Comparing the cast film and blown film processes.
The cast film process can offer finer gage control, better process stability, and greater versatility in coextrusion applications.
Several attempts have been made to compare the two most common methods of film production -- the blown and cast film processes. Comparisons have often been made by professionals with extensive knowledge of blown film but limited knowledge of cast film processing equipment. Additionally, since blown film installations outnumber cast film installations by a wide margin -- because of the lower initial cost of equipment and the familiarity of the machinery -- most comparisons tend to favor blown film. However, recent improvements in processes and product quality warrant a comparison update and an exploration of some of the little-known benefits of cast film processing. Armed with this additional information, today's film producers may be better equipped to evaluate the competitiveness of cast film vis-a-vis blown film.
For example, cast film process conversion costs are approximately 1 cent to 5 cents/lb less than those of blown film. This is a result of efficiencies in cooling the extrudate, output as a function of machinery cost, higher yield, and lower scrap rates. With this kind of economic advantage, a film producer cannot ignore possible cast film applications.
The two processes are shown schematically in Figs. 1 and 2 (based on figures originally published in Plastics Films by John H. Briston, Longman Scientific and Technical, 1986). For purposes of space, the process basics will not be reviewed here, but for comparative purposes, the following needs to be understood about the film-forming process.
With blown film, the polymer melt is extruded through an annular-type die, which uses many decreasing volume spirals to distribute the flow evenly about the annulus. Upon emerging from the circular die gap, the molten film is drawn vertically upward and simultaneously "blown" outward by pressure within the bubble. The melt is cooled by blowing air on the exterior side of the bubble by an air ring and sometimes also internally by means of a process known as IBC (internal bubble cooling). The film is then collapsed and slit prior to winding.
With cast film, the polymer melt is extruded through a flat or slot die. Occasionally restrictor bars, such as a coathanger, are used to distribute the flow evenly along the width of the die. The molten web is "pinned" against a chrome-plated, water-cooled roll by an air knife or vacuum box. The roll "chills" the film instantly and the film is slit and wound.
Table 1 shows a typical comparison of both processes. The blown film process has historically been considered to be superior in mechanical properties, width flexibility, and initial cost. The advantages of cast film include superior optic quality, better conversion efficiencies, and minimal thickness variations. Many other categories for comparison exist, but these shown are generally the most important.
The reader should be cautioned against using only the above criteria to decide on the more suitable process for a particular product. Superficial investigations can lead to an inaccurate evaluation, as it is impossible to make generalizations that will cover all film products. It is important for a converter to discuss the options with as many experts as possible. Resin and machinery suppliers who are well known in both cast and blown films are best able to assist unselfishly in the evaluation. If at all possible, laboratory trials should be performed to confirm the choice of the better process for a particular product mix.
One of the often overlooked benefits of cast film is the advantage that tighter thickness variation control can yield in process economics and product performance. However, before pursuing a comparison of the processes, a well-defined measurement standard must be established.
Gage variation is very important in any film product. Industry media and machinery suppliers often quote gage-variation percentages that can be misleading if not properly defined. For example, one can say a variation of 3% is obtainable when what is meant is that the variation between spiral ports on the die is [+ or -] 3%, while over the full die a total variation of [+ or -] 7% can be measured by a Beta gage. Similarly, an automatic cast film die can produce film with a transverse direction (TD) variation of [+ or -] 2%; however, in some applications, machine direction (MD) variation may be as high as [+ or -] 15% because of draw resonance. Often, the latter figure is not quoted, and hence it is infrequently realized by an equipment purchaser.
Plus or minus, total variation, point-to-point, between the spirals, two sigma average, and 1%, 10%, 5%, 3%, and 7% -- all of these terms are thrown about the industry in a rather careless fashion without regard for accuracy or a clear definition of the measurement techniques employed. Many techniques are used throughout the film industry for measuring thickness variation, and each of these techniques has its own degree of accuracy.
By taking a single film sample and using each technique for measuring it, a wide range of gage variations can be obtained. This is analogous to measuring liquids by using a graduated cylinder or a household bucket. The ability to accurately measure small amounts becomes more important as process technologies improve and thinner films are produced. In addition, gage variation is dependent upon film thickness. On one 25-microns film [+ or -] 5% is recognized as poor gage control, but [+ or -] 5% on 12-microns film is good. Thus "[+ or -] 5%" can have multiple meanings, depending upon the material being measured.
