Air Ring Considerations for Optimizing Blown Film Properties
The dual lip air ring for blown film cooling has evolved continuously since its widespread adoption by the industry in the early 1980’s. Early versions required elaborate baffling to ensure that the cooling air delivered to the chamber was uniformly distributed around the circumference. These “high pressure” designs provided a uniform air flow but at the cost of low blower efficiency and high power consumption. This caused heating of the air stream, which the processor had gone to the trouble and expense of chilling, and reduced the cooling efficiency.
Lower pressure drop air rings were developed through improvements in the design of the air ring chambers and lip sets. Enlarging the chamber and altering the angle from which the air was fed to the chamber, combined with an improved internal design, resulted in air rings that delivered a high volume, uniform air stream to the melt at much lower pressure drops and with considerably less frictional heating of the air.
Subsequent improvements in the dual flow air ring have provided the flexibility to cool polymers with widely varying melt strengths. Increases throughputs and/or optimization of desired properties can now be achieved by matching the air ring lip set geometry and other cooling variables to achieve a specific bubble geometry and quenching rate.
This issue of MacroLetter will focus on dual lip air rings recently developed for high stalk extrusion of HMW-LDPE, and high output, large blow-up ratio (BUR) LLDPE film.
Dual Lip Air Ring for High Stalk Extrusion of HMW-LDPE
HMW-LDPE for blown film extrusion, although not widely used, has been commercially available since the mid 1980’s. This material offers exceptional toughness, and higher impact strength than conventional LLDPE, comparable to that of HMW-HDPE, films under 25 micron thickness (decreasing as guage increases). In addition to toughness, HMW-LDPE offers much better optics than LLDPE or HMW-HDPE as well as the excellent low temperature heat sealing properties expected with LDPE.
In 1995, a customer presented the challenge to produce a film requiring high impact strength, clarity, drawdown, and balanced MD and CD (TD) elongation properties. The specified raw material was a 0.5 M.I., 0.925 density, homopolymer HMW-LDPE. The product had to be produced on an existing blown film extrusion line, comprised of the following: 63.5mm, 24:1 L/D extruder; 75 HP DC motor and drive; barrier screw at maximum RPM 125; and a 150 mm single layer spiral die, with a die gap of 0.80mm; and no IBC capability.
The molecular structure of HMW-LDPE prevents it from being drawn to thickness below 38 microns when run in a conventional, “in the pocket” bubble shape. The requirement for 20-25 micron film dictated a high stalk bubble shape. The high stalk bubble extends the distance and time available for the drawdown to occur, allowing higher drawdown ratio than those achievable with a traditional low stalk bubble. This is due to the extensional viscosity of LDPE which increases as the deformation rate increased. The relatively slow rate of melt extension in the high stalk area precludes the incidence of strain hardening that restricts the ability to draw the melt, in “in the pocket” conventional bubbles. A die gap as narrow as possible (without inducing melt fracture) should be employed to reduce the drawdown necessary to reach the required film thickness.
The processor’s initial efforts to extrude this material in a high stalk configuration were performed using single lip air ring, more commonly associated with the production of HDPE films. However, the relatively low volume of air that could be applied to the melt whilst maintaining bubble stability with the single orifice design, limited the throughput to less than economically viable rates (6-7 lbs/hr/inch of die circumference). In addition, the low extrusion rates and instability prevented the stalk from reaching the height necessary to achieve the balance between the MD and CD elongation value required.
To address these issues, a dual lip design with a significantly modified deflector lip was selected. A traditional dual lip design would not work in this instance since it would cause the bubble to expand prematurely. The dual lip configuration designed for this application provided a relatively gentle air flow from the bottom or primary orifice near the die lip exit to contact the bubble and provide the initial cooling. The geometry of the deflector established the bubble shape and prevented the bubble from expanding until just below the frost line. With the bubble now “locked” and stable, a second, higher velocity air stream was delivered from the secondary orifice providing the majority of the cooling and stabilizing the stalk of the bubble by creating a venture resulting in significantly improved cooling and throughput rates. This dual lip air ring provided extrusion rates approximately 10lbs/hr/inch of die circumference or an improvement in excess of 40% over the single lip design. In addition to thin guage and economical rates, this application required an even ratio MD/CD elongation to chieve balanced shrink properties. Using the dual lip air ring at higher production rates, the high stalk configuration (stalk height 6-7 diameters) allowed time for the polymer chains to relax to a point where the MD and CD elongation values were balanced at approximately 280% (+-5%), and at a blow up ratio of 3:1.
