Extrusion Line Automation: Extruder, Haul-off, Planetary Saw and Line Sync

A typical pipe line runs in this order: material drying and dosing, extruder (single or twin screw), die, vacuum sizing tank, spray cooling tanks, ultrasonic wall thickness gauge, marking, caterpillar haul-off, planetary saw or guillotine cutter and a tipping table or coiler. On profile lines the calibration table and haul-off are similar and the cutter is usually a saw; on compounding lines the melt leaves the die as strands, is cooled and cut in a pelletizer.

The extruder motor is an AC or permanent magnet motor running with high torque over a wide speed range; screw speed sets melt output, and melt pressure and motor torque show the state of the process. A gravimetric (loss-in-weight) feeder doses material by weighing it in kg/h. Weight per metre (g/m) is output (kg/h) divided by haul-off speed (m/min): with constant output, a faster haul-off makes the wall thinner. Haul-off speed is therefore either set in ratio to the extruder or corrected in closed loop from weight per metre and wall thickness measurement.

A caterpillar haul-off has two to twelve tracks; the tracks grip the pipe without spoiling the sized diameter and set the line speed. On multi-track haul-offs each track has its own motor and load sharing is needed: speeds are held to the same reference and torque balance is monitored. Track contact pressure is set from the recipe for pipe diameter and wall thickness.

A planetary saw cuts the pipe with a blade that orbits around it without turning the pipe; the saw carriage moves in sync with the haul-off during the cut and returns afterwards. This is the extrusion version of the flying saw principle: the cut is triggered from the position read from the haul-off encoder or a measuring wheel, and the carriage is coupled to the line with an electronic cam. Because cut time changes with diameter and wall thickness, on thick pipe the sync zone gets longer and carriage stroke sets the line speed limit.

In the ONX architecture the line is controlled by CODESYS running on an industrial PC. Extruder, dosing, haul-off and saw share one line speed reference; haul-off speed can run in ratio to extruder output or be corrected in closed loop from weight per metre and wall thickness measurement. Heating zones (barrel, die) are held by PID.

The saw carriage is coupled to the haul-off encoder with an electronic cam in CODESYS SoftMotion, and cut length can be changed while running. High-torque AC drives are chosen for the extruder motor, load sharing for multi-track haul-offs and a suitable drive structure for fast stops that need regenerative braking. Recipes (product section, output, weight per metre, temperatures, cut length) are managed from the screen.

The architecture on this page is built on the CODESYS, EtherCAT and drive platform ONX Control uses in its flying saw, load sharing and PID process control applications; extruder drive packages are part of ONX's high-power AC drive supply. We work out your line's layout together in a meeting.

A typical picture on extrusion lines: a DC motor extruder, a haul-off linked by an analog reference with a ratio set by hand, speed control with no link to the measuring system, a cutter control card with no spare parts and a screen that keeps no records. In a retrofit the screw, die, tanks and haul-off mechanics are kept; the extruder is moved from DC to AC or gets a new DC drive, and the line is synchronized under one controller.