Other TechnologiesTechnologies for manufacturing composite material elements vary according to the shape, size and characteristics required for the finished piece. According to the characteristics of a composite element and whether it must be reproduced or produced continuously, the technologies involved can be divided into those that use closed or open moulds; these can be further divided into continuous or discontinuous, manual or automatic.
THERE ARE SEVERAL TYPES OF PRODUCTION PROCESSES FOR COMPOSITE MATERIALS, THE MOST COMMON ARE:Manual impregnation without applying pressure or vacuum
This process is still widely used for large surfaces such as swimming pools and boat shells for which production is typically in small batches, and it is the most popular process for civil engineering. Reinforcements in the form of mats or fabric in the percentage laid down in the project, are draped inside a mould which, for civil engineering applications comprises a masonry or reinforced concrete support; the fibers are then soaked in catalyzed resin and consolidated manually using metal or plastic rollers to eliminate excess resin. Polymerization usually takes place at ambient temperature.
In some cases, to improve the quality of the pressed laminate the fabric is impregnated with a special tool before being put in place, so that the amount of resin for each layer is the ideal, as far as possible.
RTM resin transfer molding
This is a technology used to manufacture composites with a polymer matrix by injecting the catalyzed resin into a cavity in the shape of the part to be created which has been pre-lined with a dry reinforcement. The cavity is obtained by closing the mould and punch against each other. They can be of several types, the most common are metal or fiberglass moulds.
RTM manufacturing is divided into the following steps:
1. mould cleaning
2. application of release agent
3. application of gelcoat
4. positioning of reinforcement
5. mould closing and locking
6. resin injection and polymerization
7. mould opening and removal of piece
8. finishing operations
Resin transfer molding is extremely popular due to the automation possibilities it offers, facilitating mass production of components. It is also possible to mould very large elements.
Resin Infusion Under Flexible Tooling (RIFT)
Resin Infusion Under Flexible Tooling, a variation of RTM, is carried out using a flexible polymer bag in the place of one of the faces of the rigid mould. This is more economical and the vacuum inside pushes the resin into the dry reinforcement, reducing the operator’s contact with liquid resin and all the volatile substances emitted during the preparation process.
For low production volumes alternative resins such as epoxy or polyester with low styrene content are used. In the short term this may be an effective solution, but over the long term the process must be re-designed to acquire the advantages of lower cost systems and reduce injury hazards.
As mentioned above, RIFT is different from RTM mainly due to the use of a flexible polymer film instead of a rigid mould. The reinforcement is debulked by a vacuum created in the special valves, the resin enters through other valves and under the effect of external atmospheric pressure is carried through the distributors into the reinforcement. The flow can impregnate in two different ways: one directs the flow at right angle to the disposition plane of the reinforcement, the other employs mixed impregnation with two flows, one at right angle and one parallel to the disposition plane similar to that of RTM.
Fundamentally this process comprises winding continuous filaments impregnated with resin onto a rotating spindle, the shape of which is that of the piece involved. The resin is set by inserting the component into a furnace or autoclave.
The main essential factors in this technology are:
- type of winding;
- type of impregnation;
- type of spindle;
- type of machine;
- type of polymerization process.
The meaning of the word pultrusion is clear if we think of the basic technological scheme for this process. Whereas extrusion of aluminium or thermoplastics is performed by pushing the material through a mould, with reinforced plastics the same shape can be achieved by exerting a pulling force on the fibers so that they are forced through the mould after they have been soaked in the resin. Therefore, the pushing action typical of extrusion is replaced by a pulling action, which gives us the compound word pultrusion.
Pultrusion features continuity in production; when the system has an automatic follow-up cropper production requires minimum human intervention, reduced to starting up and checking for any breaks in the reinforcement input and the amount of resin in the impregnation tank. Good resistance to pushing and the high percentage of reinforcement possible, together with other important properties such as electric insulation, resistance to corrosion and light weight have widened the range of products to include insulator bars, catwalks, platforms and parapets, stairs, tubes for switches and fuses, motorway barriers, structural beams, and many more besides. The process requires essentially continuous fiber reinforcement and low viscosity resin, usually a thermosetting liquid. The most popular reinforcement is glass roving, as well as carbon fiber. The basic process is:
1. reinforcement feed-in
4. shaping and polymerization;
7. post forming.
Technologies that involve impregnating dry fibers, such as, for example, pultrusion, filament winding or pull winding/braiding, generally have very high production rates but lower structural performance than those which use pre-pregs such as vacuum bag and wrapping technologies and relative cure cycle in autoclave.