Vacuum Bag Moulding and Compression Moulding Technique

N.K.NagarajanRoll No: 11 MT 66

Assignment Presentation I

Vacuum Bag Moulding

This technique is widely used for high performance mouldings such as for aerospace applications.This vacuum consolidation method produces high-quality mouldings, with complete exclusion of air bubbles and improvement to the inner surface of the moulding, which is not in contact with the mould. The controlled curing conditions also improve quality and consistency and allow superior resin systems to be used, while opening the way to a more rapid cure with faster turn round of moulds.

Vacuum Bag Moulding

Vacuum bag moulding is a modification of hand lay-up, in which the lay-up (necessarily smaller) 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 mould. By increasing external pressure, a higher glass concentration can be obtained, as well as better adhesion between the layers/plies of laminate.

Vacuum Bag Moulding

The bag used in this method had two fold objectives.

It provides a means for removing volatile products during cure

It provides a means for the application of pressure of one atom which is adequate for some materials.

Vacuum Bag Moulding

The essential steps in this process areLayup, preparation of bleeder system, and the

baggaging operation. The required no of plies are cut into size and position in the mould.

Requirements for proper bagging are :Bag to be impervious to air pressureBag to uniformly apply cure pressure

Vacuum Bag molding

Vacuum Bag Moulding

Vacuum Bag Moulding

Vacuum Bag Moulding

Vacuum Bag Molding

Vacuum Bag Molding Advantages• as for hand-lamination, except– higher fibre volume fractions/lower void contents.– better fibre wet-out due to pressure/resin flow.– heavier fabrics can be wet-out.– volatile organic compounds are largely contained

during the curing stage.– the additional consolidation pressure can help the

reinforcement to conform to tight curvatures.– improved mechanical properties consequent upon

the higher fibre volume fraction.

Vacuum Bag Molding Disadvantages

• as for hand lamination, except– higher labour skills for the bagging stage.– low production rates due to bagging stage– joining bagging film for large items.– mould tool must be vacuum tight.– costs for consumables and equipment.– the vacuum pump may strip volatiles from resin– consumable material compatibility with resin

• consolidation pressure limited to 1 atm.

Compression Moulding

Compression moulding describes the process whereby a stack of pre-impregnated layers are compressed between a set matched dies using a powerful press, and then cured while under compression. This method is often used to manufacture small quantities of high-quality components such as crash helmets and bicycle frames.Due to the use of matched dies, the dimensional tolerances and mechanical properties of the finished component are extremely consistent. However, the requirement to trim the component after curing and the need for a large press means that this method is extremely expensive. Also, it is very difficult to make components where the plies drop off consistently within the component.

Compression Moulding Upper moveable mould Charge Lower fixed mould Ejector pin Compression moulding can be used to shape polymer matrix composites. The charge is placed between two steel platens and is heated and cured under pressure. This technique produces components with a high quality surface finish and also has a short cycle time, making it ideal for processes where a high throughput is required. For lower volume production, high machine costs may make this process prohibitively expensive.

Compression moulding• two matched (usually steel) mould halves

mounted in a (normally hydraulic) press• movement limited to one axis normal to the

plane of the mould• Unlike autoclave processes,

no consolidation pressure on vertical surfaces• near vertical surfaces

subject to wrinkling moulding

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Compression moulding: materials

Several materials suitable:• prepreg continuous fibres in epoxy resin • prepreg short fibres in polyester resin• prepreg short fibre in a thermoplastic matrix– continuous random orientation filament

glass mat thermoplastics (GMT)