The field of sports and leisure equipment is one where large improvements in performance have been made through the development of advanced materials. This is demonstrated well in this example, where we will be looking at the materials and construction of a modern windsurfing board.
As with all materials selection and design processes, the first step is to decide what properties we want from our component. Make a list of all these properties and discuss how you might try to achieve these with different materials.
The other important factor that needs considering in this case is how our board is to be made. The manufacturing process can have a large influence on which materials we should use and what properties we end up with. For instance, we might like to :
a) make our board from the very best materials. This, however, normally involves complex and expensive manufacturing processes, often including lab our intensive methods. This is therefore reserved for the top end of the market. or :
b) ensure that our manufacturing process does not increase the cost of the board too much. By doing this, we often have to limit our choice of materials which can compromise performance to some extent and so this forms the lower, mass produced end of the market.
On the following pages, you will see how the manufacture of modern boards illustrates these points
PROPERTIES NEEDED FOR WINDSURFING BOARDS:
a) Buoyancy, therefore low density or hollow materials are needed.
b) Stiffness; to be rigid enough to stand on without bending.
c) Strength, so as not to break, particularly with impacts due to waves.
d) A smooth surface, to reduce friction against the water and to enable painting.
e) A hard surface, to resist denting by small impacts.
In order to reduce weight, polymers and composites are the most widely used materials.
Carbon fiber composite is the optimum material, but performance almost as good at lower cost can be achieved by combining the advantages of different materials in specific locations. In most of these cases, the inner part is made from polymer foam to reduce weight and an outer casing of a fiber reinforced composite for strength and stiffness. Details on both of these types of materials are given later.
These are hollow boards with an outer casing of carbon fiber reinforced plastic. This gives the best combination of buoyancy, stiffness and strength. However, not only is carbon fiber expensive, but so are the manufacturing methods (which are a closely guarded secret) and so these are definitely the top of the range boards.
Hand made custom boards are intermediate in quality and price. They combine several different materials to give the desired properties. The inner core is a polymer foam (polyurethane) that is cut to the desired shape.
An aluminum honeycombe is then pressed into the deck to prevent denting and carbon fiber strips are laid along the length of the board to provide stiffness. This is then covered with glass fiber reinforced plastic to provide strength and a thin plastic outer layer to give a good surface finish.
Mass produced boards are made from a polyethylene moulded shell that is injected with polyurethane foam. This gives boards that are lower in stiffness and strength, but are significantly cheaper and thus form the low end of the market.
Although polymers are relatively light materials, they can be made even lighter by being made into the form of a foam. Such foams are used in many applications such as furnishings, crash helmets, packaging as well as windsurfing boards.
They are made using a ‘blowing agent’ to produce the bubbles in the foam. These blowing agents are normally volatile liquids that are added to the polymer before moulding. At the higher temperatures in the moulding process, when the polymer is molten, the liquid evaporates, forming gas bubbles.
The polymer solidifies before the gas changes back to a liquid and so the foam structure remains. Early blowing agents were CFC’s but these have now been replaced with hydrocarbons due to environmental concerns.
Polymer foam can be supplied in the form of blocks to be cut to the required shape, as a moulded article or in unreacted liquid form that can be injected into hollow spaces (e.g. cavity wall insulation). The density of these foams ranges from 250 kg/m3 to 750 kg/m3.
You might like to calculate how much foam (of density 500 kg/m3) is needed to support your own weight in the water (density 1000 kg/m3).
FIBER REINFORCED POLYMERS:
Composite materials with high stiffness and strength can be made by incorporating fibers into a polymer material. This gives a combination of the strength and stiffness of the fibers with the toughness and ease of processing of the polymers.
The most common fibers used are glass, though more expensive fibers such as carbon (higher strength and stiffness) and Kevlar (for better toughness) are also often used. for instance, for a windsurf board designed for racing on calm lakes (where rigidity is important), carbon fiber would be used, whereas for a board to be used in rough waves (where resistance to impact is more important), Kevlar fibers would be used.Number of View :11762