Thermoforming - The New Kid on the Block

December 2004-January 2005

This is an article from WaveLength Magazine, available in print in North America and globally on the web.
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by Tom Derrer

Heating the male mold.

When Dustin Hoffman was advised that “plastics are the future” in the 1967 movie The Graduate, none of us at the time had any idea what was coming. Then in 1975, I watched as the entire whitewater market did a side-slip from fiberglass to rotomolded polyethylene in the space of about two years.

Is there another revolution going on today? Yes there is, but it won’t be quite the same. As long as there is a market for low-end kayaks, polyethylene will dominate because of lower material and labor costs.

But thermoforming (also called vacuum forming), does offer some interesting options to paddlers who possess a taste for quality and can afford more than rock bottom prices.


Most of the plastic kayaks you see today are made of rotomolded polyethylene, a tough, flexible and relatively soft plastic. In this process, resin pellets are pulverized into powder and poured into an aluminum two-part mold that is closed and ‘tumbled’ in an oven. This causes the powder to melt and coat the inside surface of the mold. The tumbling and heating methods vary widely. Within minutes, a hot, rubbery kayak emerges from the mold and is supported while it cools and shrinks.

In the thermoforming process, the resin pellets are extruded into a flat sheet of appropriate size and thickness. This is where the technology takes a major departure from the rotomolding process,which is limited to a narrow selection of material types.

When such a sheet is created, two or three layers of different but compatible materials are formed into a single fused layer of even thickness. The outer layer is frequently a modified acrylic. Acrylics have long been known to have outstanding UV resistance and are often used in glazing, car finishes, paints and even waxes. This layer provides the sun protection, gloss and exterior color of the product. Sometimes another, similar layer is added for a special graphic effect, such as metallic or pearlesence.

Next comes the ‘muscle’ layer. The substrate will be much thicker and is often some form of high impact ABS. Each resin manufacturer has unique, patented formulas and they often manufacture several grades of the same material with differing properties. The multi-layered sheet can vary in the thickness of the layers, the amount of recycled content, and so on. All these variables affect price and performance.

Positioning the composite sheet on top of the mold.


Unlike fiberglass or rotomolded kayaks, thermoformed kayaks are typically formed over a mold rather than inside of one. In rotomolding, the female mold determines the surface quality, but with thermoforming (male molding), the surface quality is already in the sheet and our job is not to mess it up. There is another reason for male molding—the sheet will stretch some as it forms. A male mold will have the greatest material thickness on the bottom of the hull where the most wear occurs.

During the molding process, a sheet is clamped into a frame that holds it like a piece of glass in a window frame. The frame moves into an oven that heats the sheet to a high temperature, commonly between 350 and 400 degrees F. At that temperature, the sheet becomes quite rubbery and will stretch with ease. At this point, the frame moves out of the oven to a position over a mold. The mold moves into the sheet until the edge seals against the hot plastic. At this time, a vacuum is applied to the mold which quite literally sucks the material down around it and into any fine detail on the mold itself. Remaining steps to finish the boat are trimming, rigging, assembly and detailing.


Thermoforming provides a product similar to the quality of fiberglass in appearance and performance, but at a lower cost. A wide variety of expensive, high performance materials are available, but labor requirements are much lower than with fiberglass composite kayaks. Thermoforming technology itself is not new—it has been around as long as plastics themselves. However, recent changes in equipment, process sophistication and the plastics themselves are indeed quite new.

Plastics used in thermoformed kayaks are strong and easily repairable. Outer surfaces are harder than other plastics or gel coat, providing better abrasion resistance, no fuzz-up, better UV resistance and less weight.

Applying a vacuum to suck the sheet tight to the mold.

Another change that has brought thermoform technology to the surface is the advancement in adhesives. Modern structural adhesives have made possible the assembly of a variety of plastic parts with high structural integrity. Unfortunately, these adhesives still don’t work well on polyethylene, but they do work very well with the more expensive plastics used in thermoformed kayaks.


Fiberglass will probably have the edge for awhile yet in longevity, as will polyethylene in the extreme high impact world of whitewater. Nevertheless, modern thermoform materials offer great value and performance for recreational and performance sea kayaks, and with reasonable care will provide many, many years of pleasurable use


Since Eddyline introduced its first thermoformed kayaks in 1996, at least five other companies have introduced one or more models, and the list is growing rapidly. We will continue to see significant advances in material and forming technology. We will also see more development of hybrid technology combining plastics and composites. Ultimately, these advances will continue to lower manufacturing costs and improve performance. Last but not least, all trimmed and unused material is 100% recyclable. Nothing need go into the land fill.

© Tom Derrer began paddling actively in 1966. He founded Eddyline Kayaks in 1971 and has designed over thirty sea kayaks. He pioneered vacuum bag technology in the 70s and thermoforming in the 90s. He also served as board member and President of the Trade Association (TASK) for four years and helped create the West Coast Sea Kayak Symposium. He can be reached at, 360-757-2300. Photos courtesy of Eddyline Kayaks.