Types of Dies

There are two types of die-casling machines. The first, or air-operated machine, forces lhc material into the die by high pressure on the surface of the ntollen metal in a special ladle ur goose the second, or plunger type machine, forces the malerial into the die by means of a cylinder and piston which arc submerged in the molten metal. 

A recent paper describes the use of self-supporting disks of alumina as samples for the infrared study of adsorbed water 56). These self-supporting disks were prepared in the same type of die as is used for the pressed-salt method. The self-supporting disks will undoubtedly prove to be far superior to pressed-salt disks because the former are pervious to gas and the complications caused by the presence of salt are eliminated. Satisfactory self-supporting disks were made by pressing 0.2 gm. of alumina in a 1-in. diameter die at a pressure of 30-40 tons/sq. in. No preliminary particle-size separation is necessary, and the alumina may be impregnated with metal salts either before or after pressing in the die. 

The spiral die, commonly used to extrude tubular blown films, eliminates weld line effects and produces a thermally and geometrically homogeneous extrudate. The polymer melt in a spiral die flows through several feed ports into independent spiral channels wrapped around the circumference of the mandrel. This type of die is schematically depicted in Fig. 3.18. 

The engineering objectives of die design are to achieve the desired shape within set limits of dimensional uniformity and at the highest possible production rate. This chapter discusses both objectives, but the question of die-formed product uniformity deserves immediate amplification. To understand the problem, we must distinguish between two types of die-formed product nonuniformity (a) nonuniformity of product in the machine direction, direction z [Fig. 12.2(a)], and (b) nonuniformity of product in the cross-machine direction, direction x [Fig. 12.2(b)]. These dimensional nonuniformities generally originate from entirely different sources. The main source of the former is the variation... 

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 the cross-head type of dies, the melt is split at the inlet to the manifold and recombines 180° from the inlet. Moreover, the flow is not axisymmetric, and fluid particles flowing around the mandrel have a longer distance to travel than those that do not. 

Different types of dies are required to produce the many different shapes produced by extruders worldwide. Table 17.3 is an example of the few types of dies designed and manufactured by Extrusion Dies, Inc. 

The spinneret is a type of die principally used in fiber manufacture. It is usually a metal plate with many small holes (or oval, etc.) through which a melt is pulled and/or forced. They enable extrusion of filaments of one denier or less. Conventional spinneret orifices are circular and produce a fiber that is round in cross section. They can contain from about 50 to 110 very small holes. A special characteristic of their design is that the melt in a discharge section of a relatively small area is distributed to a large circle of spinnerets. Because of the smaller distance in the entry region of the distributor, dead spaces are avoided, and the greater distance between the exit orifices makes for easier threading.143... 

Lastly, it must be underlined that it is possible to significantly delay or eliminate the appearance of sharkskin, by considering the polymer flow through fluorinated dies [33, 39, chapter III.4. of this book]. Characterized by their particularly low surface energy, these type of dies cause the polymer shpping at the wall even in small flow regimes. Thus, the fluid could be extruded under low stresses and it doesn t crack. 

Neuial Piogenitoi CeU Fetus Basal forebrahi, cerebral cortex, hippocampus, and sphial cord Adult Subventricular zone and liippocainpal dentate gyms Limited (Can self-renew, but oiUy for a Ihnited nmnber of generations) Muldpotent (Can differentiate into at least two cell types of die dssue hi wliich diey reside)... 

There has been for some time, clear evidence that in long dies, there is considerable consolidation of the wet mass (21). This has a major contribution to the consistency of the extiudate, which is very different to the extrudates produced by short dies (i.e., screen extruders). This could well account for the differences in the performance of formulations processed by different types of dies. There is evidence, that for paste systems, the type of flow is plug flow (20). Here there is migration of the fluid to the wall of the die to form a layer of liquid, and it is the properties of this liquid layer that are very important. Thus, some migration of the fluid during extrusion may not be a bad thing. 

Hie inorganic types of die oigano-tellurium series are TeCI, andTeO, —... 

In high pressure axial screw extruders, dies and their orifice(s) can have different shapes and sizes. Their selection depends on the plasticity and flow properties of the materials to be extruded and, obviously, on the desired product shape and size. Two basic types of dies are used in extrusion. For the formation of small agglomerates, such as strands or pellets, extrusion plates are used. Such plates can be simple or more complex, the latter, so called sandwich types, consist of multiple plates that are made of different materials. Fig. 8.78 presents sketches of two extrusion plate designs. Parameters are based on manufacturer know-how as well as vendor experience from different applications and can not be discussed here. 

Extremely high safety potential peculiar to this type of die RI is characterized by the fact that even when such initial events as containment destruction and primary circuit loss of tightness coincide, neither reactor runaway, nor explosion and fire occur, and the radioactivity yield is lower than that one which requires the population evacuation. 

Figure 6.4. Types of die-attach voids. (Courtesy CALCE Electronic Products and Systems Center, Univ. of Maryland.)...

The purpose of the die is to form the extrudate into a shape that is close to its final shape. There are many types of dies and a vast number of variations within each type. In this book, the different types of blown film dies will be covered in a later section. This section discusses three common characteristics that may occur when polymer flows through any type of die die swell, melt fracture, and die drool. 

Concentric dies (Fig. 3.6) are comprised of a series of hollow spiral mandrels nested inside one another. A narrow gap remains between each mandrel and the one directly inside it. Inside these gaps is where polymer flows. The different gaps separate the various polymer layers. Just prior to the die exit, the different polymers flow together into a single flow channel. Their laminar flow behavior mainly keeps the polymers in separate layers through the final product. These types of dies have been used for many years and perform very well. A couple of possible concerns to consider when designing... 

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