Decompression screws

Screw length to diameter should be important to have uniform melt at higher output and also for uniform melt temperatures. Improved performance comes from radius between the flights and root diameter of the screw. It should be equal to or greater than the flight depth in the respective zones. Screw decompression (screw suck back) and reduction in nozzle temperature can be used to control drooling into the mold. However, it is necessary to identify and correct the cause of drooling. 

The zero-meter screw is used to reduce the viscous heat generation (power consumption) in the melt conveying zone of the extruder by having a relatively deep channel in this portion of the screw, with the depth reducing linearly with distance. Another similar approach is the decompression screw shown in Fig. 8.36. 

For example, a single screw extruder with a diameter of 32 mm and a length-to-diameter ratio of 30/1 was used to foam the polymer by continuous injection of carbon dioxide. The extruder was equipped with a decompression screw having a low-pressure zone. Carbon dioxide was injected into the low-pressure zone pressure increased as the mixture of polymer and COj was pumped along the extruder barrel towards the die. Extrusion conditions and foam properties have been listed in Table 11.12 for FEP and PFA, which are conventionally extruded at a melt temperature of370-400°C. 

The Dynamit-Nobel extmsion process (252) utilizes a volatile plasticizer such as acetone which is injected into the decompression section of a two-stage screw and is uniformly dispersed in the vinyl resin containing a stabilizer. The resulting PVC foam has low density and closed cells. 

The sharp melting poiat 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, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

The decompression section in the screw downstream from the blister was where a liquid plasticizing additive was injected into the extruder. The large channel depth in this section was used to accept the additional volume and to control pressure. The sections downstream from the first meter were not the focus of this case study and will not be discussed further. 

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

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. 

Marco, R.A.W., Kushwaha, V.P. 2009. Thoracolumbar burst fractures treated with posterior decompression and pedicle screw instrumentation supplemented with balloon-assisted vertebroplasty and calcium phosphate reconstruction. J Bone Joint Surg Am 91(1), 20-28. 

Air trapped in melt] screw decompression/back pressure too low. 

In compounding, single- and twin-screw extrusion facilitate the removal of entrapped gas or air, associated moisture or reaction byproducts which can arise in a formulation. Therefore, the equipment requires a low pressure decompression zone to enable volatiles to be removed under vacuum. Optimisation of the compounding equipment enables control of volatiles, facilitates diffusive and convective material transport and accommodates design and operational variables of the machine [4]. 

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