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.

Mold

The ski mold is one of the most critical tools of ski building. The tip, tail, camber, and rocker of the ski is created by the mold. The mold must be precisely shaped and must be able to handle pressure up to 30 psi (9' x 12' mold @ 30 psi = 19,440 lb). I have always built my molds out of wood because it is cheap and has adequate compressive strength.

To start the mold building process, I purchased two 4' x 8' x 1/2"sheets of MDF (http://en.wikipedia.org/wiki/Medium-density_fiberboard) and a sheet of 4' x 8' x 1/8" tempered hardboard (http://en.wikipedia.org/wiki/Hardboard). The molds have three separate features, the tip, tail and camber sections. These sections are individually cut out, then "woven" together to create the final mold.

The first step is to rough cut the MDF with a table saw.

Next the paper templates are glued to the sections they represent; spray glue works best.

The three MDF sections are then cut with a jigsaw using the templates. These cut sections are then used to trace the shape onto every piece that makes up the mold.

The individual pieces are then clamped together so that they can be sanded to the same shape; a belt sander works best.

Once this is complete, the sections are glued together and once dry, sanded on both sides to create a smooth contour.

In parallel the hardboard is cut so that it can be laid on top of the MDF frame work. Two 1/8" layers are used to increase the resistance of the hardboard to flex into under load. In the past I used a single layer of 3/16" hardboard; this is more challenging because the hardboard is relatively stiff and must be bent to the shape of the mold.

The hardboard is then clamped to the frame work. To get the hardboard to the proper contour, the remnants were saved from the tip and tail sections and were screwed together to press the hardboard into place.

Finally end-caps are cut out and placed on the ends of the molds to reinforce them. These ends are then rounded off with a sander to prevent damage to the air bladder which will be used to press the skis.

Finished Molds:

Base and Edges

The base of the ski is critical to the way the ski will perform since the base holds the wax on which the skis glide. The base is made from Ultra High Molecular Weight Polyethylene (UHMWPE) which has been sintered and rolled into a sheet. Sintering is a method for making object from powder until the particles bond to each other (http://en.wikipedia.org/wiki/Sinter_). The nature of sintering allows for small pores to be created in the surface; in this case the pores allow the wax to be absorbed into the surface. One side of the base material comes abraded; this process scratches the surface and allows the epoxy to mechanically bond to the plastic.

The edges are made from hardened carbon steel and are designed so that epoxy can fill in the voids and create a strong bond. The edges have also been sand blasted to create a rougher surface for bonding. Since the edges are carbon steel, they will rust over time. Stainless steel of the same hardness would be prohibitively expensive.

The base material is relatively easy to work with since it can be cut with a razor blade. The base templates created earlier are used to quickly cut out the base shape. As mentioned earlier, the base template is critical as it will give the ski its final shape.

Once the base is cut out, the edges are tacked on on with superglue. The superglue is only applied every six inches in order to leave ample room for epoxy to bond. Clamps are used to hold the edge in place until it dries.

The edges can be bent around the tip of the ski, but without engineered bending equipment, this is a cumbersome task. Since the edges are hardened steel, they can be annealed (http://en.wikipedia.org/wiki/Annealing_(metallurgy)) with a torch to soften the metal. Once the metal is cool it is more ductile and can be more easily bent. Since the metal is softened, it will wear more quickly, but since it is at the tip or tail, hardness is not as critical.

The base and edge design of these skis does not require that any edges be bent. The base material is simply brought to the edge of the ski at the tip and tail. Although this design is more susceptible to damage, it has proven to be adequate in the past, especially with a cap ski design.

I had originally ordered 12 steel edges, but was only shipped 10 so one ski does not have the edges applied yet. I have contacted the supplier (www.skibuilders.com) to find out if they will ship me the two edges they overlooked.