Section 4 Additional procedures for sandwich construction

4.1.1 The methods used in sandwich construction are, in general, to be either wet or dry core bonding techniques or by laminating directly onto the core (e.g. plug moulding).

4.2.1 The forefoot and stem of all craft of composite construction are to be moulded as required by Pt 8, Ch 3, 5.11 Forefoot and stem 5.11.1.

4.2.2 Where the core material is to be laid onto a pre-moulded skin, it is to be laid as soon as practicable after the laminate cure has passed the exothermic stage.

4.2.3 Where the core is applied to a laminated surface, particular care is to be taken to ensure that a uniform bond is obtained. Where a core is to be applied to an uneven surface, the Surveyor may request additional building up of the surface or contouring of the core to suit.

4.2.4 Where other than epoxy resins are being used, the reinforcement against either side of the core is to be of the chopped strand mat type. Additional flow coating is not to be applied to the foam core prior to laminating.

4.2.5 The submitted plans are to clearly show the staggering of successive plies in both the transverse and longitudinal directions. In general laminates are to be staggered by 50 mm per layer of reinforcement. Where very thin sandwich skins are adopted the rate of laminate stagger will be individually considered.

4.2.6 Prior to bonding, the core is to be cleaned and primed (sealed) in accordance with the manufacturer's recommendations. The primer is to be allowed to cure and is not to inhibit the subsequent cure of the materials contained within the manufacturer's recommended bonding process. The primer is to seal the panels, including all the surfaces between the blocks of contoured material, without completely filling the surface cells.

4.2.7 Where panels of rigid core material are to be used then dry vacuum bagging techniques are, in general, to be adopted. The core is to be prepared by providing `breather' holes to ensure efficient removal of air under the core. Bonding paste is to be visible at such breather holes after vacuum bagging. The number and pitch of such `breather' holes is to be in accordance with the core manufacturer's application procedure and any specific requirements of the core bonding paste manufacturer, see also Pt 8, Ch 2, 4.4 Vacuum bagging 4.4.3.

4.2.8 Thermoforming of core materials is to be carried out in accordance with the manufacturer's recommendations. Maximum temperature limits are to be strictly observed.

4.2.9 Where panels of contourable core material are to be used it is necessary to ensure that the core is cut/scored through the entire thickness such that the panels will conform to the desired shape of the moulding. The Builder is to demonstrate that the quantity of bonding material indicated in the core manufacturer's application procedure (see Pt 8, Ch 2, 2.11 Fibre reinforcements 2.11.2) is sufficient to penetrate the full depth of the core between the blocks. It is recommended that grid scored panels using a carrier scrim cloth are adopted.

4.2.10 Where the edges of a panel are to be bevelled to single skin the rate of tapering is to be not greater than 30^o. In areas where an insert (e.g. higher density foam or plywood) is to be used the rate of taper is not to be greater than 45^o.

4.2.11 In all application procedures cured, excess bonding material is to be removed and the panel cleaned and primed prior to the lamination of the final sandwich skin.

4.3.1 Backing or insert pads where fitted in way of the attachment of fittings are to be arranged so that the load can be satisfactorily transmitted into the surrounding structure. The contact area of these pads is to be suitably prepared and free from contamination.

4.3.2 Inserts in sandwich laminates are to be of a material capable of resisting crushing. Inserts are to be well bonded to the core material and to the laminate skins in strict accordance with the approved plans.

4.3.3 Where plywood inserts are to be used all edges are to be bevelled at an angle of 45°. A small gap is to be provided around each insert to ensure the passage of bonding paste during the vacuum bagging process.

4.4.1 Where wet vacuum bagging is proposed (with or without a core), full details are to be submitted for consideration.

4.4.2 The Builder is to demonstrate by visual inspection that efficient core bonding can be obtained using the proposed dry vacuum bagging process.

4.4.3 The number, size and distribution of breather holes in panels of rigid core material is to be that recommended in the core manufacturer's application procedure, see Pt 8, Ch 2, 4.2 Laminating 4.2.6. Typically, 3 mm diameter breather holes are to be provided at 50 mm centres.

4.4.4 The level of vacuum applied for initial consolidation and during the cure period is not to be higher than that recommended by the relevant manufacturer of the materials being used, to avoid the possibility of evaporative boiling and excessive loss of monomer.