Retracting screw

Fig. 1.9 Extendable/retractable screw-in lead at fluoroscopy (courtesy of Medtronic)...

Fig 1.35 Passive lined fixation lead (a). Active fixation extendable/retractable screw-in lead (b)... 

FIGURE 20.7 Close-up views of three pacemaker and/or cardioverter fi ior leads. The lead on the far left is an active fixation lead with a retractable screw embedded in the lead lip- It can be extend after implantation to attach the lead to the heart wall. The middle lead possesses a soluble ct that dissolves within a few minutes inside the body to reveal a hook or screw for tip fixation. A lined passive fixation lead is shown on the right The soft tines become lodged in the iiregular inside surface of the heart, preventing lead migratron. 

Not all active-fixation leads have an extendable-retractable screw. Some models have a fixed screw coated in mannitol which dissolves in blood prior to fixation (Guidant, Sweet Tip ). As discussed previously, a thin diameter cabled lead inserted through a steerable catheter has a fixed screw which is essentially covered by the catheter until it is attached to the endocardium (Medtronic SelectSecure Medtronic Inc., Minneapolis MN) (Fig. 1.19). Active-fixation leads can also be attached to the endocardium using a steerable stylet (St Jude... 

To protect the sample from stray electrons from the anode, from heating effects, and from possible contamination by the source enclosure, a thin (-2 pm) window of aluminum foil is interposed between the anode and the sample. For optimum X-ray photon flux on the surface (i. e. optimum sensitivity), the anode must be brought as close to the sample as possible, which means in practice a distance of -2 cm. The entire X-ray source is therefore retractable via a bellows and a screw mechanism. 

The plasticator on an injection-molding machine is a specialized plasticating single-screw extruder. The plasticator has two main differences there is a nonreturn valve on the tip of the screw, and the screw retracts as molten material accumulates between the nonreturn valve and the end of the barrel. Pressure is maintained on the accumulated material by a constant force applied to the shank of the screw via the drive system. This force is typically measured as a pressure applied to the shank and is referred to as the back pressure. During the injection step of the process, the screw is forced forward, the nonreturn valve closes, and the material is injected into the mold. Additional information on the injection-molding process can be obtained elsewhere [Ij. 

Real polymers are more complex than these simple mechanical models. Qualitatively, when a real polymer is forced to flow through a contraction or expansion in an extrusion screw, it will exhibit viscoelastic behaviour. The polymer molecules will be elongated if forced through a contraction, or they will retract when they flow into an expansion. The effect of viscoelastic behavior in a capillary rheometer is observed in the form of recirculation flow just before the polymer enters the... 

The sharp melting point and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off. For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, involves the screw retracting slightly after the screw retraction time, pulling the material back and... 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

Injection molding is a cyclic process. As an arbitrary starting point, assume that the mold is closed and the screw is in its retracted position with a shot of melt ready to be injected. This is the position at the top of Figure 14-41. The four steps that comprise the process are ... 

It is recommended that during retraction of the plasticating screw no decompression be used. If it is necessary to reduce drool, it may be used at the lowest possible level to avoid sucking back air and inducing splay. 

Figure 11.7 Schematic diagrams of a screw-type injection-molding machine (top) screw retracted, before injection (bottom) screw forward, at end of shot. (From Weir, C.L., Introduction to Molding, Society of Plastics Engineers, CT, 1975. With permission.)...

FIGURE 2.8 Cross-section of a typical screw-injection molding machine, showing the screw (a) in the retracted position and (b) in the forward position. (After Lukov, L. J. 1963. SPE /., 13(10), 1057.)... 

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