Making a Kayak through Rotational Moulding

The design and manufacture of kayaks have developed considerably through history from primitive, hand-made vessels to today’s sporting boats that are mass-produced and depend on a variety of modern technologies, including the application of heating elements.

Today, kayaking is an increasingly popular activity of choice for both leisure and sport and the kayak market has responded to demand by producing an ever developing array of boats suitable for coastal paddling, surfing, racing and manoeuvring on river rapids.

Polyethylene (often from recycled plastic sources) is the primary raw material used in the manufacturing process of a kayak. Polyethylene has a number of characteristics that make it ideal for the manufacture of kayaks; it is a tough, waxy-textured material that is unaffected by water and many chemicals, it can be repeatedly softened by heating, using heating elements, and it can be hardened by cooling. In addition to the shell, polyethylene is used to make the seats in a kayak.

The manufacturing process of the polyethylene kayak shell involves a process called rotational moulding. By applying the process of rotational moulding, the long, hollow shell of the kayak can be moulded in uniform thickness, either in once piece or as two separate pieces which can be later joined.

The process begins with a measured amount of the polyethylene plastic in granule or powder form (mixed with colouring agents) being loaded into the bottom of a two-piece, thin-walled aluminium mould for either the hull or the deck. The mould is then closed and the two halves secured tightly.

Next, a mechanical arm, which can rotate the mould in two directions at the same time, holds the mould securely and moves it to the heating station where an oven heats it to 480-840°F (250-450°C), using heating element technologies. As the heat from the mould melts the plastic, the mould is slowly rotated in two directions. Gravitational force causes the melted plastic to flow to the front and rear and up the sides of the rotating mould, eventually covering the entire surface with a uniformly thick skin of plastic. Continued heating then fuses the plastic into a solid layer that attaches itself to the mould.

After that, the mould is moved to be cooled in a chamber. By continuing to rotate the mould and gradually cooling it with carefully directed sprays of air and then water, the kayak hull or deck further hardens and shrinks away from the mould. Repeated cycles of heating and cooling may be required to form the kayak properly. After the final cooling, the hull or deck is removed from the mould. Once removed from the mould, the hull and deck are sealed together to create a single seam running horizontally along the length of the boat.

The final stage of the process involves the fitting of seat or seats to the kayak, alongwith other ancillary devices such as hatch covers, grab loops and drainage hole covers. The technique of rotational moulding using heating elements is ideal when manufacturing large hollow objects such as kayaks, and is also suitable for making items such as toys, furniture, sporting equipment, septic tanks and refuse bins.

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