It’s inevitable when two or more skiers are talking tech, the conversation will linger on ski bases: structure, grind, waxing, rilling, etc. If you listen carefully, however, you will notice the conversation is not really about base materials but base preparation. What is the base and how is it made? To learn more, I contacted Crown Plastics (www.durasurf.com) located in Harrison, Ohio. Crown is the manufacturer of the DuraSurf ski and snowboard base materials.
Base material is the correct term when discussing the surface of the ski in contact with the snow. Many skiers erroneously refer to the base material as P-Tex—a brand name for the polyethylene created by the European company International Mountain Sport.
In fact all base materials are Ultra High Molecular Weight polyethylene (UHMW)—a close cousin of the opaque milk jug—and made from number two High Density Poly Ethylene (HDPE). Both milk jugs and ski bases contain the polyethylene molecule (C2H4); however, in ski bases the chain of ethylenes is much longer.
Another way to categorize polyethylene plastics is to use the molecular weight of the chains. (This also determines what types of plastics can be recycled.) Any ethylene plastic with a molecular weight greater than 3.1 million receives the UHMW designation—the weight of the material used in ski bases.
After the molecular weight reaches one million, the material is capable of being sintered, a process of molding without melting. For example, trying to form a snowball with dry snow is a sintering process: The pressure created by your hands warms the snow, allowing the flakes to deform and lock together. For ski bases, the manufacturers begin in a similar fashion by pressing the powder form polyethylene (120-180 micron sized grains) together, forcing the grains to interlock like snowflakes.
Polyethylene also has the highest impact resistance of any petrochemical. The greater the weight of the polyethylene, the greater the resistance to deforming and remaining misshapen. Most bases, including DuraSurf, have molecular weights between 3 million and 6 million. Bases with a weight greater then 6 million do not have the pore space to accept waxes.
After sintering, base materials are formed in one of two ways: continuous compression molding (CCM) or scything. DuraSurf is the only base material made using the CCM process.
The CCM process uses two titanium belts set horizontally one above the other. The gap between the belts diminishes towards the end of the machine. The manufacturers sift the polyethylene resin powder evenly onto the lower belt; then, the upper belt compresses the resin into a continuous ribbon of base material. The pressure between the upper and lower belts raises the temperature of the material between 400 and 500 degrees Fahrenheit, and the ribbon of base exits the belt press at the finished thickness. CCM can process 100 liters per hour of resin with a varying total yield of usable base.
The scything process uses a large ram press—often a meter or more in diameter—to form gigantic puck of base material called a billet. The manufacturers sift the powdered resin into the cavity of the ram mold before the ram, or piston, closes down and compresses the powder. Similar to the CCM method, the compression raises the temperature of the polyethylene, reforming the powder into a solid. The manufacturers then place the billet on a lathe where a blade scythes off ribbons of base material.
With both scything and CCM, the manufacturers mix any base material colorants or additives with the resin powder prior to sintering. Carbon black, for example, is a common additive used to darken the ski base and hold wax. Graphite and fluorocarbon powder are also used to give the base materials different properties. The formulas are confidential.
The low surface energy of polyethylene also allows it to repel water (hydrophobic). Water wet bases have approximately half the friction of a dry base. While this low surface energy aids in glide, it also repels epoxy and other glues; therefore, to allow the top of the base to hold graphics, the manufacturers must abrade and flame-treat this part of the base to give the bonding agent something to grab onto. Abrading gives the upper side some “tooth,” and the flaming oxidizes the upper surface, giving it higher surface energy.
Graphics are either screen-printed or sublimated onto the top of the base. Printing can also be done on clear and very opaque materials.
Screen-printing places logos and words in layers, starting from the bottom of the base and moving toward its core. This process is similar to how artists paint holiday scenes onto store windows.
In contrast the sublimation process uses heat and pressure to transfer the ink of the graphics, printed onto paper, into the base material. The manufacturers seal the ink with another colored, screen printed ink that brightens the artwork. This final layer of graphics is also a primer for the epoxies used to attach the base to the ski.
Dark colored bases can also have graphics that are die-cut into the base. A section of the base is removed and then replaced by an exact match. The base and insert are run through a press where they sinter together and become a single piece.
The three types of graphics offer different advantages and disadvantages: Die cut inserts have very defined and crisp edges, while screen-printing allows sharper definition than sublimation but at a higher cost. Most graphics on Nordic ski bases are placed within 30 cm of the tip so that they don’t affect the glide but are visible in photographs.
The base material arrives at the ski factory ready to be shaped for the particular model of ski. The shape is die cut from the roll or cut using a CNC controlled knife.
How bases become part of the ski will be covered in the following installments of the series.
As always, if there are questions, comments or corrections (some of you certainly know more then I about this stuff) please feel free to post them, and I’ll do my best to find an answer.