How Rotomoulded polyethylene kayaks are madeEdit
Reproduced from Kayak fishing in SE Queensland with the kind permision of the author Nick (redro) http://members.optusnet.com.au/aus-kayak-fishing/index.html
Kayak Construction by Nick M. Ever wondered how your poly kayak was manufactured? Think of it as much like making an Easter egg. The raw material is Linear Low Density Polyethylene. Colourless PE pellets are compounded with the desired colour Master Batch via an extrusion process. The coloured pellets are then ground into a fine powder on a mill (you can't rotomould pellets).
Rotomoulding is a low pressure, high temperature process for moulding large, hollow, one piece, plastic parts — such as big water tanks. It is very cost effective for short run/large parts in comparison with injection and blow moulding, where the mould costs (and run sizes) are relatively much higher. Both of the latter are high-pressure processes. Moulds for rotomoulding are fabricated from sheet metal or cast in aluminium and are manufactured in two halves.
The metal inserts are fixed to the mould surface before the powder is loaded — the PE will form around the insert during the moulding process. The graphics are also transferred onto the mould surface before loading the powder. Once all of this is done, one half of the mould is charged, at room temperature, with a predetermined quantity of powder. The other half of the mould is then clamped on tightly.
The mould then begins to rotate in a heated environment (oven). This can either be bi-axially or, as usually in the case of kayaks, via a "rock and roll" action. The mould rolls a full 360° around the perpendicular axis and rocks about the other axis. The rotational speed is low — around 3 rev/min. The rocking angle is typically 35°. This is not a centrifugal process, i.e. the powder is not thrown against the mould wall. The powder lies in a pool at the bottom of the mould (bi-axial machine) and as the mould (which is steadily getting hotter) rotates it picks up the melting PE powder depositing it evenly on the inside surface of the mould. Once the PE has been completely distributed on the mould wall the temperature in the mould (and the oven) continues to rise to around 200°C when the part will be cured. The hot mould is then removed from the oven and the cooling cycle begins. Finally the mould is opened again and the now solid part is removed from the cooled mould. Final trimming can now take place. Hey presto, finished kayak (without fittings) — with inserts and graphics permanently integrated into the kayak!
There are many more kayaks that can be used for fishing, however choosing any kayak - including those listed above - involves balancing the pros and cons and reaching a compromise. Some kayakers are not satisfied with the commercially available kayaks and have chosen instead to build their own kayaks.