Who Measures Slip/ Friction?
Friction testing is used in the packaging industry to measures the slip resistance of a product, with the aim of predicting feeding and runnning speed on an automatic gluing, erecting, filling or packaging line.
What is slip/ friction?
A products slip resistance is characterised by it's coefficients of friction-
Static coefficient of friction = Fs/N
Dynamic coefficient of friction = Fd/N
Where Fs is the maximum static frictional force and the 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.
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, a gradually increasing force is applied to the second sample until the samples begin to slip against one another (Fd Max).
Inclined Plane Friction Testing
The Hanatek Inclined Plane Friction Tester removes this uncertainty by using a motorised platen and electronically measuring Fd Max.
Flat Bed 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 Advanced Friction Tester from Hanatek 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 filmweight.
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.
How can detailed Frictional Force measurement help improve Productivity?
The Hanatek Advanced 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.