Web Lines | Unwind Splice Reliability
- Published: October 25, 2016, By Tim Walker
To create reliable unwind flying bump splices, these 9 factors have to work together.
The unwind bump-splice process is a source of significant downtime, especially at higher speed coating of masking tape. Bump-splice reliability needs to be nearly 100% to reach coating productivity targets. During one overnight period, five downtimes were caused by the new unwinding rolls blowing open as they accelerated to bump speed.
There are several factors that all have to work together for reliable unwind flying bump splicing.
| Flying Splice Factor | Key to Reliability |
| 1. Splice Preparation |
Ensuring roll is free of damage, generally cylindrical. Avoid preparing the splicing tape in any flat spots from rolls sitting on the floor. Apply two-sided bonding tape, cut leading tail flush to tape edge, tape down leading tail with tabbing tape. Minimize buckles in prepared roll to avoid raised features that will have resistance to air (be aerodynamic, no “sails”). Align the splice to the reference position relative to the chucks or shaft, allowing the winder encoder to identify the rotational position of the splice. Some unwinder splice systems use a reflective strip to optically detect where the splice is on the rotation of the roll. |
| 2. Detect Unwind Roll Diameter |
An unwinder needs some method to know the diameter of the roll. Many unwinders use ultrasonic or laser triangulation sensors to detect the new roll diameter. This diameter is used to 1) position the turret rotation to create the proper space between bump roller and new roll, and 2) determine the initial RPMs to speed match the new roll to the web speed. NOTE: Errors in diameter measurement can create errors in both of these critical functions. |
| 3. Turret and Bump Roll Positioning |
The turret rotates to position the prepared new roll close to, but not touching the web path of the expiring roll. The bump roll moves close to, but not touching, the new roll with the prepared splice. NOTE: The turret rotation requires the expiring unwind roll to decelerate, then accelerate to let out the web length of the turret motion. |
| 4. Accelerating to Line Speed | The new prepared roll must accelerate to line speed. Faster speeds will have more air drag forces exerted on the tabbing tape. Overly aggressive acceleration may cause slippage of layer in the roll or slippage between the core and chuck or shaft. |
| 5. Bump Roller Fires and Creates Full Width Bond to Expiring Web | The bump roller must press against the speed-matched roll, making contact with the roll ahead of the splice tape position. The bump roll must remain in contact without bouncing while the transfer tape rotates
through the contact zone. This requires accurate reference of the splice position by the encoder and timing of the bump roller start and length of engagement. The bump roller also must make sufficient contract pressure across the entire width of the new roll. If the new roll has significant cross-roll diameter variations, the bump roller may only contact on the large diameter side and fail to bond to the small diameter side. |
| 6. Splice Tape Bond > Tabbing Tape Strength |
After the splice tape engages with the web, the bond of the splice tape must be great enough to break the tabbing tape. High spicing tape bond is a function of tape adhesive, bump roller pressure, contact time (which decreases with speed increases), and surface of the web (which if release coated, is more difficult to bond with). If the splicing tape bond it too low or the tabbing tape is too strong, the transfer will fail. Splicing tape adhesive is often thick, aggressive, and soft to form a bond quickly in the bump process. Bonding to release coated surfaces or low energy surfaces may require special adhesive formulations. Tabbing tape needs to be strong enough to stick to the roll, but weak enough to break so the new roll’s bumped and bonded leading tail will break away from the unwinding roll. Tabbing tapes are often “butterfly” or “dog bone” shaped to provide large areas of adhesion with a narrow, easily broken strip across the leading edge of the prepared new roll. |
| 7.Cutoff Knife Severs Old Roll |
The cutoff knife must fire into the web from the expiring roll after the splice tape runs through the bump roller contact zone. The web cutoff force is created by the web tension from the expiring roll interfering with the serrated blade position. The blade must have the proper position into the web path to enable the cut. The blade must remain sharp and free of contamination. The severed roll must immediately stop or reverse direction to keep the severed tail from being pulled into the bump roller nip point. Note: If the new roll speed is too high, slack may form after the bump and fall into the cutoff knife. |
| 8. New Roll Takes Over Tension Control | Immediately after the bump splice, the new roll’s motor must shift from speed match to tension control with dancer roller feedback. |
| 9. Splice Tape Bond > Tension (and Other Applied Forces) | The newly formed bond of the splicing tape to the web must withstand tensioning, roller contact, nipped roller or process stresses, elevated temperatures, and reach the winder without failure. |
In my next column, I'll provide more details on tabbing tape.
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; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.
