Keep your machine calender in tune with changing production

For many paper and board grades, the machine calendering process represents the last chance to affect sheet properties. Variations to production such as machine speed-up or a grade or furnish change can reduce the effectiveness of an existing machine calender.

Production variations represent disruptions to calendering balance and will likely have a negative impact on important sheet properties like caliper and smoothness. Machine calenders must be able to control CD profiles, particularly caliper, so that paper is uniform enough for successful reel build-up and for making good shipping rolls on the winder.

Possible upgrades

There are many options for upgrade to a machine calender. For example, when a machine speeds-up, typically more calendering load capacity is needed to get the same or better finish and caliper at the increased speed. Multiple nips can provide the calendering load required and deflection compensated rolls along with hydraulic load support systems help deliver uniform loads throughout each calender nip to minimize variations in caliper profile.

But multiple nips can cause problems as well. Nip pressure widens the sheet but in the short time between each nip, the sheet doesn’t have time to fully spread out. Multiple nips increase the amount of spread causing wrinkles that lead to calender induced "cuts" in the web.

Barring prevention

Calender barring is a problem common in multinip calenders as well. Barring typically begins when the sheet coming into the calender stack contains MD variations but it can also be caused by increased nip load, increased speed, paper moisture content – even factors like press section vibration and wet end flow instabilities. Machine Calender Barring Prevention from Valmet eliminates calender vibrations, thereby elevating linear load limits, providing potential for higher speeds and improved paper uniformity. Changing a pneumatic roll load system to hydraulic can also help eliminate barring problems and provide better control of nip loading and improved runnability.

Fewer nips

On certain paper grades such as recycle-content newsprint and uncoated fine papers, a conversion to a single nip calender is the best option. Theoretically, the same surface quality achieved in a multi-nip calender can be achieved in a single nip calender if the same total applied pressure is used. Fewer nips also tend to improve runnability and sheet stretch is controlled eliminating a cause of calender cuts and wrinkles.

Profiling capability

Profiling can be accomplished several ways such as through the use of hot air, induction heating or Valmet SymZ or SymCD zone controlled rolls. Benefits of using deflection compensated rolls include quick response time, less energy consumption and no additional space requirement. Applying heated roll technology to either a multinip or single nip calender can produce better web surface quality, uniformity and smoothness while at the same time allowing lower nip pressures for preserving bulk and strength properties.

Steam shower

Installing a steam shower prior to the first nip is an effective way of raising the sheet moisture content and sheet temperature before calendering. On paper and board grades, which are receptive to steam application at the dry end, it is an effective way of improving smoothness, reducing caliper and controlling two-sidedness. A profiling steam shower is an additional tool for improving CD profiles, especially caliper and smoothness.

HMI upgrade

Another key machine calender rebuild option is the integration of existing equipment with new PLC hardware and software. Valmet can provide machine calender controls upgrades such as the installation of operator interface software, or an operator interface upgrade with new I/O and hardware and software for PLC communication links.

There are limitations to both multiple and single nip machine calenders, some might call for replacement of the existing calender with soft nip calendering technology. A close look at the desired paper or board end product properties is necessary before accurately determining the optimum machine calender upgrade possibilities. In many cases, the first step is to have Valmet conduct a machine calender audit to analyze the condition of the machine calender in terms of end product quality, machine runnability and maintenance.

For more information about machine calender audits and upgrades to your existing machine calender, contact your Valmet representative.