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Developments in Blown Film Bubble Collapsing

 

 

Introduction
In blown film processing, molten polymer is transitioned from an annular bubble shape to a flat sheet. To complete this geometric transition, a collapsing frame is required. The difficulty with this transition is that the collapsing distance that the material travels will differ along the circumference of the bubble. An illustration of this transition, shown from both the front and side view can be seen in Figure 1. Due to the fact that the radius of the bubble, R, is larger than distance X, the film forming the centre of the layflat will travel a slightly longer distance to reach the nip compared to the film forming the edge of the layflat. This longer collapse distance results in an increased contact time with the collapsing frame, subsequently leading to the film experiencing more drag in this region. This increase in drag creates a non-uniform stress profile across the film width, which may lead to the formation of wrinkles and sag in the final film.



Figure 1. Collapsing theory. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
To reduce frictional drag coefficients and prevent wrinkle and centre-sag formation, both the collapsing surface and frame geometry should be optimized for the material being processed. Extensible films such as LDPE and LLDPE are more forgiving to uneven tensions and can readily adapt to the changing geometry without any negative implications on film appearance. Stiffer films such as HDPE, nylon, PET, etc., do not possess this same flexibility and are more prone to wrinkling, particularly with a shorter collapsing path. In order to minimize wrinkling with these films, the collapse angle should be reduced. One method of achieving this angle reduction is to increase the collapse frame length. However, a longer collapsing frame can result in a higher drag coefficient because of the increased material residence time. The flexibility of these stiffer films can also be improved by collapsing the bubble at a higher temperature.
Collapsing Devices
With the advancement of coextrusion technology, and the growing demand for thinner films, the selection of an appropriate collapsing device has become increasingly important in order to avoid experiencing unwanted defects in the final material. There exist many different types of collapsing devices, and determining which is appropriate for a specific application will depend on factors such as material properties, process conditions, and final material end-use requirements.
Wooden Slats
In the early days of blown film processing, the most common collapsing surface used was wooden slats. The collapse surface on these devices consists of wooden slats (typically maple), ranging from one to five inches wide, assembled in an inverted “V” at an angle between 15° and 20° (shown in Figure 2). The benefit of using wooden slats was their low heat-transfer coefficient and their low required maintenance. The drawback of using wooden slats was the high coefficient of friction that would be applied onto the film, resulting in an increased likelihood of film wrinkling. Plastic covers that snap onto the wooden slats were designed in order to reduce the overall coefficient of friction and allow for stiffer materials to be collapsed without experiencing wrinkling. This form of collapsing frame was previously used for lines producing very extensible films such as LDPE and LLDPE, however today they are typically only used for processing very stiff and slippery films.



Figure 2. Schematic representation of a wooden slat collapsing frame. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Full Width Roller Collapsing Frames
Advancements in blown film technology have given rise to faster line speeds and more complex film structures. These advancements have decreased the effectiveness of wooden slat collapsing frames, and have led to the development of roller collapsing devices. Initial roller collapsing frames incorporated a series of rollers extending the width of the frame, arranged perpendicular to the direction of film travel (shown in Figure 3). These rollers collapse the bubble with significantly less friction than wood and are more suitable for IBC applications where higher line speeds are achievable. The coating of the rollers can be optimized for the material being processed. For example, a non-stick coating is commonly applied to all roller surfaces when processing EVA film structures containing a high vinyl acetate content in order to prevent them from adhering to the rollers during collapse.
Although an improvement over traditional wooden slats, these full-width roller frames did have some disadvantages. The contact time between the film and the rollers is not uniform across the entire bubble circumference, and this variable contact time can cause localized cooling in the film, especially when using uncoated metal rollers. This localized cooling can result in the formation of sag and wrinkles. To mitigate this issue, rollers are often covered with heat-insulating materials. Common materials include cotton, felt, Velcro, and other similar substrates. Recent advancements in roller technology have led to the replacement of metal rollers with lightweight, composite carbon fibre rollers. Another issue which may occur when using full width rollers is undesirable scrubbing between the roller and the film, creating the potential for sags, wrinkles, and scratches to form within the final product. This scrubbing occurs due to the difference in path length and roller contact time that the film experiences during collapse, resulting in the rollers being driven at different speeds. The immense popularity of roller collapsing frames has led to the development of new variations to the original full width roller collapsing concept.



Figure 3. Schematic representation of a full width roller collapsing frame. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Segmented Roller Collapsing Frame
Segmented roller collapsing frames were developed as an improvement on the original full width roller collapsing frame design. These collapsing frames contain a series of short, small diameter rollers rotating on steel rods arranged perpendicular to the direction of film flow (shown in Figure 4). This modification improves on the original full-width roller design by allowing for the rollers to rotate independently and maintain surface speeds equivalent to the local film being collapsed. This ability to rotate at speeds independent of one another effectively eliminates any scrubbing and friction induced sag which may be caused by rollers rotating at speeds unequal to that of the passing film. The width of the segmented rollers typically increases progressively and symmetrically about the centre line of the film as you travel towards the primary nip. This was designed to allow for better contact between the rollers and the film as the bubble flattens and the surface area of the film increases.
The preferred surface material for these rollers is Teflon due to its low friction, high temperature resistance, and low static charge generation properties. However, other materials such as nylon and polycarbonate can be used as well. The size of these rollers typically ranges between 12.5 – 25 mm in diameter and 1.2 – 5 cm in length. This smaller roller size results in a lower inertia at startup and reduced momentum when spinning. The size of the collapsing frame itself can range from as narrow as 30 cm to as wide as 6 m.
The use of segmented rollers provides considerable improvements in the appearance of the collapsed film. Film collapsed using this device is likely to experience less wrinkling, centre sag, and scratch marks, all of which are common defects associated with wooden collapsing frames. One disadvantage with using segmented roller collapsing frames is that the roller system can sometimes become jammed with film debris and additive buildup, preventing the rollers from rotating freely and increasing the likelihood of scratching and wrinkling to occur. The rollers should be properly maintained and periodically inspected in order to prevent such issues.



