Oscillating Nip System for Improved Blown Film Gauge Randomization
Although the blown film process will yield film with superior and more balanced physical properties, it is virtually impossible to produce film with a constant uniform thickness. There will always be deviation from a target thickness and it is t he t ask of the film manufacturer to establish the acceptable tolerance or variation from the mean. This is commonly referred to as guage variation. There are numerous influences and factors that will collectively conspire to destroy good roll geometry.
If some method of randomizing these irregularities is not incorporated prior to winding, they will accumulate in localized areas, resulting in unacceptable roll geometry. With the converting industry facing the challenge of higher speeds in printing and laminating operations, the need to minimize downtime has led to demands for larger roll diameters. This, combined with the general industry trend to down guage, has heightened the demand for good geometry of the wound rolls. High speed converting requires wound rolls with uniform hardness and cylindericity that are straight edged, free of guage bands or creases. Film manufacturers employ various methods of randomizing or distributing the guage irregularities and imperfections originating in extrusion or bubble cooling. These can range from oscillation/rotation of the extruder and die, oscillation of the die and air ring lipset, oscillation of the nip through to oscillation or rotation of the winder, either up in the tower or at floor level.
Gauge Distribution Methods
Rotating or Oscillating Die
Die rotation or oscillation has proven to be the most popular means of guage randomization. In recent years, oscillation of the die has supplanted rotation as it eliminates the need for maintenance of collector ring assemblies and greatly simplifies temperature control. This method of guage randomization is relatively simple and cost efficient for single layer dies. However, it becomes significantly more expensive and difficult to maintain when utilized in co extrusion applications which require complex interlayer sealing devices that wear out in time. It also leads to more elaborate and costly IBC systems, as opposed to stationary dies. An additional consideration in oscillating/rotating dies is the increased height required for the requisite hardware. This is particularly evident in IBC situations where a rotating air distribution/collection chamber is required. The net result is a higher die top with reduced operator access to the die and air ring for adjustment, cleaning and thread up. A further disadvantage of the additional height is increased polymer residence time and pressure drop versus a stationary die.
The oscillating or rotating die will randomize guage variations originating from the extruder, die or air ring lips. Unfortunately, variations that occur due to drafts, cages or irregular air distribution in the air ring chamber will not be distributed.
In this instance, the extruder and die are mounted on a common platform which oscillated through 360 degrees. This method achieves better results than oscillation of the die, as all irregularities are randomized except drafts or stretching/sagging from collapsing. A disadvantage with rotating the extruder is that it only works with a small machine. To rotate a large extruder would be both impractical and expensive, requiring a considerable amount of valuable floor space. Also, as the hopper rotates with the extruder, the resin can be fed from only one position. Consequently, in emergency situations, the resin must be hand fed. Another drawback to this system is that equipment must be tightly packed into a small platform, restricting access and making maintenance awkward.
Rotating Tower Platform with Winder
The practice of mounting a winder with the nip on a rotating platform atop the tower is the most effective means of guage randomization. This method will distribute all guage irregularities except sag in collapsing. However, this method is really only feasible when converting occurs on the top level of a multi-storey facility and the extrusion operation is in the basement. Otherwise, the processor is presented with the task of transporting heavy rolls of film from the top level of the tower to the converting level. This is an expensive proposition if dome with an elevator, or relatively dangerous if a hoist is used. Since blown film extrusion is rarely done in multi-storey buildings, this method is not widely used.
Floor Level Rotating or Oscillating Winder
This method of downward extrusion places the winder on the floor and the extruder on the top of the tower. First employed in Europe 20 years ago, this method is not common today except for some specialized biaxially oriented film applications. Randomizing at the winder is the optimal point and placing the winders on the ground floor solves the roll handling problems. However, placing the extruder and die thirty feet above the floor complicates accurate process monitoring by the machine operator. Also, it is difficult to extrude lower melt strength materials downward, due to melt elongation. In addition, the stress applied on the melt by the weight of the bubble reduces dart drop and tear strength of higher molecular weight polymers, due to the lesser degree of molecular relaxation. It is for this reason that downward HDPE extrusion has largely been replaced by upward extrusion.
Oscillating Nip Methods
Introduced to markets approximately 25 years ago, the oscillating nips system eliminated most of the disadvantages that existed with other methods of guage distribution. The extruder and the winder both mount on the ground floor. Extrusion occurs upwards and the oscillating nips randomize all guage irregularities except those that occur in collapsing. None of the other methods discussed will randomize collapser induced flaws either. Oscillating nips allow for easy, trouble-free, multi-layer extrusion and with the exception of the oscillating die, are the most popular method of guage randomization.
