Screw decompression zone

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]. 

Decompression zone de-k9m- pre-sh9n zon. In a vented extruder, the zone of deep flights immediately forward of the first metering zone and beneath the vent, where unwanted gases and vapors are released from the (typically) foaming melt. In some vented single-screw machines and most twin-screw machines, the two zones are separated by a dynamic value, whose resistance to flow helps to increase the superheating of the melt prior to decompression. 

Vented screw n. A two-stage screw with a screw vent in the decompression zone, permitting volatiles to escape through the screw core. 

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. 

Vented barrels typically have slower screw recovery since there is a decompression zone in the middle that impedes resin conveyance forward. (See machine/screw supplier guidehnes for optimum barrel profile to minimize the effect of the vented barrel.)... 

General purpose screws can be used, for example, with an L D ratio of about 18 1 and a constant pitch of ID. The feed zone should be approximately 0.5L, the compression zone 0.3L and the metering zone 0.2L a compression ratio of about 2 1 should be sufficient. The screw is usually fitted with a nonreturn valve but valves are not essential on the nozzle unless high back pressures are involved. The best results have been obtained by using spring loaded or hydraulically operated types decompression or suck-back is also used. High speed low torque screw motors are often preferred to achieve short screw recovery times. 

The screw configuration is as follows thread depth ratio, approximately 1 2, and L/D ratio, 17/1 to 23/1 (standard three-zone screws feed, transition or middle, and metering or feed zones). Screws are equipped with a nonreturn valve to prevent backflow. Decompression-controlled injection-molding machines have an open nozzle. A short nozzle with a wide bore (3-mm minimum) is recommended in order to minimize pressure loss and heat due to friction. Residence time should be as short as possible, and this is accomplished with barrel temperatures at the lower limits of recommended settings. ... 

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. 

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