Coring, screw

Internal plastic thread The strength of plastic threads is limited, and when molded in a product involving either an unscrewing device or a rounded shape of thread similar to bottle-cap threads, they can be stripped from the core. Screw threads, when needed, should be of the coarse type and have the outside of the thread rounded so as not to present a sharp V to the plastic that can produce a notch effect. 

L., and Gaskill, D. (1995) Open-core screw dislocations in GaN epilayers observed by scanning force microscopy and high-resolution transmission electron microscopy Appl. Phys. Lett. 61, 2284. Copyright 1995, American Institute of Physics. 

Cored screw n. An extruder screw bored centrally from the rear to permit circulation of temperature-controlled hquid within all or part of the screw s length. 

Constant-taper screw n. A screw of constant lead and uniformly. Cored screw. A screw with a hole bored along its axis for circulation of heat-transfer medium or insertion of a heater. The core may extend only through the feed section or further, even to the screw tip. 

Extruder, single-screw n. An extruder with one tubular barrel within which a solid or cored screw rotates. Strong AB (2000) Plastics materials and processing. Prentice-Hall, New York. Pittance JC (ed) (1990) Engineering plastics and composites. SAM International, Materials Park, OH. James F (ed) (1993) Whittington s dictionary of plastics. Technomic Publishing Co. Inc., Carley. 

Extruder, Single-Screw An extruder with one tuhular harrel within which a soUd or cored screw rotates. 

Natural Deposits. Natural deposits, eg, minerals and fossil fuels, are located by drilling operations. An auger, eg, a screw or worm, is turned in the earth and pulled out, and material is scraped from the auger for analysis. Alternatively, samples can be taken by hoUow core drills which, when withdrawn, enclose a core of the earth that is representative of the strata through which the drill has passed. Such core samples are used in geological surveys for fossil fuels. As the drill drives deeper into the strata, each core is extracted and placed in a shallow box and coded so that a complete cross section of the geological strata can be reconstmcted. From this, the relative thickness of coal and mineral seams can be directly measured. 

Another PVC siding is an extmded product of an internally ribbed, dual-wall profile. A conventional screw extmder, with a vacuum sizer cooled with water and air, forms a product about 20 cm wide with 160 mm walls and 80 mm ribs on 25 cm centers. The total thickness is about 0.64 cm. The ribs provide rigidity and strength and the dual-wall constmction adds thermal insulation. No backing or core is used. 

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

Dislocations are characterized by the Burgers vector, which is the exua distance covered in traversing a closed loop around die core of the dislocation, compared with the conesponding distance traversed in a normal lattice, and is equal to about one lattice spacing. This circuit is made at right angles to the dislocation core of an edge dislocation, but parallel to the core of a screw dislocation. 

A scram causes the control rods to drop into the core, absorb neutrons and stop the chain reaction. Some rods perform both controlling and scram functions. The control rods are raised to increase the neutron flux (and power) or lowered to reduce it by magnetic jacks (W and CE) or a magnetic "clamshell" screw (B W). The chemical volume and control system (CVCS - not siiown) controls the water quality, removes radioactivity, and varies the reactivity by controUing the amount of a boron compound that is dissolved in the water - called a "poison." Thus, a PWR coiiirols reactivity two ways by the amount of poison in the water and by moving the control rods. 

Figure 3 Twin-screw extruded PP-LCP blend processed at a melt temperature of 290°C with low- (left) and high-draw ratio (right). Upper micrographs are taken from the core and lower ones from the skin region.

The core structure of the (100) screw dislocation is planar and widely spread w = 2.66) on the 011 plane. In consequence, the screw dislocation only moves on the 011 glide plane and does so at a low Peierls stress of about 60 MPa. 

Figure 3 Core configuration of the (111) screw dislocation. The Burgers vector distribution is calculated for a 211 cut and clearly shows a compact dislocation core.

Fig. 2. Schematic pictures of the core structure of the 1/2 [110] screw dislocation in TlAl. (a) Planar core spread in the (111) plane, (b) Non-planar core spread in both (111) and (111) planes.

Fig. 3. Schematic pictures of the core structure of the [101] screw superdislocation.

Rod Anodes These consist of solid or copper-cored titanium rod, portions of which can be platinised. They range from 6 to 25 mm in diameter and are normally supplied pre-assembled in a steel or unplasticised p.v.c. mount ready for screwing into prepared bosses on the plant under protection. Electrical connections are made via cables in the usual way. The anodes are spaced to give even current distribution. 

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