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CHAPTER-5

FABRICATION METHODS INVESTIGATIONS

5.1 Fabrication techniques of composite materials

Various processes are available for making composite materials. The different

processes available for the fabrication of fiber-reinforced composites are

1) Hand lay-up

2) Vacuum bag moulding

3) Pressure bag moulding

4) Autoclave moulding:

5.1.1 Hand lay-up

Hand lay-up is the simplest process for making the composites laminates. The

selected fibers are wetted with resin and placed in the mould and entrapped air is removed

with rollers. Layers of glass and resin are added to build up to desired thickness and it is

normally allowed to cure at room temperature.

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Figure 5.1: Hand lay-up process

5.1.2 Vacuum bag moulding

In this method vacuum is used to eliminate voids and force out entrapped air and

excess resin. The component is first lay up in the mould with the resin, over the layers. A

series of bleeders is placed, to provide a permeable space between lay-up and bag for

escape of evacuated air. A suitable sealing material such as cellophane of nylon is placed

over the lay-up and sealed at the edges. Vacuum is draw-in on the bag formed by the

film, and a laminate is formed. In this technique pressure less than atmosphere is

possible.

5.1.3 Pressure bag moulding

In pressure bag moulding, usually a rubber bag is placed over the lay-up and

then at the pressure is applied to eliminate voids; force out entrapped air and squeezes

the excess resin. In pressure bag moulding higher pressure of the order of 100-mpa are

possible. Laminate with better mechanical properties are obtained.

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5.1.4 Autoclave moulding

Autoclave moulding is similar to vacuum bag moulding and it is a modification

of pressure bag moulding. Autoclave moulding refers to the process of lying of

reinforcing materials and resin matrix in required shape and quantity in suitable open

moulds and effecting the polymerization of the product with simultaneous application

of pressure, heat and vacuum. The entire operation is carried out in special equipment

called autoclave, which is essentially a pressure vessel with heating and evacuating

equipment.The autoclave moulding can be employed where,Large contoured, odd

shaped parts are moulded.Preparation of moulds or dies are difficult to make or

expensive in construction.

5.2 Method adopted for fabrication

Usually the laminates can be prepared by lay-up techniques. But, the laminates

produced will have voids, cracks and may delaminate easily, applying pressure or

applying vacuum can overcome this. This can be achieved through by bagging the

laminates. This process is called vacuum bagging. A vacuum bag provides both pressure

up to 14.7psi, depending on your altitude and vacuum.

5.3 Accessories

5.3.1 Peel ply

One the laminate is in place; its time to apply the bag the first item to go down is a

peel ply. Peel plies are a tightly woven fabric, often nylon, and impregnated with some

type of release agent. The peel ply will stick to the laminate, but it will pull away without

to much difficulty.

Peel ply is optional. Most often it is used to give the laminate a rough, rather than

smooth, finish. Many engineers consider this a bondable finish, and it usually passes a

wet out test.

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Figure 5.2: Modeling methods and tooling

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5.3.2 Release film

After the peel ply comes a layer of release film. This is a thin plastic, which has

been treated so it won’t bond to the laminate. It is highly stretchable so it can conform to

complex geometries.

Peel ply can be either a solid sheet, or it can have perforations (in the latter case, it

is often called peel ply). The perforations might be like pin pricks, or they might be small

holes, which are punched out. The spacing can also vary from 2 inches to 8 inches.

Choose spacing based on the amount of resin that needs to be bled out: wet lay-ups can

use close spacing; prepared manufactures can recommend spacing for their particular

products; and net resin systems of course use imperforated release films.

Not all release films are compatible with every resin system. a few years ago, they

were preparing some cyan ate-ester test coupons, and the release film we normally used

for epoxies bonded to the coupons. You can also get release film treated so it will bond to

the laminate (bondable one side, bos, or bondable both sides, bbs). Bos can be used to

create a permanent release layer on composites tools, or as a moisture barrier on

laminates.