rotomolded vs. blow molded

[truckeeboater]

can anyone tell me the difference/ benefits/ detriments of each method?

Thanks!

(sorry if this is a stupid question )

[TheKrikkitWars]

Blow moulding allows for almost perfect consistency of thickness, as practiced by the now defunct Eskimo it produced boats of legendary strength and stiffness...

How much of that was integral to the blow moulding process and how much of that was down to the way that the plastic used for their parissons* was composed or pre-treated is however anyone's guess.

I think that it limits the complexity of shapes that can be moulded (ie the moulded in grooves that retain the centra pillars in LL boats, or the largely superfluous deck mouldings and textures of most boats), this seems to be more of an issue making kayaks than canoes.

If this is related to the Pheind/Phiend/Fiend/Phein/whatever else you feel like spelling it as; then the most honest answer is you'll have to wait and see how it turns out, Mohawk are only the second paddling company to use blow-moulding and whilst it's obvious to compare with Eskimo there's no guarantee that the results will be the same.


For my part I'm curious as to why no-one has tried Vac-Forming PE canoes, whilst it would probably require a more complex form that could be dissembled and lifted from within the boat once complete for whitewater designs the simpler flatwater ones could probably be achieved quite easily.

*Blanks and/or blank tubing which is cut and sealed as the hot mould closes over it.

[sbroam]

Prijon doesn't blow mold?

[jlc22]

Blow molding produces a more consistant product and uses a tougher plastic, but the drawbacks are it's much, much more expensive to produce the molds. I'd also assume the investment in the machine is much higher as well. You can't control the thickness of the boat. As I understand it a tube of molten plastic is dropped vertically into the mold and expanded by a shot of air. This can result in thin areas around the cockpit combiing and other areas where you have edges or complex designs.

Roto molding is much simpler but doesn't produce a product with the same consistancy. Imagine taking a plastic bottle and and putting a handfull of sand in it. You spin the bottle clockwise for a little bit then counter clockwise, while doing that you tilt it back and forth like a seesaw. That's basicly how a rotomold works. The mold is filled with plastic powder and ran into an oven and cooked for about 15-20 minutes while the mold tilts and turns. As the mold heats the plastic sticks to it. Areas that get hot the fastest have plastic stick to them first. The mold will run boats differently in the morning when it's cold vs in the afternoon when it's hot. However, someone who knows what they're doing can work with the mold to change how the plastic lays up, meaning you can put more plastic in the high wear areas and less in areas that dont' matter. The molds are much cheaper and quicker to produce.

[Shep]

For my part I'm curious as to why no-one has tried Vac-Forming PE canoes, whilst it would probably require a more complex form that could be dissembled and lifted from within the boat once complete for whitewater designs the simpler flatwater ones could probably be achieved quite easily.

This might not be what you are asking, but the Taureau is either vacuum or press-formed. The issue, I think, is with having to build the boat in two halves, as with the Taureau, or with having to overcome gravity to get the shape, which I don't think would be possible without turning the plastic into soup. I can tell you from experience that the seam line in the Taureau is the weak spot, and it has bloodied many a knuckle from new Taureau paddlers.

[AJ]

Wildwasser, the Prijon distributor for USA, is located in Boulder, Colorado. At one point there was some relationship between Prijon and Eskimo and still might be? Wildwasser handled both brands and I still see Eskimo labled boats in their wearhouse. Prijon blow molds, I believe in Germany. Here is a nice video of the HTP process and equipment. http://www.youtube.com/watch?v=LkXiBFceq4I

The Taureau was innovative, but a poor design from talking with a friend in the plastic business and also from my experience. The sheet formed plastic tends to get thinner on sharp bends like a chine. Think about a flat plastic sheet bent at a sharp angle, the plastic is going to be thinner at the bend point. The Taureau was the only boat, I wish I never purchased and was glad to part with.

[truckeeboater]

Such a cool video AJ, thanks for posting! I really had no idea that most plastic boats (kayaks anyway) start out as just a big molten plastic straw that gets inflated like a balloon! That's so wild. I'm beginning to understand why our plastic counterparts are as spendy as they are.

Has anyone had any experiences where their PE boat has failed catastrophically in some way or another (besides the seam on a two-piece boat) due to inconsistencies in the hull thickness? It seems to me that regardless of manufacture technique, these boats can sure take a beating!

