Pity the blown film processor. Compared to molders or extruders, blown film operations combine multiple extruders, complex dies, cooling systems and takeoff apparatus in a system which would seem inhe...
Pity the blown film processor. Compared to molders or extruders, blown film operations combine multiple extruders, complex dies, cooling systems and takeoff apparatus in a system which would seem inherently difficult to control. While the number of control parameters has gone from large to larger with the popularity of multilayer films, advanced materials and slimmer margins, optimum throughput is still subject to knowledge of the basics of blown film. Surprisingly, some companies contacted by Canadian Plastics for this article expressed the “knowledge gap” as a primary inhibitor to higher productivity. Unlike other mainstream processes, a good place to start is in the middle.
WATCH THAT FROST LINE
The frost line, the point at which the resin cools through its softening point, is an excellent indicator of a properly set line speed. The frost line is especially easy to measure when running polyethylene where it presents a well defined opaque hue. Other polymers have a less obvious line, and in some cases, the line may be invisible. As a general rule of thumb, the frost line can be assumed to be the point at which the final bubble diameter is established. The critical frost line measurement is the height above the die.
Control of frost line height is important for several reasons. The two most important relate to gauge control and physical properties. The highest line speeds raise the frost line, often causing problems with gauge consistency. When viewed at eye level, the frost line should be level and consistent. Higher line speeds, however, exacerbate unevenness in the frost line, resulting in poor gauge control.
The second reason for maintaining tight control over frost line involves the structure of the resin itself. Altering frost line height by changing takeoff speed changes the ratio of linear expansion between the longitudinal and transverse directions. The molecules of polymer are reoriented, influencing physical properties such as tear and tensile strength. If extremely anisotropic (different in each direction) physical properties are required, however, throughput improvements will require additional cooling.
Raising the frost line also results in a smoother film with higher clarity and gloss by allowing the polymer additional time to solidify. The drawback of a very high frost line, however, is the film’s tendency to block or stick to the nip rolls. Sticking can be countered by altering the nip role temperature, but blocking may require reformulating the resin additive package.
Increasing line speed while retaining correct biaxial orientation, then, can take two forms. In one, increased takeoff speed combined with higher extruder output involves either costly modifications or an entirely new line. The other alternative is enhanced cooling, such as Internal Bubble Cooling (IBC). IBC supports the primary cooling offered by the air ring, provided air exchange is steady and smooth, and bubble geometry remains consistent.
IBC FOR FASTER LINE SPEEDS
Auxilliary cooling can involve chilled nip rolls, cooling bars in the collapsing frame, and auxilliary air rings. These differ from IBC given that they cool the bubble externally, while IBC operates inside the bubble through a lance mounted on top of the die. A new innovation from Brampton Engineering Inc. (98), called Anti-block IBC, further enhances the throughput advantages of conventional IBC. Standard IBC systems introduce cool air below the frost line and remove it somewhere above the frost line. Air above the return stack port is essentially stagnant and hot. Anti-block IBC introduces cold air in two places – below the frost line and just below the nip. The hot air at the top of the tower is replaced with cool air thereby lowering the temperature of the inside surface of the film.
The system is particularly useful for processors running very tacky film and where blocking limits output rate due to hot ambient conditions, or a restricted tower height. Peter Bicak, mechanical designer and product specialist with Brampton Engineering, tested the system on the firm’s three layer lab line head to head against a conventional IBC system. Bicak reports that where blocking began at a bubble temperature at the nip roll of 55C, the new system reduced the nip roll bubble temperature to 35C at identical output rates. When the output rate was increased to 125 percent of the baseline, the temperature stabilized at 45C, well below the blocking temperature. The system is retrofittable to most IBC dies and offers the potential for faster production rates, lower tower requirements and lower loadings of anti-block additives.
KNOW YOUR RESIN
What should film processors know about the materials they blow? There are three primary factors which processors should keep in mind, according to Dow Plastics’ Jeffrey Wooster, Value Chain Manager, Food & Special Packaging Group for Dow’s Polyethylene & INSITE Technology Technical Service & Development Group. (99) The first essential is a sound knowledge of the base resin’s required physical and optical performance. A common problem, Wooster reports, occurs at the front end of the process: “in reality they are not processing it correctly or there’s some other material in the system that’s interfering with the function…so it’s important to know how the material should perform to get the full value of the material”.
A second point to remember is simply that different resins run differently. Each resin requires its own combination of temperature and pressure settings for its screw type and die parameters. Different resins also often have unique preferred web handling methods.
Wooster’s final recommendation addresses a primary throughput killer: material changeover and startup. Difficulties in purging or alignment can negate faster line speeds and trigger quality problems at startup. If the processor “chases” the problem with running adjustments, the process and quality may become difficult to control. Over time, good documentation of setup parameters may reveal trends not obvious to even experienced setup technicians.
While seemingly obvious, substantial differences may exist in the “same” resin sourced from different suppliers, resulting in melt flow problems and bubble instability, usually early in the production run. Differences are generally caused by additive packages. Individuals sourcing new or alternate resin supplies should, therefore, consider additional machine test time, even where the supplier provides a “drop-in” replacement.
Purging compounds are another potential target for productivity improvements. The performance of purging compounds vary widely, with some requiring substantially more time to clear than the resin they are intended to remove. As a result, cost savings in purging compounds can be quickly negated even with small increases in extruder downtime. Purging compounds may require individual matching to incoming resins for optimal changeover.
Maximum throughput at the front end of the system means little if the line is rate-limited by winding equipment. Modifications to extruder, die and cooling systems can increase throughput rates to the point where instability at the winder causes poor roll edges and inconsistent tension. This instability can often be traced to tension control systems which exhibit poor damping or erratic feedback when run at very high spindle speeds. One solution is tighter microprocessor control. Advanced technology winders are available from several suppliers. Alpha Marathon Technologies Inc. (100), for example, offer an improved winder incorporating an advanced programmable logic control, digital drive systems and electronics. Automatic tension control is provided through a load cell. A flying cutoff knife, a pneumatically-assisted changeover system, and automatic core shaft transfer allow the direct-drive winder to run unattended for long periods of time. Finished rolls are transferred automatically to shipping pallets.
Another state-of-the-art winder is Battenfeld Gloucester’s Gloucester 1011
(101). The Model 1011 uses computer integrated tension-sensing and a closed loop-servo to optimize incoming web tension, spindle drive speed, and air entrainment levels. Standard operating parameters for each product are accessed by a touch screen interface, with maximum linear speeds exceeding two thousand feet per minute.
Although no one article can cover the multiple parameters which must be understood for optimum throughput in blown film operations, a “back to basics” approach pays dividends. For example, while a surprising number of blown film processors deemphasize the role of base resin formulation in higher productivity, a good working knowledge of resin properties can eliminate much trial and error. A good working knowledge, however, of extruder performance, bubble dynamics and cooling as well as downstream equipment is also essential for successful maximum speed operations. Experience is valuable, but theory has its place, too. Knowledge is not only power, it’s profitable. CPL