Inline color measurement directly in the extruder

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  • Color measurements takes place directly in the polymer melt
  • Real-time information about color values
  • Self-cleaning: material shear forces clean measuring probe

Color values are an important indicator for product quality. When it comes to extrusion, all parameters such as temperatures, pressure and production speed have a direct impact on the color values. Raw material also influences the color values, reflecting different lots or ratio of mixture. In order to determine the color valus, the ‘off-line’ color measurement method is commonly used. But this method has several drawbacks. Thanks to a new technology, it is now possible to make measurements directly in the melt without the need for sampling. Immediate adjustments of the process parameters are therefore possible.

The drawbacks of ‘off-line” color measurement

Color measurement is usually performed ‘off-line’, and includes many, time-consuming steps: take a sample, send the sample to the laboratory, shape the material into a measurable sample, make the ‘off-line’ measurement with a spectrophotometer and report results back to the production. Based on the off-line measurement results, production can adjust the process accordingly. The time between sampling and obtaining the results from the laboratory can take several hours. In addition, a significant disadvantage is that only a single measurement is generated, and the quality of the color during, before and after the sampling point is unknown.

RPMP with Sapphire window

Picture 1: RPMP with Sapphire window and Dynisco thread ½”-20 UNF (source: COLVISTEC AG)

Color measurements takes place directly in the melt

To eliminate the drawbacks of off-line color measurement, the German company COLVISTEC AG has introduced a new system for the inline measurement of color values. The inline color measurements takes place directly in the melt. As a result, immediate intervention can take place when color variations occur. The measured results are available real time.

RPMP in the extruder die-plate

Picture 2: RPMP in the extruder die-plate (source: COLVISTEC AG)

Melt probe with sapphire lens

This technology deploys a probe directly in the extruder. The Reflection Polymer Melt Probe (RPMP, picture 1) is installed at the exit of the extruder by using ½”-20 UNF thread. As illustrated in picture 2, it is screwed into position on the exit of the extruder. The RPMP consists of the threaded sleeve and fiber optics. The Sapphire lens on the tip of the probe is the observation window and suitable to withstand the harsh environment in the extruder.

Trend charts of EquiColor Software

Picture 3: Trend charts of EquiColor Software (source: COLVISTEC AG)

Self-cleaning: material shear forces clean the probe

The melt stream must completely cover the Sapphire surface. The self-cleaning function of the Sapphire is ensured by the shear force of the material flow. Ideal installation is directly into the melt stream at the die-plate, in an adapter or flange. Measurements are performed on opaque as well on transparent materials.

Illumination of molten material

Illumination of the molten material through the sapphire window is achieved by 6 circumferentially positioned glass fibers which convey the light of a Xenon flash lamp. The reflection from the illuminated surface is detected by one centre positioned fiber optic. The reflected light is then interpreted by the spectrophotometer. Color values such as L*, a*, b* C*, h are calculated from this spectral curve (for the range of 380 through 780 nm), and displayed as trend charts, picture 3. The frequency of the measurements is freely adjustable. For a continuous extrusion process, a measurement interval of 10 to 60 seconds is recommended. The calibration of the spectrophotometer can be carried out without interruption of the process.

Stainless steel NEMA4 box with touch-screen

Picture 4: Stainless steel NEMA4 box with touch-screen (source: COLVISTEC AG)

The hardware of the system

The UV-VIS spectrophotometer is integrated in a NEMA4 box (equal to IP66) with a panel PC and touch-screen (see picture 4). The appropriate spectral range is 220 through 820 nm (resolution 1 nm). The NEMA4 box is made from stainless steel. The box contains a thermoelectric cooling and heating device to eliminate the influences from ambient temperature by keeping the temperature inside the box at a constant level.

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