Polyethylene rotomoulding
Here's a challenge: Make polyethylene rotomoulded kayaks and canoes on your own, in your barn or garage.
I'm not saying it would be economic to start from zero and rotomould one kayak, or even twenty kayaks. However, if you want to make a hundred you could do so in a fairly small space; and if you are able to sell them, you would make a very good return on your investment. All you need is a metal mould; a few drums of polyethylene powder or granules; a home-made oven, made by folding and riveting steel sheet; a few butane or propane cylinders, pipe and 4 blowtorches to heat the air inside the oven, and an electro-mechanical way to make the mould and oven rock end to end while slowly rotating (think rotisserie). And you're in business.
The expensive part is the mould. Unlike a fibreglass mould for making fibreglass kayaks, a tool for rotomoulding needs to be able to withstand temperatures up to 350°C without distorting or degrading. Or melting or catching on fire! The actual cost of your mould will depend on the size and shape of the object you want to make. The geometry of some lovely 5.2 metre sea kayaks consists entirely of "developable surfaces", which means they can be sewn together from panels cut out of stiff plywood. For example, the classic Greenland kayak. This kind of shape can be made just as well from thin sheet steel, quite cheaply, by your local steel fabricator, so there's your mould.
If you want to make kayaks or canoes with compound curves for a really rounded hull shape, and if you're not too fussy about having exciting details or a perfectly symmetrical hull, you can get your local panel beater to make you a mould. You'd need somebody who is an artist at naking wheel arches for classic cars from sheet steel, using a simple device called an "English wheel".
But if you're going to manufacture a really professional-looking kayak or canoe, with a perfectly symmetrical shape, a great finish, surface texturing, moulded-in logo, hand grips and so on, and if you want to manufacture in quantity, you will want a specialist company to make you an two-part or three-part aluminium mould. This was traditionally milled out of a solid billet of aluminium 4, 5 or 6 metres long, but new 3D printing technologies may provide alternatives. Either way, a big, massively heavy, two- or three-part aluminium mould isn't going to be cheap. If you're in Europe, you will probably get your alu mould from India or China. If you're in the USA, you'll probably get it from a US plant.
Time for a confession: I don't yet know very much about rotomoulding, but I want to! I have an outrigger canoe design that I want to manufacture. Why in polyethylene? Well, it all comes down to manufacturing hours:
- It takes me 5 months to make a wooden outrigger canoe (main hull, laminated crossbeams, two outriggers, outfitting, sailing rig, hull and outriggers sheathed in fibreglass), plus and another 5 or 6 months to make it perfectly fair and put a Ferrari-glossy finish on it.
- If I call that a "plug" and take a fibreglass mould off it, I can make fibreglass canoes in a fraction of the time it took to make the wooden original. In fact, I can reduce the manufacturing time from 10 months in wood to 10 days in GRP (longer than for a sea kayak, but then the outrigger has three hulls).
- If I then switch to polyethylene manufacture in a one-man-and-a-garage setup, I can reduce manufacturing time to 1 day.
Once you're set up for rotomoulding, you aren't confined to kayaks and canoes. Rotomoulding is particularly suitable for fairly large closed shapes, and especially for any "developable" shape where the mould can be simply made from thin steel sheet. That means cubes and other box shapes, with or without radiused edges; cylinders; and sections of a cone. For example, water tanks up to 50,000 litres, oil tanks, diesel tanks, septic tanks, pontoon floats, hard buoys, municipal planters, water butts, wheel wedges for motor caravanners... also fairly large open shapes such as barrels, coolers, pallets, fish boxes, crates, litter bins, traffic cones, playground equipment, mounting blocks for horse riders ... the list is practically endless. In Britain, for example, we buy a lot of outboard runabout boats from Norway. These are mainly developable shapes that can be made in a sheet-steel mould. The Norwegians import polyethylene powder and, since they have a lot of mountains, they are able to use electricity from hydro-electric power to heat the moulds. But then they have to shrinkwrap and crate the finished boat for tranport to the end user in Britain, France or Italy. After a time, the remaining big expense is that of shipping a very bulky, relatively fragile product. It might make more sense just to ship the powder and mould the boat locally.
