This month, I continue looking at normal and abnormal behavior of extruders, picking up with more abnormal behavior and common fixes.
Problem. Temperature persists a few degrees above setpoint when the heater current is off.
Here, the problem could be that closely adjacent zones are set to run hotter and pass their heat across. This suggests that an unrealistic temperature profile is set up.
Problem. Temperature indication is extremely high, but the zone doesn’t feel overly hot and the current is off. The controller may show a broken-sensor message.
Disconnect the thermocouple from the controller and check the thermocouple and wiring continuity all the way back to the thermocouple. The most probable cause is open circuit thermocouple or wiring. The resistance measured from the controller input terminals should normally be under about 20 W. If the controller is good, it will indicate around room temperature when you connect a wire link or double alligator clip across its thermocouple terminals. If the thermocouple and wiring are good, change the controller.
Problem. Heater current is on and the temperature indication stays abnormally low, but the zone is clearly overheated.
Check for the following:
- Whether the thermocouple pulled out of its hole and is sitting at room temperature.
- Stray wire strands bridging the thermocouple connector or wiring.
- Thermocouples looking after the wrong zone.
If the temperature indication goes below room temperature, check for crossed thermocouple wires.
Trap: Inconsistent Wire Colors. On North American Type J thermocouple wires, white is positive and red is negative. It is an understandable mistake to connect the red wire to the positive input terminal. Thermocouple wiring errors are common when installing or rewiring imported machines.
On plastics machines, thermocouples are almost invariably type J (Iron/Constantan) and the iron wire is positive, so if you don’t trust wire colors, you can identify the positive wire with a magnet. Also, note that the small pin on the thermocouple plug is positive, and the plastic body on Type J is black.
The internationally agreed color code standard for thermocouple wire is IEC 584-3, dated 1989 -- but as of 2005, few users in North America have adopted it or even heard of it.
Problem. Zone temperature comes up normally at startup, but after a few hours, the zone overheats. The controller indication stays normal.
Reason 1. The run between the thermocouple head and the controller is wired in copper instead of thermocouple extension wire. As the head warms up, being close to the heat, the presence of copper wire causes a shortfall of temperature signal. The controller receives a signal lower than true temperature and turns up the heat until it makes up the shortfall.
The result is a zone running hotter than indicated, by an amount equal to the head-to-controller temperature difference.
Reason 2. The run between the thermocouple head and the controller has the correct thermocouple extension wire but it is crossed at both ends. This is even worse than simply using copper wires near the heater. The controller receives a signal lower than the true temperature by twice the head-to-controller difference. This is because the thermocouple extension wire subtracts instead of adding its signal to the controller. Again the controller turns up the heat to compensate for the shortfall, and the indication looks normal.
This error can vary during the day from not noticeable to some 90oF (50oC) too hot, depending on how near the thermocouple head is to the hot zone and how long the zone has been on.
Heater Problems And PrecautionsEnsure clean heater mating surfaces, well clamped down (snugly inserted, or using a hairpin design, in the case of cartridge heaters) for good heat transfer. Poor thermal contact or air gaps can cause hot areas on the heater and premature local burnout.
Use the largest contact-area heater you can get into the space. Lower watts per square inch ratings yield longer life. Tighten down the heater occasionally. Infrared or thermocouple contact thermometers can help locate too hot or too cold areas on a heater surface.
You will always see disagreement between your contact thermometer on the heater, barrel or die surface, and the controlled temperature from the zone thermocouple deep in the barrel or die. This is to be expected due to deep-to-surface temperature differences in the zone. With thermocouple contact probes, another error comes from incomplete transfer of all the surface heat to the probe tip. Measurements taken on the surface are of little use in knowing the polymer processing temperature.
I'll have more on extruder problems next month.
Damage and Corrosion: A Check ListAmong the problems that can be encountered are the following. A little knowledge and planning can help you avoid these hazards.
- Wires that are frequently bent or moved due to maintenance or die changes can break or disconnect.
- Corroded or dirty studs, nuts and washers can overheat, spark, buzz
and lose contact. Keep them tight and replace with bright new hardware
if needed at maintenance time when you have the barrel covers off.
- Unplated copper cable oxidizes readily. Use high temperature connecting cable.
- Bad connections, loose fuse clips and oxidation can appear anywhere
in the heater wiring. These points behave like an electric arc welder
and fire hazard. Fuses and breakers do not protect against bad
- Watch for polymer or oil dripping onto connections or between segments of ceramic band heaters. It can decompose into conductive carbon and flash over.