This concluding part on ‘How to Solve Issues With Calender Gauge Control?’ is a continuation from Part 2 on force induced variations in calendered gauge (or thickness) and focuses on the three tips.

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

3. Limit the temperature rate of change at all times to a maximum of 3 degrees per minute.

Because faster rates can damage or warp the rolls.

The control system on the TCU should be designed and operated to limit the rate of temperature change for each calender roll water circuit to 3 degrees per minute. Faster rates of change do not allow for equalization of mechanical expansion throughout the rolls, bearings, and calender frames. Differential expansion can stress various components of your calender. In extreme cases, they can actually crack or warp the rolls, or even damage the bearings.

Heating usually is limited by the available heating capacity but the cooling system frequently has the capacity to rapidly cool the rolls and therefore also has the potential to cause damage if not limited to 3 degrees per minute.

Again, it is important to keep the calender running during warm-up and during cool down. Stopping
the calender and turning off the TCU circulation pumps before the calender reaches ambient is not considered good practice.

If the temperature control system has rate of change limits properly set, cooling down is easy, since all the operator must do is to lower the temperature set points to ambient and then 60 to 90 minutes later return back to the operator console to turn off the TCU.

4. Minimize speed changes.

Because changes in your calender speed result in gum wall gauge thickness variations that are undesirable. 

When the calender is running there are forces generated that try to separate the rolls, as explained earlier. And these
separating forces are proportional to calender speed (although the relationship is not linear).

The hardness of your rubber stock and bank temperature determines the magnitude of separating forces. The
thickness of the gum wall being calendered also affects the separating force – thinner gum walls generate higher separating forces. Separating forces of 160,000 to 300,000 lbs at each actuator literally stretches the calender frame. When the forces vary, the amount of stretch varies. This further results in the roll-gap varying that in turn causes the gum wall gauge to vary.

Hence,  speed changes should be avoided to the extent practical. A speed reduction from 45 mpm (meters per minute) to 9 mpm can result in a gauge decrease of approx. 0.002 inches. Similarly, the vice-versa holds for a gauge increase with roll speed increase.

Calenders are of different types. And different calendars have different degrees of frame stiffness so the quantum of the gauge change for a given speed change will vary accordingly.

5. Mill management

Maintain consistency

You can substantially reduce gauge variations by feeding the calender consistently.

A consistent, uniform, dwell times on the cracker and feed mills, as well as consistent bank size, will give consistent rubber stock temperatures and work history. The stock temperatures, bank sizes, and work history directly influence calender roll-separating forces.

When you minimize variations in these 3 important process parameters, you will minimize variations in roll separating forces, and this in turn will minimize gauge variations of your calendered sheet.

Further, good mill and bank control practices also give improved uniformity for other final calendered gum and/or fabric properties.

When you feed to the calender, you must also ensure that the rubber bank does not have areas that could stagnate. Because, if rubber remains in the bank on the calender for an excessive amount of time it will partially cure and become much harder. Edge trim process that returns to the bank and stagnates is a common source of this problem.

Such cured and hardened compound will introduce gauge variations as well as negatively influence other properties of your calendered material. In extreme cases, burnt lumps will appear in the calendered gum.

Presence of hard stock makes it difficult to properly penetrate the cord and may cause cord disturbance and improper cord distribution.

In such cases the cushion force causes lateral movement in the nip and this further results in cord displacement that is visible. Paired cords are often the first indicator. You will observe that the calendered thickness will be heavy and there will be cord displacement or irregularities. However, the weight of test samples will not be significantly affected.

Localized cord disturbance in cord distribution does not usually affect fabric weight, whereas redistribution of the cord does. Overall cord distribution problems can affect fabric weight in the case of fabric calenders.

For example, incorrect cord distribution may reduce the average EPI (Ends Per Inch) in the center of the fabric and increase the EPI (Ends Per Inch) on the outer edges. This will affect fabric weight and thickness – the weight of the center would be light, while the outer edges would be heavy.

In steel cord calendering, visible cord displacement and cord pairing after the cushion nip is to be expected in the areas where the cord EPI count is excessive.

