The Melt Pressure Transducer is a measuring instrument that is subject to damage if mishandled, improperly installed or dropped.
The ISI fluid transducer is a sealed system. The sensing diaphragm is one of the most important components of the transducer. The standard diaphragm is a one piece construction, machined out of type 15-5PH stainless steel and is coated with Armoloy to increase the surface hardness. It has a nominal thickness of 0.0045 inches. ISI offers other types of diaphragms suited for use in harsh or abusive applications - see diaphragm material chart.
If the sensing diaphragm becomes damaged; ie: dented, gouged or deformed in any way, the transducer will no longer provide accurate readings and should be returned for repair and/or replacement.
A bad transducer mounting hole is one of the key causes of damage to the transducer. Extruder barrel wear should be monitored. As the extruder barrel wears, the transducer tip will get closer and closer to the barrel bore until flush with the sidewalls. Once the tip is flush, it will then wear at the same rate as the extruder barrel. To alleviate this problem, ISI recommends that the transducer be recessed 0.015" from the barrel bore. A copper shim can be manufactured (to seal on the 45 degree seating surface) that will act as a spacer backing off the transducer tip from the inside barrel bore.
If the transducer tip protrudes into the extruder barrel, it is subject to unnecessary wear and is vulnerable to damage by a passing screw flight.
Another common mounting hole problem that is a result of barrel wear is roll over. Over time the opening into the barrel is reduced from the recommended dimension of .312" / .314". If a transducer is installed in this smaller hole, the tip becomes crimped or forced into a cone shape, deforming the sensing diaphragm and damaging the transducer.
CLEANING & GAGING THE MOUNTED HOLE
The mounting hole should be cleaned and gaged prior to the installation of a new transducer or replacement of an existing transducer. A 3 piece cleaning tool kit is available from ISI to clean the mounting hole and also includes a gage plug to check the integrity of the hole. Part number is CT-3.
The transducer mounting threads should be 1/2-20UNF-2B and should be cleaned and inspected prior to the installation or reinstallation of a transducer. The use of a high temperature anti-seize compound will help in prevent galling and will allow easy removal of the transducer. Copper graphite anti-seize compounds should be used such as C5-A by Felpro.
Although the transducer is capable of being installed at a mounting torque of up to 500 inch/pounds without damage or zero shift, an adequate seal is made with 100-200 inch/pounds. This can be achieved by installing finger tight and tightening a quarter turn with a wrench. Excessive mounting torque will cause seizing and make the transducer difficult to remove.
The transducer tip is capable of operating at temperatures up to 750 degrees F. The transducer electronics are capable of operating at temperatures of up to 160 degrees F. It is important that the electronic housing not be placed on top of a heater or heat source.
One of the most common causes of diaphragm damage is cold starts. A combined temperature and pressure transducer can be used to insure that the extruder barrel is up to temperature prior to the rotation of the extruder screw.
REMOVAL / REPLACEMENT
The transducer should only be removed or replaced while the machine is at operating temperature and the polymer is liquid. Removal of the transducer from a cold extruder will cause diaphragm damage due to polymer adhesion. A transducer should never be installed into a mounting hole where there is solid polymer. If the diaphragm is forced against a solid, the transducer can easily be overloaded and left with a very high zero offset, too high to be rezeroed by the zero potentiometer on the instrument.
If the transducer is removed warm, the tip can easily be cleaned with a soft cloth. If there is solid polymer present, a fluidized bed cleaning system can be used. Never use a wire brush, knife, or touch to clean the tip of a transducer.