Tip #1: Mount them correctly
Damage to a melt pressure transducer is commonly caused by its installation in an improperly machined hole. In forcing a transducer into a too-small or eccentric hole, the transducer diaphragm may be crushed and the instrument will not function.
Tool kits available for machining the mounting hole will help to make sure the holes are properly sized. A mounting torque of 100 to 200 in.-lb (for 1/2-20 UNF threads) is essential to form an adequate seal, but excessive mounting torque will cause seizing. To prevent seizing, apply a high-temperature anti-seize compound to the transducer threads prior to installation. Transducers installed at a mounting torque above 500 in.-lb will be difficult to remove even when anti-seize compound is applied.
Tip #2: Check hole thread size
Abrasion from screwing a transducer into a mounting hole with an incorrect thread size will damage the instrument’s threads.
This may prevent a tight seal, resulting in melt leakage, and the instrument will not function properly or safely.
Correct dimensions for the mounting hole must be used to avoid thread galling.
(The threads are generally the industry standard of 1/2-20 UNF 2B.) A mounting-well gauge plug should be used to verify that the mounting hole is correctly machined and cleaned.
Tip #3: Keep mounting holes clean
It is important to keep transducer mounting holes clean and free of any plastic build-up. Before an extruder is cleaned, all of the transducers should be removed from the barrel to avoid their being damaged.
When they are removed,plastic is likely to flow into the mounting holes and harden. If this plastic residue is not removed,
extensive sensor tip damage will result when the transducers are re-inserted.
A cleaning tool kit can be used to remove the plastic contamination.
But note that repeated cleaning may produce “too deep” holes and result in damage to the transducer tip.
If this occurs, spacers should be used to raise the transducer.
A Few Simple Rules
· Threads must be machined to correct tolerances.
· A metal-to-metal seal on the 45° seating surface is necessary.
· The recommended recess for mounting holes is flush to 0.010 in.
· Apply a high temperature anti-seize compound to the threads before installation.
· A minimum 100 to 200 in.-lb of torque are recommended to achieve a firm seal.
· Do not torque above 500 in-lb.
Tip #4: Select a good location
Transducers may be located in the barrel, before a screen changer, before and after a melt pump, or in the die.
When a transducer is positioned too far upstream in the barrel, unmelted pellets may abrade the transducer tip, resulting in damage.
If a transducer is positioned too far back in the mounting hole, a stagnant pool of melted plastic will build up
between the transducer tip and the screw flights. Over time, this plastic will degrade to carbon,
which will prevent transmission of an accurate pressure signal. On the other hand,
if the transducer extends too far into the barrel the screw flights can shear off the sensor tip.
Tip #5: Clean with care
All the transducers should be removed before cleaning an extruder barrel with either a wire brush or special cleaning compounds.
Either one of these can ruin the transducer’s diaphragm. The transducer should be removed
while the barrel is hot and its tip wiped clean with a nonabrasive cloth.
The transducer hole should be cleaned at the same time with a cleaning drill and guide sleeve.
Tip #6: Keep transducers dry
Although a standard transducer’s electronic circuitry is designed to withstand the rigors of the extrusion process,
most transducers are not watertight and will not operate when wet.
Make sure there are no leaks in the water-cooling jackets on the extruder barrel, which could damage the transducer.
If you know that the transducer will be unavoidably exposed to water or moisture, specify watertight transducers.
Tip #7: Avoid cold starts
Both the transducer and extruder can be damaged if the extruder is not brought up to operating temperature before production begins.
A sufficient “soak time” must be provided for the plastic to go from solid to molten state. In addition,
if a transducer is removed from a cold extruder, material might adhere to the transducer tip, causing the diaphragm to tear off.
Before removing the transducer, make sure the barrel is warm enough that any plastic present will be soft.
Tip #8: Don’t overpressure it
Even though transducers are designed to withstand 50% overpressure (in excess of the rated maximum),
avoid risk by making sure to use a modeldesigned for your range of extrusion operating pressures.
A good rule of thumb is to use transducers that are built to withstand twice the rated pressure in your process.
If so, the extruder would have to run at an extremely high (and unsafe) pressure level for the transducer to fail.