FACTS Tech Blog

The following relates to fabric calenders, both textile and steel cord only.

Localized cord disturbance in cord distribution does not usually affect fabric weight, whereas overall cord distribution problems can affect fabric weight, sheet gauge, and quality.

Read more…

A consistent feed to the calender will substantially reduce gauge variations. Consistent, uniform, dwell times on the break down and feed mills, as well as consistent bank size, will result in consistent
stock temperatures, work history, and reduced variations in roll separating forces.

The stock temperatures, bank sizes, and work history all directly influence roll-separating forces. Minimizing variations in these 3 important process parameters will minimize variations in roll separating forces, and this in turn will minimize gauge variations. Good mill and bank control practice also results in improved uniformity for other final calendered gum and/or fabric properties. Read more…

When the calender is running there are forces generated that try to separate the rolls. These separating forces are proportional to calender speed, although the relationship is not linear. The hardness of the rubber stock and bank temperature determines the magnitude of separating forces. The thickness of the gum wall being calendered also affects the separating force, with thinner gum walls generating larger separating forces. Read more…

Limit the temperature rate of change at all times to a maximum of 3 degrees per minute F. Faster rates can damage or warp the rolls.

The control system on the TCU should limit the rate of temperature change for each roll water loop 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 and in severe cases actually crack or warp the rolls, or even damage the bearings. Read more…

Increase the temperature by 20 to 40 degrees F when the rolls are stopped to maintain a more uniform roll surface temperature.

The TCU (Temperature Control Unit) controls the temperature of the water exiting the calender not the roll surface temperature. This is an important distinction. When the calender is not actively processing rubber, i.e. during warm-up and when the calender is stopped, the roll surface is losing heat to the ambient and thus the TCU is actively heating the water loop. In this condition the roll surface temperature is between 10 degrees to 20 degrees F below the water temperature. Read more…

Severe thermal run-out can be introduced during warm-up of the calender if it is not kept running. Remember that the calender gap will be reduced as the rolls grow (get larger) as their temperature increases. Good practice is to open the calender at least 100 mils (0.10”) 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.

Keep the calender running – stops cause the rolls to become egg shaped and introduce significant gauge thickness variation.

When the calender is stopped, roll heat loss is not uniform around the roll. Heat loss along the circumference near adjacent rolls is minimal while heat loss in other areas is much higher. This leads to different temperatures and therefore different degrees of roll expansion. A few degrees on a 24” to 30” diameter will result in measurable “out-of-roundness” of each roll. This is true when the calender is empty but even more so when there is a hot bank of rubber between the rolls. The longer the calender is stopped the worse the condition. Read more…

Force induced variations result from

• the way the calender is operated and
• from previous process steps such as feed mill operation
• compound consistency from mixing

In rubber calendering, separating forces generated are quite high; typically 160,000 lbs to 300,000 lbs at each actuator for a 66 inch active roll face producing a 60-inch wide rubber sheet. Those forces literally stretch the calender frame. Variations in these forces will therefore vary the amount of frame stretch and thus vary roll position and calendered gauge. Read more…

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 experience significant wear even with proper lubrication.

As the actuators wear, back lash develops. As the back lash increases, the response to small changes or corrections deteriorates. Also with increased backlash the number of corrections required would increase, which in turn causes increased wear.

In addition, most calenders still have fixed speed electric motors with mechanical brakes. The breaks particularly need periodic maintenance. Sticky brakes will adversely affect gauge control results.

Most modern calender rolls are the drilled type. This means that water passages are drilled across the roll face under the surface. This effectively forms a radiator-like heat exchanger around the periphery of the roll.

If these passages become plugged, then non-uniform heat transfer will occur and this results in a thermal ‘out-of-round’ condition for the roll or rolls. This has the same effect as if the rolls were machined ‘out-of-round’.

To avoid this condition only treated water should be used and the rolls should be periodically flushed.

How do you check for plugs? Compare the roll run-out at ambient temperature and at operating temperatures.