Melt Pressure Transducer Care
from ONEhalf20

ONEhalf20 Melt Pressure Transducers are measuring instruments that are subject to damage if mishandled, improperly installed or dropped.

The ONEhalf20 Melt Pressure Transducer is a sealed system. The sensing diaphragm is the most important component of the transducer. The standard diaphragm is of a one piece construction, machined from type 15-5PH stainless steel, with a proprietary coating designed to increase durability. It has a nominal thickness of 0.0050 inches. If the sensing diaphragm becomes damaged in any way i.e.: dented, gouged or deformed, the transducer will no longer provide accurate readings and should be returned to ONEhalf20 for repair and/or replacement. See the repair and re-calibration information in the Technical section of our website for further details.

A poorly prepared or machined transducer mounting hole is one of the main causes of damage to the transducer. Extruder barrel wear should be monitored periodically. As the extruder barrel wears, the transducer diaphragm will move closer and closer to the barrel bore until it is flush with the sidewalls. Once the diaphragm is flush with the extruder barrel, it will wear at the same rate as the extruder barrel. To alleviate this problem, ONEhalf20 recommends that the transducer be recessed 0.0175" from the barrel bore. A copper spacer can be manufactured or purchased (to seal on the 45 degree seating surface) that will act as a separator backing off the transducer diaphragm from the inside barrel bore. If the transducer diaphragm protrudes into the extruder barrel, it is subject to unnecessary rapid wear and is subject to permanent damage by a passing screw flight.

The mounting hole should be cleaned and gagged prior to the installation of a new transducer or when replacing an existing transducer. A ONEhalf20 Cleaning Tool Kit is available from ONEhalf20 to permit cleaning of the mounting hole. The kit also includes a gage plug to check the integrity of the mounting hole.

The transducer mounting threads should be 1/2-20UNF-2B. See the mounting hole dimensional drawing in the Document Library section of our website for further details.  The mounting hole 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 are recommended.

ONEhalf20 Melt Pressure Transducers are capable of being installed at a mounting torque of up to 500 inch/pounds without damage or zero shifts; an adequate seal is generally achieved with a mounting torque of 100-200 inch/pounds. This can be accomplished by first installing finger tight and then tightening the transducer a quarter turn with a wrench. Excessive mounting torque will cause seizing and make the transducer difficult to remove.

The ONEhalf20 Melt Pressure Transducer diaphragm is capable of operating at temperatures up to 750ºF. The transducer electronics are capable of operating at temperatures of up to 150ºF. It is important that the electronic housing not be exposed to temperatures above 150ºF.

One of the most common causes of Melt Pressure Transducer diaphragm damage is cold starts. A ONEhalf20 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.

The transducer should only be removed or replaced while the machine is at operating temperature and the polymer is liquid. Removing 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 the presence of solid polymer. If the diaphragm is forced against a solid, the transducer can easily be overloaded and left with a very high zero offset, often too high to be rezeroed.

If the transducer is removed warm from the machine, the diaphragm can be 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 torch to clean the tip of a transducer.