Troubleshooting Extruders, Part 1
If you are lucky, you will have played a part in specifying and procuring the extruder. You will have spent time with the people installing and commissioning it and have possession of all the manuals and drawings.
Unlucky? You have inherited the machine and its care as part of your new job. Now you have to find or create drawings and manuals and get to know your machine. At first look, it will be hard to imagine how your wiring and piping drawings bear any relation to the actual equipment.
A typical arrangement would be six barrel zones of some 4 kW each, with a liquid-cooling jacket cast into the same shell as the heater (figure 1). Alternatively, the cast shell would have fins for blown-air cooling. For the die, adapter, screw, roll stack and other zones, there could be several heat-only control loops.
Lucky or not, be sure that you can identify the various controls, indicators and parts such as:
- Temperature controllers, thermocouples, plugs and sockets.
- Heaters, heater contactors, ammeters, cooling solenoid valves.
- Melt pressure transducers and indicators.
- Motor drive controls, their meters, switches and push-buttons.
- Electrical disconnects, fuses, blowers, their contactors and indicator lights.
This includes terminal and wiring identification both around the machine and inside the control enclosure.
Write down or use a marker to denote normal readings or settings on gauges, indicators and adjustable valves.
For controllers, list settings that vary with different jobs; e.g., temperature, speed and current. With an integrated system, these may be stored for you by way of built-in recipes shown on the operator’s color screen. Record adjustable parameters such as proportional band, integral time, derivative time and alarms. These do not normally need resetting for different jobs unless you change polymer, controller or die.
Before you run a real job where disturbances can be costly, do a practice or dry run so that you know what to expect of any changes or adjustments you may have to make to the process. Get familiar with the wiring and piping drawings; keep them marked up after changes. Know how to open control enclosures. If you cheat a door-latch safety-disconnect, be aware of live and exposed parts inside the panel. Use your AC voltage tester.
Put a permanent sticker on control enclosures showing names and phone numbers of contractors and parts suppliers who can help with parts or troubleshooting advice.
Identify spares. Attach a “what’s wrong” label on any suspect parts that come off the machine and put them in for repair right away. This way, you keep only good parts on the spares shelf.
Separate heat-only from heat-cool temperature controllers. They look the same. If you plug a heat-only (e.g., die controller) into a heat-cool (barrel) zone, you will be unable to bring the temperature down when screw shear heat overheats the zone.
Your Tool Kit. After hand tools, your diagnostic aids will include a clamp-on ammeter, a digital multimeter, a digital thermometer with a matching Type J thermocouple probe, a temperature calibrator and an AC voltage tester (figure 2). This is a plastic probe that lights up when its tip touches a live cable, even through the insulation. It’s useful for distinguishing live from neutral or grounded cables and terminals.
Normal Behavior of Temperature-Controlled ZonesWhen observing the control loop in action, you will be watching temperature; a current indication for each heater is a great help. Otherwise, check heater currents with a clamp-on ammeter. Other useful aids are lights showing the on/off state of cool solenoids or blowers and coolant flow indicators.
- If the temperature is well below setpoint, the heater current will be on steady at its maximum value and not pulsing. Cooling will be off.
- If temperature is well above setpoint, the heater current will be off. Cooling will be full-on (barrel zones).
- If the temperature is around setpoint (a few degrees either way), the heater current could be coming on and off -- typically every 10 sec or so (faster with solid-state contactors) -- or the cooling solenoid valve (or blower) could be cycling equally slowly.
This can try your patience and it looks illogical sometimes when you see pulses of heat coming on when temperature is a little above setpoint or cool coming on a bit below setpoint. That's all right: the controller will slowly work it out and settle down delivering just enough heat or cool to hold the correct temperature. The amount of heat or cool to expect when the temperature is near setpoint is not easy to predict because of the dynamics of PID control.
Abnormal BehaviorProblem: Temperature is well below setpoint and heater current is off. Check for loss of line voltage, open heater fuses or breakers or heater open circuit. If the contactor coil is energized, usually at 120 VAC (some solid-state contactors are switched by a DC signal around 10 V), check that its contacts have closed. If not, change the contactor. If the contactor coil is not energized, voltage could be failing to get to the controller’s internal relay, or that relay (or triac) could be failing to close. A useful aid here is an AC voltage tester.
Trap. Triac switches inside controllers need an AC voltage on them and at least a 50 mA AC load to check their operation. DC continuity tests do not work like they do on magnetic relays.
Another Trap. Suppose you disconnect the contactor coil and use a voltmeter to check whether a controller’s internal relay or triac is switching voltage through. You will probably find a voltage there all the time, even when the controller is not calling for heat. Why? A snubber (spark suppressor) is often fitted internally across the triac or the relay contacts. This can pass a tiny current, enough to show full line voltage on your meter when the relay or triac is open circuit to normal loads. This current is nowhere near enough to pull in a contactor coil. Even without the snubber, a triac switch could sometimes leak enough current to show on the voltmeter. A 120V 5 W filament lamp load works well as a substantial test load.
I’m out of space, but I’ll continue with common abnormal behaviors, and their fixes, next month. PH