The ‘Other’ Web, It’s Not Just a Spider’s Playground

KEY WORDS

Process control & instrumentation

Process control systems

Advanced control

Quality assurance

Sidebars: A Look at Web Quality

Web processing encompasses a myriad of products and materials that are manufactured in flat sheets or rolls. Web process industries produce many products including textiles, paper, and plastic film. They can also perform any number of secondary operations on these base materials. Countertops, flooring, sandpaper, and food packaging materials such as potato chip bags, candy bar wrappers, and juice boxes are a few products produced using web-processed materials.

Basic processes

In the textile industry, the term “web” is synonymous with fabric. Separate fabric manufacturing processes produce the web and modify it after forming. The two major classes of textile fabrics are woven and nonwoven.

In the paper industry, web (or “sheet”) processing refers to the portion of the papermaking process that begins with raw material discharged onto the paper machine forming fabric. The web forming process, pressing, drying, coating, calendering, and rereeling are all part of this continuous process. Cutting and rewinding are also included in paper web processing.

In plastic film manufacture, a continuous sheet is created by heating plastic resin pellets in an extruder and shaping through a flat die. The product is then chilled to “cast” the sheet, preheated for stretching in the machine direction, and stretched in the cross-machine direction to increase stiffness and improve the moisture barrier. Further treating or coating, slitting, and rewinding are also manufacturing options.

Obtaining quality/consistency

Issues critical to all web manufacturing processes are overall quality, repeatability, processing efficiency, and process information tracking. A web processing operation must have repeatable control over product quality. Production efficiency and quantity is important, but that focus should only be pursued after proper quality is ensured (see sidebar on web process industry quality).

Many different control technologies can be combined to improve both product quality and overall productivity. Web process manufacturers must “see” more of the web as it is produced, then use the additional information to lower variations in web quality.

Proper handling of web products is done by machinery and transport systems, more commonly referred to as “drives.” While the nature of machinery impacts the amount of tension a web experiences, the drive system really controls the stress applied to the web. Advances in technology, such as encoderless torque controlled drives, now provide a less expensive direct control element. In the past, the speed of the process was set and tension was controlled as a secondary function. Now, with torque or tension control as the primary focus, the system can consistently control web tension at varying speeds.

Advances in open system technology, integrated with improved drive systems, have enabled these systems to effectively collect, store, and correlate parameters of the process, machine condition, and web quality. Operators now can compare web quality over time and machine and process conditions simultaneously.

Additionally, the availability of fully integrated manufacturing execution systems (MESs) into web control provides information to all plant levels for real-time operational decisions. Quality engineers can now assess the web, as it is produced, and feed decisions forward to downstream web processing systems. An MES eliminates functional boundaries between the production floor and front office. For example, in a plastic film manufacturing application, an online view of resin usage per hour allows quality control to track and tag a resin lot number on individual rolls of film. Widely available product tracking applications can now be used to capture process and product information and integrate it with sales forecasting, order management, and financial and accounting systems. This allows throughput tracking from raw material to customer delivery.

Advanced analysis/control

Advanced analysis tools, such as Fast Fourier analysis (a diagnostic tool that can help minimize off-spec product) can be applied to web quality and machine condition data to identify quality variations introduced by specific process or machine conditions. Once identified, these conditions can be corrected to enhance product quality. What’s more, the availability of “smart” machine diagnostic systems can help to identify specific machine conditions (excessive vibration, motor and bearing temperatures, etc.) affecting web quality and be used to target both corrective and preventive maintenance efforts.

Process efficiency can also be enhanced by advanced control and modeling techniques. Different approaches making their way to the production floor include fuzzy logic control, neural networks, and expert systems. These approaches use one or more related inputs to predict or model the output of overall processing where no direct measurements are available—allowing relatively accurate control of a process parameter in a situation where direct measurement is nonexistent or cost prohibitive.

A control strategy is available that allows manufacturers to minimize waste by enabling them to change web parameters during product grade changes with the need for equipment shutdown. Auto-grade change controls maximize production efficiency by minimizing transition time from one product grade to the next. For example, in a paper-making operation auto-grade change controls enable grade changes requiring a different finish to be anticipated at the paper machine wet end by adjusting the refining and stock blending operations, allowing stock changes “on the fly.”

