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Wednesday, December 28, 2011

Type of FRC Moulds : Matched-Mould

    Where matching moulds are required for processes such as resin injection, foam reservoir moulding or cold press moulding, a full size pattern of the final moulding is generally required. If this is a vaila ble the two halves of the mould can be made in a similar way to making a split mould but incorporating appropriate injection and vent tubes along the join between the two halves. The dictates of the process will govern which attachments are necessary. Locating dowels must also be accurately positioned.

    With thin mouldings an alternative procedure is to make a pattern of the outside of the moulding and then make the negative mould. This negative mould, after full cure, is then used as a base on which to construct a model of the moulding using sheets of pattern maker's wax. When the required thickness of wax has been applied the positive mould is constructed on top. During construction of the wax pattern due consideration must be given to the provision of drainage channels and vent/injection points. The positive mould must be accurately made, allowing for resin shrinkage, so that the mould cavity is of the correct size. An oversize mould will only waste expensive materials each time a moulding is made.

    For cold press moulding the back of the mould should be filled with a material capable of withstanding continuous loading in a press. One such material is concrete, although a filled resin system may also be used. Here, after the final layer of glass reinforcement has been applied to the mould, a further layer of resin is applied at a rate of about 400 gjm2, into which is sprinkled a layer of broken stone chips. After the resin has been fully cured the back of the mould is filled with the concrete or resin mix which bonds around these stone chips. Both halves of the mould should be similarly treated.

    Where fibre reinforced plastic (FRP) moulds are used for cold press moulding the moulding cannot be trimmed as part of the moulding cycle. To maintain pressure on the resin and prevent it from being squeezed out leaving air bubbles in the moulding, the mould should be constructed with a pinching area. During final closure of the mould this allows air to escape but retains the resin. For mouldings up to about 5 mm in thickness, the pinching area should be sufficient to compress two layers of glass mat in a gap of 0-4-0· 5 mm. For thicker mouldings the pinching area should accommodate three or four mat thicknesses. In addition, it is useful to incorporate a drainage channel into which surplus resin can drain. An example of such a design is shown in the image.

    When not in use, moulds should be stored flat to prevent distortion and protected from dust and moisture. In use, continuous scrutiny is necessary so that any imperfections which occur can be immediately rectified.

Sharp instruments must always be kept away from mould surfaces.

Properly treated, FRP moulds can give excellent service. 

Pinch Area for Cold Press Moulding

Wednesday, December 21, 2011

Type of FRC Moulds : Split Mould

     Where deep draw mouldings or ones with undercuts are to be produced which would be difficult or impossible to remove from a one-piece mould, split moulds can be used. Here a temporary barrier should be fitted to the pattern so that the first half of the mould can be made with a flange (image below). 

      The flange area should be about 50% thicker than the mould shell to ensure adequate life. The first half of the mould is left in place, the temporary barrier removed and the second half of the mould manufactured using the suitably released flange as former. A metal plate can then be laminated onto either side of the flange to assist in supporting the bolts used to clamp the flange halves together. Once the resin has cured, holes can be drilled to take fixing bolts. These should be spaced at about 150 mm intervals.

     The mould should not be removed from the pattern until all necessary work has been carried out. Mould release can be assisted by using compressed air carefully applied between the mould and the pattern. Release can also be assisted by filling the gap between the mould and the pattern with water to soften and dissolve the polyvinyl alcohol release agent. If the mould has to be struck in any way, extreme care should be taken to ensure that this does not result in star patterns forming in the gelcoat.

     Any imperfections in the mould surface can be removed by rubbing with fine abrasive such as grade 600 wet emery paper followed by a fine cutting paste or by using a metal polish. Before use, the mould surface must be thoroughly polished to a high gloss finish using a silicone-free wax polish applied in several thin coats.

Method of  Constructing a Split Mould
PVA Release Agent

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.