Thickness Measurement Techniques
A comparison of the three most common thickness measurement techniques --mechanical (snap) gage, nuclear transmission gage, and capacitance gage -- is shown in Table 2. The comparison is made by looking at contact distortion, number of data points, spot size, statistical noise, and on-line capability. How do these methods compare based on these criteria?
Contact distortion occurs when a mechanical (snap) gage is used. When a film is measured by a common snap gage, the force of the gage on the film distorts the sample, preventing accurate measurements. Fortunately, the other two methods do not requires contact on both sides.
Each measurement technique also determines differing qunatities of data points. It is important to get as many thickness data points on a sample as possible. Typically, by using a snap gage only, a few data points are taken. But nuclear and capacitance gages approach a true continuous reading by taking many data points. Wide variation can be seen over areas less than 2 inches apart, so it is important that continuous measurement be done. Similarly, the size of the area measured should be as small as possible to limit averaging over a wide area. Mechanical and capacitance gages examine a very small area while a nuclear gage may examine an area as large as 2 inches. Recent development in Beta (nuclear) gages has lowered this spot size on newer units; however, because of the inability of the radiation detector to react instantaneously, the effective measurement area is increased. This effective area is a function of scan speed and the time constant of the gage measurement head. Typically, trade-offs are made between scan speed (system response) and gage accuracy, resulting in Beta measurement, which evaluates larger areas than capacitance or mechanical measurements.
Beta gages are also limited by statistically noise, which is an inherent property of nuclear source decay. This effect becomes significant when trying to measure thin films (less than 0.001 in) to an accuracy below [+ or -] 2%. At these levels the signal-to-noise ratio becomes excessively high. Noise can be eliminated through complex software and signal averaging. Response time, however, does not permit this luxury on most film lines. Even with this limitation, Beta gages are used throughout the industry, primarily because of their on-line capability and insensitivity to production environments.
Our efforts have shown that the percent variation readings seen on a single film sample can vary dramatically depending upon which method is used. As a rule of thumb, the percent variation using a Beta gage is twice that of a mechanical gage. It should be noted that the capacitance gage gives a variation that is six times that of mechanical gages and three times that of Beta gages (Table 2). In order to make a fair comparison of these gages, the method of measurement needs to be clearly defined. A film producer should be cautioned to get proper definition on measurement techniques with quoted gage variations from the equipment supplier prior to reaching any conclusions.
Thickness Variation Control
Table 3 shows some major factors that contribute to thickness variation in the day-to-day operation of film lines. When comparing the processes, one needs to look at on-line production and not laboratory-type environments. Questions to be asked include, "Can the film-forming section of the line consistently produce tight-tolerance film?" and "Can the process equipment accept changes in resin rheology, process parameters, and environment, and continue to produce a consistent film?" To answer these questions, we must look at each process and the factors that can contribute to gage variation.
First we need to examine how each system can respond to changes in materials and resins. It is becoming more common that production equipment must produce a wide variety of film products. This is especially true in coextruded films. The concept of dedicated lines for dedicated products is a luxury that only few can afford. Therefore, the chosen process must be capable of running a wide range of products with consistency and tight thickness control.
Each process has a unique method of distributing flow across the film width. Blown film dies generally have spiral-flow passages while cast dies have coathanger-like manifolds. Computer flow modeling has shown that a cast die can accommodate changes in resin rheology and products much easier than blown film dies, and continue to produce consistently flat film. The reason is that the spirals are more sensitive to changes in rheology.
The flow distribution of each process (spirals or coathangers) requires that a final adjustment be made at the die lip. On a blown film die, the die gap adjustment is difficult because it is made on the concentricity of the annulus. On a flat die, adjustment is done on the slot opening only and is more easily controlled.
Another factor influencing gage variation is the effect of the cooling medium on the film. Variations in cooling by using the flow from air rings and IBC are much higher than the more even rapid cooling of a chill roll. Cast film is also less sensitive to in-room drafts.
Up to this point in our comparison, cast film has had the edge in production environment and thickness control. However, LLDPE cast film produced at very high line speeds can experience a phenomenon known as draw resonance. Here, 5% to 10% variation in machine direction is possible. Blown film, because of lower line speeds, does not exhibit this phenomenon. Continuing work, however, may eliminate draw resonance from cast film in the near future.
Day-to-day production environments can be expected to produce the following gage variations as the best result:
* Blown film manual die, [+ or -] 5% to 8%;
* Cast film manual die, [+ or -] 3% to 5%;
* Cast film Thermoflex die, [+ or -] 1% to
(Measurement method: 25 microns LDPE film; two sigma statistical limits; Beta gage on-line measurement; 10-millisecond time constant; fine line sensing; minimum software filtering.)
