Words of Wisdom: Pellet to product: Thermoforming 100,000-plus units/hr



Frank Nissel

is the president of Welex Inc., a supplier of single-screw extrusion equipment used to produce thermoformed containers, as well as pipe, profile, tubing, sheet, film, and other items.

Extruded sheet has always been a precursor to thermoformed products. In offline processing, sheet is extruded, cooled, rolled up, and put into storage. The rolls are subsequently retrieved, unwound, and fed to a thermoformer. There the sheet is reheated, thermoformed, and trimmed into finished parts.

However, the inline combination of sheet extrusion and thermoforming offers many advantages to high-volume producers of packaging and disposables. Integrating extrusion and thermoforming into an inline process yields improved production and energy savings. Several alternative methods offer the processor advantages with specific polymers and end products.

Both sheet extrusion and thermoforming have developed into highly precise operations in the 50 or so years since their inception. Sheet extrusion systems provide practically unlimited output capacities. Larger and larger thermoforming machines are being developed to satisfy the insatiable markets for packaging and single-service items such as drinking cups and lids.

Matching Capacities
Integration of extrusion and thermoforming into an online system requires closely matched capacities to provide the best economies of scale. Smaller-capacity sheet extrusion systems are relatively expensive, so integration favors larger thermoforming machines. Smaller systems are best served by a large sheet extrusion line that can supply two or more thermoformers. For a single-layer sheet extrusion line, doubling capacity only increases the investment by 30 percent.

While Welex has a number of integrated systems running in the 400- to 500-kg/hr range overseas, most U.S. operations run from 1000 to 2000 kg/hr. Such capacities require markets with large appetites for a given thermoformed product. Cost and efficiency considerations also require that the extrusion capacity exceeds the thermoforming capacity so that the thermoformer is never limited by extruder capacity.

Sheet Extrusion Technology
Sheet extrusion has evolved into one of the most precisely controlled and automated of plastic processes. Extremely tight tolerances are commonly maintained with ease. This high degree of uniformity reduces product weight and minimizes scrap, resulting in substantial material savings.

Sheet extruders are usually vented to avoid the need for predrying materials and to eliminate air entrapment from recycled thermoforming skeletal scrap. The melt is always filtered by a screenchanger to remove contamination. Cassette-type slide plate changers are commonly used, but highly contaminated material is best handled with a fully continuous changer that does not interrupt the flow during changes.

While extruders are precise polymer pumps, the variation in feedstock from pellets to various levels of less dense regrind adversely affect output stability. Perfect volumetric output stability is ensured by the use of a gear pump system after the screenchanger. This is supplied at an automatically controlled constant pressure by the extruder, which becomes simply an efficient melting device. This is usually followed by a static mixer to provide perfect melt temperature and viscosity uniformity.

This Welex/Irwin rotary inline forming system produces 100,000 lids/hr. The hot sheet is extruded directly onto the cooled vacuum forming drum, eliminating the need for a sheet takeoff.

Altogether, these provide the sheet die with a constant supply of uniform volume and viscosity melt such that the die is easily adjusted to very close profile tolerances. This can further be enhanced with automatic thermal die bolt thickness control. Well-developed feed block coextrusion technology permits the production of an unlimited variety of multilayer structures to obtain desired appearance or performance properties.

The temperature of the sheet emerging from the die is generally higher than the most desirable thermoforming temperature, and the sheet often requires surface polishing to achieve the desired product finish. In the case of polypropylene, rapid cooling is essential to prevent crystallite formation, which results in poor clarity and reduced stiffness.

Roll-fed Thermoforming
In offline processing, the sheet is surface-finished and cooled on a three-roll stack that applies the desired surface finish under roll pressure while the heat is removed. Three rolls with two nips are required to control the surface finish and to cool the sheet adequately. The first roll provides only a momentary pressure point and serves mainly to eliminate air entrapment between the middle roll and the sheet. The middle roll provides the surface finish on one side while the third roll applies the finish to the other side during cooling. The rolls require exacting precision since any deviation from flatness, roundness, or speed uniformity adversely affects sheet thickness uniformity.

After cooling, the sheet is edge trimmed to exact width and wound into rolls for subsequent thermoforming. Edge trimming is essential to winding good rolls because untrimmed sheet tends to have small beads on the edges that upset the roll shape. While the edge trim is granulated and reused, it represents a 5 to 10 percent waste of production capacity.

Rolls should be stored for several days prior to thermoforming to ensure complete temperature uniformity throughout. This is particularly critical for polypropylene, which has a very narrow temperature range for thermoforming.

