Melt pumps and screen changers for blown film extrusion
Author:yisainuo Time:2016-03-03 09:53:25
Melt pumps in combination with screen changers can produce a number of positive benefits in new blown film lines or as retrofits on existing lines. Melt pumps and screen changers used in the blown film industry will be reviewed in detail. Also reviewed will be the benefits of each along with the potential challenge with each piece of equipment.
Melt pumps are positive displacement devices, producing a consistent flow regardless of discharge pressure. Installed between the extruders and blown film die, the pump takes on the job of generating pressure and does so at efficiencies approaching 98% versus approximately 40%f or an extruder. This leaves the extruder responsible for only feeding, melting, and mixing. The melt pump’s tight clearances also greatly avoid the pressure pulsations generated by the extruder. This provides a much more consistent die pressure, enabling closer tolerances, which in turn permits higher yields. The melt pump will also cause a lower net product temperature by lowering the extruder head pressure. This temperature drop will more than offset any temperature rise through the melt pump.
PUMP BENEFIT REVIEW IN BLOWN FILM
The three key benefits of adding a melt pump to an extrusion line are:
1.reduction of extruder head pressure fluctuations
- isolation of downstream equipment from the extruder
-increased level of regrind usage possible
-less scrap due to off spec at start up
-raw material savings
2. inceased throughput
-for every 1000 psi reduction in head pressure a 1-4%increase i throughput can be achieved
-rate increases of up to 25% have been seen
3. reduction of melt temperature to the die(less residence time+lower temperature=high rates)
-potential increase in throughput can be found for situations where output in limited due to melt temperature.
- reduced extruder barrel and screw wear, resulting in lower maintenance costs
- faster start up, requiring less manpower
- reduced energy requirements. Typically appx. 20%
PUMP CHALLENGES IN BLOWN FILM
1 INCREASES IN THROUGHPUT CAN VARY, DEPENDING ON HEAD PRESSURE REQUIRED TO PRODUCE A GOOD HOMOGENEOUS MELT.
2 Downstream limitations: on a retrofit can the equipment be moved?
3 gauge variations may already be at a optimum level with the use of the right combination of cooling. Screw design and take off equipment
4 added floor space
5 initial cost of pumps and auxiliaries.
Continuous screen changers are recommended for blown film filtration. A two bolt or piston design is usually preferred. The standard two piston screen changers incorporates two pistons, each containing one filter pack. It is used for continuous filtration. At the inlet, product is divided into two streams.
During normal operation each screen operation pack handles on e half of the total flow. After exiting the filter packs, the two streams, now filtered, are recombined at the outlet. During screen change, one piston moves out of the housing for screen pack removal, blocking the melt stream. After the dirty screen pack is replaced, the off-line piston is moved to a venting position to eliminate process upsets and then returns to the normal position. Automatic controls are recommended for the venting and screen change procedure.
BENEFITS REVIEW OF CONTINUOUS SCREEN CHANGERS IN BLOWN FILM
1. Throughput, viscosity and temperature remain practically constant
2. The screen surface available is regulated so that the pressure can also be kept fairly steady
3. Filtration is continuous
4. Screen changers and pumps can be combined on one common stand for optimum space efficiency
Screen changers challenges in blown film
1. Continuity does not refer to pressure, speed of flow or viscosity, it refers to the process
2. Care must be taken in the venting process as not to create bubble issues(as in loss of )
Generating suction on the inlet side
Transport of the pumping medium from the inlet to the suction side
Displacement of the medium at the outlet side
Sealing between the inlet and the outlet side
Transmission of half of the drive toque from the driving to the driven shaft
Effect of pressure:
P1: stress on housing, covers and bolts for covers. It is also the pressure across the seal
P2: stress on the outlet flange and its bolts
Dp: axial and radial load on the bearings(lubricating film). Load on the teeth flanks bending stress on the teeth, bending and torsion stress on the shaft as a whole