Instrumentation advances

Full-sheet imaging provides a more complete look at a web product as it is formed. Advance sheet imaging systems have increased the number of full scans of web parameter by a factor of 40,000 compared to conventional scanner technologies. This allows a papermaker, for example, to “see” virtually 100% of production and allows for accurate determination and control variation of actual web quality during manufacture.

Film thickness measurement now makes use of improved nucleonic techniques said to reduce errors induced by in-plant environmental conditions. Older nucleonic methods did not account for error caused by variations in air volume and temperature next to the web. Additionally, this technology provides real-time compensation for scanner alignment tolerances, increasing measurement accuracy and repeatability over conventional ultrasonic systems. Accompanying advances in edge control in the film extrusion process allow flat die design to increase the controllable width of the process and allow a better process map of sensor to actuator.

Of many technologies available today to help improve the quality and consistency of web processing products, developments in open-control technology, cross-enterprise software solutions, drives, and advanced measurement and control have provided much of the process improvement potential. According to web technology suppliers, they are only harbingers of things to come.

For more information on ABB Industrial Systems, visit www.controleng.com/info .

A Look at Web Quality

Proper quality is not always the same as highest quality. To be competitive, web processors must repeatedly achieve the quality required for the product, and then to produce the product as efficiently as possible. Efficient production is generally determined by a ratio of the time in quality production divided by the time desired to be in production. However, the “opportunities” are different for each of the major web industries.

There is a distinct balance between quality and cost of textile fabric production. A related issue to processing efficiency and quality is style change. Textile fabric processors must quickly accommodate a staggering number of changes to process setup. Additionally, process points that are monitored in upstream processes (bleaching, mercerization, etc.) may impact downstream processes such as dyeing and finishing. It is important to identify potential problems and take corrective action as soon as possible.

The key production goal of papermaking operations is to produce the maximum volume of on-spec paper for shipment, given the process/machine conditions existing at the time of manufacture. Production issues surrounding this goal include the minimization of paper-grade change time (the transition time between manufacture of on-spec paper for one particular grade and the achievement of quality specifications for a new grade of paper). The paper produced during this transition time is considered off-spec and is discarded to the “broke” system for repulping. Of equal importance to on-spec manufacture is documentation of the quality specifications of the paper produced. The manufacturer quickly identify and correct process and machine problems that introduce variations to web quality.

Quality and waste control

Many web products require a coating or series of coatings be applied to form the finished product. Food packaging films are coated to provide heat sealability, increase its oxygen barrier, and control coefficient of friction for subsequent wrapping operations. Photographic films require coatings to produce and guarantee color characteristics. Fabric is coated with glue and abrasives to make sandpaper, paper is coated with other substrates to make countertop material. In each of these examples, off-spec products cannot be recycled as is. Manufacture of these products requires exacting controls and extensive material tracking to minimize off-spec production that contributes to both scrap expense and solid waste.

Distortion of a web’s structural properties is also undesirable. The most common web distortion is stretching caused by uncontrolled tension during processing. At this point, many web products stretch outside of specification. In textiles, the bow or bias of the fabric’s yarns indicates change of weave pattern by inconsistent tension across the fabric’s width. In quality fabric production, dimensional stability must be maintained.

What variables are important?

Some directly controlled variables in fabric manufacture are tension, moisture content and consistency, web width, surface temperature, and percent wet pickup for dye and chemical applications. Indirectly controlled variables that impact processing results are washing and steaming temperature and flow rates, process steam quality and flow rates, nip roller pressures, dry can temperatures, and dye/chemical concentrations.

In papermaking, key measurements include basis weight, coat weight, moisture content, and caliper in both the machine and cross-machine directions, opacity, brightness, formation, ash, and color in the machine direction. Also included is the identification of web defects such as holes, light and dark spots, slime deposits, coating streaks, picks, scratches, and tensile, burst, and tear strengths.

For all web manufacturing, control of machine conditions is important to the production of on-spec product. These include a wide variety of variables including operating speeds, vibration, and wear of components (especially bearings and roller diameters).

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