Extrudate flow

Waeschle (Fig. 13) has developed a pressure sensing plug system. The boosters in this system are actuated by pressure rises in the line. The Fluid-Schub system of Buhler, Fig. 14, uses valves to inject air from an auxiliary air line. A system that produces extruded flow of long plugs can also be devised as shown in Fig. 15. 

A 203.2 mm diameter smooth-bore extruder running a PS resin experienced low rates when extruding to a complicated downstream sheet line. The extrudate flowed from the extruder to a screen pack system and then to a sheet die. The line was able to produce acceptable product but at reduced rates. The reduced rates increased the cost of manufacturing the product and caused the plant to miss shipment dates to its customers. At a screw speed of 110 rpm, the rate was measured at 1750 kg/h for a specific rate of 15.9 kg/(h-rpm). Under all processing condi-... 

The extruder flow model suggested by Carley et al. (9) is chosen, by virtue of its simplicity and good agreement with experimentally determined residence time distributions, throughput and power requirements, to illustrate the methodology of this approach. 

The characteristic curve of extrudate flow including adherence to the walls, and hence representative of shghtly to moderately entangled polymer flow in sudden two-dimensional or axisymmetrical contractions [7, 32], is represented in Fig 2. It shows a slope discontinuity above a certain pressm-e level, which depends on the pol3uner-die pair considered. With low flow rates, the flow is stable. Indeed, for these regimes, allowing for entrance effects, the flow curve is in fact representative of the shear rheometry of the polymer imder consideration, at low shear rates [34]. The slope discontinuity of the head loss curve indicates a modification in the structure of flow. It will be seen that this corresponds to the triggering of a hydrodynamic instability upstream of the contraction. 

Extrudate flow at the die outlet accelerates suddenly and the sharkskin cracks disappear immediately (Fig. 12a). This may be attributed to the fact that slip considerably reduces the stretch stresses at the die outlet and the fluid no longer cracks. Thus, as has been shown in numerous works [18, 24, 25] the extrudate is then opaque and irregular and has practicedly no fiirther swelling (Figure 12a). The appearance of the extrudate is the result of slip effects superimposed on melt fracture effects. 

Profiles. The types of profiles that can be produced are determined by the die design and the secondary shaping that occurs in the haul off of the extrudate. Flow in the die occurs by pressure flow only as indicated by Equation 6. 

Twin-screw extruder Flow of Newtonian fluids in kneading disc elements and (right and left handed) screw elements. Szydlowski et al., 1987... 

In flat-film extrusion (particularly at high takeoff rates), there is a relatively high orientation of the film in the machine direction (i.e., the direction of the extrudate flow) and a very low one in the traverse direction. 

At present, there is no theory able to predict polymer drop deformation in viscoelastic medium. Furthermore, the extruder flow is complicated by the complex temperature and deformation history. Thus, the microrheology may only serve as a general guide for variation of morphology during compounding in a twin-screw extruder. 

High amylose starch can have a more stable extrudate flow, increased melt tenacity (bubble strength), and reduced die clogging by increasing the moisture content, increasing the compression ratio of the screw, and increasing the screw revolutions per minute (rpm) (Thuwall et al. 2006). 

To illustrate the dynamic operation of a smart blender and use of the morphology map, the designated value of N on the process control computer can be changed to restore State 4 from State 5 in Fig. 19.5 if desired. Additionally, screw extruder flow rates can be changed to increase to 30% and reach state 6. Multilayers at State 7 can be attained by reducing N, as shown. Such dynamic operation facilitates cost and property optimization of extruded plastics and can lead to more rapid research and development for new products. 

Lastly, each multiplication increases the instabilities in the flow field and the likelihood that the layers will either break-up or have non-uniform thickness. Increasing the number of layers produced in the feedblock can reduce the number of layer multiplications required. The Sandia feedblock has been designed to separate the incoming extruder flow into four layers. Thus, eight total layers exit the feedblock. Since the flow division is accomplished before the layered structure is produced, we expect that there will be no negative impact on the layer uniformity or flow homogeneity. 

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