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Slip & Friction FAQs

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Who measures slip/friction?

Friction testing is used in the packaging industry to measure the slip resistance of a product, with the aim of predicting feeding and running speed on an automatic glueing, erecting, filling or packaging line.

Other industries that test for slip include the paper industry (e.g. for the automatic feeding of envelopes and banknotes) and plastic manufacturers (the frictional properties of packaging films).

What is slip/friction?

A products slip resistance is characterised by its coefficients of friction:

Static coefficient of friction = Fs/N

Dynamic coefficient of friction = Fd/N

Where Fs is the maximum static frictional force and Fd is the average dynamic frictional force. N is the Normal force, ie the force of gravity acting on the sample and test sled.

In practical terms, the static slip relates to the force required to get two resting surfaces moving, dynamic slip is the smaller force that is required to keep the surfaces moving once this initial “inertia” is overcome.

These values are expressed as ratios and do not have units, they are usually quoted as a decimal value between 0 and 1, for example, a surface might have a static slip coefficient of 0.35 and a dynamic slip coefficient of 0.18.

How is Coefficient of Friction measured (COF)?

All methods of COF measurement involve preparing a sample into two flat pieces, the samples are placed together and a constant weight is applied to represent the normal force. One piece is fixed, and a gradually increasing force is applied to the second sample until the samples begin to slip against one another (Fd Max).

How to measure friction

Horizontal Plane (flatbed) Friction Testing

To measure Static and Dynamic coefficients of friction it is necessary to use a fixed bed instrument. These instruments use a motor to pull a sled across the sample, using a load cell to measure the forces.

Original instruments were converted tensile testers that used a cord to pull the sample, the use of a cord has now been removed from many friction measurement standards due to the uncertainty added by its own elasticity and problems with sample positioning.

The Universal Friction Tester uses mechanical linkages to apply the force and uses automatic sled placement for very accurate sample positioning with variable dwell time before testing.

How can Coefficient of Friction (COF) values relate to packaging speeds?

COF can often be related to the feeding and running attributes of products, for instance, U.V. varnished food cartons have a slip coefficient that is related to the formulation of the U.V. coating its cure and film weight.

Cartons that have a very low static coefficient of friction may have handling difficulties as they will tend to slide apart and are difficult to place into feeding hoppers.

In contrast, products which have a high coefficient of friction will tend to stick together and can be prone to misfeeding due to multiple cartons entering the packaging line.

Different packaging lines will often require products with specific surface frictional profiles to achieve their highest running and feeding speeds, it is only by measuring and specifying these values that a manufacturer can achieve maximum productivity.

What parameters affect Coefficient of Friction (COF) values?

COF is primarily influenced by the chemical composition of the surface and its surface profile, in packaging, this is often a coating applied to the packaging. Chemical additives are used to adjust the slip resistance, these additives are often waxes or silicones which change the profile on a molecular level and alter the atomic attraction of the surfaces.

Other important factors that affect COF are test speed, the normal force (mass of the sled), contact area and geometry of the sample, these values are often specified in the test method (ASTM D1894 and ISO 8295).

How can detailed frictional force measurement help improve productivity?

The Hanatek Universal Friction Tester allows the user to measure and store the full force curve which graphically illustrates the frictional characteristics in addition to providing the static and dynamic COF values.

These force curves give more detailed information about the surface of the product allowing us to better understand how the surface will perform in the production environment.

The graphical analysis shows inconsistencies in the surface and identifies other characteristics such as stiction, an attribute during dynamic slip when the surface “judders”.

The unique strength of the Hanatek Universal Friction Tester is that profiles can be overlaid for comparison, allowing identification of substrate or coating changes that can cause problems with product runnability.

This powerful feature can highlight subtle differences in substrates or coatings that allow the user to fine-tune their product for their production conditions giving optimum feeding, running and packing speeds.

FAQ Category: Slip & Friction