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In-Line Color Measurement Directly in the Melt - …

35 COMPOUNDING VKunststoffe international4/2009 FUAT EKERC olor Measurement constitutes ameans of verifying Color quality. Themeasurement is generally conduct-ed on the finished end product using sam-ples that have been taken manually, di-rectly from the production line, or bymeans of automated measuring distinction is drawn between sever-al different types of Color the off-line method, the measure-ment results are not available for a mat-ter of hours, which is why this method isnot suitable for intervening in the online Measurement method sup-plies the Measurement results for the fin-ished product after just a few seconds.

35 COMPOUNDING V Kunststoffe international 4/2009 FUAT EKER C olor measurement constitutes a means of verifying color quality.The measurement is generally conduct-ed on the finished end product using sam-

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Transcription of In-Line Color Measurement Directly in the Melt - …

1 35 COMPOUNDING VKunststoffe international4/2009 FUAT EKERC olor Measurement constitutes ameans of verifying Color quality. Themeasurement is generally conduct-ed on the finished end product using sam-ples that have been taken manually, di-rectly from the production line, or bymeans of automated measuring distinction is drawn between sever-al different types of Color the off-line method, the measure-ment results are not available for a mat-ter of hours, which is why this method isnot suitable for intervening in the online Measurement method sup-plies the Measurement results for the fin-ished product after just a few seconds.

2 Butat a much later stage in the process se-quence. The online method is thus onlysuitable to a limited extent for timely in-terventions in the process. With an in-linemeasurement, by contrast, the results areavailable within a matter of seconds and,since the Measurement is taken directlyin the melt upon output from the ex-truder, this method is particularly suit-able when it comes to making immediatecorrections to the process [1].Different ColorMeasurement MethodsOffline Color Measurement is a methodfrequently used in industry.

3 This meas-uring method involves the followingsteps: Removing the sample from the pro-duction process, transporting the sample to the labora-tory, converting the sample material into ameasurable shape (e. g. Color plaques,sheet), cooling the sample to room tempera-ture, measuring the sample with a spectralphotometer, notifying the Measurement results tothe production department, deciding on a potential modificationof the process, and where appropriate, repeating theprocess for a further individual is clear from the different steps involvedthat several hours can elapse between themanual removal of the samples and thenotification of the Measurement resultsto the production department.

4 A furtherdrawback is that only one measuringpoint is established during this time. Thecolor quality prior to this measurementpoint, or at a later point in time, still re-mains unclear. Color deviations can,however, occur abruptly as a result of one-off events and may also come about grad-ually (as happens in the majority of cas-es). Effects of this type are not taken intoconsideration by the off-line measure-ment Measurement systems make itpossible to measure Color quality onsheets or semi-finished products, for ex-ample at the intermediate stages duringproduction.

5 These systems are used torecord the actual status of already-fin-ished products. All the problems that haveoccurred in the process up until thenwhich led to Color deviations are simplydocumented here. Given the time lag inrelation to the earlier process steps, thismethod is not suitable for supplying use-ful feedback for genuine process Color Measurement Directly in the MeltProcess shade differences result from a change in the processparameters, the material, or the material feed. Continuous In-Line Color measure-ment Directly in the melt as it leaves the extruder permits gap-free control of theprocess as a whole, with the possibility of immediate intervention in the runningprocess in the event of deviations.

6 Measuring tip of the extruderprobe with a sapphire lensPausch Messtechnik GmbHNordstrasse 53D-42781 HaanGermanyTel. +49 2129 9396-0 Fax +49 2129 from Kunststoffe 4/2009, pp. 54 56 Article as PDF-Fileat ; Document Number: PE11003235-37_PE110032_PE4 9:26 Uhr Seite 3536 COMPOUNDING Carl Hanser Verlag, MunichKunststoffe international4/2009An In-Line Color Measurement per-formed Directly in the melt upon outputfrom the extruder not only permits con-sistent documentation but also offers theopportunity of intervening Directly in theextrusion process in the event of Color de-viations.

7 Direct In-Line Color measure-ment dispenses with the first six steps ofthe off-line method and does not involvea time lag like the online method. Mea-suring the Color Directly during the ex-trusion process supplies the decisiveprocess information within just a few sec-onds, at the earliest possible time in theproduction In-Line MeasurementTechniqueTo record the measured values for the col-or, the Measurement probe (Title photo)is screwed into a threaded hole at the ex-truder exit (Fig. 1).The so-called extruder probe compris-es a sleeve with a robust head, which hasa sapphire lens set in it.

8 As the second-hardest material after diamonds, sapphireis particularly suitable for use in the ex-trusion process where it is subject tostressing through high temperatures andpressures and also to friction through aconstant flow of probe is placed in the extruderhousing in such a way that it does nottouch any of the movable parts, such asthe screw elements (Fig. 2). The melt,however, must cover the sapphire lensduring the flow process. The sapphire lensis self-cleaning, which ensures that nomaterial sticks to melt flowing over the lens is lit upby a xenon flash lamp.

9 To do this, the lightis transported to the sapphire lens via sixflexible glass fiber conductors, arrangedin a circle. The reflected light is fed backto the spectral photometer via a central-ly-positioned glass fiber. This operationis repeated every ten seconds, and a con-tinuous Color Measurement performed inthis and StructureThe special design of the extruder probemeans that the spectral photometer canbe calibrated without any interruption toproduction. To do this, the fiber optic isunscrewed from the probe sleeve and cal-ibrated on the basis of the black and whitestandard.

10 A green control standard is em-ployed to check the validity of the cali-bration. The calibration is a function ofthe surrounding conditions. The systemcan retain its stability for up to threemonths before a renewed calibration actual Color Measurement is per-formed in a spectral photometer usingCCD technology. The spectral photome-ter employed covers the wave range be-tween 360 and 780 nm, i. e. it includes thevisible part of the spectrum. By combin-ing a monochromator with the CCDtechnique, it is possible to achieve a finerresolution than with the standard colormeasurement 1.