EoW November 2010

Compounds & colourings

Ultrasonic temperature measurement system for

polyethylene melts by Dr T Clausen of Sikora AG and Dr P Huotari of Maillefer Extrusion Oy

For this purpose various methods are employed, such as the use of shielded thermocouples or infrared pyrometers, but each of these methods has at least minor imperfections. Using a thermocouple for contact, single-point measurement has a relatively slow response time. If the thermocouple does not penetrate into the melt, only the melt surface temperature or the temperature of the metallic extruder flow channel is measured. If the thermocouple penetrates into the melt shear heating errors can occur, presenting a local increase of temperature. Furthermore the thermocouple influences the melt flow properties and premature cross-linking takes place. Using an infrared pyrometer a contact-free temperature measurement with an improved response time is possible though, still, mainly the surface temperature of the PE melt is measured because the penetration depth of infrared radiation in LDPE (low density polyethylene) amounts to only several millimetres. Furthermore the presence of specific fillers in the PE melt can drastically decrease this penetration depth. The ultrasonic velocity in plastic compounds strongly depends on the temperature of the material. Therefore it is essential to compensate this influence if, for example, the dimensions of plastic insulated power cables (eg insulation wall thickness, eccentricity) are measured using an ultrasonic technique. On the other hand, a known temperature dependence of a material’s ultrasonic velocity can be used to determine the material temperature by measuring its ultrasonic velocity. One of the challenges in the measurement of ultrasonic velo- city in plastic melts is the high ultrasonic attenuation of these materials.

Melt temperature plays a critical role in the insulation phase of medium voltage (MV), high voltage (HV) and extra high voltage (EHV) XLPE insulated power cable extrusion, with organic peroxides used for cross-linking with both insulation and semi-conductive materials. Acceptable extrudate temperature range is limited since proper melting and homogeneity set the lower limit, while avoiding premature cross-linking sets the upper limit.

Adapter with measuring ▲ ▲ gauge ▲

Schematic view of the adapter in the extrusion line ▼ ▼

Extrudate temperature can affect both quality of the cable and the productivity. Premature cross-linking before or on the screens creates pressure build-up in the extruder and reduces running time. Premature cross-linking after screens, on the other hand, may lead to ambers or scorch in the insulation itself. In continuous vulcanizing (CV) lines the common practise is either to use thermocouple sensors or to measure extrudate temperature before start-up using a hand-held meter after crosshead. Before the introduction of an extrudate temperature gauge based on ultrasonic velocity there has not been an online solution available that is both non-contact and reliable. In the majority of cases extrusion lines are not equipped with an online melt temperature measurement system to record and to control plastic melt temperature during the extrusion process. Normally this temperature is measured only during the commissioning of the extrusion line at various line speeds and pressure conditions or later, during servicing.

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EuroWire – November 2010