Vacuum bag seals

Mold temperature selection the mold temperature control is very critical for (1) resin curing management, (2) resin gel time control, (3) resin viscosity control, (4) material selection of vacuum bag, sealing tapes, flow distribution medium layer, flow distribution tubes, resin flow inlet and outlet tubes, peel ply, mold release agent and the construction material of the mold itself. 

A development of the matched-die moulding process is to vacuum assist the resin into the tool. Dry reinforcements are placed in an open tool. The layup is sealed by a flexible vacuum bag and vacuum is applied. The resin is forced under pressure into the... 

VARTM In this process (which is a variation oftheVARI process), the preform is placed in an open rigid tool (Figure 3.6). The layup is sealed in a flexible vacuum bag. As a vacuum is applied, resin is drawn into the preform which is compacted on the tool surface. The general advantages of vacuum processes such as VARTM are discussed below. 

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. 

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. 

A VIP is obtained by packaging a microporous low conductivity filler material inside a highly impermeable gas barrier bag. The filler is then evacuated to a proper vacuum level and the bag sealed. A gas adsorbent, normally referred to as a getter, inserted in the bag before sealing, is also necessary in most panel designs to ensure the proper vacuum level during the lifetime of the panel. 

Vacuum Bag Moulding—A process in which the lay-up is cured under pressure generated by drawing a vacuum in the space between the lay-up and a flexible sheet placed over it and sealed at the edges. 

A porous release fabric is applied to the lay up to allow the passage of air and excess resin. This is followed by bleeder/breather layers which allow any air to be drawn from the laminate when the vacuum is applied. A vacuum bag is then placed over the laminate, sealed down and the vacuum is applied. The curing cycle now starts either in an oven or in an autoclave which applies both heat and pressure (typically 180°C and 10-20 bar). The action of the pressure from the autoclave or vacuum consolidates the laminate and the individual layers become fused. 

Vacuum Bag Molding. To improve compaction of hand or spray laid assem-bhes, the wet product is covered with a release film such as polyvinyl alcohol, nylon, or silicone rubber. The edges of the film are clamped and sealed to the edge flanges of the mold. An internal vacuum is pumped, so atmospheric pressure presses the film down onto the product. Hand roller pressure helps the process. This squeezes out air bubbles and excess resin and results in a denser product with a better inside surface. The extra work adds to the cost, so this is used mainly in the aerospace and military fields, where the improved quality is worth the cost. 

Employing a vacuum-bag technique, atmospheric pressure can be used to hold the skin to the honeycomb during cure. A sandwich of the new skin, a film adhesive and polyester non-woven monofilament fabric (3M-AF 3306 is satisfactory) is placed on the honeycomb. In this case, a film adhesive layer of 0.085 Ib/ft or two layers of 0.06 Ib/ft should be used. The thickness of AF 3306 should not exceed 0.003 in. The vacuum bag is sealed around the damaged area and evacuated, the non-woven fabric providing a conduit for the air pumped out. Elevated pressure may also be required to prevent honeycomb/skin disbond adjacent to the repair area. Complete rebuilding can be done in this way." ... 

A much better approach to the elimination of uneven pressure application is the use of a relatively new technique isostatic lamination. In this process the layers are stacked on a fixture plate and sealed in a vacuum bag. The bag (or several bags) are then immersed in a pressure chamber that is filled with heated water or a mixture of water and glycerin. Most isostatic pressure chambers operate between 34.5 and 68.9 MPa (5000 and 10,000 psi). Typical temperatures range from 70 to 90°C and the time for the complete cycle is from 3 to 10 minutes. The manufacturers state the following advantages of isostatic lamination ... 

Large, relatively flat surfaces, such as aircraft components, are commonly bonded by means of autoclaves. Initial pressure to hold the substrates in the proper position is achieved by means of vacuum bags. An impervious flexible membrane is placed over the parts to be bonded and sealed to the bonding tray or platen. Air is evacuated from within the assembly and the external air pressure then clamps the parts. 

Apply the sealing tape, which is a double-sided tacky tape for adhering to the mold surface and the vacuum bag together, surrounding the preform assembly. 

Carefully lay up the vacuum bag on the assembly and secure it against the sealing tape on the mold. 

Vacuum bag, flow distribution medium, peel ply, sealing tape and resin tubing may not be reusable. These consumables will need to be prepared for each individual VARTM process every time. 

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