Extruder output

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

Equation (4.12) enables the die characteristics to be plotted on Fig. 4.12 and the intersection of the two characteristics is the operating point of the extruder. This plot is useful in that it shows the effect which changes in various parameters will have on output. For example, increasing screw speed, N, will move the extruder characteristic upward. Similarly an increase in the die radius, R, would increase the slope of the die characteristic and in both cases the extruder output would increase. 

At the operating point, the die output and the extruder output will be the same. Hence... 

During extrusion blow moulding of 60 nun diameter bottles the extruder output rate is 46 X 10 m /s. If the die diameter is 30 mm and the die gap is 1.5 mm calculate the wall thickness of the bottles which are produced. The flow curves in Fig. 5.3 should be used. 

The exit region of a die used to blow plastic film is shown below. If the extruder output is 100 X 10 m /s of polythene at 170°C estimate the total pressure drop in the die between points A and C. Also calculate the dimensions of the plastic bubble produced. It may be assumed that there is no inflation or draw-down of the bubble. Flow data for polythene is given in Fig. 5.3. 

When the polystyrene leaves the dryer, it is in strands of Ys in. diameter. These strands are cooled in awaterbath, and then slicedinto Vs in. lengths. The cutter must be synchronized with the extruder output so that the correct length is obtained. 

A web of molten plastic is pulled from the die into the nip between the top and middle rolls. At the nip, there is a very small rolling bank of melt. Pressure between the rolls 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 extruder output rate and the take-off rate of the pull rolls. A change in either the extmder screw speed or the pull-roll 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. 

Extruder output Melt temperature Die inlet pressure... 

Bulk density and packed bulk (or tapped) density are important properties. The bulk density determines the weight of resin that can be stored in a vessel and the amount a mixer can hold. It also has a major influence on extruder output rates. The bulk density of a resin depends upon its porosity, particle shape and particle size distribution. For suspension resin, bulk density is typically in the range of 450-650 kg nr3. 

Figure 18.9. The effect of talc particle size on extruder output. [Adapted, by permission, from Ishibashi J, Kobayashi A, Yoshikawa T, Shinozaki K, Antec 96. Vol. I. Conference Proceedings, Indianapolis, 5th-10th May 1996,386-90.]...

Extruder Output Rate. Flow and output rate of an extruder may be predicted from Equation 1, which is derived from fluid dynamics (2-... 

Extruded products, such as pipe, sheet or complex profiles for window frames, have a constant cross section. Figure 5.8 shows part of a pipe extrusion line the continuous output must either be coiled, if it is sufficiently flexible, or cut into lengths and stacked for distribution. All parts of the line must be carefully controlled to keep the product dimensions within the acceptable limits. We will concentrate on the analysis of the extruder output. 

The metering section of the screw controls the extruder output. It should contain 100% melt at a nearly constant temperature. Only the velocity components parallel to the flight (Fig. 5.10a) contribute to the output the other two velocity components are part of a circulatory flow that mixes the polymer. The output is a combination of a drag flow and a pressure flow. The drag flow is due to the motion of the screw surface with circumferential velocity... 

The average output of a 2-inch diameter extruder is around 50 kg per hour (at a power of 15 kW) but in larger extruders outputs of more than a hundredfold are reached. The machine efficiency is measured as units of output per kilowatt of the motor, ranging from 3.4 to 8.7 kgZ(hr-kW). 

Fig. 3-4. Illustration showing that a higher temperature input for plastics entering the barrel throat will increase the extruder output capacity with an increase in RPM.

In-line thermoforming production lines, particularly those being fed directly from an extruder, have to be completely synchronized, or inferior parts result, and/or the cost of the operation goes up. For example, if the trimmer operation has to be slowed down, the extruder output has to be reduced. In fact, a slowdown can result in a shutdown if the extruder cannot operate at the slower speed required. All functions and stations have to be properly interrelated. 

By maximizing feed/barrel friction and minimizing feed/screw friction, solids conveyance is maximized. Typically, a threefold increase in extruder output is obtained using a grooved feed throat instead of a smooth one at the same screw speed. 

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