Polyethylene manufacturing is kinder to the environment than fibreglass manufacturing. Fibreglass is great for users in that it lasts a very long time (my favourite sea kayak is 50 years old), but that's kind of the problem: there will still be abandoned fibreglass boats littering planet Earth in a thousand years' time. Polyethylene boats, though, you can grind up, melt and recycle at the end of their useful life. And unlike fibreglass manufacture, which releases a lot of styrene solvent into the atmosphere, the only by-products from polyethylene manufacture are heat from the oven, CO2 from whatever form of oven heating you use, and maybe a little water vapour.
Now, I have a memory (at least I think it's a memory .... it was a loooong time ago so maybe it was just a dream!) of going to the actual factory in New Zealand to buy a couple of Dancer river/GP kayaks so we could paddle the Wanganui, the Rangitikei and the Whakapapa rivers, amongst others. Late 1996, this would be. So off we went in our gigantic Holden estate car to somewhere in the south end of North Island. The "factory", if I remember right, was down a dirt track, and consisted of a roof standing on four posts, sheltering a tin oven that was just about big enough to make one kayak at a time. And they made our two Dancers for us while we waited, just like making a couple of omelettes (only blue, not yellow).
25 years later, I'd got into the habit of thinking of rotomoulding as a serious high-tech industrial process, involving deeply serious men in white coats, with ovens the size of a cruise ship inside a factory the size of a Zeppelin hangar, and everything controlled by computers. And there certainly are rotomoulding factories like that, where they can pop a racing dinghy out every 20 minutes.
However, when I started to look into it, I found that my memory of early-days manufacturing in New Zealand might actually be accurate. The rotomoulding process doesn't involve advanced knowledge (it's slightly easier than baking a cake). Humidity control is a good idea, but rotomoulding requires no manual skills or dexterity, and there's no need for high pressures, or vacuum, or solvents, or EM radiation, or a dust-free environment, or a special atmosphere, or much in the way of protective clothing.
You don't even need particularly high temperatures. High- and medium-density polyethylene powder melts at 120 to 130°C (and starts to burn at about 350°C, so that'd be way too hot).
That's not a huge amount hotter than making an omelette. In fact, let's imagine you are making an omelette, but you want it to be hollow ... maybe the shape of a jam doughnut or a rugby ball. You could do that by putting a lid onto your frying pan, attaching the lid so that it doesn't fall off, and then heating up the whole thing and swinging it around so the egg mix spreads evenly over the inside of the pan and the lid. That's more or less how you make a rotomoulded kayak, except that you swing it around not by hand but with a "rock and roll oven".
The rotomoulding oven should be evenly heated; you don't want flame playing directly on the mould. The oven should be thermally insulated to the point where you can safely touch the outside of it. It may be not much bigger than the mould, and is probably mounted over a pit, because otherwise it'll be pivoting on a point 8 feet up from ground level, which would be inconvenient. The professionals use a carousel device to rock and roll one, two or four moulds at the same time. A carousel can slowly rotate a 6-metre long mould end over end continuously, in an oven shaped like a coin standing on edge, while also rotating the mould on its long axis.
Roll speed: The mould rolls (rotates constantly on the mould's long axis, like a chicken in a rotisserie oven), completing a roll every 5 to 10 seconds, depending on the size and shape of the mould. The time to complete a roll might be as long as 18 seconds when moulding a 12-foot sailing dinghy.
Pitch speed: The mould also rocks end for end, as when you're emptying the water out of a flooded kayak. It does this quite slowly, going from one end up to other end up every 20-30 seconds, depending on the length of the mould.
For the artisanal setup in your barn or garage, it will be enough to have a total of 50-90 degrees of pitch (in other words 25-45 degrees from horizontal each way). A simple rock and roll mechanism is nothing an agricultural engineer couldn't run up for you in a day or so (once they've figured out a way to have the electric drive motors outside the oven, or parts of them are going to melt).
Once your omelette is made, you need to remove the mould from the oven. The mould has to keep slowly rolling, although not pitching, until the liquid polyethylene has cooled enough to set, or it will set into pools and stalactites of plastic. To shorten the natural cooling time to a more convenient 15 minutes, you can use six or eight electric fans in a different places to blow ambient air over the metal mould until it is cool enough to open. Some bolder manufacturers cool the mould with a water spray, but that risks damaging it. You can take the hot mould out of the oven, but it's usually easier to remove the oven from the mould (by opening one side of it and then wheeling it away along tracks).
