Developments in Blown Film Bubble Collapsing
Although superior properties can be achieved by the blown film process, most laminating film and film used for high-speed packaging is still made using the cast process. The main reason for this related to the problem of flatness and surface imperfections, inherent in blown film when compared to cast film. Much effort has been made to eliminate this problem.
The gauge irregularities and imperfections originating in extrusion or bubble cooling can be effectively randomized by various methods. These methods include oscillation or rotation of the die, oscillation of the nips and floor level oscillation or rotation of the winder. Not one of these methods in effective for randomizing guage variations that occur when collapsing takes place.
This article will focus on the problems of wrinkling and sagging in bubble collapsing and methods of minimizing this problem.
Imperfections, wrinkles and sagging of the film are commonly caused by the following:
Irregularities in gauge, melt temperature, cooling and mixing
Scratching and stretching in collapsing frame
Irregular vertical and horizontal bubble stability; the bubble wanders, creating wrinkles
The film at the centre of the bubble, contacts the collapsing surface for the longest duration. This longer contact creates more drag. Thus, the film in this area is stressed to a higher degree and can be permanently deformed creating a centre sag.
The inherent compatibility in collapsing a tube of plastic material from a tubular (circular) cross-section to a flattened form
Relatively extensible films, such a LLDPE or LDPE, are more forgiving and readily adapt to the changing geometry of the collapsing bubble. This is accomplished with minimal adverse effect on film flatness and gauge uniformity. Stiffer films, like HDPE or many co ex structures, do not possess this flexibility. If not handled correctly, these films can present the processor with unacceptable wrinkling and/or sagging.
There are a number of different collapsing surfaces commonly in use today. By far, the most popular is the maple slat collapsing frame. This usually comprises 1”-5” wide maple slats with slightly crowned faces, arranged in an inverted “V” at a total angle of between 24 and 30 degrees. The typical collapsing length is 1.5 x nip roll face width. This style is frame is most commonly found on LDPE and LLDPE film lines where the stretch and yield characteristics of the film are sufficient to minimize the effect of the differential length in collapsing.
A recent refinement to this collapsing method is the use of plastic covers that snap onto the maple slats. Typically made of nylon or similar material, these covers offer the benefit of reduced friction, over the maple surface. It has been found that film jerking or stuttering due to friction in collapsing, can be a limiting factor in line speed.
Roller Collapsing Surface
The processing of stiffer films or co extruded structures requires more attention be paid to the method of collapsing. This has left to the introduction of a variety of new collapsing frames, one of which incorporates a series of parallel rollers, each extending the width of the frame. These rollers collapse t he bubble with significantly less friction than wood. They are more suitable for IBC applications with low tower heights and film which is still hot when collapsed (and more susceptible to the effects of drag).
Although an improvement over wood, this full-width roller collapser does have some limitations. If uncoated metal rollers are used, they can cause localized cooling, as a result of the varying contact with the film, producing sagging bands and poor roll configuration. Additional problems can occur, due to the variation in (the length of the line of) contact of each other roller with the film, as the linear speed of the film increases progressively from the centre outwards. The result is that different parts of the film are attempting to drive the roller at different speeds and some scrubbing must occur. Frequently, this will still result in an undesired amount of wrinkling, sagging and even scratches, as the hot sticky film engages the collapsing surface. To prevent differential cooling with large rollers, some companies cover the rolls with heat0insulating material, such as cotton, cork or velour.
A logical progression from the full-width roller is the introduction of the segmented roller collapsing surface. This is comprised of a series of short, small diameter rollers, rotating on tensioned steel rods within a structural frame. The segmented roller collapsing surface presents a substantially continuous surface that comes in contact with the bubble. The individual rollers are free to rotate independently. They maintain the same surface speed as the collapsing film. This surface speed will vary from section to section, effectively eliminating sag in the centre of the film.
Typically, the width of the segmented roller section increases progressively and symmetrically about the centre line, in the film direction. As the bubble moves between the converging frame, a larger surface portion of the tube is contacted. Ideally, the rollers are in a size range of 12.5 to 25 mm diameter by 1.2 to 5cm long.
The preferred material for these rollers is “Teflon” because of its low friction, high temperature resistance and low generation of static electricity, so as not attract dirt. However, other materials, such as nylon or polycarbonate can also be used. These short, small diameter, low friction rollers have a low inertia during start up. Correspondingly, the rollers have a low momentum when spinning. Therefore, they will not damage the film.
The size of these collapsing frames can vary widely, from as narrow as 30 cm to as wide as 6m. the usual range is from 1.5 to 3.0m. The axles are typically spaced 3 to 4cm apart, in the direction of the film movement. If the axles are longer than about 1.2 m, then intermediate supports should be provided to prevent sagging.
