Composites - An Overview
Press moulding is a mass production technique of processing Fibreglass Reinforced Plastics. It can be used to manufacture complex parts to close tolerances with a good finish on both sides. It incorporates matched metal moulds between which the material is cured at elevated temperatures and high pressures, usually applied by hydraulic press. Although equipment and mould costs are higher than in contact moulding, the added expense can be justified by the larger number of parts which can be produced in a shorter length of time, by the weight saving due to the higher strength of the composite thus produced and by being able to achive a smooth and consistent finish.
When glass is drawn into fine strands its strength increases considerably over that of bulk glass. There are two types of glass which are used for Fibreglass manufacture. These are "E"glass, which is a low alkali-containing borosilicate glass, and"A" glass, a high alkali content material with a composition similar to that of window glass. From the composition and general properties of these two glasses, it can be seen that "E" glass is the stronger and this is the material usually specified for reinforcement purposes,unless the operating stresses are relatively low.
The Fibreglass reinforcement is supplied in a number of forms,
which are designed for use by the fabricator of reinforced plastics components. These include random reinforcement materials known as chopped strand mats and needled mats, bi-directional materials such as woven rovings and glass fabrics, and rovings which are used for chopping into random lengths or as high strength directional reinforcements.
Resin is an essential component of Fibreglass Reinforced Plastics. The most popular is polyester resin, which can be cured to a solid either by catalyst and heat, or catalyst and accelerators can be used at room temperature. The ability of polyester resin to cure at room temperature into a hard material is one of the main reasons for the growth of the reinforced plastics industry. It was this which led to the development of the room temperature contact moulding methods which permit production of extremely large integral units. Epoxide resin is also used for applications where its greater cost is offset by desired properties. Both polyester and epoxide resins, by suitable choice of the basic resin and the method of cure, can be made to give a very wide range of physical, mechanical, chemical and electrical properties.
Where particular properties are required, it is recommended that the resin manufacturers are approached for their advice on the best system to use.
Furane resins are included because they have excellent chemical resistance. They can be used in conjunction with Fibreglass materials, but it is more usual to incorporate Fibreglass reinforced polyester resins as the backing material for a surface of Fibreglass reinforced furane.
Polyvinyl chloride and polypropylene are extensively used in chemical plant. Their use is frequently limited by their low mechanical strength rather than their chemical resistance. There is a growing use of these materials in conjuction with Fibreglass polyester laminates where the latter do not have the chemical resistance required but can contribute strength. The P.V.C. or polypropylene is used as an inner skin which is backed and armoured by a Fibreglass/polyester laminate.
Fibreglass Reinforced Plastics are a family of materials, and not only does the type of Fibreglass reinforcement and resin used have an influence of the combined material, but their relative quantities also have a major effect.
By varying the properties of resin and Fibreglass reinforcement and controlling the orientation an extremely wide range of mechanical properties can be achieved. The strength and modulus of a laminate in any direction is, in fact, proportional to both the total amount of glass fibre that is present (by volume) and the proportion present in that particular direction. The Fibreglass provides the majority of the mechanical properties but the resin and Fibreglass are interdependent and it is not, therefore, possible to consider either of them separately. The quality of the adhesion of the resin to the glass fibre at the resin/glass interface is thus an extremely important factor in Xtra Reinforced Plastics Pvt.Ltd .
Strength is not the only factor which is important to the designer, but also the physical factors of thermal expansion, thermal conductively and heat resistance. The co-efficient of thermal expansion of glass is itself relatively low-less than most metals. However, when combined with resins which have a high co-efficient of expansion, the overall result is usually a co-efficient of expansion greater than that of metals. By selecting the correct glass content, it is possible to match the co-efficient of expansion of steel and this is sometimes done where it is necessary for both to expand at the same rate. Thermoplastics have high co-efficients of expansion and glass fibres can be a very important factor in stabilising them.
All plastics materials have relatively low thermal conductivity. Compared to metals, the conductivity of most plastics materials is only 1/500th of metals. This means that reinforced plastics incorporated in chemical plant has an inherent insulating effect. Their low thermal conductivity also makes possible at higher temperatures than would be expected. Where the heat is only applied from one side the steep temperature gradient across the laminate ensures that the outer portions of the component are well within the limiting temperature and, therefore, not seriously weakened.
The maximum service temperatures possible for these materials shows that temperature resistance is a problem with reinforced plastics components, although the presence of the Fibreglass helps considerably in stabilising and improving heat resistance. Where higher heat resistance is required than shown, it is sometimes possible to utilise phenolic or silicone resins to give improved performance.
The property of reinforced plastics of most interest to the chemical and plant engineer is their resistance to chemicals. This is one of the most difficult properties to define accurately. It is strongly recommended that either the resin manufacturers are consulted or practical trials are carried out to ensure that the material selected is satisfctory for the intended purpose. In most cases, at least one resin would be capable of withstanding completely each of the specified conditions at normal temperatures. In most cases elevated temperatures together with strong oxidants are the main factors influencing chemical resistance. It should ;be noted that 200'F is, in fact, outside the recommended operating temperature for both polyvinyl chloride and polyethylene and it is at these high temperatures that the thermosetting materials, particularly epoxide and furane, really come into their own. These figures illustrate why the combination of P.V.C. or polypropylene as the inner corrosion resistant skin backed by structural Fibreglass reinforced polyester is ;becoming more widespread, as the resistance to strong oxidants and strong acids of P.V.C. and polypropylene is considerably better than that of polyesters.
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