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,
- Mechanical Induced – variations when your calender is not operating under any load and is at ambient conditions.
- Force Induced – variations when your calender is in operation or is programmed to follow a set-process for your product under manufacturing.
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
- Grind Profile of Calender Roll (Roll Crowning)
- Eccentricity of Calender Roll or Run-out at Ambient Temperature
- Condition of Calender Roll’s Bearings
- Condition of Calender Roll’s Water Passage
- Condition of Calender Roll’s End Actuator
Let’s briefly examine each one.
- 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.
Do you agree?
(In Part 2, we will cover the force induced variations)
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