[AJ]

Just to be clear that video was for a blow molded boat.

Here is a Jackson video for a roto molded boat http://vimeo.com/30179595. The process starts at about 1:58 in.

Interesting stuff.

[sbroam]

at 5:14, isn't that the C-1 console?

[AJ]

at 5:14, isn't that the C-1 console?

It sure is, good eye!

[PatrickOC1]

so in that Jackson Video looks like they heated the mold with the plastic powder in it, then spun it after the fact before it cooled down.... I always assumed you had to spin the thing while in the oven like as everything was melting? They didnt have the see-saw motion either, or at least in the video.....

[RodeoClown]

so in that Jackson Video looks like they heated the mold with the plastic powder in it, then spun it after the fact before it cooled down.... I always assumed you had to spin the thing while in the oven like as everything was melting? They didnt have the see-saw motion either, or at least in the video.....

They did, it's just not obvious. Once the oven is closed, it's pretty boring to look at, so they didn't really focus on it. At 3:15, the machine on the left is rocking- it's a lot slower than most people think. The shot of the mold cooling is also sped up.

[ohioboater]

Here's a Liquid Logic video that, while being shorter, does a better job of explaining the rotomolding process:

http://www.youtube.com/watch?v=lj9WOnrg ... 4aIzn2ts9M

[PatrickOC1]

oh alright ive never seen the process before that clip....but that makes more sense, that spray-glued in powder was just for color then? Interesting process and you start to understand the costs that go into it seeing all the equipment that has to be involved..... that blow moulding sure looks pricey

[plasticman]

The main factor in choosing blow molding vs. roto molding basically comes down to sales volume. Generally, blow molding is only appropriate for sales volumes of thousands of units due to the very high cost of the machine itself (compared to roto-molding machinery), as well as generally higher costs for the molds.

Eskimo boats are (or were) made by another German company that specializes in blow molding and had already invested the money in the machinery to blow mold other products, making Eskimo kayaks is (was) a tiny side-business. To the best of my knowledge, Eskimos molds are high-production models and were very expensive to make, relative to the general run of boat molds made for the roto molding process.

Blow molds for low unit volume products like canoes and kayaks can be made for not all that much more than roto-molds (look close at the Prijon video at the molds, not the parts coming out of the machine). The capital cost is not so much in the mold, it is in the giant blow molding machine. A custom blow molder (an outfit that owns the machinery and basically rents out time on the machine to some other outfit that owns molds but no machine, custom roto-molders exist as well) with a machine big enough is going to charge per part something that generally makes is uneconomical for a boat maker to offer blow molded boats even if the cost of the blow-mold tooling is close to the cost of the same mold for roto.

Not sure how Mohawk is expecting to make the blow-molding project work economically, I guess they are expecting/hoping for us to buy a lot of canoes.

That is why roto molded boats are so much more common than blow molded ones- the reasons are economic and driven almost exclusively by volume, not really because one process makes a better boat than the other (that's not to say I don't have an opinion...).

The above being said, the blow molding process inherently eliminates some of the possible strength issues that are common defects in roto-molded parts (NOTE THAT I AM NOT SAYING THAT THESE DEFECTS ARE COMMON, JUST THAT OF THE DEFECTS THAT IT IS POSSIBLE TO PRODUCE, THESE ARE THE MOST COMMON) because the plastic material is guaranteed to be thoroughly and evenly melted through the entire wall thickness of the part. This is one of the most common defects in a roto molded part- the material in some (usually thicker) area was not melted completely (the cycle time was too short and time=money for a custom molder). The strength of the hull in any given area of a blow molded part is dependent only on the wall thickness in that area and the strength of the PE material itself, the blow molding process pretty much guarantees that the plastic material is melted homogeneously the whole way through any wall section.

Blow molding does put a few more restrictions on part design though. As someone else mentioned, generally the wall thickness of a blow molded part is a more function of the mold shape and the limitations of the process than it is in roto. There is more possiblity in the roto molding process to tweak the tumble/tilt/temperature/time variables to change the distribution of plastic material inside the mold to where it is wanted than there is in the blow molding process.

Okay, hope that didn't bore everybody to death. If it did... well i guess the victims aren't really going to be in any shape to whine about it.