Section 5 Additional Procedure for Sandwich Construction

501. Cores

1. Core material used in lamination structure of hull and exposed deck is to be of unicellular con- struction where it is a foam plastic.

2. The polystyrene foam is to be covered as a protection of resin containing styrene.

3. Where the cores which are composed of hard plastic foams are temporarily set by nails, care is to be taken to ensure that the cores are free from dent, misalignment and other defects due to nailing. And, no penetrating clearance not less than 1 mm is to be left between the cores.

4. Where balsas are used for cores, care is to be taken to have the balsas thoroughly impregnated with resins. The clearance between the balsas is, as a rule, not to be more than 4 mm.

502. Treatment of Cores

1. The surface of cores is to be properly treated in order to obtain sufficient bonded connection be- tween the FRP layer and the core.

2. The hull structures should not be of abrupt change of thickness or strength discontinuity between sandwich lamination and adjacent single plating lamination and the core should be reduced in thick- ness differences by 1/2 taper or less.

3. The connection of structural cores are to be of scarf connection or similar ones. In general, scarfs are not to not be steeper than a 12:1 taper. Scarf joints may be either ground or stepped and may be single or double taper. Where single taper scarf joints are proposed, a sealing laminate is to be provided, details of which are to be submitted. Where stepped joints are proposed care is to be taken to ensure that over-cutting does not occur. All joints are to be arranged so that they can be reinforced internally to maintain structural continuity of the laminate.

4. For non-structural cores made of seat material, the butt or seam of panel need not be bonded but should be crossed each other.

5. The inserts for structural material to withstand the crushing are to be fixed by using bolts and fix- ing steel and the inserts are to be sticked to the core and surface of sandwich.

6. Resin applied to the core material or its protective sheathing/coating should be compatible with its surface.

7. Embedded plywood should be of the waterproof and boilproof type and should have a surface that bonds easily to the resin or adhesive.

503.

Sandwich construction using female moulds

1. Core surface cavities and other irregularities should be removed or coated with filler, resin or sand- wich adhesive according to the material manufacturer's specification and depending on the following skin lay-up. When using scored core material, a sufficient amount of resin or adhesive should be used in the bond to fill the gaps.

2. When bonding core material to a wet laminate, sufficient resin should be in or on the laminate to achieve a bond between the laminate and core material without resin deficiency of the laminate.

3. The materials should be kept in contact while curing to ensure a structurally sufficient bond and to avoid air entrapment.

4. Deviations from these procedures may be made, provided that the structural requirements of ISO 12215- 5 are met.

504. Sandwich construction using male moulds

1. Joints, scores and voids in the core material should be filled or fixed to each other before the skin laminate is applied.

2. When laying the core material, it should not be bent or deformed to such an extent that the prop- erties of the core are adversely affected.

3. Irregularities on the core surface and the joints should be removed.

4. The core surface should be primed where required before the laminate is applied.

505. Laminating of Sandwich Constructions.

1. Efficient bond is to be obtained between the skin laminates and the core and between the in- dividual core elements. Approved tools for cutting, grinding etc. of various types of core material should be specified in the production procedure. The bond is to be verified by shear or tensile testing.

2. All joints completely

between skin laminates and core and between the individual core elements are to be filled with resin, glue or filler material.

3. Core materials with open cells in the surface, should normally be impregnated with resin before it is applied to a wet laminate or before lamination on the core is commenced.

4. When the core is applied manually to a wet laminate the surface should be reinforced with a chopped strand mat of 450 gÕmĪ in plane surface and 600 gÕmĪ in curved surface. If vacuum is ap- plied for core bonding the reinforcement type in the laminate surface may be considered in each individual case.

5. The reinforcement next to the core is normally to be a chopped strand mat of at least 300 gÕmĪ. A lighter mat may be accepted provided proper bond is documented by testing.

6. When a prefabricated skin laminate is glued to a sandwich core, measures are to be taken to evac- uate air from the surface between skin and core.

7. The core material is to be free from dust and other contaminations before the skin laminates are applied or core elements are glued together.

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

9. 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.

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