Annular dies

Although plastic sheet and film may be produced using a slit die, by far the most common method nowadays is the film blowing process illustrated in Fig. 4.20. The molten plastic from the extruder passes through an annular die and emerges as a thin tube. A supply of air to the inside of the tube prevents it from collapsing and indeed may be used to inflate it to a larger diameter. 

Example 4.3 A plastic shrink wrapping with a thickness of 0.05 mm is to be produced using an annular die with a die gap of 0.8 mm. Assuming that the inflation of the bubble dominates the orientation in the film, determine the blow-up ratio required to give uniform biaxial orientation. 

Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it to take up the shape of the mould. This is illustrated in Fig. 4.21(a). Although this process is principally used for the production of bottles (for washing-up liquid, disinfectant, soft drinks, etc.) it is not restricted to small hollow articles. Domestic cold water storage tanks, large storage drums and 2(X)... 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

Polyethylene at 170°C passes through the annular die shown, at a rate of 10 x 10 m /s. Using the flow curves provided and assuming the power law index n = 0.33 over the working section of the curves, calculate the total pressure drop through the die. Also estimate the dimensions of the extruded tube. 

Figure 11.6 illustrates the general configuration of a film blowing operation. Molten polymer from the extruder is pumped into an annular die, where it is distributed around a tubular melt channel before emerging vertically as a relatively thick-walled molten tube. The top of... 

AMMONIUM HALIDE, 293 ANISOTROPY, 38 57 153 174 406 ANNULAR DIE, 311 ANTIOXIDANT, 385 ANTI-SLIP PROPERTIES, 208 ANTISTATIC PROPERTIES, 91 302 426 427 444 447 450 APPEARANCE, 124 186 241 293 AQUEOUS DISPERSION, 103 ARGON, 185 233 235 321 339 433 ARRHENIUS FACTOR, 243 ARTIFICIAL HIP, 52 ARTIFICIAL KNEE, 52 ARTIFICIAL SEAWEED, 455 ASPECT RATIO, 314 419 ATOMIC FORCE MICROSCOPY, 149... 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

In more recent work, talc-filled polystyrene compounds, with various filler volume fractions, have been processed by compression moulding and through a variety of slit, capillary, rectangular and annular dies [37]. Particle orientation has been characterised using wide angle X-ray diffraction, then expressed in the form of pole figures, and by scanning electron microscopy. It was concluded that... 

Film blowing. In film blowing, a tubular cross-section is extruded through an annular die, normally a spiral die, and is drawn and inflated until the freezing line is reached. Beyond this point, the stretching is practically negligible. The process is schematically depicted in Fig. 3.55 [14],... 

Film blowing. A tubular 50 pm thick low density polyethylene film is blown with a draw ratio of 5 at a flow rate of 50 g/s. The annular die has a diameter of 15 mm and a die gap of 1 mm. Calculate the required pressure inside the bubble and draw force to pull the bubble. Assume a Newtonian viscosity of 800 Pa-s, a density of 920 kg/m3 and a freeze line at 300 mm. 

Cox and Macosko (19) have reported experimental results on measurements of the melt-surface temperature upon exit from the capillary using infrared pyrometry, which senses the radiation emitted by the hot polymer melt surface. Their work also included the numerical simulation of viscous heating in a capillary, a slit, and an annular die, using a method resembling that of Gerrard et al. (13). They used a boundary condition at the die wall in between the isothermal and adiabatic case, —k(dT/dr) = h T — To) at the wall, where 7o is the die temperature far from the melt-die interface as well as the inlet melt... 

Plastic tube and tubular films are formed continuously by extruding a polymer through an annular die. The annular flow channel is formed by the outer die body and the die mandrel. A number of annular die designs are currently employed. In the first, the mandrel is supported mechanically onto the outer die body by a number of fins called spider legs Fig. 12.41 illustrates this type of die. The flow is axisymmetric, and the only serious problem encountered in the cross-machine direction uniformity of the extruded product is that of weld lines and streaks caused by the presence of the spider legs, which split the flow. 

In converging or diverging sections of annular dies, the fluid elements are subjected to axial and radial accelerations. Neglecting the radial special accelerations (for small tapers), the -component equation of motion reduces to... 

Fig. 14.25 Schematic parison die designs (a) convergent (b) divergent, and (c) straight. [Reprinted by permission from A. Gracia-Rejon, R. W. DiRaddo, and M. E. Ryan, Effect of Die Geometry and Flow Characteristics on Viscoelastic Annular Die Swell, J. Non-Newt. Fluid Mech., 60, 107-128 (1995).]...

N. Orbey and J. M. Dealy, Isothermal Swell of Extrudate from Annular Dies Effects of Die Geometry, Flow Rate, and Resin Characteristics, Polym. Eng. Sci., 24, 511-518 (1984). 

An alternative way of producing film is the blowing process. An annular die is used, from which the material emerges as a thin-walled tube, which is immediately blown-up by internal pressure to a much larger diameter (Figure 11.26). 

The BM extruder is almost always arranged in a horizontal attitude, so the first task for the parison head is to turn the melt flow stream through a right angle. This is basically difficult and undesirable but necessary to achieve in a way that meets the essential requirement for a constant flow rate at every point in the annular die gap. A second and related requirement is that the parison should carry as little evidence as... 

Extrusion A plastic processing technique to produce pipe, film or sheeting. The plastic is fed through a flat or preformed annular die, which gives the object its definitive shape. 

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

An annular die is used on the end of the secondary extruder. Die diameters range from 76 to 254 mm and die gaps range from 0.25 to 0.76 mm. Nucleation of the cells occurs near the exit of the die because of the pressure drop in the die lips. Controlled foaming continues outside the die lips as the material is stretched over a forming mandrel (Figure 11.7). The forming mandrel diameter... 

Figure 11.6 A tandem extrusion process for the manufacture of polystyrene foam sheet. (1) Primary extruder (2) blowing agent addition system (3) screen changer (4) secondary extruder (5) annular die (6) cooling mandrel (7) S-wrap (8) winders...

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

Both pipe and tubing are made in dies with an annular die exit. A pipe product is defined as being greater than 1 in. in outer diameter and a tube less than 1 in. Dies for these products are made in two styles 1) in-line dies (also called spider dies) shown in Fig. 4A and... 

The melt parison is extruded from an annular die. The wall thickness of the parison depends both on the annular gap setting and on the shear rate of the melt in the die. The melt will swell after the parison exits from the die and the die swell Increases with increasing shear rate. The die swell is also a function of the temperature, the type of polymer, and its elastic melt properties. Usually the shear rate in the die varies from 10 to as high as 700 s . As a rule of thumb one uses a die land length 8 times the annular gap. The parison should normally not be blown up beyond 3 1 (i.e., bottle parison diameter). 

The drawdown ratio, DDR, characterizes a tubular die and is defined as the ratio of the cross-sectional area of the annular die to the cross-sectional area of the finished insulation, as seen in Fig. 6.20 and Eqs. (6.2) and (6.3). Each resin has a characteristic range of DDR shown in Table 6.5. A comparison of drawdown ratios for a number of polymers is listed in Table 6.6. 

---