Force-controlled nip rollers or winding nip magnitude is too high, too low, or zero.
Problems with nip loads or pressures too high:
- Creates roller, core, or winding roll deflection and associated problems
- Rubber coverings wear more quickly, overheat, or come apart.
- Product will be overly compressed and may be permanently damaged.
- Web may break.
- Rolls may wind too tight, forming hard roll defects.
Problems with nip loads or pressures too low:
- Drive point slips with small tension differential.
- Drive point air lubricates and slips.
- Laminating forms low bond strength between laminate layers.
- Laminate will have air bubbles.
- Wound rolls will have air entrainment problems.
- Embossing and microreplication process will not reach intended feature depth or height.
- Product density is not changed enough.
- Roll fails to wind tight enough, forming soft roll defects.
- Nip winding forms slackness ahead of nip point, potentially forming accumulation wrinkles.
- Calculate the load created by roller assembly effective weight, counter weights (if used), and pneumatic (or hydraulic) loads.
- Adjust supply pressure to determine minimal pressure required to open and close the nip.
- Measure pressure-dependent product properties.
- Monitor roller wear, re-grinding, and re-covering rates.
- Calibrate nip load with force gauge.
- Calculate roll weight vs final roll size and nipping geometry.
- Check geometry of nipping roller closing direction aligns to fixed roller rotational axis.
Position-controlled nip roller forces are too high, too low, or zero.
Problems with nip loads or pressures too high:
- Creates roller, core, or winding roll deflection and associated problems
- Rubber coverings wear more quickly, overheat, or fall off roller core.
- Product will be overly compressed
- Web may break.
- Rolls may wind too tight, forming hard roll defects.
Problems with nip loads or pressures too low:
- Drive point slips with small tension differential.
- Drive point air lubricates and slips.
- Laminating forms low bond strength between laminate layers.
- Laminate will have air bubbles.
- Wound rolls will have air entrainment problems.
- Embossing and microreplication process will not reach intended feature depth or height.
- Product density is not changed enough.
- Roll fails to wind tight enough, forming soft roll defects.
- Nip winding forms slackness ahead of nip point, potentially forming accumulation wrinkles.
- Calculate load, force, or pressure from rubber compression model.
- Use dial indicator or LVDT to calibrate the “zero” / minimal contact and measure the difference between zero position and controlled nip gapped or indented position.
- Ensure position-controlling stops are equal on both sides of nipped process.
- Check to ensure closed nipping roller reaches and stays on gap controlling stops.
- Measure the force to contact with gap controlling stops.
The force or pressure created by nipping rollers (or nipping against a winding roll) varies with time or process events and creates problems from being too high or too low.
- Nip-controlled product properties will have MD variations
- Drive points may have intermittent slip if the low nip load is insufficient to maintain the desired tension differential.
- High and low nip load problems occurring over time. See Problems of Force-Loaded Nip Variations.
- In nipped processes, nip transients may be measurable in product properties, such as bar marks or thickness variations.
- Short of bouncing open and closed, it may be difficult to measure nip load transients in nips at drive points.
The web tension distribution varies cross machine (or crossweb) direction (CD) from operator to drive side of the web.
- Pressure-dependent product properties will have CD variations.
- Web will have CD shift, bending or converging and wrinkling.
- At drive points with larger tension differentials, the web will slip through in low nip load lanes and not in high load lanes, shearing, shifting, and wrinkling the web.
- Both high and low variations will occur across the nip width. See Problems Force-Controlled Nipping Too High or Too Low.
- Check to see if a bright light can be viewed all along the CD length of the ‘closed’ nip’s contact zone.
- Sample and measure product pressure-dependent properties vs width.
- Measure nip MD footprint length vs width by any of several methods.
- Measure pressure vs MD and CD position with electronic pressure mapping system.
- Install load cells to measure and confirm the forces of pressurized cylinders.