Continued from Part 1, this post is on force induced variations in calendered gauge (or thickness).

Force Induced Variations

Force induced variations result from the way your calender is operated. This is also an outcome from previous process steps such as your feed mill operation and compound consistency from rubber mixing.

The separating forces generated during calendering are quite high! For example, if you have a 66 inch calender roll and producing 60 inch wide rubber sheet, the separating forces are in the range of 72,575 Kg (approx. 160,000 lbs) to 136,000 Kgs (approx. 300,000 lbs) at each actuator.

Such high forces literally stretch your calender frame. Variations in these forces will therefore vary the amount of frame stretch and thus vary roll position and calendered gauge of your rubber sheet.

Here are FIVE tips on how you could solve issues of calender gauge control from force induced variations.

1. Keep the rubber calender running
2. Increase the temperature by 5 degrees Celsius when you stop the rolls.
3. Limit the temperature rate of change at all times to a maximum of 3 degrees per minute.
4. Minimize speed changes.
5. Mill management

Let’s examine each one.

1. Keep the rubber calender running.

Because stoppages cause your calender rolls to become egg-shaped, that in turn introduce significant gauge thickness variation.

When you stop the rubber calender, the heat loss from the rolls is not uniform around their surface. Heat loss along the face of the rolls near adjacent rolls is minimal, while heat loss in other areas that are open is higher. This differential heat transfer leads to different temperatures on your calender rolls. And therefore different degrees of calender roll expansion.

If you have a calender with rolls that is 24 -30 inches in diameter, even a few degrees of expansion will result in “out-of-roundness” of each roll that you can measure. This is true when the calender is empty but even more so when there is a hot bank of rubber between the rolls. If you stop the calender for longer period, this condition worsens further.

Your calender rolls run at different speeds. This means that periodically the high and low spots on the rolls match up. When high spots of adjacent rolls match up, you get thin gauge spots on your calendered sheet. When the low spots match up a thick spot is observed. So calender stops induced variation results in an effective doubling of any roll “out-of-round” or run-out error.

Typical short-term thermal out-of-round gauge variations are (+/-) 0.0005 to .002 inches. Though thermal run-out is gradually reduced as the calender runs, it takes 15 to 25 minutes for variations to be eliminated.

Experts recommend you to adopt the following steps to minimize “thermal run-out” of rubber calender,

• Keep the calender running during warm up. If not done, severe thermal run-out can be introduced. You need to remember that the calender nip gap between adjacent rolls will be reduced as the rolls enlarge with their temperature increases. So, a good practice is to open your rubber calender at least 0.10 inches (2.54 mm) before starting to increase the roll temperature above ambient. Keeping the calender rolls turning during warm-up and cool-down also eliminates the possibility of warping the rolls.
• Minimize calender stops.
• When you stop the calender for longer time, remove the rubber from the banks. This is a good operating practice for the rubber compound as well.
• When leader is going thru the calender, dropping off tension and allowing the uncoated fabric or leader to go slack will permit running the calender during delays and personnel breaks. This will dramatically reduce thermal run-out.

2. Increase the temperature by 5 degrees Celsius when you stop the rolls.

Because this helps to maintain a more uniform roll surface temperature.

You need to realize that the TCU (Temperature Control Unit) of your rubber calender controls the temperature of the water exiting the rolls and not the ‘roll surface temperature’. This is an important distinction.

Visualize these two scenarios –

Scenario 1 – When the calender is not processing rubber (i.e. during warm-up and when the calender is stopped), the roll surface is losing heat to the atmosphere. Here, the TCU is actively heating the water loop. In this condition the roll surface temperature is below the water temperature.

Scenario 2 – When the calender is running and processing rubber, heat is being generated. Here, the TCU is cooling the water loop. In this condition the roll surface temperature is above the water temperature.

From the above, you will realize that for a constant water circuit temperature, the roll surface temperature swings (above & below the water temperature) between the calender normal running condition and when the calender is stopped. This difference in roll surface temperature means that your rubber compound is being processed under varying conditions. Shrinkage and other properties of your processed compound will therefore vary.

To minimize the differences from such processing condition variations, the roll temperatures should be increased whenever the calender stops. The temperature increase should be gradual. Your specific value should be experimentally determined by comparing roll surface temperatures in normal operation and after the calender has been stopped for 20 minutes.

In the concluding Part 3, you will see more on the other tips

• Limit the temperature rate of change at all times to a maximum of 3 degrees per minute.
• Minimize speed changes.
• Mill management

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