In the old days, PE kayaks were a single monolithic layer of polyethylene; the plastic was (let's say) 3mm thick and the same all the way through. Today it's possible to make a three-layer laminate (with, let's say, three 1mm layers) which will perform very nearly as well as GRP composite in terms of stiffness and gloss, while being far more impact-resistant. The three layers are built up during a single heating-cooling cycle. Start with enough powder in the mould to make the outer layer, latch the mould shut, start the rock and roll mechanism and the heating system, which can be inlammable gas or infra-red electric panels. This will coat the inside of the mould with a relatively dense, shiny layer of super-linear polyethylene that is strong in compression. This will be the outer layer of the finished hull. Then add a second batch of powder: a foaming PE which melts over the inside of the still-sticky shiny layer to leave a mid-layer of PE that expands into a foamy core material. Finally, add a third batch of powder to line the whole thing with a layer of general-purpose PE that is softer than the outer layer but stronger in tension. That gives you a hull that is very nearly as glossy as a GRP composite, and much lighter, stiffer and more scratch-resistant than the early PE hulls from the 1980s. You can either blow the second and third batches of powder into the rotating heated mould along pipes with a blast of compressed air, or release the second and third batches inside the mould at intervals of a few minutes by opening thermally-insulated metal boxes inside the mould using either a compressed-air or mechanical lid release.
The mould will be in two halves (usually hull and deck, sometimes port and starboard mirror images). Each half has a flat, rigid flange along the edge, which attaches to the flange of the other half with latches at 24-30cm intervals. Since this gives you a shape that will be hermetically sealed by molten plastic, while undergoing big temperature changes, it needs a pressure relief pipe, with a diameter of 13mm for each cubic metre of mould volume. The pipe connects the inside of the mould with the outside air and must stick far enough into the mould that it doesn't get blocked by plastic.
If you want a very simple canoe hull with no closed deck, you can save on plastic by putting rockwool thermal insulation over the deck part of the mould, so that part doesn't get hot enough to melt the powder. Or you may want a sophisticated shape where the deck part of the mould dips down until it almost touches the inside of the hull, moulding a horizontal floor in your canoe which almost touches the hull ("almost kiss off"), or which actually touches the hull ("kiss off") so as to link the hull and deck, giving much greater rigidity and forming bulkheads, baffles or reinforcing ribs in a single moulding process. It is also possible to embed brass or stainless steel nuts (for later attachment of footrests, rudders, decklines, rigging wires, etc) inside the hull plastic. Holes are drilled through the skin of the mould where these skin fittings are required. Bighead fittings (a nut with a wide, thin flange that will be entirely covered by the melted polyethylene as it flows over the inside of the mould) are temporarily held in place by bolts through the drilled holes so that when the bolts are removed, and the mould is lifted off the cooled moulding, the bighead fitting stays with the moulding.
Once the mould has been opened, the moulding has to be removed. If the mould has a smooth shiny surface and was designed and made with plenty of draft (flaring angle) then your new moulding should come out quite easily. If the mould does not have much draft, it may necessary to use all sort of levers, mallets and brutality to persuade it. (And if the mould was designed or made with any undercut, you will never get the moulding out and should probably consider a career in some other line of work.)
Your new moulding is, of course, an airtight bottle with no openings except maybe through the bighead nuts. A sharp knife is now used to cut away the waste plastic blocking the cockpit(s) and and deck hatches. Then drain bungs, hatch coamings, rudders, skegs, seats, foam buoyancy and other outfitting can be bolted, riveted, welded, glued or just plain rammed into place.
I have two big questions at this stage:
- What are the names of some reliable mould-making companies with long experience of milling fully rounded hull shapes (and a solid reputation for not quietly running of a second mould for themselves, so they can quietly add the customer's boat to their own product range and sell it for half price on AliBaba...)
- Or is 3D printing ("additive manufacturing") of aluminium now an affordable, reliable alternative way to produce a high-temperature mould of this size?
If you have the answers, please get in touch.
I'm sure there are hundreds more questions I should be asking: PE density, UV additives, ESCR (environmental stress crack resistance), application of decals, etc .... and I would really love to hear from somebody who has been involved in rotomoulding of kayaks or canoes in PE. And if they'd like to join me in this, I'm definitely listening.
Go to next page for:
• Build your own wood kayak
 |
 |
 |