by Stein, Eric's Son
Ski Core Construction Materials
Skis are built of a variety of materials, and different
materials are used in the different parts of a ski. Generally
speaking, skis consist of a core, structural layers, a
topsheet and a base. In this segment we take a look at
the materials used in the cores of skis. The core may
make up as much as 85 % of a ski. It may be composed of
wood, foam plastic, or honeycomb materials.
Wood was once widely used in ski core construction. Its
advantages include that it is available in a wide range
of densities and mechanical properties (depending on the
species used), bonds well, and can be easily worked. Disadvantages
include increasing scarcity of desirable types of wood
and wood's weight. An example of how wood is used in a
ski comes from the legendary Rossignol Strato downhill
ski, designed by Roger Abondonce a cabinetmaker. He built
the Strato with hickory and okume (an African wood) on
top, because these woods have small cell-structure and
will therefore minimize compression. On the bottom he
placed a long-cell, and therefore flexible, wood like
ash. When laminated together, the core possessed properties
of contraction and extension that helped keep the ski
base on the snow better than any other ski then in production.
Today, however, only a few skis are made with wood cores.
Foam plastics, like wood, are available in a variety
of densities and mechanical properties. All are polymers.
The physical properties of foam depend on its type and
the size of the cells inside the foam. The larger the
cells, the lower the density of the foam and the weaker
the foam. Four types are commonly used: 1) polymethacrylimide
(PMI); 2) polyurethane (PU); 3) polyvinyl chloride (PVC);
and 4) polystyrene (PS). PMI is a lightweight, white,
stiff foam that is often used to stiffen cambers. PU is
a heavier, more rigid, gray foam. It is easier to work
than PMI, but weaker by weight. It is the most commonly
used foam in injected cores. PVC is light in weight and
is generally used as a filler in the channels of wood
core skis. PS foam is the weakest of the foams and mostly
used for filler in the channels of skis built of other
materials.
Honeycomb materials are used in the cores of extremely
lightweight skis. Two types of material are used to make
the honeycombs, aluminum and resin-impregnated cardboard.
Honeycomb materials are a spin-off of the aerospace industry
and are extremely strong.
Some foam core skis have longitudinal strands of fiberglass,
wire or carbon fiber in them for reinforcement.
One of the most common manufacturing methods for inexpensive
skis is injection molding. In the injection molding process
ski cores are produced by injecting the constituents of
PU foam into a mold where they react. In the reaction
process carbon dioxide is formed. The amount of carbon
dioxide present and the rate at which it forms determines
the cell size of the foam, which in turn determines its
strength, flexibility, etc. In addition to varying the
foam's density, the amount of foam used can also be carefully
controlled because the foam can be injected from any or
many places.
