The Structure That Does Not Actually Exist
A great deal has been written about structures—perhaps so much that it has become difficult to make sense of them. Let us therefore look at them from a different perspective and show that, at least on well-prepared skis, a structure does not actually exist as a separate and unchanging geometric form.
What We Mean by Structure
By structure, we mean a system of grooves of different shapes, pitches and depths ground into the surface of the ski base.
The resulting pattern may be linear, cross-hatched, chevron-shaped, multi-level or otherwise combined. It is produced by a defined relative movement between the grinding stone and the ski base; its form is determined primarily by the stone-dressing method, stone rotation, ski feed speed, contact pressure and other grinding parameters.
We will use a simple linear structure as a reference. The same principles apply to more complex patterns, although their geometry and the interaction between individual grooves are more complicated.
Groove pitch and depth are generally related. The transverse profile often resembles a V, but it may also be rounded, asymmetric or irregular. A greater distance between the peaks is usually associated with a deeper structure, while finer structures tend to be shallower. In foreign-language materials, the ridges are referred to as PEAKS and the distance between them as PITCH.
For the structures considered here, the ratio of pitch to depth is approximately between 10:1 and 12:1. A groove with a pitch of 0.5 mm may therefore be about 40 to 50 micrometres deep. This is not a universal constant, however. In practice, both parameters may also be affected by the grit and condition of the stone, the ski-base material and the settings of the particular grinding machine.
The word “ground” is crucial. We are not referring to a structure that has been pressed or engraved into the surface, but to geometry created by removing material, most commonly by stone grinding. This distinguishes it from imprinted structures (IMPRINTS), which are produced mainly by plastic deformation of the surface.
The ground structure is the initial, relatively stable geometry. Its actual form is subsequently altered by brushing, waxing, thermal and mechanical loading, ski break-in and wear. This article therefore deals exclusively with stone-ground structures.
Structure Does Not Exist Without Material
A structure cannot be described accurately without considering the material into which it is ground. The same geometry created in different materials does not necessarily produce the same surface or the same functional properties.
We will limit the discussion to bases made of UHMWPE, or ultra-high-molecular-weight polyethylene, used particularly in high-quality cross-country skis. In addition to UHMWPE, they may contain carbon black and other graphitic, paraffinic or oil-based components.
Carbon black does not consist solely of perfectly separated primary nanoparticles, but also of aggregates and larger agglomerates. The properties of the ski base are therefore determined not only by the declared particle dimensions, but also by their concentration, dispersion and the nature of the interface with UHMWPE. In racing bases, molecular weight is typically around 5 million g/mol or higher, although the specific formulations used by individual manufacturers vary considerably.
Formation of Fibrils During UHMWPE Grinding
UHMWPE has exceptionally long and highly entangled molecular chains. During machining, shear deformation, chain orientation and chain pull-out occur, together with the separation of parts of the semicrystalline structure and, to some extent, chain scission.
This may result in fine surface fibrils formed by bundles of oriented chains. Higher molecular weight and greater entanglement density promote the formation of longer and mechanically more stable fibres.
Carbon-black aggregates may act as local inhomogeneities and as sites where fracture initiates or propagates. They can therefore alter the way the material separates and limit the pull-out of polymer fibrils. There is consequently no simple rule according to which a higher carbon-black content automatically produces a better surface.
With a suitable concentration, uniform dispersion and optimum aggregate size, a ski base may therefore exhibit a less “hairy” surface after grinding. Larger agglomerates, however, may instead act as defects and reduce homogeneity.
A Ground Structure Is Not Yet a Functional Structure
A freshly ground surface is not the final functional surface ready for skiing. In addition to the intended grooves, it may contain fibrils, burrs, sharp peaks, locally deformed material and irregular transitions.
Before the skis—especially racing skis—are used, the surface must therefore be mechanically finished, cleaned and stabilised. The first stage is thorough brushing, which removes loose particles and fibrils, cleans the grooves and softens the sharpest edges. It does not change the basic ground geometry, but it does change its actual surface profile.
This is followed by repeated hot-wax applications, scraping and brushing. These mechanical cycles continue to remove protruding micro-irregularities, while heat and molten wax affect the immediate surface layer of the ski base. Racing skis may then be broken in on clean, moderately abrasive snow. Contact with snow crystals gradually abrades the highest protruding micro-irregularities and stabilises the surface under real gliding conditions.
When molten wax is applied, the grooves are filled to varying degrees. After scraping and brushing, most of the excess wax is removed, but not equally from every part of the profile. The peaks and upper portions of the grooves are the most exposed, whereas more wax remains in deeper areas. At the same time, the wax does not penetrate all locations equally, and its subsequent removal is limited by the accessibility of the individual parts of the profile.
The degree of removal depends on groove geometry; the stiffness, length and diameter of the brush fibres; the shape of their tips; the applied pressure; the brushing direction; and the properties of the wax. The functional profile is therefore no longer identical to the UHMWPE profile immediately after grinding: the valleys are partly filled, sharp transitions are covered or softened, and the peaks are gradually rounded. This change can generally be described as partial flattening and rounding of the functional structure profile.
A distinction must be made between the more permanent alteration of the UHMWPE surface caused by brushing, scraping, heat and break-in, and the variable alteration caused by an unevenly distributed layer of wax.