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7.1 FRP Applied to Core Material

FRP can be used other than with a mould. For example, if an owner required a vessel with a non-standard deck layout, his requirements could be accommodated by moulding that part of the deck which remains standard and using it as a foundation to which new structures could be attached either in wood or FRP. On a simpler level, fish tanks, hatches, ice containers, etc., can all be made by coating a plywood or plastic foam core with resin and reinforcement. Balsa wood can also be used when it is prepared for such a purpose but is more prone to water absorption than other cores.

The requirements for use as a core are that the material have a surface to which the FRP will form a mechanical bond, and that the core material is dimensionally stable so that it will not distort and consequently throw off the FRP sheath. New plywood is the only form of wood which can be expected to give a reasonably sound bond which will last. The bond is mechanical (gluing effect between two dissimilar materials) rather than a chemical bond which implies a joining of the materials on a molecular level. To assist the bond, surfaces should be roughened so that resin can grip as well as possible. Ply and balsa should be pre-coated with an unreinforced resin to which the reinforced layers may obtain a good bond.

Corner joints may be simplified by butt jointing and coating the internal and external angles with a single layer of glass tape. This is then left to cure. Such a joint will hold sheets of ply and foam in shape while the main reinforcing sheath is applied. In lightweight construction it may be strong enough to be all that is necessary. Another material to which FRP forms a good and reliable bond is PVC in solid form e.g. PVC pipe. This property is useful when making tanks and the like, as PVC pipes and ducts, etc., can be bonded directly to the FRP.

The construction of on-board insulated fish containers for canoes is a good example of a small production run where the insulating material itself (sheet polyurethane foam) has been bonded together to give the basic container shape and then coated with a protecting and strengthening FRP skin.

7.2 Making a Simple Plug and Mould

Simple moulds may include small portable moulds for items such as buckets, hatch covers, chairs or fish boxes. The simplest mould is a flat panel, FRP mouldings from which can be used for a customized wheelhouse for example. Any mould:

The plug should always be regarded as disposable and only to be used once; so repair work to cover any blemishes made during construction is governed by smoothness of the finish and not by any visual or long lasting quality, e.g., a little resin putty would grip slotted screws long enough for the mould to be made. When it has been made smooth the whole unit can be given 3 coats of wax and a coat of PVA (Poly Vinyl Acetate) release agent before the gelcoat is applied.

By convention, plug and mould gelcoat is usually black and is a harder finished surface than hull gelcoat. This is because it is subject to repeated overcoating, but does not have to be as pliable. Once the mould gelcoat is cured, a surface tissue is used to strengthen it and then FRP is built up to twice the thickness of the moulding to ensure rigidity. It is usually CSM only to remove the risk of roving print being transferred permanently to the mould surface. After the FRP laminate has been completed and has cured beyond the gel stage, stiffening can be bonded to the outside before release from the plug. This may include some legs to raise it off the floor. The stiffening can be of wood, foam or rolled newspaper to give a former for the FRP framing. The whole assembly should then be left for up to 3 weeks to become fully cured before release. Small wooden wedges gently hammered between the mould and plug should achieve release, a few blows to the flat areas with a rubber mallet can also help on occasion. Once the plug has been removed, the mould can be inspected for flaws and damage and any repairs made and blemishes removed before it is polished with 5–7 coats of wax to render it serviceable (see simple mould - Figure 22).

Figure 19

Figure 19 Insulated FRP/foam fish container

Figure 20

Figure 20 Free standing insulated container

Some moulds need waxing each time they are used, high quality moulds may need waxing after every third cycle. A wax build up shows as a dull sticky area and should be washed off.

Figure 21

Figure 21 Release angles

7.3 FRP Practice Panel

This exercise allows practice in basic skills such as gelcoating, laminating and repairs.

  1. Obtain flat sheet 800 mm × 1 000 mm which will be the mould for the exercise. Formica covered plywood is best but hardboard or other smooth surface will do. Polish surface with silicone-free wax.

  2. Apply white gel coat to one half of surface. Use masking tape to give a straight line, remove it during gel stage and apply clear gelcoat to other half.

  3. Check for flaws when cured and laminate one layer 300 g CSM on top.

  4. Check for air bubbles, sand smooth if necessary and laminate one layer 300 g CSM and one layer woven cloth AT SAME TIME. Use rollers.

  5. Repeat checks and judge whether resin ratio is correct by sheen. Laminate one layer 450 g CSM AT SAME TIME as one layer 800 g WR using squeegee. All edges should be trimmed with a sharp knife to shape of mould when each layer is at the gel stage.

  6. Leave for at least 24 hr and release panel from mould using wooden wedges. Trim edges by hand sanding.

  7. Cut panel to 800 mm × 800 mm, using jigsaw with metal cutting blades or padsaw with hacksaw blade.

  8. On non gelcoated side scribe lines at 150 mm from two opposite edges so that scribed lines cross gelcoat divide line at rightangles.

  9. On one line place sheet of newspaper rolled to 30 mm diameter and hold in place with 2 – 4 strips of masking tape.

  10. Sand offcut to remove gloss from both faces and place on-edge on other line. Prop in place and cement with dabs of resin putty.

  11. Along gelcoat join line (seen from working side) place dabs of resin putty and bed-on foam stiffener (cross section shown below). Arrangement now provides working practice for two types of panel stiffener and panel-to-panel bonding.

  12. Work on foam former first. Coat with resin. When cured, prepare piece of CSM to cover foam and give 25 mm overlap onto FRP panel. When cured roughen surface with sandpaper and prepare CSM and woven cloth to give overlap 25 mm beyond first overlap. Repeat with CSM and WR.

  13. Vertical web. Round off internal angle at panel/web joint with resin putty. The radius is obtained by running the rounded end of flat stick through the putty. Reinforce joint both sides with CSM only, first layer 5 cm width, second 10 cm, third 15 cm.

  14. The paper stiffener cannot be expected to be as resilient as the first two. It is included to demonstrate the non-structural nature of cores and the use of paper when a stiffener is needed urgently. The first problem occurs in that being of a circular nature, the lower half of the paper tube cannot provide any support for wet reinforcement. To overcome this, a tailored piece of CSM should be wet out on top of a corresponding piece of woven cloth away from the tube, then carefully laid on the tube and gently smoothed out to remove air bubbles. To prepare the WC/CSM lay the cloth down first with the CSM on top and wet it out. To apply it, the cloth should be uppermost to facilitate smoothing out probably by hand. Continue lay up when first layer is hard.

  15. There should now be two areas above and below the crossbar of the “H” formed by the framing. These are for practicing repair techniques. One with a white gelcoat and one with clear. Holes about 150 mm dia should be made in these areas and conventional and blind hole repair techniques practiced according to methods described in Chapter II. An effective method to obtain holes is for the student to attempt to punch a hole through the laminate with a hammer. This will adequately demonstrate the strength of his creation. If a hole is not obtained the cracked area can be cut out to give an asymmetric hole. Impact testing of the bonded web can also be carried out.

Figure 22

Figure 22 Simple mould

Figure 23

Figure 23 Practice panel showing working face and gelcoat face

Figure 24

Figure 24 Panel with framing reinforced and hole repaired

Figure 25

Figure 25 Repairing one-side access hole

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