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Positive moulds and boating The usual proceeding to realize the mould for hulls (bottom) of a boat provides the building of a POSITIVE model, on which they create the negative mould, later used to give shape to the boat hull. The first step of this proceeding consists of gluing polystyrene blocks (EPS) to obtain a gross structure of what will be the volume of the bottom. Subsequently blocks are milled to obtain an almost definite shape; then in order to give structure to the model, called armour, they are covered with a fiberglass mat and soaked with epoxy resin (or vinylester resin, but with low styrol content in order not to affect the polystyrene below); this armour gives stability, resistance and avoids flexions. On this armour the common method provides that a thixotropic, polyurethane or epoxy paste of density 700 kg/m3 is distributed. In order to be smeared, paste needs a machine which is quite expensive as it must be equipped with a force pump; frequently yards cannot equip themselves and they resort to external “services” equipped with professional machines and operators. By the customer’s yard, external service teams smear 3-4 centimeters of product so that the whole structure is covered. After approximately 24-36 hours, paste can be milled with a 5 axys milling machine, that gives the exact shape of the desired hull, and can be covered with putty and gelcoat, and then painting and polishing. The obtained positive model is then covered with detaching agent and used for a successive fiberglass stratification, that will be the shell of the mould. The obtained mould, reinforced with a metal structure, is very resistant and can be reused many times, but it is easily guessed that times and costs of such a method are very high, both because of the costs of the products and of the required manpower. Polyurethane foam innovation It is exactly in the moment where external service is required that the last innovation of DUNA-Corradini enters into the field; DUNA-Corradini recently supplied some of the most known Italian boat builders with a spray polyurethane system to be sprayed replacing paste. First trials of this solution foresaw a spray polyurethane of density 160 kg/m3, afterwards abandoned because the density difference with the polyester putty made a uniform and planar sandpapering very difficult. DUNA-Corradini spa introduced then the DUNAPOL™ T 501 (polyurethane system with high density, as pastes and putties), that can be distributed with a normal spray machine similar to the ones used in building field, and much less expensive than the ones with force pumps required by paste. Yards can equip themselves with a much cheaper machine, without the need to resort to external services. Spraying of a layer of 2/3 centimetres of polyurethane gave very positive results, as the following advantages have been found: • Sensible reduction of the waste quantity in comparison to the use of paste.
• Reduced cost of the application in comparison to service. The economy is given by the possibility of realizing work internally.
• Reduced cost of DUNAPOL™T 501
• Release from service supplier and work according ties and methods decided by the yard.
• Possibility of realizing small pieces.
• Application speed (max 8 hours versus 2/3 days of the paste)
• Excellent behaviour of DUNAPOL™T 501, that originates chip and not powder.
• Possibility of equipping with a complete solution (product, machine and assistance. Product in fact can be applied with a machine developed by DUNA-Corradini specifically for this application
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