vacuum bag molding

Vacuum bag molding

• Vacuum bag molding

• Also known as vacuum bagging.

• Open mold techniques for thermoset composites.

• Hand lay-up: The application of reinforcement along with a polyester or epoxy resin by hand.

•  Vacuum bagging: The use of a vacuum bag to apply an even pressure over the composite to consolidate the material.

Hand layup

Hand Layup

Hand Layup

What is vacuum bagging?•Vacuum bagging (or vacuum bag laminating) is a clamping method that uses atmospheric pressure to hold the adhesive or resin-coated components of a lamination in place until the adhesive cures.

•(When discussing composites, “resin” generally refers to the resin system— mixed or cured resin.)

Vacuum Bagging - Advantages

• Even Clamping pressure

• Control of resin content

• Custom shapes

• Efficient laminating

Vacuum bagging -Equipment

Vacuum Bagging - Materials

• Release fabric

• Perforated film

• Breather Material

• Vacuum bag

• Mastic sealant

• Plumbing system

• Mold release

Release fabric:•Smooth woven fabric – not bond to epoxy.

•Used to separate breather and laminate.

•Excess epoxy can wick through release fabric.

Perforated film:•Used in conjunction with release fabric.

•This film helps to hold the resin in laminate, when high vacuum pressure is used with slow curing resin system (or) thin laminates.

Breather material:•(or) bleeder cloth.

•Allows air from all parts of the envelope – to be drawn to a port (or) manifold by providing slight air space between the bag and laminate.

Vacuum bag:•If vacuum pressure less than 5psi(10 hg) at room temperature – 6mil polyethylene plastic can be used.

•Clear plastic material is preferable as compared to opaque - for easy inspection

• For high pressure and temperature applications – specially manufactured vacuum bags can be used.

• Vacuum bag should always larger than mould.

Mastic sealant:• Provide a continuous air tight sealant between bag

and mold.

• Also used to seal the point where the manifold enters the bag and to repair leaks in the bag.

Plumbing system:•Provides an airtight passage from vacuum envelope to vacuum pump – allowing pump to remove air.

•A basic system consists of flexible (or) rigid hose pipe, a trap, a port that connects pipe to the envelope.

•Vacuum hose designed specially for this.

•Vacuum port connects the exhaust tubing to vacuum bag.

•Control valve – control of airflow at the envelope.

• Trap incorporated into the line as close as possible to the envelope.

• Vacuum gauge – is necessary to monitor the level.

The reading of negative pressure inside the bag is equal to the net pressure of the atmospheric pressing on the outside of the bag.

Mold release:

•Necessary to prevent epoxy from sticking.

•Three types of mold release:

1.Carnauba based paste wax

•(usually 5 layers for new mold and atleast one layer before each new part is molded)

•Also use PVA in wax.

•Fine detail gloss level obtained by the use of wax.

2. Semi permanent formulation:

•Liquid release system.

•Fine detail and gloss level are obtained as well as texture.

3. General contaminant variety:

•Grease, vaseline, toilet bowl wax, hair spray, hair gel.

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