Collapsing Frames

Power lines severed m Controls damaged n Block walls fail Frame collapses Frame deforms Case damaged Frame cracks Piping breaks... 

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

One of the latest innovations in collapsing frames is using carbon-fiber rollers rather than aluminum ones. Because of the much lower heat transfer rate of the carbon-fiber, the rollers do not act as a significant heat sink, which reduces wrinkling problems. In addition, the carbon-fiber rollers produce less inertia so there is less drag on the film. 

As the bubble moves upward and approaches the nip rollers, it is preflattened by the collapsing frame (Fig. 3.17). This device provides a smooth transition from a round tube shape to a flattened tube shape. Collapsing frames utilize wooden slats, metal rollers. Teflon-coated rollers, or an air cushion to perform the shape transition. 

Figure 3.17 The collapsing frame is shown transitioning the bubble from a circular shape to a two-layer flat film...

In addition to flattening the tube, the collapsing frame also helps eliminate wrinkles in the final product. These devices are generally adjustable for both height and entry angle. Proper positioning of these two adjustments is often used to correct wrinkling problems. 

When the bubble has cooled sufficiently, it is flattened in a collapsing frame and pulled through a set of nip rolls to its top. From there, the layflat film is guided over several idler rollers to the winder where it is rolled up over a core. 

One of the featmes of tnbnlar film process is the roll formation obtained. It is common to rotate the tnhe or collapsing frame. This will distrihnte caliper variations in the film across the woimd np roll. Soft areas and hard hands may he eUminated in the output roll with this process. This process requires low labor content as compared to larger fiat film fines. As all the film is imiformly stretched, the yield of this process may he very high. 

Blown Film. The blown film process (Fig. 7) uses a tubular die from which the extnidate expands in diameter while traveling upward to a film tower. The top of the tower has a collapsing frame followed by guide and pull rolls to transport the collapsed film to subsequent slitting and windup rolls. The tubular bubble from the die is inflated to the desired diameter by air passing throngh the center of the... 

Fig. 7. Extrusion of blown film A, blown-film die B, die inlet C, air hole and valve D, plastic tube (bubble) E, air ring for cooling F, guide rolls G, collapsing frame H, pull rolls and I, windup roll (20).

Figure 7-41. A schematic of a basic (vertical-up) blown film line. The hot-melt bubble exits the die with an air tube in its center that provides internal air pressure to the bubble that is pulled by the nip rolls via the bubble collapsing frame.

The series of photographs shown In Figure 8 demonstrate the change In shape of the oll-alr meniscus Immediately prior to collapse. Frame 1 was taken at a surface speed of 0.12 m/s which was known to be close to the Instability condition. The elongation of the meniscus relative to the stable situation at a lower surface velocity shown In Figure 7 Is already apparent. The remaining frames (2-5) were taken at Intervals of (15/64) second, however, the precise velocity associated with each situation could not be determined. The Increase In surface speed was slight and collapse occurred precipitously. 

Wrap-Around Collapsible Frame. This forms the most basic type of wire-mesh container by using a standard pallet as a base. 

The tubular film continued upwards where it encountered a second collapsing frame that directed the bubble through nip rolls. The collapsed bubble was flattened and passed idler rolls and finally wound up on a collection core. 

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