- September 01, 2010, By Timothy J. Walker Contributing Editor
“Good morning, Mr. Phelps. Many converting processes rely on uniform nip pressure to create a uniform product (and satisfied customer) independent of crossweb position. However, our best agents have been foiled in finding an inexpensive method to infiltrate the dangerous combat zone between nip rollers. Your mission, should you decide to accept it, is to determine a repeatable, yet inexpensive methodology to assess whether any nipped process meets our goal of crossweb uniformity or has fallen to the forces of variations.”
Nipped processes are under assault from many evil forces trying to thwart our best efforts at providing consistent uniform nip pressures. The perfect nip system has cylindrical, nondeflecting, aligned rollers pressing evenly on uniform webs and coatings without temperature variations.
However, the perfect nip is a rarity. All efforts for uniform nipping are threatened by diameter variations from wear or poor machining, deflection from gravity and end-loading, misalignment or uneven loading from nipping mechanisms or gap control, uneven thickness in webs and coatings, and uneven thermal expansion from poorly designed heating or cooling rollers and uneven product thermal loading.
With a goal of measuring what happens between two nipped rollers, there are many options available to you ranging from more than $40,000 to less than 25¢.
There are two approaches to measuring nip uniformity. The more difficult and expensive option is measuring pressure between the rollers using a resistive, capacitance, or strain-based sensor. The far simpler option is to measure the machine direction contact length, also known as the nip's footprint length. A forensic pathologist can estimate your weight from your footprint by modeling the combination of your shoe size, footprint depth, and compressibility of the ground. A web handling pathologist can estimate nip load from roller material and geometry and by measuring the contact length between nipping cylinders.
For small indentations, the footprint length will increase proportional to the load (in force per width) raised to the 1.5 power. A 20% footprint contact length increase reveals a 31% load change. A 50% or 100% footprint change represents an 84% or 183% load change. If you have a 3x footprint variation, you have a greater than 5x load variation.
How do you measure nip variations for 25¢? With tape! This method works best if one of your rollers is fairly smooth and you can see through the tape so you can easily identify where there are entrapped air bubbles between the tape and roller or a bubble-free wetted contact. I have found many polypropylene splicing tapes work well. Follow these steps:
- With the nip open, carefully place several strips of thin splicing tape on one of the nipping rollers without pressing out the bubble trapped between the tape's adhesive and the roller.
- Rotate the tape to the nip contact zone and close the nip with the desired nip load.
- Open the nip and rotate the sample out of the contact zone.
- Now measure the machine direction length of the “wetted” contact area where the bubbles have been pressed out.
- Plot the footprint length versus crossweb position and check for uniformity.
- Repeat the test at different loads to ensure your system provides uniformity at all your typical process conditions.
If you wondered who Mr. Phelps is, you will have to search the web for old “Mission: Impossible” episodes. This column will self-destruct in five seconds.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.