One further benefit of the high stalk was the high impact strength that results from the orientation created from the rapid expansion of the bubble immediately below the frost line.
Dual Lip Air Ring for High Output of LLDPE at High Blow-Up Ratios
LLDPE resins are found in an ever increasing variety of flexible packaging applications. Their economy, combined with superior mechanical properties, make them particularly well suited for blown film extrusion. LLDPE resins can be generally categorized into three co monomer types; butane, hexene and octane. For this study, a Unipol hexene based resin, 1.0 M.I., 0.918 density was used.
It is quite evident that cooling efficiency in blown film increases with blow-up ratio. Obviously, heat transfer is improved when the surface area exposed to the cooling air-flow is increased. However, blow-up ratio and its relation to film properties must be carefully considered in efforts to maximize extrusion rates. Whereas some desired properties may be enhanced by large blow-up ratios, other equally important properties may be diminished.
Much research has been conducted in examining the freeze line area and the relationship between the strain history of the bubble and the development of specific film properties. The degree and speed of orientation and crystallinity, to a large extent, determine the mechanical properties of the film.
It has been demonstrated that increasing CD orientation via BUR or output rates or a combination of the two will result in higher MD tear values. The low frost line heights associated with rapid quenching of the bubble also contribute to higher MD tear values.
The objective of this exercise was to maximize extrusion rates without sacrificing bubble stability or guage uniformity, while optimizing MD tear properties, at a 3:1 blow-up ratio and a normal film thickness of 33 microns.
The laboratory equipment used for the trial was comprised of the following: 90mm, 3-:1 L/D grooved feed, air cooled extruder; 150 HP DC motor and drive; maximum screw RPM 100; 200 mm single layer die, with a 2.25 mm die gap; and a three orifice conical IBC pancake diffuser.
The external cooling was provided by a conventional dual lip air ring with four supplementary adjustable collars mounted above the chimney. To obtain the desired quench rates, chilled air (10c) was supplied to the IBC and air ring.
Assuming that the extruder and the downstream equipment did not restrict extrusion rates, the limiting factor in producing thin guage films at high BURs has typically been the onset of bubble instability which may lead to unacceptable film variation and a tendency to wrinkle. If bubble stability could be improved, the bubble could tolerate a higher volume and velocity of cooling air which would translate into greater throughputs for the producer.
In general, stability has been improved by the venture created high velocity air passing over a fixed surface. By creating a high velocity flow, a lower pressure is created which draws the bubble uniformly towards it.
Based on this experience, it was believed that by stacking progressively larger diameter adjustable collars above the deflector lip of the air ring, the venture force would increase and the bubble would be very quickly drawn to the desired large blow up ratio. This would achieve the rapid quenching necessary for optimizing MD tear properties without compromising bubble stability or guage uniformity.
Actual results achieved with four stacked adjustable collars were 240 kgs/hr at 53 screw RPM, melt temperature of 210c, lay flat width of 960mm (3:1 BUR), at a nominal film thickness of 33 microns with a minimum 30.5 and a maximum of 35.5 measure on an off line thickness guage for a variance of + - 7%. The frost line height was 1050 mm and MD tear values in excess of 450g were measured. The suitability of this configuration of air ring is limited to larger blow up ratios, not less than 2.5:1 for this 200mm diameter model. However, the air ring is easily and quickly modified to run lower blow up ratios by simply removing the supplementary collars. Removing the top three collars, leaving only one in place will permit the processor to run blow ups as low as 1.5:1, albeit at lower throughput rates, typically 17-18 lbs/hr/inch of die circumference, dependent on the melt strength of the extrudate (assuming chilled air is used for the IBC and air ring).
Summary
Although the inherent properties of the raw material selected remain the primary factor in determining the physical properties of blown films, these properties can be greatly influenced by many variables in the fabrication process. The external cooling of the bubble is one variable that can be tailored to optimize very specific film properties. The cumulative experience with polymer behavior combines with advances in the design of dual lip air rings provides the processor with much greater flexibility today. The air ring lip set geometry can be customized to suit the specific needs of the application, instead of the application and bubble geometry, output rates, and ultimately, film properties being compromised to suit prevailing air ring designs.
