Materials such as paper, plastic film, foil, and cloth often are produced in long, continuous sheets that are rolled up for more-convenient handling and transportation. These rolls of material vary significantly in size and weight—ranging from 2- to 200-in. wide and weighing as much as several tons.
The Converting Industry takes these continuous rolls of thin, flat materials—known as webs—threads them though processing machines (such as printing presses, laminators, coaters, slitters, etc.), and changes the web of material into an intermediate form or final product. For example, a converter’s equipment might take a web of plastic film, cut it into lengths, and fuse their edges, thus converting it into plastic bags. This activity is known as web processing.
Keeping the Web on Track
A moving web of material has a tendency to tend to track off course and wander out of alignment during converting processes for any of several reasons. When a web is out of alignment with the equipment processing it, the quality of the finished product may suffer, and scrap rates can escalate. To avoid these problems, designers have developed a variety of automatic web-guiding systems that ensures production accuracy and reduces waste.
Web-guiding systems typically are positioned just before a critical stage on a converting machine (for example, just before a print station on a printing press). Each type of web guiding system uses a sensor to monitor the web position for lateral tracking, and each has an actuator to shift the running web mechanically back on course whenever the sensor detects movement away from the set path. Actuators may be pneumatic or hydraulic cylinders, or some kind of electromechanical device.
Because the web may be fragile—particularly at its edge—non-contact sensors are used. These sensors may be pneumatic, photoelectric, ultrasonic, or infrared. The system’s controls must put the output signals from the sensors into a form that can drive the actuator.
Many controls today are electronic, typically using an amplifier to convert signals from the sensor, then commanding a special servomotor incorporating a lead or ball screw for guiding actuation. Some electromechanical guiding systems also utilize comp