Note: Descriptions are shown in the official language in which they were submitted.
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~OECHST ARTIENGESELhSCHAET ~OE 89/F 224 Dr.DA/sch
Description
Proce~s for the preparation of a colored plastic molding
m~terial
S The invention relates to a process for the preparation of
a colored plastic molding material, in which the color of
the molding material is continuously monitored and the
test signals are used indire~tly ~o control the mixing
ratio of plastic and color pigment.
In the known process for the preparation of colored
plastic granules, a mixture of polymer and pigment, the
latter usually in concentrate form, and optionally
auxiliaries, is introduced into a granulator and pro-
ces~ed into granul~. Sample~ of these granules are taken
and molded into test piece~, the shade of which is
assessed ~isual~y or colorimetrically. If appropriate,
the mixing ratio of polymer to pigment is then chanyed.
Change~ are made to this ratio until the shade of the
granules coincides with the ~pecified shade.
Owing ko the time con~umed in preparing and assessing the
test pieces, discrepancies such as those which are
caused, for example, by differences between ~he batches
of pigment, can only be detected after a lengthy period.
A~ a consequence, the proportion of sub~tandaxd product
can amount to ~everal percent. In particular, ~he produc-
tion of end product~ for wh~ch individ~lal compone~ts of
identical shade are ~ade from granule~ from differe~t
production run~ or di~ferent production unit~, requires
continuou~ tracking of the ~hade during polymer pigm~nta-
: 30 tion to e~sure that the varian~ of ac~ual shade from ~he
control approa~he~ zero. ~o achieve thi4, the batches of
granules mu~t be homogenized ln a costly additional
prscP~s ~tep.
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The object is therefore to determine the shade of pla~tic
molding material~, in particular plastic granule~,
continuously during their production and to use the
results as rapidly as possible to control the metering
of polymer and pi~ment.
It has been found that a continuous monitoring of the
~hade of the molding material is possible if the molding
material particles are fed past the monitoring apparatus
in a suitable manner.
The presen~ invention accordingly provides a proces~ for
the preparation of a colored plastic molding material by
~dmixing at least one pigment with ~he plastic~ which
comprises continuously removing samples from the dry
colored molding material and feeding the~e past a light
lS source, the colors green, blue and red of the light
reflected from the surface of the molding material
particles being monitored and the test signal~ being
converted into control signal~ for controlling th~
metering apparatus for the pi~ment and the pla8~ic .
Furthermore, the invention provides a process for the
continuous monitoring of the shade of a colored pl~ BtiC
molding material by illuminating the ~urface of the
molding material particle~ by means of a light source and
monitoring the color~ green, blus and red in the light
reflected fr~ the ~ur~ace of the moldin~ material
particle~, wherein the clo~e-packed molding ma~erial
particle~ ara fed through tha monitoring apparatus past
the mea~uring window.
Finally~ ~he invention provide~ the appara~us for the
c~n~inuou~ moni~oring of the shade of a colored plastic
molding material, this apparatus compri~ing a feed hopper
(61), a gr~duated t~be (62) connected ther~to in which a
glazs mea~uri~g window (63) ha~ been ~et, a light source
(h4) out~ide the tuba (62) and in front of the measuring
window (63), and ~ ~en~or b~nk (663. Light source ~643
2~a9~rl
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and sensor bank (66) can be arranged next to ~ach other.
PrefQrably they are combined by using an optical fiber
bundle (67) to in~roduce li~ht and to lead off the
reflected light. In this case, light source (64~ and
sensor bank (66) are lvcated in a separate housing.
In the process according to the invention, samples are
~aken continuously from the stream of dry colored
material downstream of the m:ixer. The molding material
can have been obtained by various processes, the es~en-
tial points being that it must be dry on the surface andthat no water condensation can form during monitoring.
Sampl~s can be taken compl~tely continuou~ly by diverting
a small portion of the material flow, or else they ~an be
taken continuously at tLme intervals. Preferably s~mples
are taken from the material flow at regular time in~er-
vals, for example by suction. The samples of material are
passed from the sampling device to the ~eed hopper of the
monitoring apparatus.
The monitoring apparatus consi~ts of a tube of round or
square cros~ section in whose wall a window sealed with
a glass plate has been fitted. At one end, the tube has
the form o~ a feed hopper and at the other end it i~ con-
nected to an apparatu~ which uniformly empties the t~be
and simultaneously forms a barrier to pos~ible vaporR ox
ga~es emanating from the main ~tream of the molding
material. Preferably a star fe~der i~ used.
