11 Lay-up of preimpregnated plies placed between a heated plaque and the bleeder cloth.

Autoclave moulding is typically used in the aerospace industry for the production of high-value composites from prepregs. The laminate, which is covered on both sides by a fine polyester cloth peel-ply (for enhancing the surface effect), is built up on the mould surface. The top surface of the laminate is covered by a porous release film and bleeder cloth. The whole assembly is then covered with a non-porous membrane, which is sealed to the mould, and then placed inside an autoclave as shown in Figure 6.19. 

A bleeder cloth, a nonstructural layer of material such as polyester felt, is used to allow the escape of excess gas and resin during cure. This material is removed after the curing process is completed and the excess resin taken with it. Next, a barrier film is... 

Probe 1 Nylon Bagging Bleeder Cloth Release Film... 

This is followed by a perforated release film to allow for transfer of the excess resin to the bleeder cloths and to separate the bleeder cloths from the actual laminate sample. Such films are readily available commercially, such as the FEP teflon 0.025 mm type film. Flowcver, a polypropylene or nylon sheet extensively perforated with a large pin will also suffice. Bleeder cloths are then added to absorb the excess resin, and also to provide a path for the vacuum to evacuate the volatiles and voids within the lay up. It is important to ensure that the bleeder cloth is extended continuously from the vacuum probe to the lay up Industrial type absorbent paper tissue can also be used, but the better cloths would be the thick wool type cloths used for water filters, etc. The number of cloths is sometimes used to control the resin content of the cured sample. If maximum fiber content is... 

Bleeder cloth Usually made of glass fabric or non-woven, used to absorb a calculated amount of excess resin from prepreg when pressure is applied in a vacuum bag lay-up. 

Flat panels of uniform thickness are frequently bonded in heated platen presses. However, most bonded aerospace assemblies are contoured. For this reason, bonding is usually carried out in heated, pressurized autoclaves. In the autoclave process the assembly is supported by a tool of the required contour, wrapped with several layers of glass or polyester bleeder cloth, and then enclosed in a impermeable heat resistant bagging film. This layup is then placed inside a pressurized autoclave and the inside of the bag is vented either to the atmosphere or connected to a vacuum pump. The autoclave is then pressurized to the desired level by compressed air, nitrogen, or other relatively inert gas. 

The function of the bleeder cloth is to absorb adhesive flash and provide an unrestricted path so that entrained air or reaction volatiles can be removed. In the case of condensation reaction polyimides the bag should be vented to relatively high vacuum conditions. This is critical because it is necessary to depress the boiling point of the high boiling point solvents used to flexibilize the film before gellation of the adhesive occurs. 

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