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The Barwell Gear Pump Preformer
Combining the plasticizing properties of a cold feed screw extruder with the volumetric flow control properties of a gear pump together with the proven Barwell cutter system.
This combination increases the process capabilities of difficult materials including tight dimensional tolerances and overcomes temperature to output restrictions. Although there are three basic types of Gears, Barwell uses only the very latest “Herringbone” design as this has been proven to be the most efficient on the market today.
A significant advantage in using a Gear pump is that the output is linear relative to the Gear Pump rpm. Therefore outputs are much more predictable. The use of Herringbone gears greatly compliments this effect by ensuring even flow through the pump and maintaining precise extrusion. Effects of screw extrusion pulsing is therefore eliminated and quality controlled production is assured.
Principle of Operation
Material is delivered to the Gear Pump by a screw extruder which masticates the material prior to entry into the Gear Pump.
The masticated material is pulled into the Pump by the intermeshing gear teeth at a constant rate, irrespective of screw rotation speed, ensuring the Gear Pump cavity remains full at all times. Minimal shear is generated in the Gear Pump so increases in material temperature are kept to a minimum, especially important when processing temperature sensitive materials.
The material is then compressed at the outlet point at a constant rate enabling precise control over extrudate dimensional stability. The constant rpm of the Gear Pump and constant monitoring and regulation of inlet pressure help overcome extruder pulsing. This is important when considering the effects of strip feed change-over. The Gear Pump is independently driven but controls the screw rpm by means of inlet pressure monitoring.
Without a Gear Pump the pressure build up and extrusion uniformity is directly influenced by the screw rotation and compound variations. When a Gear Pump is incorporated into the system line these variations are monitored at the Pump inlet and adjustments made automatically to maintain constant output flow.
Whilst the pressure at the inlet varies (due to material changes, variations in feed etc.), the pressure at the outlet of the Gear Pump remains constant. This ensures improved extrusion quality and tight control of extrudate dimensions.
By keeping the Gear Pump rpm constant, it is possible to influence the plasticizing effect on materials by changing the screw rpm. Increasing the screw rpm increases the inlet pressure and also the compound temperature as greater mastication is achieved during the dwell time in the extruder barrel.
This can have significant advantages when processing difficult materials that need additional mastication to achieve output requirements. Care should be taken to ensure that material temperatures remain within normal process limits.
Material flow through the gear pump is generally linear and therefore entirely dependent on the gear pump rotation speed. Increasing the gear pump rpm will automatically adjust the screw feed accordingly, maintaining the inlet pressure and cavity fill.
This linear effect is not influenced by varying material characteristics. As long as the screw is capable of achieving adequate plasticizing the throughput will remain linear.
Very little energy is put into the material by the gear pump which means the system can be used at lower temperatures or alternatively, higher outputs can be achieved at normal process temperatures.
A metal detector is fitted as standard to all Barwell complete systems. This greatly reduces the possibility of foreign objects or materials contamination to cause serious damage to either the screw or the Gear Pump.
Barwell can offer complete Gear Pump Preforming systems, Profile Extrusion only systems or alternatively offer Gear Pump retrofit packages to suit customers existing extruders. Although the Gear Pump has an outlet pressure up to 350 bar, Barwell offers two head options, a standard 210 bar GP (or SH) head for normal compounds or alternatively a 350 bar high pressure GP head for tougher compounds.
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