Figure 4. Schematic representation of a segmented roller collapsing frame. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Spreader Rollers
The development of segmented roller collapsing frames has proven beneficial for minimizing wrinkle formation during the bubble collapsing process. However, even with the integration of segmented rollers, some film structures still experience varying degrees of wrinkling. To address this issue, the concept of a spreader roller was developed and introduced into collapsing frame technology. Spreader rollers were designed to gently spread the film out across the length of the roller to prevent centre sag and wrinkling. Figure 5 shows two of the most common types of spreader rollers, the helical contour and downward incline spreader rollers.



Figure 5. Schematic representation of a helical contour and a downward incline spreader roller. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Helical contour spreader rollers were designed to have a raised helical contour on the surface of the roller. The helical contour diverges outward from the middle of the roller in opposite directions. This diverging contour helps spread the film out along the length of the roller. A downward incline spreader roller (also known a as Bowed Roller) consists of a series of standard segmented rollers mounted on a rod that is slightly angled toward the direction of film travel on either end of its longitudinal axis. This angled orientation helps the passing film spread out to either end of the roller. Both of these rollers have been proven effective in reducing the likelihood of wrinkle and centre sag formation. In some applications (particularly with very stiff films), a brush roller may be used. These rollers consist of a brush surface which is both flexible and compressible, allowing for a more effective collapse of stiff films, while also providing the added benefit of a spreading action. However, one issue with this type of roller is that it can become contaminated with buildup of additives that bloom from the film. Each of these rollers can be retrofitted into existing segmented roller collapsing frames.
Air Collapsing Surfaces
An alternative collapsing frame surface to both wood and rollers is air. Air collapsing devices operate by floating the film on a cushion of air between two converging frames. This method of collapse applies little to no friction on the material due to the minimal contact between the film and the collapsing frame. An example of a typical air collapsing frame is shown in Figure 6. The converging surfaces are typically rectangular or trapezoidal in shape. The surface facing the film consists of holes through which air is forced through to create a cushion of air. The air is supplied by a blower, and the air chambers are baffled to ensure uniform airflow distribution. Air collapsing frames are typically used for very tacky films (such as ones containing PVC and EVA), as well as stretch films containing cling additives. When collapsing these types of films using a wood or roller surface, residue from the film tends to build up on the frame surface, negatively impacting the collapsing process over time. An additional benefit of using an air collapsing device is the enhanced cooling provided by the airflow, which helps minimize any blocking tendencies of the film. One drawback of this collapsing frame is the potential for bubble movement, caused by the essentially unrestrained collapse of the bubble. If present, this issue can be mitigated by incorporating side stabilizers into the frame to help entrap the bubble and prevent it from wandering. However, the bubble may still be able to twist during the collapsing process, even after the incorporation of side stabilizers.



Figure 6. Schematic representation of an air collapsing frame. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Side Stabilizer Adjustments
The stability of the bubble is extremely important to maintain throughout the collapsing process. In some cases, side stabilizers are required to help minimize unwanted bubble movement during collapse. Side stabilizers can be integrated into wooden, roller, and air collapsing devices. Side stabilizers used with air collapsing frames are specially designed with holes along their surface (similar to the air collapsing frame itself) from which air is blown through to help contain the bubble. An example of a conventional side stabilizer with adjusting points installed on a wooden slat collapsing frame is shown in Figure 7. The adjustment points are typically located at the top and bottom of the stabilizer, along its centre line. These adjustment points are powered by synchronized motors to allow for both side guides to maintain an equal distance from the centre line, regardless of adjustment position.



Figure 7. Schematic representation of a side stabilizer with adjusting points installed on a wooden slat collapsing frame. Adopted from: Stobie, J., Tamber, H., “Film Stabilization, Forming, and Collapsing Systems” in Film Extrusion Manual, 3rd ed. Peachtree Corners, GS, USA: Tappi Press, 2020, ch. 3, sec. 3, pp 105 – 113.
Summary
The most common and widely accepted forms of collapsing frame technologies have been discussed in this article. Each device has its benefits and drawbacks, and selecting the appropriate collapsing device (along with collapsing surface material and geometry) is crucial in order to prevent any unwanted defects from forming in the final film. Process characteristics, material properties, and end-use film requirements all need to be considered by the film manufacturer in order to select an appropriate collapsing device and create the conditions required to successfully collapse the bubble and produce an exceptional final product.

 

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