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April 3, 2003

11 Min Read
Words of Wisdom: Choosing sheet extrusion equipment to meet packaging market demands


Chris Eaton is sales manager at Processing Technologies Inc. (PTi) and has 26 years of experience in extrusion.

Several new trends in the sheet extrusion industry influence the process of specifying and purchasing new sheet systems. The days of the company owner or head buyer requesting a quotation and making a quick decision for a line to produce a particular product are gone. Companies typically assemble a team consisting of engineers, managers, and manufacturing personnel to research the project, visit potential vendors, and eventually select equipment. Most lines today are purchased for flexibility for new products, resins, and changing applications.

Changes in packaging materials have had a major influence in the equipment selection process. As a result of relative material pricing levels and improvements in overall performance, polypropylene (PP) is finding its way into applications such as food storage containers, meat trays, and dairy containers.

Polyester (PET) is also growing in usage for items that were the domain of oriented polystyrene (OPS) or clear polyvinyl chloride (PVC). Examples include deli containers, fruit and berry boxes, baked goods packages, and even some blister pack applications. PP is also gaining ground in the large volume car-carry beverage cup business, traditionally a polystyrene (PS) product.

These changes do not drastically alter the way sheet is extruded. However, machinery configurations are influenced by demands of flexibility and output expectations. Processors often rely on the equipment supplier to assist in machinery selection.

Systems can be designed to permit processing a variety of resins. This allows the user to compete in various markets when packaging designers change raw material, whether for cost, aesthetics, or physical properties.

Variables related to each resin include screw and barrel designs, cooling roll diameters, conveyor lengths, and roll drive systems. Flexibility is often designed into a system to allow production of different materials in the future.

For example, if a line is purchased to run olefins, which typically operate with nonvented screws, the barrels should be vented for the future possibility of running polymers that require a vacuum system to draw off volatiles. The vents can be plugged when not needed.


This coextrusion sheet system has a pivoting base on the main extruder for easy screw removal. The system also has a continuous screen changer, a gear pump, a static mixer, and a flex-lip sheet die positioned in front of a compact, vertical down-stack roll stand with an antistatic roll-coater. This system works for three-layer inline sheet production directly feeding a thermoformer.

Another way of providing flexibility is to design the extrusion system for the future addition of coextrusion, even if the current line is for a monolayer product. This is accomplished with simple adapters that can be removed to install coextrusion feed-blocks at a later date. The line can be installed with floor space reserved to allow for future coextruders. Providing for retrofit of coextruders at the time of the initial installation minimizes coextruder installation costs later.

Inline vs. Roll Configurations
A major consideration is whether the sheet system will run inline with a thermoformer or run roll stock. The choice is a given for most companies due to the nature of their business, but for others who extrude sheet and also thermoform in house, the decision may take more thought. In most cases inline systems are best applied for runs where changeovers are infrequent.

An inline arrangement where the sheet is conveyed directly into the thermoformer chain rails does provide significant savings in energy, labor, and floor space. The sheet takeoff unit is very compact, so the sheet is delivered to the forming machine at an elevated temperature, and less heat is required to bring the sheet to forming temperature.

Compact systems associated with inline processing offer additional benefits. These include a shorter overall length (less floor space used), and there is no need for an unwind station that feeds roll stock into the chain rails of the thermoforming machine. Additionally, the overall length of the pre-heat oven can be reduced since the sheet is entering at an elevated temperature.

Some disadvantages in overall efficiencies occur with compact inline systems that require frequent tool changes. Tool changeovers require that the entire line be down for a while. However, the advantages typically outweigh the disadvantages, and more applications are turning to inline processing. Some common applications include beverage cups, lids, and high- volume packaging trays.

If an inline system is specified, the designer of the tooling must estimate the cycle time for a product. With this information and the overall dimensions of the tool, the sheet system designer can determine the required output for the given sheet thickness, then properly size the extruders. The extruders are typically sized to run at approximately 85 percent of full capacity.

The trend among the end-users includes procuring systems that have relatively high capacity rates, which in turn yield higher rates of production efficiencies. Systems with rate capacities in excess of 6000 lb/hr are becoming more common.

Design concerns exist for something as simple as removing a feed screw on an inline system. The roll stand cannot typically be retracted far enough away from the extruder so the screw clears the end of the barrel since the thermoformer is positioned immediately downstream from it. In this case, the addition of a pivot point and lateral casters at the rear of the base of the main extruder allows the extruder to be rotated slightly askew to the line. The screw is then easily pulled into the adjacent aisle.


PTi?s Titan Model Computer Control System shows a plan of a coextrusion system on the left screen and a thickness gauging system display on the right.

A line running roll stock is sized to fulfill orders for various volumes in a range of materials, structures, widths, and gauges. If demands are relatively large, multiple sheet lines may be needed. In thin-gauge applications, line speeds affect operator safety when the web is cut and transferred to the new winding shaft. Since down-gauging is prevalent in the packaging markets, sheet winders are beginning to resemble film winders, with automatic transfers and accumulators accommodating multiple webs simultaneously.

In any case, heat transfer software programs and processing experience enable machinery suppliers to determine the required cooling roll diameters, water flow rates, and conveyor lengths. Resin data are collected to predict melt temperature and determine roll temperature settings that are typical for a given polymer. These tools are especially valuable when used to evaluate design configurations for various cooling rates associated with processing multiple resin selections. With this information, one can also determine if auxiliary cooling/reheating rolls are necessary or if a longer conveyor is required for a roll stock application to increase ambient cooling. The temperature of the sheet as it is wound onto a winder is predictable, which helps determine the length of time needed to let the roll come to ambient temperature before feeding it into a thermoformer.