The application of a segmented roller frame leads to a considerable improvement in the appearance of the flattened film. The film exhibits much less wrinkling and there is an absence of scratching, caused by wooden or full-width roller frames. More importantly, it is found that the collapsed film is much flatter. This is most probably due to the improved support of the film in the frame, by the small multiple independently rotating rollers, resulting in less drag on the film.
The friction between the film and each roller is substantially higher, parallel to its axis of rotation than in the direction of the film. It is not reduced by sideways scrubbing movements of the film on the rollers. The result is that the film “tracks” more positively with each of the small individual rollers. The film is held more stable in the frame against transverse movement so that uneven gauge, marking and damage to the film is avoided. This improved flatness is particularly important with co extruded films which are used on high-speed machines that require the use of sufficiently flat film. Also, it is found that the rate of extrusion and line speed can be increased substantially. One of the reasons is that the film can be much hotter, when it is contacted and collapsed by the frame.
With the previous collapsers, the maximum speed was limited to a threshold above which the still hot and sticky film begins to jerk in its contact with the frame members (to avoid the danger of the bubble being torn and disrupted). This does not happen with the segmented rollers because the film is contacted by low friction free-rotating rollers. However, the improvement is not fully explainable by this effect alone.
It is also due to efficient heat removal that is provided by the spinning rollers which constantly present a fresh effective heat transfer surface to the hot film. By their rotation, the rollers produce a highly turbulent airflow over the remainder of their surfaces that are not contacted by the film and also over the adjacent film surface. This turbulent flow is effective in cooling both the film and roller surfaces.
Spreader Roller Concept in Bubble Collapsing
The collapsing frame with parallel rows of small segmented rollers perpendicular to the film flow has proven to be a substantial improvement for minimizing wrinkling during flattening over prior methods. However, some films may still not be as wrinkle free as desired. To address this deficiency, the principle of the “spreader” roller has been introduced into the collapsing frame geometry. Several slightly different manifestations of the concept have been used, depending upon the circumstances.
The rollers on the left-hand side of the longitudinal centre line have a raised helical contour on the outside diameter surface (comparable to a left-hand thread). This design tends to move the tube at an outwardly inclined angle to the centre line, as the tube moves upwards over the frame. Similarly, the rollers on the right-hand side of the centre line have a helical contour, comparable to a right-hand thread. Again, this directs the film outward.
In addition to reducing the tendency of the collapsing tube to wrinkle, the helical contour enhances the purchase of the collapsing tube to the rollers and further improves bubble stability. This style of roller is easily retrofitted into existing perpendicular segmented roller frames. An alternative arrangement is the use of unitized rollers with a smooth, cylindrical surface. On each side of the longitudinal centre line, the rollers are mounted on axles which are downwardly inclined to the direction of travel of the film, approximately 7 degrees from perpendicular. This configuration has proven even more effective in eliminating wrinkles. If bubble stability is a concern, the helically contoured rollers can also be utilized.
Air Collapsing Surface
One further method of film tube collapsing that should be mentioned is the “air collapsing” surface. In this design, the film is floated on a cushion of air, providing an almost frictionless surface. This system is comprised of converging rectangular or trapezoidal structures with two opposing surfaces facing the film. Air is forced through a series of non-repeating holes. The (filtered) air is supplied from a suitable blower and the chambers are baffled to ensure a uniform distribution of air flow.
An additional benefit of air collapsing is enhanced cooling of the film. This cooling effect can help alleviate blocking tendencies and winding shrinkage. Air collapsers are particularly well suited for use on tacky, cling film extrusion lines, such as PVC and EVA, also, stretch films containing PIB tractifier that can quickly build up on wood or roller collapsing surfaces are benefited.
The very low friction associated with this method of collapsing can result in undesired bubble instability. This can be partially overcome with the use of air collapsing side guides or stabilizers that entrap the bubble. The air collapsing surface is the most expensive methods discussed and is also the most difficult to retrofit.
In selecting a particular method of bubble collapsing, the film manufacturer must address the processing characteristics of the particular substrates in the process. Also, the film manufacturer must evaluate the requirements of the film end user.
A film line dedicated to producing monolayer trash can liners from 100% LLDPE for example, would benefit little from a sophisticated method of film collapsing. In this case, a simple maple slat system would be adequate and considerably less costly. Conversely, a producer of complex co extruded structures for use in high-speed laminating applications must devote a lot more attention to ensure a stress and wrinkle-free lay flat. This type of application will require a more elaborate collapsing devise. The key is to match the film requirements to the collapsing surface.