Layer Preparation

Every modern ski is made up of a host of layers. They typical ski has the following:

1. Base and Edges
2. Composite layer (fiberglass, metal, etc.)
3. Core (wood, foam, etc.)
4. Composite layer (fiberglass, metal, etc.)
5. Graphics Layer
6. Top Sheet

The skis I am designing have the same layers, except that I add an additional layer. A layer of rubber is placed between the steel edges and the composite layer.

Base and Edges

The base and edges were cut out and joined together earlier. The final preparation for the base is to apply a layer of clear tape (packing tape). The tape serves two purposes, it prevents epoxy from sticking to the base and it aids in keeping the edges attached to the base during the pressing process. The tape is applied to the base in a series of three strips since the tape is not wide enough to cover the base in one pass.

The tape should be applied so that the strips do not overlap; overlapping will cause an unevenness in the base after pressing and more work would have to be done to flatten it. A razor blade is then used to trim the tape along the edges. I don't worry about trimming the tape around the tip or tail as it will be cut off later and doesn't hinder the pressing process.

Rubber Layer

A thin strip of rubber, treated for bonding, is placed between the ski edge and the composite layer. This is done because the bond between the steel edge and fiberglass is weak. The rubber provides a good bonding surface for both and will flex/stretch with shear stress. The shear stress would otherwise potentially delaminate the ski. The rubber comes in a long strip which is cut to the length of the ski and then cut in half. A thin yellow strip of rubber can be seen in last picture in the composite layer section.

Composite Layers (Fiberglass)

The composite layers of the ski give the ski greater torsional strength (imagine grabbing the tip of the ski and trying to twist it around the long axis of the ski). The wood core is excellent for bending and shock, but the twisting of the ski is strengthened by one or two composite layers. Two layers of fiberglass will be used for each ski, one on top of the core and one on the bottom of the core.

For these skis, I am using a 19oz triaxial braided fiberglass (http://en.wikipedia.org/wiki/Glass_fiber). Triaxial means that the fiber bundles are woven in three directions. This will give the layer additional strength. 19oz means that a square yard of the material weights 19 oz (1lb 3 oz). The material comes in a 25 inch width which means that it must be cut into four separate pieces (6.25" wide).

Other material can be used for the composite layers. Last year I used a 5oz carbon/Kevlar fiber weave(http://en.wikipedia.org/wiki/Kevlar ; http://en.wikipedia.org/wiki/Carbon_fiber). The advantage to this is that it is higher strength and so the ski can be made lighter (see the photo at the end of this section).

Many ski companies are using metal in place of the fiberglass layer. The material of choice is Titanal (http://www.amag.at/AMAG-Titanal-R.1193.0.html?&L=0) which is an aluminum alloy made from aluminum, zinc, magnesium, copper, and zirconium (no titanium). As epoxy needs a mechanical structure to bond, the Titanal must be treated to create a porous surface, this is typically done by anodizing (http://en.wikipedia.org/wiki/Anodize) the surface.

For strength and density comparison see the following list (tensile strength is the breaking strength of a material):

• Carbon Fiber: 570 ksi (tensile), 1.78 g/cm^3
• Kevlar Fiber: 435 ksi (tensile), 1.44 g/cm^3
• Fiberglass (E): 250 ksi (tensile), 2.57 g/cm^3
• Titanal: 102 ksi (tensile), 2.70 g/cm^3
• Carbon Steel: 36 ksi (yield), 58 ksi (tensile), 7.85 g/cm^3

If I were to add cost to the list above, you would find that fiberglass is, by far, the most affordable. Glass (and glass fiber) is made from Silicon (Si) and Oxygen (O), with additional additives. Silicon is the most abundant element in the earths crust (just a random fact I learned on Jeopardy). To note, Titanal is both weaker and more dense (heavier) than fiberglass; additionally, the epoxy bond is not as strong. The advantage of Titanal is that it does not rely on epoxy for stiffness, as fiberglass does, but can be placed directly onto the core. This will save epoxy (expensive) and weight.

Top Sheet

The top sheet is the top layer placed on the ski. It is used to protect the core from damage which could be caused the by the sharp ski edges or other obstacles. The top sheet is made from P-Tex, which is an brand name for ultra-high-molecular-weight polyethylene (http://en.wikipedia.org/wiki/P-Tex). The top sheet is 0.75mm thick and comes in a 33cm wide sheet which must be cut in half. The top sheet is what most people see when they look at a ski; it covers the graphics layer; the graphics layer can even be printed onto the top sheet. In order to protect the top sheet during pressing from damage and epoxy, packing tape is applied.

A chalk line can be used to make a straight line to cut. A razor blade will cut the material easily. Precision is not critical here as the top sheet will be trimmed to size after the ski is pressed.