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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