- March 01, 2010, By Timothy J. Walker Contributing Editor
Web guiding done right is easy to forget. The principles of web guiding are well understood. With good decisions in the design phase of a process and a couple adjustments at startup, many automatic guides are in the “set it and forget it” mode.
To help more processes into the good guiding groove, review this four-part series, which starts this month with “What is lateral registration, and where do you need it?”
Lateral registration is the left-right or crossweb relative positioning of two or more features of your web or equipment. To most people, registration is something used in printing and packaging to align one print or die-cut pattern to another. In these cases, registration has both machine and crossweb direction components.
Webs without machine direction patterns still need lateral registration. Whereas printing and die-cutting may have tolerances of ±3-10 mils (thousandths of an inch), most other lateral registration needs have much lower tolerance (but equally important).
Let's review the most common needs and ranges of lateral registration:
This seems like a simple one, but the web should stay on the face of the rollers and not track over the end of the roller. For most idler rollers, this is usually 2 in. or more. For a 58-in.-wide web, the rollers typically would be 62 in. wide or greater. This gives you ±2 in. of lateral shifting without tracking off roller, assuming the rollers are centered on the machine centerline, which often they are not.
If you want to see how good your tracking is, try to run a 61.5-in. web on your 62-in. rollers. I think you will find ±0.25 in. of clearance isn't enough.
- Equipment Slots
The entrance and exits of most oven, curing, dip tank, or vacuum processes require the web to pass through a slot designed to minimize the leakage of gas, heat, or radiation. Slots may be either generous (±1 in. or more) or tight (I've seen slots with less than 1 mm clearance).
- Air Nozzles, Turns, and Flips
Tracking off the edge of any forced-air web handling device usually leads to crashing into a stationary object and web breaks. All air nozzles, turns, and flips should have the same lateral buffer as rollers (±2 in. or more), unless they are preceded by a well-tuned web guide.
- Coating and Laminating
Most coating processes coat something short of full web width, leaving a small uncoated margin at the edge of the web. If the web tracks off more than this margin of error, usually between 1/10-½ in., the coating will go onto the back-up roller or someplace where it shouldn't be, again often leading to web breaks. Lamination processes apply one web to another (and may include a coated layer), again with some tolerance to misalignment, beyond which creates waste.
- Winding Core and Sidewall
The winding should be centered on the core or at least not hanging over the core's end. For flush cores, where the web and core width are essentially the same, there always will be a slight error in lateral position. (I consider less than 50 mils to be well aligned.)
More challenging is a roll's layer-to-layer alignment with an ideal sidewall having the shine of a phonograph record. When making a roll for internal use, many wound rolls do not have or need this level of perfection, they just need sidewall alignment that is not easily damaged in shipping or handling. However, whether for internal or external customers, a good-looking roll sidewall, like a fresh coat of paint, will always sell better and lead to happier customers.
Now that we know our guiding needs, we can move on over the next three months to understand why our web isn't where we want it, what force is required to bring it back, and where automatic guiding is needed.
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; email@example.com; www.webhandling.com.