Effect of Ski Base Material on Structure Quality - Part No. 1

 Effect of Ski Base Material on Structure Quality

Ski base material affects the quality of a stone-ground structure primarily in two ways:

1.     through the formation of unwanted fibres and hairs during grinding,

2.     through the dimensional stability of the material during machining and subsequent loading.

A conventional racing ski base consists of an ultra-high-molecular-weight polyethylene (UHMWPE) matrix reinforced and modified with functional particles, most commonly carbon black, lamellar graphite and/or graphene.

How a Stone-Ground Structure Is Created

A stone-ground structure is most commonly produced using a rotating grinding stone whose surface has first been dressed to the required profile with a diamond dresser. The dresser therefore does not create the structure directly in the ski base; instead, it shapes the working surface of the grinding stone. When the stone subsequently comes into contact with the base, it transfers the prepared pattern into its surface by progressively removing material.

The resulting structure, however, is not a simple geometric imprint of the stone. It is produced by a dynamic machining process in which individual abrasive grains penetrate the UHMWPE surface, locally compress it, subject it to shear stress, deform it and ultimately separate material from it.

The actual shape of the grooves therefore depends not only on the profile of the grinding stone, but also on:

• the rotational speed of the stone,
• the ski feed speed,
• the applied pressure,
• the condition and grit of the stone,
• the dressing method,
• cooling and lubrication by the grinding emulsion,
• the mechanical properties of the ski base itself.

A soft and elastic material may partially yield in front of the abrasive grains and recover elastically after they have passed. The resulting groove may therefore be shallower, wider or less sharply defined than the geometric profile of the grinding stone would suggest.

A stiffer and more dimensionally stable material, by contrast, permits a more accurate transfer of the tool geometry. At the same time, however, it may be more susceptible to local matrix damage or particle pull-out.

Consequently, the same grinding programme and the same grinding stone may not produce an identical structure on different ski bases. The geometry of the grinding stone defines the intended shape, but the actual form of the structure is also determined by how the particular material responds to pressure, shear, elastic deformation and material separation.

The grinding stone therefore defines the structure geometrically, while the ski base material determines how accurately, cleanly and repeatably that geometry can be produced.

Formation of Unwanted Fibres

UHMWPE is a very tough material composed of exceptionally long polymer chains. During grinding, the material is not removed solely by clean cutting. Some polymer chains may be pulled out and stretched by an abrasive grain before they finally break. This produces unwanted fibres and hairs of varying lengths on the surface.

Their formation is also promoted by the internal heterogeneity of UHMWPE. The material contains alternating crystalline, amorphous and transitional regions with different strength, stiffness and machining behaviour. The length, density and distribution of the resulting fibres are therefore not entirely uniform.

Carbon black particles alter the internal structure of the polymer matrix and influence the way the material separates during grinding. With a suitable particle type, concentration and quality of dispersion, they can limit the extensive drawing-out of long polymer chains and promote shorter, cleaner separation of the machined material.

Carbon black may therefore create preferential sites for material separation. Damage does not necessarily propagate along a long section of a polymer chain, but may be locally interrupted in the region of a particle or particle aggregate. This limits the formation of long fibres protruding from the surface.

The result may be a less hairy surface, more precise groove edges and an overall cleaner cut. The quality of the result, however, is not determined solely by the amount of carbon black. Particle size, aggregate structure, uniformity of dispersion and the quality of bonding to the polymer matrix are equally important.

Dimensional Stability During Grinding

The second important parameter is material stiffness, expressed primarily by the modulus of elasticity, or E-modulus.

UHMWPE itself is tough but also relatively elastic. Under the action of an abrasive grain or the pressure exerted by the grinding stone, the surface therefore deforms elastically at first. Some of the material yields in front of the tool and, once the load is removed, returns at least partially to its original shape.

The more the material yields elastically during grinding, the more difficult it is to produce a geometrically precise structure. Groove edges may be less sharp, groove depth less uniform, and the final shape may deviate from the geometry of the grinding stone.

In this context, carbon black acts primarily as a reinforcing component of the polymer matrix. Its overall effect depends not only on the total amount added, but also on particle size and structure, aggregate size, the quality of dispersion within the polymer, bonding between the particles and UHMWPE, and the pressing and sintering conditions.

A well-designed compound can increase the material's modulus of elasticity, reduce deformation under load and thereby improve grinding accuracy. A stiffer ski base retains the shape of the grooves more effectively and allows the geometry of the grinding stone to be transferred more accurately to its surface.

The Structure That Does Not Actually Exist - Part No. 1

 

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.

How often should be competition skis re-grinded?

There are two extreme positions related to the wear and life time of ski base structures existing next to each other.

First we need to define what is the ski base structure for the purpose of this article?

Ski base structure is any type of stone-grinded structure for the purpose of this article. We do not question that also other manufacturing methods exist to produce ski base structures as e.g. imprinting, belt-grinding, rilling... For us these methods are - however - so unstable and create so varied results that we do not want to discuss them here.

Stone-grinding

If a ski base is stone-grinded, old structures, small defects and oxidated or thermally damaged layer of the ski base are removed first. Normally a few tens of micrometers are removed in this preparation steps.

In the second step the ski base is flattened with a very fine stone to achieve the best structuring result. In the last step the required structure is grinded in many movements depending on the shape, width and depth of the selected structure. Multiple structures are not considered here...

Best performance of stone-grinded ski base

If competition skis are stone-grinded, they need to pass another long trip before they achieve the best performance...

First the ski base needs to be cleaned to remove cooling and greasing agents, grinding residues and dirt. Then the ski base needs to be waxed several times to clean the cavities inside the ski base. After waxing, ironing, scrapping off excess wax and brushing the skis need to be used on - if possible - fresh crystalline clean and abrasive snow to remove the “unwanted” hair and homogenise the ski base surface.

Here we are already touching our main topic slowly...

Ski base wear

Crystalline cold snow is abrasive and can remove the unwanted hair which is the side effect of the grinding process where molecular chains of polyenthylen are cut and material is removed to create grooves of different shapes. If unwanted technological hair can be removed by abrasive snow, it means automatically that also “wanted” hair of the ski base is subject of abrasion and removal.

If both “unwanted” and “wanted” hair of the ski base is subject of abrasion, it means automatically that ski base wears when used. The more abrasive the snow condtions are, the faster the ski base wears off.

Two contradicting positions

Position I.

Even if ski base structure is protected by ski wax which penetrates inside the ski base, the ski base and the structure on its TOP wears quite fast. The optimal gliding features of a specific structure are less than ca. 100 km on abrasive snow. In other words: after ca. 100 km of use on abrasive snow the structre is worn so significantly that grinded structure loses its optimal gliding features.

Position II.

If skis with a specific ski base structure performes optimally for certain snow and weather conditions, they need to be protected with ski wax application but never re-grinded. Even if the ski base structure wears slightly after each use, the optimal conditions should not be changed. Re-grinding will never restore the original quality.

What do you think, what is true and what is superstition only?