®
GRAVITROL
Extrusion Control
Closed control loop: GRAVITROL®
By the loss in weight in the weigh hopper over a time interval GRAVITROL ® measures the extruder
output. The GRAVITROL® computer and the extruder drive module ensures that the extruder‘s output
is always equal to the debit.
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computer  …
…  computer
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Applications:
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Blown film extrusion
Cast film extrusion
Sheet extrusion
Cable extrusion
Profile extrusion
…
All extrusion processes which allow to measure the extruder‘s
throughput gravimetrically (flow fed extruders)
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Parameters, that cause problems without gravimetric
extrusion control:
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Tolerances due to screw and
barrel wear
Changes in screen pack condition
Changes of temperature
Varying extruder power
Polymer density
Bulk density
Melt temperature
Polymer viscosity
Different conditioned polymer
Change in blend reciepe
•
Different line speeds
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This is GRAVITROL® able to do :
• Conversion of an extruder into a gravimetric metering element
with exactly defined mass throughput (accuracy of ± 0,5% )
• Ensures exact layer ratios
• Simplifies ramping up and down a line
• Smooth changeover from manual to automatic
• Synchronizes the extruder with the haul-off (also while
ramping up and down a line) with an accuracy of ± 0,5%.
Ensures product quality and economic
operation
GRAVITROL® is like „cruise control“ for
extruders
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Processes and product variables that can be controlled
with GRAVITROL®:
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Increase in profit with GRAVITROL® by:
• Reduced labor costs and reduction of human-made errors
• Automatic ramping up and down a line results in
o Processes optimised in time
o Final product is always in specification
• Use of material can be adjusted very close to the minimum
tolerances
• Reduction of waste
• The system learns the extruder‘s ramping curve by its own
(normally this takes labor and time and errors can be made)
• Can be used instead of a melt pump (much more expensive);
except in fiber processes
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GRAVITROL® : typical application
Production of high energy cables:
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GRAVITROL® : typical application
Blown film extrusion
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Ramping up and down a line:
Typical extruder throughput curve  not linear
m’
0
Extruder speed in %
100
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Ramping up and down a line
Extruder
speed
When ramping up a line for the first time the system learns the extruder curve by
creating a step ramp. The evaluated function will be saved to the GRAVITROL®
computer memory and is base for the exact control of the extruder output.
time
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Throughput control with GRAVITROL®:
GRAVITROL® adjusts the extruder speed in a way that the
throughput is always equal to the debit.
m‘
[kg/h]
Extruder
speed
[min-1]
time
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Components of a GRAVITROL® system
EXB GRAVITROL® Computer
Features and function:
• 32-Bit Mikrocomputer
(contains all necessary control
algorithms for an accurate
extrusion control)
• Operator panel (for the operating
personnel) with user-friendly
surface
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Components of a GRAVITROL® system
HGB weigh hopper
Features and function:
• different hopper sizes for
throughput up to 2.000 kg/h
• High resolution load cell
(resolution 1:1.000.000
• DSP (Digital Signal Processor)
weigh module
• material drain valve and door
(depending on the model)
• loading platform for vacuum
reciever or swing gate
• steep wall hopper for regrind
material available
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Components of a GRAVITROL® system
The function of the HGB weigh
hopper can be replaced by
either:
• a continuous gravimetric
blender with weigh
downcomer
or
• a GUARDIAN ® gravimetric
batch blender with HGCweigh hopper
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Components of a GRAVITROL® system
features and functions:
• 2 inputs for encoders
• 8 outputs for:
1 EXL + max. 7 EXD
or
max. 8 EXD
• connection to Mini-Op via
ethernet port.
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Example: GRAVITROL® system
• Mono-extrusion
• output control only
• for the use of raw material (i. e. PE, PP, …) or pre-blend
EXB
GRAVITROL®
Computer
Material conveyance
HGB
weigh hopper
EXD
Extruder
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Example: GRAVITROL® system
• Mono-extrusion
• output control and haul-off control
• for the use of raw material (i. e. PE, PP, …) or pre-blend
EXB
GRAVITROL®
Computer
Material conveyance
HGB
weigh hopper
EXD
Extruder
EXL Line speed module
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Example: GRAVITROL® system
• Co-extrusion with 3 extruders
• output-, layer- and haul-off control
• using X-series continuous gravimetric blenders
Material conveyance
X-series
continuous
gravimetric
blender
EXB
GRAVITROL®
Computer
EXD
Extruder
EXD
Extruder
EXD
Extruder
EXL Line speed module
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Example: simplified GRAVITROL® system with a GUARDIAN® blender and a
HGC weigh hopper*
• Mono-extrusion
• output control
• Installation follows the haul-off speed
• use of a GUARDIAN® blender and a HGC weigh hopper
EXB
GRAVITROL®
Computer
*: This GRAVITROL® system
represents a simplified low-cost
solution which is often chosen in
lines using gravimetric batch
blenders.
The haul-off has to be controlled
manually by the operator. The
GRAVITROL® system takes care
of the correct extruder throughput
referring to the current line speed.
The system always follows the line
speed.
GUARDIAN®
gravimetric
charge blender
HGC weigh hopper
for GUARDIAN®
Customer supplied haul-off
control +/(or manual operation
control)
EXD
Extruder
EXLOX
Line speed encoder
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Data recording for preparing a quotation:
In this data sheet
all necessary data
concerning throughput and
extrusion process
are caught
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