Composites are quite distinct in their manufacturing process, since from the primary materials we get the final product in one step. It is a sharp contrast to the steel or aluminium construction, where we have the original plate that needs cutting, forming and welding in order to get the block assembly, before the block. There are indeed pre-pegged reinforcement sheets, that resemble slightly the metal plates or prefabricated stiffeners, to be attached to the shell structure, but predominantly the majority of composite products are cut, shaped and glued at the same stage.
In respect of the structure itself, it can either be single skin or sandwich panels. As single skin we refer to the plate that is solely made by the resin and fibre reinforcements, i.e. several layers of roving or chopped strand mat depending the laminate prescribed in the scantlings. In the sandwich panel there is a core material in-between two single skins, which increases the rigidity of the structure, by adding to the inertia of the plate. Sandwich laminate can result to a very light plating, depending on the core material used, with strong qualities. Yet it is more prone to de-lamination, there is the possibility of core dysfunction depending on the material used and is it not supporting penetrations or connections with other parts. Generally below the waterline, single skin laminate is preferred, as well as around bolting arrangements, while sandwich is ideal for bulkheads, decks and superstructures with added benefits like noise absorption or insulation for interior climate conditions.
The shape of the finished product is determined by the mould, or moulds in the case of larger items that won’t be defined in one piece. The necessary precursor of the mould is the plug, very simply a sculpture of the finished product made by a solid material and necessary considerations for the mould itself, like tapering that facilitates the separation of the mould with the finished product. It is made of wood, foam, MDF, clay or balsa and when it is made by porous materials, like the first two listed, it needs to be further sealed with resin or primer. It is very important to have a finished surface equivalent, if not better, with the one of the finished product. There are CNC machines that make these plugs, but it is not uncommon to have carpenters prepare the same. When it is a large item, it is solid and requires extra reinforcements to ensure that it stays in one piece till the completion of the mould. As you may by now realise, it is made to be discarded as soon as we create our mould. Therefore, and always in connection with the material used, it may all end up in the landfill or at best be reused in another application.
So we are at the point where we will be creating the mould selecting between a male, female or compression mould, the latter being a combination of the two former types. The male mould can be made very easily, it won’t even require the plug, but it is not for many production pieces, somewhere in the range of 5 to 10 pieces. Also the finished product will require additional work in way of the finished surface, that will never reach the respective surface made from a female mould. On the other hand the female mould is very costly and can’t justify the investment with less than 10 pieces of the finished product. The compression moulds are required for very finite components, with high end finish and price.
The moulds are made similarly to the finished product, with a main differentiation point that they are about 5 times thicker than the finished product, in order to be able to hold together through the numerous applications and support the separation process. For their longevity they also require extra stiffening from composites and in some cases steel that also assist in the transport and storage. As their fabrication emulates the end product, they will be investigated together.
By now we’ve only established the foundation of our structure, with very little reference on the environmental impact, but it is not possible to explain the consequences unless we set the backbone principles. In the following post, items like waste, energy consumption and recycling will be described.