Moisture in Paper Webs

Paper converting is important in preparing paper manufactured from wood pulp at mills to be converted to usable paper products from note paper to paper cups, tags, tapes, labels and more.  Paper is the most widely used wrapping, writing, printing and packaging material in the world.

Paper converters need to control moisture to maintain flatness and eliminate edge curl.  If the top and bottom of a paper sheet are at different moisture levels, the sides expand or contract when equilibrated causing curl.  The curl can cause malfunctions (jamming) in the subsequent printing, slitting and packaging unit operations that increases scrap and reduces efficiency.  Converting companies vying for the high-quality performance paper market focus on flat and level sheets.

Summary of Process

Re-Moisturizers

Paper can gain or lose moisture based on the surrounding humidity when in transit or storage.  Many converters measure the incoming paper stock prior to coating the paper, to ensure that the coating will be properly applied to the paper substrate.  If the paper moisture is too low, they will run it through a re-moisturizer.  Re-moisturizers can be roll, steam, disk, spray or another implementation technique.  Roll re-moisturizers can be optimized by adjusting applicator roll speed, gap and skew.  Steam re-moisturizers are adjusted by increasing the volume of steam applied to the paper.  Disk re-moisturizers can be adjusted by water volume or disk rotation speed.  Spray re-moisturizers are controlled by water volume or the number of micro-sprays applied per second.

Water Based Coatings

Converters employ different applicators for water based coatings dependent on their needs.  These applicators can be roll, spray, doctor blades, dip, air knives, and other techniques.  These coatings will have a fixed liquid/solids ratio depending on the product.  The wet end water based coating can be measured based on the moisture absorption in the Near Infrared region of the light spectrum.  This measurement will provide the amount of dry end coating deposited on the paper after the dryer in engineering units such a #/ream, GSM, mils or other.

Dryer Control

Dryers can be convection, radiant, infrared or other technique.  Paper moisture can be controlled by web speed through the dryer or dryer temperature.  Typically, web speed is the preferred method as dryer temperature control can be slower to react to system upsets.  Dryer cans are essentially steam heated cylinders where the temperature and contact time control the heat driving off moisture.

 

Quality Parameters and Measuring Points

Moisture Measurements:  Paper is cellulose based and hydroscopic.  Paper moisture is usually between 3 and 6% moisture in its finished state dependent on surrounding humidity.  NIR transmitters are useful at every unit operation of the converting process including incoming raw material, re-moisturizing stations, coating, drying and data archiving of finished product.

The MCT series of NIR stand-alone transmitters are usually mounted in the center of the web or in series on the left, right and center of the web.  This is of great value to the converter however it doesn’t cover the complete web and as such if a steam valve or disk rotation clogs or fails and it’s not in the path of the fixed transmitter, it will not be detected.  Similarly, if dirt or specks of coating attach to the roll in places where the web isn’t being monitored, streaking can result.

The Guardian Cross Direction Web Profile System supplies a cross direction zoned web profile and machine direction trend for system operation.  The color-coded CD profile visually displays in-spec product as green zones, alerts in yellow and red alarms.  This is often connected to a color-coded Stack Light with annunciator to alert operator when product is in the alarm condition.  Machine Direction Trends allows troublesome zones to be diagnosed and corrected.

Value and Quality

Implementation of Near IR technology to measure moisture at unit operations speeds start up, increasing production, reduces scrap and allows for data archiving by roll or run.  Without an on-line measurement system, the line must be started up, sample collected for laboratory analysis, system optimized and started up again with another sample collected to confirm the optimization.  With an on-line system this process is stream lined resulting in greater efficiency.  The data archiving, whether from fixed or scanning transmitters via Ethernet communications protects against customer complaints and allows data histories to be associated with modifications to unit operations or work shifts.

Guardian Multi Display Profiles 072916 USE

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