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Wednesday, December 14, 2011

Type of FRC Moulds : Opened Mould

In this type of mould, only one mould surface is used in open mould process. This single mould represents either the positive (male plug) or negative (female cavity) surface as shown in image below.

Types of open mould: (a) Positive and (b) Negative 
In order to produce large fibre reinforced plastic and composite components and structures, (for instance swimming pools, boat hulls, etc.) very large moulds are usually used. The main matrix materials used are thermosetting resins of epoxy and polyester, while E-glass fibres are the most widely used reinforcement material. Depending on the desired thickness, matrix resins and reinforcement fibres are applied to the mould surface layer by layer. The fibre reinforcement can be used in the form of mats, woven roving or yarns. The use of prepregs may simplify the laying process. After the lay-up process, curing treatment will be necessary for rigid thermoset matrices. Depending on the type of resin used, little or no pressure will be necessary during curing.

Open mould processes have several advantages over closed mould composite manufacturing processes. Since a single mould is used in open mould processes, mould costs will be much less than using two moulds in the closed mould processes.

Another advantage is that very large and complex structures of fibre reinforced plastic and composite, may be produced in open mould processes, which is difficult in closed mould processes. Depending on the component to be produced, a variety of materials (e.g., metals, plaster, woods, fibre reinforced composites) are available for cheaper open moulds, whereas expensive metallic moulds should be designed for closed mould processes. Therefore, it may be concluded that open mould processes have better design flexibility compared with closed mould processes. However, open mould processes have some disadvantages as well. Firstly, only one surface of the product will be finished and smooth. This is because the other surface will be not in contact with the open mould surface. Moreover, to achieve a good surface finish on at least one surface of the component, the surface of the open mould must also be very smooth.

The second disadvantage of open mould processes is that they are very labor intensive. Therefore, for the production of components with higher quality, the personnel working in the process should be adequately skilled. There have been many advances in the automation of open mould processes, which helps to solve the skilled personnel problem. Automation in open mould processes is increasing not only the quality of the product, but also the number of the parts manufactured per unit time. Another disadvantage of open mould processes is the much longer curing periods required compared with other methods. Normally, application of heat will decrease curing time. However, it is difficult to heat treat components which are very large. Open mould processes are usually classified according to the methods of resin and reinforcement application to the mould, or according to the curing methods. If the matrix and reinforcement is applied by hand, then it is named hand lay-up, if it is by a spray gun, then it is called spray-up. Similarly, if the curing is accomplished in a bag, then it is called bag moulding, if it is performed in an autoclave, then it is termed autoclave moulding, etc. However, in order to use the advantages of each method, generally two or more of these methods are combined during manufacturing.


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