Most surfaces, even those that are considered polished, are extremely rough on a microscopic scale. When two characters are brought into contact, the tiny peaks that compose them determine the actual contact area, a small proportion of the apparent contact area (the area of the base of the block). The real contact area increases with increasing pressure (the normal force) as the peaks deform as they rub against each other due to friction, so if you want to know how strong two materials are, a scuff test is ideal.
A scuff coefficient test identifies the friction values of the materials and how the different surfaces of the materials relate to each other. For example, if the coefficient is too high, the material cannot be fed through all the machinery efficiently, causing jams.
The coefficient of friction is the ratio of the sliding force to the holding pressure exerted by two surfaces in contact. This coefficient is, in fact, an evaluation of the difficulty with which the surface of one material will slide over another material.
There the following types of coefficients of friction :
- The static (or starting) coefficient of friction is related to the force required to initiate the sliding motion. This is usually the maximum force of the test.
- The kinetic (or sliding) coefficient of friction is the average force measured during the movement.
A friction table mounted on a motorized testing machine is usually used to measure coefficients of friction in packaging. Motorization is necessary to have a constant speed and to avoid variations due to speed during the measurement.
The friction table consists of several elements:
- A support table to place and hold the sample.
- A block of known mass
- A training system
A scuff testing system is capable of measuring the static and kinetic coefficient of friction values. The scuff testing is performed by pulling a flat block of known mass through the material placed on a flat table. The maximum force required to initiate motion (static) and the average power needed to maintain movement (kinetic) are accurately and repeatedly measured to determine the coefficients.
Scuff testing is performed by sliding the block of known mass over the specimen. The partnership is driven by the motorized frame, which measures the continuous tensile forces.
The force is recorded throughout the test, and the following results are noted:
- The maximum force (F0) corresponds to the static coefficient of friction. This is the force required to initiate the movement of the block.
- The average force (Fm) in the regular part of the curve corresponding to the dynamic friction coefficient.
Scuff testing allows determining the first maximum force (static friction coefficient) and the average between two times (dynamic friction coefficient), which helps optimize the production line’s operation. This solution is used in several factories of significant industry players.
Scuff testing usually employs a system that is controlled by a PC. The software allows a graphical representation of the test to be plotted on the screen, with automatic calculation of static and kinetic friction coefficients. This provides further data on the material’s surface structure and frictional properties, including “friction” (when the surface vibrates during dynamic sliding).
Scuff testing will provide the company with valuable information to optimize their processes, such as machine synchronization. Anomalies that may have caused poor performance in the past can be easily identified, resulting in increased production line efficiency while minimizing the risk of jams.