Die entrance angle

White and Roman [94] measured the effects of L/D and drawdown on die swell of polypropylene. Figure 3.13 shows the die swell as a function of L/D for Hercules Profax 6523 and four other melts at 180°C. The d/D ratios were measured on frozen extrudates. Figure 3.14 shows the die swell as a function of take-up velocity. The relationship between die swell and die entrance angle for a polypropylene was determined by Huang and White [95]. Figure 3.15 presents their data. 

The swell of the extrudate as it leaves the extrusion die is an important phenomenon in polymer melt extrusion [14]. Die entrance angle plays an important role in the die pressure in the extrusion pressure. Die pressure is affected by flow rate of polymer melt, cross section of the die, die temperature, and material viscosity [15]. Controlling the die pressure can achieve a maximum production rate and an optimal application of extruder. Therefore, the entrance angle in die should be optimum to minimize the pressure drop [16]. Die entrance angle has no considerable effect on mechanical strength of the product, but the land length has some effects [17]. 

The viscous contribution to the total entrance pressure loss is very small. [C. D. Han, Influence of the Die Entry Angle in the Entrance Pressure Drop, Recoverable Elastic Energy and Onset of Flow Instability in Polymer Melt Flow, AIChE. J., 17, 1480 (1970).]... 

Fig. 12.16 Entrance flow patterns in molten polymers, (a) Schematic representation of the wine glass and entrance vortex regions with the entrance angle. [Reprinted by permission from J. L. White, Critique on Flow Patterns in Polymer Fluids at the Entrance of a Die and Instabilities Leading to Extrudate Distortion, App/. Polym. Symp., No. 20, 155 (1973).] (b) Birefringence entrance flow pattern for a PS melt. [Reprinted by permission from J. F. Agassant, et al., The Matching of Experimental Polymer Processing Flows to Viscoelastic Numerical Simulation, Int. Polym. Process., 17, 3 (2002).]...

The ribbon was placed between two split billet halves of the same polyethylene, and the assembly coextruded through conical brass dies which had an included entrance angle of 20 and nominal extrusion draw ratios, EDR, of 12, 25, and 30. No lubricant was used. The EDR calculated from the displacement of the line mark was in good agreement with the nominal EDR defined as the ratio of entrance to exit cross-sectional area of a die. The extruded films were used only for the evaluation of the effect of deformation components on the resultant morphology and properties. The semiperipheral coextrudates obtained simultaneously from the extruded assembly were used for the x-ray study of the deformation mechanism for extrusion drawing. 

FIGURE 3.15 Effect of capillary entrance angle on die swell for polypropylenes. (From Huang, D. White J.L. Polym. Eng. ScL, 1980, 20, 182. With permission.)... 

Hydrostatic extrusion of polypropylene has been conducted by Nakamura et al. [223] and by Williams [224], Williams extruded cylindrical billets of undrawn polymer with essentially no orientation. The billets were forced through an extrusion die with a 30° entrance angle and a diameter of 0.3 in. Silicone oil was used to transmit pressure to the billet as well as to... 

Entrance angle (entry angle) n. In an extrusion diem the total included angle, never more than 180°, of the main converging surfaces of the flow chaimel leading to the land area of the die. 

Flat-entry die n. An extrusion die in which the approach to the land has no taper, i.e., one having a 180° entrance angle. 

The angle of approach (or entrance angle) is the maximum angle at which the molten material enters the land area of the die, measured from the centerline of the mandrel. The angle of approach for any extruded material is an inverse function of its viscosity as it passes through the die. If the land is too short, it is difficult to maintain the shape being extruded. If the land is too long, it creates an excessive... 

The processing and properties of HDPE rods produced by solid state extrusion will be briefly described for more detail, the reader is referred to the original papers (6-8) and an extensive review by Zachariades, et al. (9). The polyethylene used was a duPont grade Alathon 7050 Mw = 57,000, Mw/Mn = 3. Melt crystallized billets were prepared by melting at 160 C and subsequent crystallization under the combined effects of temperature and pressure. The billets were extruded through a conical brass die, with an included entrance angle of 20°, in a 1 in. bore Instron capillary rheometer. 

The barrel size was 6.35 mm radius. Dies No. 1 and 3 have 90 degree entrance angles die No. 2 is very short and the entrance geometry from the barrel to the die is wine glass shaped. 

Shear viscosity-shear rate curves were measured using a Goettfert Rheograph 6000 triple bore capillary rheometer at 210°C. A die of 30/1 aspect ratio and 180° entrance angle was used. Apparent values were determined by controlling the drive speed and recording the entrance pressure. No corrections were made to the data to determine the Bagley and Rabinowitch factors. 

The capillary dies were attached after the heat exchanger, with die entrance pressures recorded in the reservoir (Fig. 2(b)). All the dies were the same except for the length of the capillary. The capillary ratios (L/D) were 3, 5.5, 20.1, 40.2, and 51.7. The converging angle between the reservoir and the capillary is 120°, which helps to suppress recirculation. 

It is doubtful, however, if melt fracture can be predicted from shear rate criteria alone, without taking the geometry of the apparatus into account. Especially the form of the channel at the entrance of the die is very important. In using extrusion dies with a conical entrance, flow instabilities are suppressed by decreasing the cone angle. This effect has been found experimentally by Tordella (1956), Clegg (1958) and Ferrari (1964). 

To avoid melt fracture in the potyetltylene extrusion described in Problem 7.18, the maximum elon tional strain-rate in the die must not exceed 10 s The die is therefore provided with a conically tapering entrance channel of half-angle a. Estimate the maximum allowable value of a, and comment on aity approximations used (see 7.N.1). 

The capillary dies must be very smooth, accurately machined, and hard. They are often made of tungsten carbide. Accuracy is especially critical. In a round-hole capillary, for example, the shear rate is a function of the radius cubed [see Eq. (7)]. A 1% error in the radius will result in a 3% error in the shear rate determination. Some dies have entry angles machined into their entrance to ease the abruptness of the transition from barrel to die and reduce the entrance pressure losses (see Bagley correction). Typical dies have diameters in the range of 0.5 to 2 mm, but specialty dies are available in almost any practical dimension. Depending on the piston speed and die selection, a wide range of shear rates can be achieved on capillary rheometers (see Fig. 9). 

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