Tandem Inline Systems
A tandem inline system combines a complete sheet line, minus the edge trimmer and winder, with a complete thermoformer. The sheet is cooled well below its softening temperature and a short hanging loop of sheet serves as an accumulator between the continuous sheet line and the cyclic thermoformer. Synchronization is achieved with an optical loop detector system that advances the thermoformer each time sufficient sheet has been accumulated. A small amount of residual heat can be maintained in the sheet for some energy savings, but a complete thermoformer with a full reheating oven follows.

This is the easiest system to operate and is suitable for all materials. If necessary, an intermediate roll winding and unwinding station can be inserted to permit short independent test runs of either the sheet extrusion system or of the thermoformer. The main advantages are consistency of feed and freedom from handling rolls. As in all inline systems, the edges do not need to be trimmed, so 100 percent of the extruder capacity is used.

The tandem inline system is the essential process for clear materials. It is also possible to make test runs of each half of the system independently by installing an intermediate roll winding/unwinding station.

The main disadvantage of the system is that a great deal of energy is wasted cooling and then reheating the sheet. Even at a modest 1-ton/hr rate, this amounts to about 75,000 Kcal/hr of heat removed and later replaced. This is equivalent to 90 kW, which, at $.10/kWh, costs at least $50,000/year.

The tandem inline system combines a sheet extrusion line with a thermoformer. The system is easy to operate and suitable for all materials, but a great deal of energy is used to cool and reheat the sheet.

The hot inline system feeds partially cooled sheet into a thermoformer. The small rolls in the sheet takeoff minimize heat removal. The process uses energy efficiently but is limited to large-capacity and thick-sheet applications.

The rotary inline system extrudes hot sheet directly onto a cooled vacuum forming drum, eliminating cooling and sheet takeoff components. The system is used to produce shallow draft items.

Hot Inline Systems
The thermal losses from cooling and reheating the sheet can be eliminated or minimized by feeding partially cooled sheet to the thermoformer. The ideal thermoforming temperature is well below the extrusion temperature, at which the sheet is too fluid for free suspension over large areas by its edges. The excess heat content would also require longer thermoforming cycles for adequate cooling?a serious disadvantage.

Welex uses a special three-roll sheet takeoff for this application that uses relatively small rolls to minimize heat removal. This is followed by a fully supported short accumulator loop system in a closed housing that ties directly into the thermoformer transport chain.

Under ideal conditions, such a system would be capable of running without any further heat input. Presently, this is not achievable because of the long strokes of large thermoformers that create cyclic temperature differences from the beginning to the end of each forming frame. Such hot inline systems are therefore run with shortened reheating ovens operating at low power to stabilize the sheet temperature prior to forming.

This process is suitable for all polymers except for clear polypropylene products. Clear PP and APET require rapid cooling immediately after extrusion to avoid the formation of crystals. Hot inline forming would result in milky-looking containers with inferior stiffness. This process offers the most efficient use of energy but is limited to very large-capacity and fairly thick sheet applications, such as cups and containers.

Thin sheet and small capacities are not suitable for this method since it would not be possible to maintain the elevated sheet temperature under these conditions. These are better served by tandem inline operation. In any case, the equipment costs are essentially the same. The main advantage is in the energy savings.

Rotary Inline Systems
For lids and other shallow draft items, the hot sheet can be extruded directly onto a cooled vacuum forming drum. This completely eliminates the need for a sheet takeoff. In some cases, a small intermediate pair of polishing rolls is used to apply a surface finish to the sheet and to cool it slightly prior to forming. Rotary formers of this type have been custom-made in-house by lid producers for many years, and had not been commercially available until recently.

Previously this system was understandably of little interest to either extruder or thermoformer builders. The extruder builder loses the opportunity to sell a sheet takeoff unit, a major part of the system. The thermoformer builder loses even more, since it is relegated to selling a trim press. The tooling manufacturer is indifferent, since it makes the molds in either case.

The die of the Welex/Irwin rotary hot inline system is usually tilted to match the direction of pull from the drum system. This angle is critical to avoid buildup of extrudate on the die lips. Such buildup can make die lines on the sheet and slough off periodically. The angle adjustment is achieved either by using an end-fed die that can be rotated about its inlet axis, or by using a center-fed die with a universal jointed adapter system.

The forming drum should have at least eight facets of molds to minimize its chordal action since this tends to induce cyclic thickness variations. This problem can also be corrected by programmed cyclic peripheral speed variations. A commutator system applies vacuum to the mold facets at the initial sheet contact point and turns it off at the stripping point.

After stripping, the thermoformed web is transported to a synchronized trim press where parts are trimmed and stacked in the usual manner. The process capacity is limited only by the trim press capability.

This system produces large volumes of shallow draft parts. It isn?t suitable for smaller capacities because of the large number of cavities required. For high volumes it offers economy in investment and operational costs.

Contact Information

Welex Inc., Blue Bell, PA
Frank Nissel
(215) 542-8000
www.welex.com

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