Direct on-line color measurement in the hottest me

2022-10-13
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Direct color measurement in melt

color testing constitutes a way to verify color quality. This kind of test is usually carried out on the final product obtained directly from the production line or by using automatic measurement methods

the differences between several different types of color measurement are outlined. Using the off-line method, the measurement results can not be obtained within a few hours, which is why this method is not suitable for intervention in the processing process. The measurement method can provide the measurement results of the finished product within a few seconds, but it is at a very late stage in the processing sequence. Therefore, in this process, this method is only applicable to the situation of intervention within a limited range. On the contrary, when using measurement, the measurement result can be obtained within a few seconds, because the measurement is directly carried out in the melt output by the extruder. This method is especially suitable for direct correction of processing

different color measurement methods

offline color measurement is a method commonly used in industry. The utility value of this measurement method in the aerospace field lies in the following steps of metal replacement:

take samples from the production process

deliver samples to the laboratory

change the sample material into a shape that can be used for measurement (such as color plate, sheet)

cool the sample to room temperature

measure the sample with a spectrophotometer

inform the production department of the measurement results

formulate a possible processing improvement measure

repeat processing and production correctly for further individual product measurement

it is clear from the different steps involved that it takes several hours between manually taking out the sample and notifying the production department of the measurement results. Another disadvantage is that only one measurement point has been confirmed during this period. The color quality is still unclear before or after the measurement point for a certain period of time. However, chromatic aberration may occur suddenly at one time and may also occur gradually (as in most cases). Such effects are not considered by the offline test method

the measurement system makes it possible to measure the color quality of sheets or semi-finished products, such as in the intermediate stage of production. These systems are used to record the actual status of finished products. All the problems that have occurred during the processing until then that lead to color difference are simply recorded at this time. Considering the time interval associated with early processing steps, this method is not suitable for providing useful feedback information for real process control

a color measurement directly measured in the melt output from the extruder not only allows consistent recording, but also provides an opportunity to intervene directly in the extrusion process in the case of color difference. Direct color measurement omits the first six steps of the offline method and does not include the time interval existing in the image method. Direct measurement of color during extrusion provides clear process information in seconds at the earliest possible point in the production line

measurement technology

in order to record the measured value of color, the measurement probe is screwed into a threaded hole located at the outlet of the extruder

the so-called extruder probe consists of a solid front-end sleeve with a sapphire prism device inside. As the second hard material after diamond, sapphire is especially suitable for the extrusion process of withstanding the large stress formed by high temperature and high pressure and the friction formed by the stable flow of materials

The

probe is installed in the extruder housing. In this way, it will not contact any movable elements such as screw unit. However, during flow, the melt must cover the sapphire prism. Sapphire prisms are self-cleaning, which ensures that no material sticks to them

the fluid flowing through the prism is illuminated by xenon flash lamp. For this reason, light is transmitted to sapphire prism through six flexible glass fiber conductors arranged in a circle. The reflected light is reflected to the spectrophotometer through a glass fiber fixed in the center. This operation is repeated every ten seconds, and continuous color measurement is carried out in this way to optimize the bottle bottom

calibration and structure

the special design of the extruder probe means that the spectrophotometer can be calibrated without any interruption of production. To do this, the optical fiber is unscrewed from the probe sleeve and corrected based on black and white standards. A green control standard is used to check the effectiveness of calibration. Corrected as a function of surrounding conditions. The system can maintain stability for up to three months before recalibration is required

the actual color measurement is carried out on the spectrophotometer with CCD technology. The wavelength range covered by the spectrophotometer is between 360 and 780 nm, that is, it includes the visible part of the spectrum. Through the combination of monochromator and CCD technology, it is possible to achieve a better resolution than the standard color measurement unit

all technical equipment, that is, spectrophotometer, xenon flash, control unit, a complete computer with Windows XP operating system and a touch screen encapsulated in a box called NEMA 4. This box is made of 1. Stainless steel with measured thickness according to gb6672 and is designed to be used in industrial environment to avoid the influence of dust and splashed water. The built-in air conditioning equipment ensures the stable operation of the equipment even in the environment with large temperature fluctuations

evaluation of measurement

the measurement results are depicted in the form of a trend chart (as shown in Figure 1). This indicates that the measured color value is comparable to the standard color difference of the traffic light display. The current measured values are assigned separately as numerical values to be compared with the trend chart. The measured color values within this tolerance range are displayed in green. The value displayed in yellow has reached the warning limit of the allowable tolerance. Moreover, if the allowable tolerance is exceeded, the color will change from yellow to red. These obvious signals are not only displayed on the screen, but also can be displayed in the same color through the signal lamp

figure 1

according to the display on the screen, it is possible to choose from four freely selectable trend charts. All standard color ranges and white and yellow formulas can be obtained from similar light source types

process control

the extrusion process with a large number of influencing factors affecting the final result is a highly complex operation. In addition to the process parameters of temperature, pressure and output speed, the extruder itself and its components have a great impact on the results. The materials used increase the difference of results, such as raw materials of different batches or mixing ratios

the influencing factors caused by single or combined action have a direct impact on the color difference of extruded products. By using measurement methods to measure color, it is also possible to draw conclusions about improving machine or process parameters

The correlation between the color value and the cause of the defect has actually been clarified. For example, the addition of excessive titanium dioxide can be confirmed by the brightness value (as shown in Figure 2). The fluctuation of color difference may be attributed to the positive pulse pump or the precipitation of pigment in liquid colorant. In addition, color measurement can also be used to diagnose mixing problems, homogenization or metering problems in extruders, or other fluctuations in process stability

Figure 2. Too much titanium dioxide is added. From the perspective of future demand changes, the brightness value (middle line)

color difference may not necessarily be a key aspect of the quality of finished products. However, measuring color is an excellent way to achieve stable process control. On the whole, the instability in any production process is reflected by the difference in color. These deviations can be eliminated directly through corresponding intervention in the production process

on the whole, because of the continuity of records, direct color measurement in the melt establishes a particularly good way to analyze and optimize the production process. It can be used to prevent the production of unqualified products, prevent high-quality products from being polluted, keep defective batches of products within a minimum, optimize formula design, save raw materials/fuels, and ensure accurate measurement and compliance with product specifications, so as to avoid returning products

all this leads to considerable savings in raw materials and production costs

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