Roll Stand Drives
Approximately 50 percent of all sheet lines sold in the U.S. still employ a drive system consisting of sprockets on each roll and a serpentine chain driving them. However, over the past several years, there has been a move towards independent drives for each roll. The individual drive system permits each roll to be precisely speed-regulated, which lets the operator make incremental adjustments to the surface speed of each roll.

There are many advantages to individually driving each roll, including: compensating for minor variations in roll circumferences; compensating for various shrinkage rates of the web on each roll; and producing a higher-quality, defect-free sheet. Chain drives can produce chatter marks from the chain engaging with the teeth on the roll sprockets. Managing varying shrinkage rates, which are prevalent when processing heavier-gauge sheet, is the primary advantage of implementing individual roll drives. Most packaging sheet applications can be run on a chain-driven stack. PP is better controlled on individually-driven rolls as it takes more time to cool and stabilize.

ww3.jpgCooling & Polishing Stands

Roll stand designs continue to evolve at an even pace with the ever changing demands seen in packaging markets. Roll stand configurations, cooling roll sizes, auxiliary cooling/reheating equipment, drive systems, and control functions are all taken into consideration with each application. Extruder throughputs, hot melt strength, and sheet gauge are factors that are included in the design criteria for optimum roll-stack arrangements.

The workhorse of the sheet industry continues to be the vertical three-roll stack, running sheet either in an up-stack or down-stack manner. This type of design has been used throughout the history of the sheet industry for its simplicity and ease of thread-up and general operation. Some materials that have low hot-melt strength, such as PP and PET, are often run on stands that are inclined at a 45° attitude to allow the extrudate to flow into the primary nip easier, as these resins tend to droop or sag toward the floor when exiting the die. The incline design reduces the effects of gravity on the sheet. In the vertical stack configuration, the problem of the sheet sagging is lessened by contouring the bottom die half, permitting the die to be positioned closer into the nip.

A hybrid of the aforementioned designs is PTi?s J-Stack Model design. This roll stand configuration has the bottom roll positioned at a 45° attitude and the top roll positioned vertically above the center roll. The benefits of this arrangement include improved handling of low melt-strength resins while permitting an additional 25 percent web wrap around the chrome rolls compared with the normal wrap achieved with vertical or inclined arrangements. As a result of the higher residence time associated with the increased web wrap, this feature allows higher throughput without increasing the roll diameters.

Another roll stand seen in the packaging world is a horizontal three-roll stack. As the title implies, three rolls are configured on a horizontal plane with the sheet die arranged vertically above casting downward into the nip. This is also used for low melt-strength materials. High quality versions of the horizontal configuration can produce sheet materials as thin as .007 inch. More commonly used and less expensive, these thin-sheet gauges may be produced using an air knife to impinge the sheet onto the primary roll without using a steel-to-steel nip. Drawbacks of the horizontal configuration: The line is difficult to thread, and the back side of the sheet is not visible as it wraps the center roll.

The majority of sheet extrusion systems sold today feature a PLC/computer control system (see photo, opposite page). The trend for these systems is to use commercially available hardware for the major components. The machine supplier acts as the system integrator by closing loops and programming software for OEM supplied components such as feeding and blending systems, gauging systems, and automatic die adjustment.

There are many operator-convenience features that can be installed to run more efficiently and control quality. On complex multilayer coextrusion systems the gravimetric blending system controls the thickness of individual layers. The required outputs are entered either in throughput (lb/hr) or as a percentage of a total throughput. The system will then control the speed of the individual extruders to extrude at the proper speed in order to meet these set-points. The entire system is programmed to ramp up to the desired total output with controlled acceleration until maximum output is reached. The sheet take-off unit will also automatically ramp up to the correct line speed for the preset sheet thickness. These features are cost-justified, as is the ability to trend operating parameters, store recipes, diagnose problems, and protect equipment and personnel with safety alarms and shutdowns.

Automatic die bolt control is traditionally used for systems that run thin-gauge film or in extrusion coating systems where gauge is tightly held. Online adjustments are best performed automatically rather than by an operator with an adjusting wrench. High spots can be quickly adjusted out, saving material. The absence of gauge bands improves overall quality. Automatic dies can control thickness to within ±1 to 1.5 percent of the target. Many companies extruding sheet in gauges of .1 inch and higher are now using automatic die bolt control technology.

These processors also get the same benefit of material savings through tighter gauge control and overall quality improvement. Equally important is the advantage of automatically controlled adjustments, avoiding the need to train operators in the fine art of die tuning. Plus, in coextrusion arrangements where adapters connected to the coextrusion feedblock make the area congested and access to the die bolts difficult and hazardous, operators can stay out of the sheet die area altogether.

Supplier Specified
Given the tough economic times that we are enduring, most extrusion companies are not adding to their in-house engineering staff, and in many cases, these departments are being reduced. The result is dependence on the equipment supplier to perform the engineering tasks to properly specify, size, and design the correct components and systems for today?s market demands that emphasize high outputs, reduced labor, and conservation of floor space and energy, while still producing high-quality sheet. The technology exist to meet these demands. As machinery manufacturers, processors, and resin companies work together, packaging products?whether fabricated or thermoformed, from case-ready meats to cosmetics?will improve in performance, quality, and capacity due to an in-depth understanding of the processes and end use.

Contact information
Processing Technologies Inc., Aurora, IL
Chris Eaton
(630) 585-5800 [email protected]

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