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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.

A Calendar Line at Bharaj Machines

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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Calender Gauge Control related variations was first in the list of of my earlier post 8 Common Defects In Rubber Calendering. Calender control issues directly affect your ability to properly control the rubber gauge or to produce a rubber sheet of consistent thickness. Rubber Calendering is the mechanical process by which rubber is pressed into textiles (cloth, fabric, tire cord) forming composite sheets.

While these sources are closely related, for quicker understanding, you may visualize and attribute the two main sources of variations in calendered gauge (or thickness) as,

1. Mechanical Induced  – variations when your calender is not operating under any load and is at ambient conditions.
2. Force Induced – variations when your calender is in operation or is programmed to follow a set-process for your product under manufacturing.

India’s First Fully Automatic Tyre Cord Calender Line From Bharaj Machineries.

In this post, I will touch on Mechanically Induced Variations

Mechanically induced Calender Gauge variations relate directly to your rubber calender’s mechanical condition. Thus it’s basically a maintenance issue. While there are many maintenance issues that could affect your calender performance, the following FIVE directly effect your rubber sheet’s calendered gauge

1. Grind Profile of Calender Roll (Roll Crowning)
2. Eccentricity of Calender Roll or Run-out at Ambient Temperature
3. Condition of Calender Roll’s Bearings
4. Condition of Calender Roll’s Water Passage
5. Condition of Calender Roll’s End Actuator

Let’s briefly examine each one.

1. Grind Profile of Calender Roll (Roll Crowning)

You must periodically grind the calender rolls to establish the proper profile. Your selected crown profile is based on
the range of your rubber compound hardness, type of center compensation such as cross-axis or roll bending, and roll width.

The forces that separate the rubber calender rolls to bend apart in the center are partially compensated for, by grinding the center to a larger diameter. When the grind profile is incorrect it will be difficult or even impossible for you to achieve a consistently flat-calendered rubber sheet.

2. Eccentricity of Calender Roll or Run-out at Ambient Temperature

Your calender rolls must be round and free of run-out at ambient temperature. Any run-out of the individual calender rolls will be amplified as the high spots and low spots periodically match up between the roll pairs. You can easily check this with dial indicators when the calender is running without rubber and with a small nip gap between the pairing rolls.

3. Condition of Calender Roll’s Bearings

You must ensure that your Rubber Calender roll bearings does not have excessive run-out. Excessive bearing clearance will result in the roll shifting during operation because the roll separating forces vary. This could appear as calender roll surface run-out and will adversely affect your ability to control calendered gauge.

4. Condition of Calender Roll’s Water Passage 

Most modern calenders have drilled rolls. A drilled roll has water passages across the roll face beneath their surface. These water passages effectively forms a radiator-like heat exchanger around the periphery of your calender rolls.

In an event when these passages become plugged, then non-uniform heat transfer will occur. And this results in a thermal ‘out-of-round’ condition for your calender roll which subsequently has has the same effect as if the rolls were machined ‘out-of-round’.

You may avoid this situation by using only treated water and ensuring that the rolls are periodically flushed. You could also can check this condition by comparing the roll run-out or eccentricty at ambient temperature with that at normal operating temperatures of your Calender.

5. Condition of Calender Roll’s End Actuator

Of the different variants – manual, motor-driven and hydraulic nip gap adjustments – electric screw actuators are the most common type of calender roll end-positioning-system. The screw and nut operate under very high loads and thus, you will observe, that they experience significant wear even with proper lubrication. As these actuators wear, backlash develops.

When the backlash increases, the response to small changes or corrections deteriorate. Further with increased backlash the number of corrections required would increase, which in turn causes additional wear.

Rubber calenders that have fixed speed electric motors with mechanical brakes require periodic maintenance, particularly the brakes. Because sticky brakes will adversely affect your gauge control results.

Summarizing, proper maintenance of all the above systems is crucial to your rubber calender performance and solve issues with calender gauge control that are mechanically induced.

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(In Part 2, we will cover the force induced variations)

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