Cast Film Advantages
The single largest benefit from a film with minimum thickness variation is the increased yield of footage per pound. An economic benefit can be realized when one incorporates the use of adaptive target control (ATC). ATC is performed on-line by means of a microprocessor. The microprocessor will steadily lower the average film thickness while maintaining the minimum thickness above a certain preset limit -- for example, ensuring that no section of film will be less than 25 microns in thickness. This example is illustrated as follows:
Blown Cast Minimum thickness 25 25 Gage variation [+ or -] 5% [+ or -] 1% Target thickness 26.25 25.25
It is seen that a cast film producer can produce the same amount of square footage with 4% less resin.
Another benefit of cast film is in mechanical strength during elongation. Since any product is only as strong as its weakest link, the producer realizes a benefit resulting from less variation in thickness (when comparing cast and blown films of the same nominal thickness). We have seen a cast film stretchwrap product with fewer breaks per roll, when the film is wrapped around a pallet, than a blown film product of similar thickness.
Another advantage is that higher line speeds are achieved on the end user's machinery. Given a flatter film, the producer can also wind flatter rolls. This film tracks truer and can be run faster on diaper, packaging, printing, and laminating machines. A development project under way at Egan on the third-generation automatic die shows that perfectly round rolls can be produced from this new die system. Circumferential variation on a 800-mm-diameter roll is less than 0.2%.
In addition, advantages can be seen with cast film in process stability. For example, the cast film line will run with fewer web breaks than a blown film line. Often, uptimes of over 98% are realized, which dramatically reduces overall scrap production. Scrap rates under 0.5% are common.
The stability of the cast film process is due to the relative insensitivity of the film-forming section to gel and carbon contamination, and the comparatively streamlined flows and high melt velocities. In addition, with today's sophisticated edge pinning and vacuum box technologies, the molten web is actually held in place during the drawdown, which occurs over a very short distance. In blown film the melt is abused much more through blowing the bubble and cooling with high-velocity airstreams over a long distance. The blown film process will show more web breaks, downtime, and scrap.
With tighter overall thickness control, the same comparative analogy may be applied to the barrier layer in coextrusion. Transverse direction layer variations in cast film are generally about [+ or -] 3% while in blown film they are [+ or -] 10% or more. Lower variation means that the nominal layer thickness on the expensive barrier resins can be reduced. By using on-line layer measurement and the ATC technique, the barrier layer thickness can be reduced.
With the increasing use of coextruded structures in flexible packaging, it is becoming necessary for flexible packaging converters to use on-line measurement of individual layers. The need for this is demonstrated by the use of expensive barrier resins in flexible packaging and also by critical functional resins in medical packaging. Typically, the barrier layer in a coextruded structure must be held to an absolute minimum thickness because of cost, but it is necessary to know that at least a minimum amount exists to ensure product performance.
Product barrier performance can be assured if two barrier layers are used in the product. This means that the structure needs a minimum of seven layers. Only the cast process can achieve this seven-or-more-layer structure.
Additionally, increased versatility in coextrusion can be realized from cast film over blown film for the following reasons:
* Change of a selector plug in the feed block makes it easy to go from two to five or more layers with the same die setup. In blown film, usually very expensive dies need to be inventoried and maintained.
* It is easier and cheaper to add coextrusion to existing cast film extrusion dies.
* It is possible to produce from two to over 100 layers (microlayer extrusion).
* There is the possibility of sequential casting and coating, which produces complex but economical structures for barrier packaging. These structures have nine or more layers.
* It is easier to purge and clean a cast film system during a product change. Cast film dies often run for years without disassembly for cleaning.
* Low residence times for heat-sensitive layers are orders of magnitude less than the best spiral mandrel die.
Summary It is noted that each process -- blown and cast -- is viable for most applications, and that generally one method will be preferred. Before choosing the proper method, it is important to evaluate the benefits of each process, and, where possible, to run product on production-scale equipment.
Only companies (resin, machinery, or consulting) actively involved in both blown and cast film technologies can be assumed to be reliable sources of comparative information. It cannot be over-emphasized that a measurement standard needs to be established for definition of quoted thickness variations. [Tabular Data 1 to 3 Omitted] [Figures 1 and 2 Omitted]
R. Keller John Brown Plastics Machinery Egan Machinery Division Somerville, New Jersey
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|Date:||Aug 1, 1989|
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