Use of melt probes is indispensable in extrusion processes as they contribute:

• To the safety of the system when used to prevent an uncontrolled rise in machine pressure.

• To improving the performance of production with their ability to keep flow-rate stable and optimal.

 

Melt sensors are normally used in pressure reading

• along the cylinder to check its performance during the development and design of the screw

• in the filter changer to check its cleanliness

• before and after the gear pump to keep the flow rate constant

• in the head for closed-loop pressure control.

 

The innovative IMPACT sensors (series I) are pressure transmitters, without transmission fluid, in which the medium pressure is transferred directly to the silicon sensing element through a thick membrane. The transduction of the physical stress is entrusted to a Wheatstone bridge made by means of 4 piezoresistors. The IMPACT series, Gefran’s proprietary technology, is characterized by:

• Remarkable robustness (up to 15 times that of a conventional sensor)

• Notable response speed

• Extreme ease of installation thanks to the modularity of the sensor

• High safety standards (compliance with Machine Directives and RoHS)

 

The operating principle is based on the hydraulic transmission of pressure by means of filling liquids with a low compressibility coefficient: sodiumpotassium NaK mixture (K series), FDA approved diathermic oil (W series) and mercury (M series, available only in cases permitted by European Directive 2011/65/EU - RoHS II). The entire structure is therefore designed to transfer the pressure exerted by the medium on the contact diaphragm to the transduction part, i.e. the measuring diaphragm on which the strain gauge is located, taking care to keep it away from the heat source. The strain gauge then translates the pressure into an electrical signal. Using the “screen printing process” technique, the insulating layers (dielectric), the conductive layer (cermet) and the resistive layer are deposited on the steel membrane to create the “Wheatstone bridge”. The thickness of the membrane determines the measurement range, and the step-by-step transition from 200°C to 900°C makes the sensor extremely robust and reliable.

 

The GTP+ coating is used in turbines of jet engines, as this material is ideal to withstand high levels of temperature and pressure.

MELT PROBES WITH ADAPTERS AND FLANGES

Gefran also manufactures melt pressure probes with dedicated flanges, made to measure for plastic polymer production plants. Gefran was the first company to create a probe for this sector complete with Atex and SIL2 or PLd certifications featuring the HART digital communication protocol, dedicated mechanical flanges and IMPACT fluid free technology.

IO-LINK DIGITAL COMMUNICATION

Digital solutions for the transmission of values measured in plastics production are becoming increasingly important. IO-Link is the worldwide standard for connecting sensors and actuators. Thanks to this technology, melt probes with IO-Link output are able to measure not only the pressure but also the temperature of the extruded material, for example by storing the maximum pressure and temperature peaks and hours of operation under pressure. This allows the user to recognize in real time when the system is operating improperly or when the temperature of the point where the melt probe is installed has reached the permitted limit for production. The Gefran probe with IO-Link digital output version 1.1.3 completes this unique range, available in SIL2 or PLd certified versions with relay output, or, alternatively with analogue output scalable up to 1/3 of the full scale value.