Advancements in COF Tester Technology


Coefficient of Friction, commonly referred to as COF, is a ratio of the force required to move one surface over another to the total force applied normal to those surfaces. In more common words – COF measures how slick or slippery a surface is. In this post we will talk about COF within the context of aluminum foil based packaging.

COF is broken down into two readings in COF tester, Static and Kinetic. Static COF is the ratio of force required to move one surface over another from the instant motion starts while Kinetic COF is that same ratio once the surface is already in motion. To once again break it down in more common terms, Static COF is the force required to get moving while Kinetic is the force required to keep moving.

Why COF Matters in Packaging
COF can be vital to the performance of a wrap on a packaging line, particularly on high speed packaging lines. Any time a wrap or wrapped item is pushed, pulled, dragged, or grabbed throughout the wrapping process its COF comes into play. A variety of wrapping problems through modification of COF has been solved. When the wrap isn’t wrapping tight enough around a customer’s chocolate pieces, lower the slip on the wrap to allow the end folds to slide more easily over one another leaving a tighter overall seal. When errors in another customer’s lines are observed while chocolate piece is transferring from one grabbing mechanism to another, lower the COF to allow a cleaner release from the grab. Although less common, issues can also arise if COF is too low on a wrap. In certain packaging lines, wrapped items are moved from one stage to another using wheels or belts and a wrap with too low of a COF might be difficult to grab or move.

Measuring COF
There are a variety of methods for measuring COF and a variety of machines designed to do so. Use a TMI Slip and Friction Tester (Fig. A below) which uses a calibrated load cell to determine both static and kinetic readings for our products. Our procedure for using COF tester is to lay a long strip of the material on our device, then use the machine to drag a smaller square of that same material across the strip while the machine gathers data.

Choosing the Right COF
When it comes to selecting the right range for COF on your foil packaging, we have found that at the end of the day the best determinant of selecting an appropriate COF tester is wrap speed. The faster your machine, the lower the COF spec should be. We have put together the chart below as general guidelines for choosing both a Kinetic and Static COF Spec.


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