In front of the mea~urin~ wi~dow a light source i~
arranged having a virtually con~tant color tempera~ure,
; for example a halogen lamp. ~his i~luminate~ the moldin~
material particles which are located in ~he tube. The
light reflected fxom the particles is then receiv2d by
three ~en ors which ~re re~ponsive to the colors green,
blue or red.
The harmoniæed functioning of the feed hopper and the
emptying apparatu~ ensure that the graduated tube is
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always kept full and the molding material particles are
always close-packed. This is essen~ial for evaluating the
test signal~ from the tranducers. Current averaging oYer
the individ-lal te~t signals in a measuring and control
unit averages out the variations in the mobile molding
material surface (silhouetting and interstices). This
"current average" is compared with a reference mean which
has been obtained as a reference value by monitoring and
storing of a reference sample. The difference between the
desired value and the actual value is used to control the
metering apparatuse~ of the mixer.
The Figure~ ~how the process according to ~he invention
as a flow diagram and the monitoring apparatus employed
for the process in the preferred use with pla~tic
granules. These granule~ have a round or rectangular
cross-section, a diameter of 1 to 5 mm and a particle
length of 0.5 to 5 mm.
In accordance with Figure 1 a constant stream (23 of dry
colored granules low~ from the granulator (1). The
sampling apparatus (3) regularly takes ~ample~ via line
(4) and pas~e6 them thrvugh line (5) into the monitoring
~pparatus (6) from which thay are withdrawn by the
emptying apparatus (8) through the line (7) and are again
introduced into the ~ranule ~tream ~2) via line (9). The
si~nal3 o~tained from the monitoring apparatus (6) are
proc2ssed in the computer (10) to give control input~ and
are u~ed to actuate the metering control appar~tus (11).
The control ~ignal~ coming from the control apparatu~
(11) are pas~ed via cables (12) and (13) to the granu-
lator (1).
Figure 2 ~how~ a preferred form of a monitoringapparatus. The sampling app~ra~u~ (33, a suction fan and
the emptying apparatus ~8), a star feeder, are mounted
directly on the monitoring apparatus (6). The lines ~S)
and (7) are thus eliminated and line (4) enters directly
into the ~eed hoppar (61) of the monitoring apparatus
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(6). The graduated tube (62), in which the glas~ maasur-
ing window (63) is set, is connected to the feed hop-
per (61). Throu~h this measuring window (63), a light
source (64) illuminates, via the optical fiber bundle
(67), the surface of the granules (65) as they are fed
past. The reflected light is captured by the sensor bank
(66) and the green, blue and red color components are
supplied separately in the form of electrical signals to
the computer (10), not shown.
The correct color monitoring of the granule iB hi~hly
dependent on the velocity at ~hich the granules flow past
the measuring window. A minLmum velocity of 1 cm/~ec has
been established. The color temperature of the halogen
lamp is kept constant using an appropriate brightne~
~5 control which tracks the lamp vol~age (in the present
case about -6V). I~he outpu~ voltage range of each of the
three color sensors is O...lOV. The signals are
digitalized in an A/D-converter (12 bit3, smoothed by
averaging and fed to the granule brightnes~ computation..
Thus, 4 values are available for determining the color;
: 3 measured values from the 6elective color frequency
monitoring and the computed brightness. The~e are c~m-
pared in a computer with the stored values of the desired
granule color and made ready for further processing.
High resolution in the range of ~he desired value i8
necessary to detect even the sma~lest varia~co~ of the
actual value. ~hi~ is achieved u~in~ ~ignal ~preading.
Normally, th~ voltage ~ignal i~ at + lO0 mV. The
control input thus obtained for ad~u~ting the color i3
fed into an appropriate control apparatus which again
ad~usts the control si~nal~ for t~e installed metering
apparatu~. The~e correspond to ths valu 5 which ara
customary in measurement technology, 0~2...10V or
0/4...20mA.
Controlling the me~ering ra~io of pol~mer to coloring
polymer-piyment mixture( B ) give~ a cons~ant product
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quality wi~hin the product stream, thi~ being based on
the constant and reproducible measured values from color
monitoring of the continuous sample stream.
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