In a typical oscillating nip system, the collapsing frame, nip roll assembly and platform oscillate continuously through 360 degrees, in relation to the stationary film die. The oscillating platform should be driven by a gear rack or similar system to ensure smooth, steady movement and to avoid jerking or shaking of the bubble which can cause wrinkles. Power for oscillation is provided by a small (1hp) variable speed DC motor and drive.
Oscillation speeds can vary dependent upon throughout but are typically in the range of one rotation every 120 to 300 seconds (0.5-0.2 rpm). The oscillation speed should automatically slow considerably with the use of limit switches, a short distance prior to the completion of each full oscillation. Four-sided collapsing using side-stabilizers is essential to prevent bubble movement which may cause wrinkles. Simultaneous adjustment of all opposing collapsing surfaces will ensure that the bubble is centered and symmetrical at all times. The use of a regenerative DC drive on the primary nip with tachometer feedback for speed control of the secondary nip will ensure that consistent tension is maintained through to the windup.
There are basically two types of oscillating nips: those with horizontally mounted turning bars and those with vertically mounted turning bars.
Vertical Oscillating Nips
In the vertical oscillating nip, the collapsed tube exits vertically upward from the nip rolls and passes over a diagonally oriented 45 degrees turning bad. This turning bar, mounted on the oscillating platform, redirects the tube to move horizontally. The hollow tube turning bar is coated with a low-friction, abrasion resistant covering, typically chrome. In addition, the tube turning bar has a series of small holes on the upper side, in which air is forced through. The forced air provides a cushion between the turning bar and the film, as the film passes over it. The horizontally moving film passes around several vertical rotating idler rolls (mounted on the oscillating platform) to a fixed vertical roller located off of the oscillating platform. The film is then directed to a second diagonal 45 degrees turning bar (also mounted off the oscillating platform) which changes the direction to vertical. The film is then directed down the tower toward the winder/secondary nip assembly.
Some unique benefits of the vertical turning bar system are that it can accommodate the incorporation of an edge guide between the inclined turning bars. The significance of this being that any web travel that may occur during the first 45 degrees turn is corrected, prior to final redirection which would otherwise tend to accentuate the error. This will aid roll geometry by assuring better web tracking down the tower. This consistent tracking will help to reduce the potential for the web to wander downstream, due to undesired polymer memories.
An additional consideration is the fact that, although the hual-off may be oscillating completely through 360 degrees, the collapsing bubble (depending on the elasticity of the material and the tower height) will lag somewhat behind the equipment. The film tube may in fact only oscillate through 270 degrees to 300 degrees. This will be of particular concern to processors who are producing large diameter rolls of single wound sheeting with in-line slitting into multiple rolls, on a common core. Without complete oscillation of the bubble, they may find unacceptable guage variation and roll geometry on the outer rolls. With the vertical systems, this can be overcome by providing 720 degrees of oscillation. This can be achieved with relatively modest mechanical modifications to the structure that will not unduly compromise the ease of threading. These include additional idler rolls for directing the web and preventing it from fouling the collapsed tube, as it exits from the nip rolls.
The advantages of using vertical turning bars are that they are relatively compact, simple to thread and readily maintain alignment. A disadvantage is the greater head room required. Additionally, with winder webs in excess of 90 inches (2300mm), there is a risk of the unsupported web hanging or wandering. With vertical oscillating nip systems, it is essential to avoid long unsupported spans in the web path, as these may result in sagging and wrinkling of the film.
Horizontal Oscillating Nips
In this system, the film remains parallel to the floor. The turning or skew bars are horizontal and the film is directed in a scissor like manner through the 180 degrees of 360 degrees of rotation. The fixed horizontal turning bars, like those in the vertical process, use an air cushion for lubrication. The benefits of the horizontal nip system include a lower head room requirement than the vertical system. Also, the elimination of the need to stand the film on its edge, reduces the risk of the web wandering or sagging.
A drawback of the horizontal system is a restriction to a maximum of 360 degrees of oscillation. As discussed previously, this can prove inadequate when winding multiple rolls of in-line slit sheeting. In addition, the threading operation is more complex with the horizontal system. Any slight misalignment of the turning bars will create wrinkles in non-stretchable materials.
The Last Word
Relatively minor thicknesses irregularities will inevitable occur in the blown film process. These flaws can appear in the extrusion process itself or above the die and air ring system. Unfortunately, these imperfections do not always rotate around the bubble and if not addressed will create thinner and thicker areas across the wound roll face, as they continuously overlap. As roll diameters grow, the severe effect of even these minor thickness irregularities increases, later showing up as hard or soft bands in the roll and limiting its application.
There are many variations of oscillating nip system commercially available. Presently, it is the most favored and trouble-free method of guage randomization particularly in co extrusion, high quality sheeting and IBC applications. The numerous advantages of the oscillating nip combine to offer a system that improves quality, reduces downtime for cleaning and maintenance, minimizes floor space requirements and ensures production of high quality jumbo rolls of films.