Restrictor bar

Control flow Contains adjustable matching inserts, no restrictor bar Has restrictor bar or flow dividers in each polymer channel but with blown film dies control is by individual extruder speed or gearboxes... 

Fig. 12.1 Schematic representation of a sheet die, including manifold, approach channel, and die lip regions. The restrictor bar is incorporated so that the die can be used with different polymers of varying viscosities.

The flat dies, or slot dies as they are sometimes called, are used to produce webs in a variety of processes. They all have an interior manifold for distributing the plastic and lips for adjusting the final profile of the web (extrudate). Some dies have movable restrictor bars for changing the manifold for proper melt distribution (Figure 17.10). All flat dies have flexible lips that can be adjusted by bolts to remove humps or bumps in the web s profile. Die lips can have their adjustment bolts push only, where internal plastic melt pressures are adequate to keep the lips positioned against the bolts, or can be push/pull for low pressure applications. Direct acting or differential thread designs (for minute adjustments) are available. Profile variations of at least 3% or less can be achieved with flat dies. 

Many characteristics of cast film dies carry over into sheet dies but because of generally thicker materials, die lip openings are much larger and do not generate enough back pressure for accurate distribution of melt (Figure 17.6). Therefore, many sheet dies have a restrictor bar.143... 

The dies are generally equipped with flow modifiers such as restrictor bars and adjustable lips, to adjust for processing variations such as changes in resin, extrusion temperature, and flow rates. As mentioned, sometimes the lip adjustments are computer-controlled and linked to in-line thickness measurements. 

Die adjustments do not significantly influence the extrusion rate the lip opening, restrictor bar clearance, die heat, and overall die design have small to moderate effects on the back pressure. However, the extrusion rate is only slightly affected by substantial changes in back pressure. Thus attempts to open the lip or increase the die heat to increase the extrusion output rate can be exercises in futility, as well as the source of many additional problems. 

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. 

H. Gross, W. Michaeli, F. Pohler, and J. Ullrich, (Membran Statt Slaubalken) Membrane instead of Restrictor Bar, Kunststoffe, Oct, 1352-1358 (1994)... 

Restrictor bars are used in most manifold designs to aid in flow tuning if the lip opening of the die is greater that 2 or 3 mm. These bars can be either fixed or adjustable, and are either profiled or bent with external adjustments to change the distribution in the flowstream. 

Chill RolL In the chill roll cast film process, a plasties web is extruded from a slit die (Fig. 8.6) against the surfaee of a water-cooled chill roll. The die is arranged to extrude vertically or obliquely downwards so that the film web is delivered approximately tangentially to the roll surface. The die is similar in principle to a sheet die but will usually not inelude a restrictor bar. Film thickness is partially regulated by the gap between the die lips but also by the rotational speed of the chill roll which is arranged so as to drawdown and thin the melt web. Consequently, the die gap... 

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