Vacuum bag molding is a technique that uses vacuum pressure to remove air and excess resin from laminate layers, resulting in higher fiber volume fractions, better wet-out and adhesion between layers, and improved mechanical properties. It involves laying up plies, adding a bleeder system, bagging, and applying a vacuum to consolidate the laminate. Compression molding uses matched steel molds to compress and cure pre-impregnated layers under pressure, allowing high-quality finishes but requiring expensive presses and trimming. Both techniques can produce composites but vacuum bagging has lower costs while compression molding enables short cycle times and high throughput.

1. Vacuum Bag Moulding and
Compression Moulding Technique
N.K.Nagarajan
Roll No: 11 MT 66
Assignment Presentation I

2. 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.

3. 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.

4. 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.

5. Vacuum Bag Moulding
The essential steps in this process are
Layup, 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 pressure
Bag to uniformly apply cure pressure

6. Vacuum Bag molding

7. Vacuum Bag Moulding

8. Vacuum Bag Moulding

9. Vacuum Bag Moulding

22. Vacuum Bag Molding

23. 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.

24. 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.

25. 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.

26. 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.

27. 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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28. 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)