GEFRAN’s high-temperature melt pressure sensor without filling medium IMPACT IE / PL’c ‘is reliable and automatically shuts down the monitored installation when the pressure is exceeded. Thus, it meets the requirements of the EN 1114-1 standard in accordance with the PL’c ‘safety level and is ideally suited for protection against overpressure arising from the processing of plastics in the extruder. These advantages have already been appreciated by Clariant Masterbatches, which has included this sensor in the specification accompanying the supply of twin-screw extruders for the production of color and additive concentrates.
Plastic products fill our daily lives, whether it is functional industrial components, packaging, attractive design objects, household appliances, textiles or colorful toys. An infinite variety of colors and the required properties of products are provided by the concentrates of dyes and additives that are added to the base polymer material. One of the world leaders in the production of such concentrates is Clariant Masterbatches, one of seven divisions of Clariant International AG with headquarters in Switzerland. The company produces masterbatches for polyolefins, engineering and medical plastics, as well as multi-purpose dye concentrates. The company offers hundreds of standard colors, custom formulations or masterbatches of colorants and additives.
For the production of color and additive concentrates, Clariant uses twin-screw extruders with unidirectionally rotating, parallel, meshing screws. Only two German masterbatch factories use 50 such units. All extruders are equipped with melt pressure sensors in the extruder head. This is important because extruders push the polymer melt through a die at the end of the die under high pressure and temperature. At the same time, the pressure in the twin-screw extruders used at Clariant reaches 200 bar. Other similar installations are capable of developing pressures up to 1000 bar. When the die is clogged, a sudden pressure increase occurs in the extruder. If not responded in time, it can cause damage to machine parts and the splash of hot polymer material from the degassing and loading areas. “To clearly demonstrate this danger to our employees, we once bridged the sensor of a laboratory extruder and at the same time plugged the head. It made a lasting impression, ”recalls Thilo Kind, Head of Manufacturing Technology at Clariant Masterbatches.
Overpressure and destruction protection
The purpose of the melt pressure sensor is to shut down the extruder when a critical pressure is reached. In the past, Clariant used a potassium-sodium or mercury-filled melt pressure transmitter. However, filling pressure sensors with their relatively thin diaphragms can easily break down and go unnoticed. For example, when a shrinking mass of polymer solidifies, the sensor membrane may rupture. The problem is that with such a breakdown of the melt pressure sensors it can happen that they show the first measured value without a fault message. If this happens during operation and the pressure builds up imperceptibly as a result, the die head could burst. Careless or improper cleaning of the sensors can also quickly damage the relatively thin diaphragm and thus prevent reliable overpressure protection. Only experienced personnel are able to clean the sensor head without affecting its performance.
To eliminate the risk of equipment damage, the EU Safety Directive was amended in 2014 to replace one conventional sensor with two redundant sensors or one sensor with a self-diagnosis function.
Clariant Masterbatch chose to use self-diagnostic sensors and added GEFRAN’s IMPACT melt pressure sensor without filling medium to the specification for extruder suppliers. “Two redundant sensors per extruder means you have to buy twice as many sensors. Not to mention, the risk of destruction still remains. Therefore, the reliable and durable IMPACT sensor is the best choice. There is currently no alternative to this sensor on the market, ”explains Thilo Kind. Therefore, since 2014, every new twin-screw extruder at Clariant Masterbatches has been equipped with one IMPACT (Innovative Melt Pressure Accurate Transductor) type IE / PL’c ‘sensor from GEFRAN. Machines that have worn out conventional fill medium sensors are replaced with melt pressure sensors without fill medium. They independently recognize the malfunction and reliably shut down the extruder or prevent it from starting. In addition, the melt pressure sensors provide important information for setting up the production line, as the geometry and quality of the granulate is determined by the melt pressure in the die. The pressure in it must always be kept within a certain range in order to ensure the exit of the granulate with the correct geometry. Thus, the sensors used help to guarantee the high quality of the products.