Choker bar

A web of molten plastic is pulled from the die into the nip between the top and middle roUs. At the nip, there is a very small rolling bank of melt. Pressure between the roUs is adjusted to produce sheet of the proper thickness and surface appearance. The necessary amount of pressure depends on the viscosity. For a given width, thickness depends on the balance between extmder output rate and the take-off rate of the pull roUs. A change in either the extmder screw speed or the puU-roU speed affects thickness. A constant thickness across the sheet requires a constant thickness of melt from the die. The die is equipped with bolts for adjusting the die-gap opening and with an adjustable choker bar or dam located inside the die a few centimeters behind the die opening. The choker bar restricts flow in the center of the die, helping to maintain a uniform flow rate across the entire die width. 

In a 200 mm OPS line, the die is usually 1.0-1.3 m wide. These dies are generally of the coat hanger manifold design with separate choker bar and... 

Fig. 1 Coat hanger-type sheet die concept (A) (1) central inlet port (2) manifold (distributes melt) (3) island (along with manifold, provides uniform pressure drop from inlet to die lip (4) die lip (die exit forms a wide slit) and schematic of sheet die (B) (1) upper die plate (2) lower die plate (3) manifold (4) island (5) choker bar (6) choker bar adjustment bolt (7) flex die lip (8) flex lip adjustment bolt (9) lower lip (10) die bolt (11) heater cartridge.

Choker bar adjustable along with the width of the die and serves to tune the flow balance across the width of the die. 

Fig. 8 Coextmsion feed block manifold and sheet die (A) (1) sheet die with flow restriction (2) adapter plate (3) feed block asembly (4) core material layer inlet (5) upper material layer inlet (6) lower material layer inlet and coextrusion multimanifold sheet die (B) (1) lower melt channel (2) upper melt channel (3) lower choker bar (4) lower choker bar adjustment bolt (5) upper choker bar (6) upper choker bar adjustment bolt (7) flex lip.

Each melt channel has its own choker bar for flow control. 

Choker bar (restrictor bar) n. A bendable metal bar incorporated in a sheet-extrusion die for controlling flow distribution and lateral sheet thickness, and for reducing stagnation in the melt. The shape of a flow passage between the choker bar and the lower die body is altered by turning bolts coimected to the bar. 

Figure 5.43 Die lip designs (a) flex bar die and (6) flex bar die with a choker bar. ...

One special feature of this design is the capability to adjust the local gap by means of a choker bar, similar to the gap adjustment in a flat sheet die, see Section 9.2. The choker bar in this drum extruder is activated by adjustable hydraulic oil pressure. Drum extruders have not been able to be a serious competition to the single screw extruder over the last 50 years. 

The examples shown in Fig. 9.12 are methods for local balancing of the land length. Another method of adjusting the shape of the die is to incorporate moveable elements in the channel such as choker bars, flex lips, and membranes. These elements are frequently used in film and sheet dies. However, they can be used in other types of dies as well. 

The first is the flex lip adjustment. A number of bolts along the width of the die allow local closing of the final land gap. This allows fine adjustments of the extru-date thickness at discrete points. The gap can be adjusted by as much as 1 mm or more if the flex lip is properly designed. The choker bar is not used as often as the... 

The upper layer can be adjusted with a choker bar, while the final combined layer thickness can be adjusted with the flex lip. 

In this design, the layer thickness can be adjusted by die vanes. The internal vane adjustment allows a greater degree of adjustment in overall actual layer thickness however, it does not enable local thickness adjustment as is possible with the choker bar. 

Fig. 2.7. Flat die of equipment for 5 m wide HDPE geomembranes. The die is fed by two extmders, connections far right The arrangement of the countersink screws on the top indicates the line of the coathanger-shaped duct. The choker bar can be adjusted by tlie nuts of the protruding bolts, and the die lips can be adjusted by the hexagon socket screws on the far left. The carpet of molten material leaves the die lip horizontally and is pulled into the gap between the two bottom rollers of the polishing roll stack. This and the following two figures courtesy Naue GmbH Co. KG...

Liu et al. [20] have proposed two novel approaches to eliminate flow nonuniformities caused by production variations instead of rebuilding dies to meet production variations, Liu et al. suggested first that a specially tapered cylinder can be inserted into the manifold to absorb flow nonuniformities. Here we shall discuss the second approach that is, to construct a specially designed choker bar for each production variation, the flow nonuniformities can be properly adjusted by inserting this choker bar into the extrusion die. 

Traditionally, a choker bar has a rectangular cross section and is used manually through a trial-and-error manner to correct flow nonuniformities for an extrusion die. In this chapter, we shall present design formulas that can be used to construct tapered choker bars so that trial-and-error approaches can be eliminated and an extrusion die should be much more flexible to handle different production requirements. 

A mathematical model that is based on the lubrication approximation is developed to analyze the effect of these four types of variations, and choker bars with specially tapered shapes can be constructed to correct flow nonuniformities owing to these variations. 

---