Cross Direction Web Profile

Liquid Applications Systems (LAS) saturate the web with water or water based chemical solutions through an applicator and metering roll system.  These systems sometimes incorporate a third roll to saturate the entire sheet.  The MCT360 Moisture Transmitter is used to measure the moisture and output a 4-20mA analog signal that is used to control applicator roll speed and achieve set point.  The digital output is sent to a separate PC for product roll archiving.

The Liquid Application System is typically used to re-moisturize the paper to eliminate edge curl or to impart a chemical component into the paper such as a mold inhibitor (food products) or fire retardant (cigarette papers).  The mold inhibitor applied in food products may subsequently be coated with a wax to seal the paper.     

Points of Control

There are typically three points of control for LAS re-moisturizers.  The gap, the skew and the applicator roll speed.  The gap limits the amount of liquid applied, the skew adjusts the roller alignment, and the applicator speed controls the amount of liquid applied for a given gap.

The gap is set according to manufacturer’s specifications and trial runs.  The skew is manually adjusted by the operator for an even profile.  The applicator roll speed needs to be adjusted as web speed changes.  Incorporating a moisture transmitter after the LAS greatly improves quality control, speeds startup, and ensures smooth operation independent of web speed.

LAS Moisture Control

The moisture transmitter measures the moisture content in % or the chemical additive in engineering units and outputs a 4-20 mA analog signal to a PID controller or PLC (Programmable Logic Controller).  The controller compares the real time measurement to the set point target value.  The controller has adjustments for Proportional Band (Gain/Amount of Change), Rate (Speed of Adjustment), and Reset Rate (How often it compares real time value to target value).  As the target set point is approached, the reset rate slows the adjustments down so as to not overshoot the target, and avoid a yo-yo effect chasing set point.  The controller sends a proportional signal to the applicator roll speed actuator to increase or decrease roll speed to maintain set point independent of web speed.

Basically, if you increase web speed, the applicator roll speed is increased to maintain the appropriate moisture or chemical additive addition to achieve set point.

Approaches to LAS Moisture Control

We recommend our Guardian Cross Direction Web Profile System as your first consideration.  The Guardian provides a complete cross direction web profile that assists operators in adjusting skew control and provides the outputs needed for applicator speed control.  The Guardian includes the MCT360 Moisture Transmitter, Automatic Scanning Frame System with position encoder, Industrial PC and Display and Profiling Software.

A less expensive approach is to mount the MCT360 Moisture Transmitter on an Auto/Manual Scanning Frame System.  This allows continual scanning of web, or auto jog to a fixed position.  It does not provide a web profile, but operators can use it to adjust skew, and the output can be used for control.

A third option is a stand-alone MCT360 Moisture Transmitter mounted above the web to measure the moisture % or engineering unit.  The operator is responsible for the skew adjustment and implements control when comfortable that the profile across the web is uniform.

Helpful Hyperlinks

MCT360 Brochure

Guardian Brochure

Guardian Scanning Frame Operation Video