Influencing factors of ABS resin color detection
The tristimulus brightness (L) and chromaticity (a, b) values of the sample and the standard plate were measured with different types of spectrophotometer respectively. By analyzing the statistical differences of L, a, and b values under different background plates, different sample thicknesses, and different measurement positions of the same sample, the effects of background plates, sample thicknesses, and different measurement positions of the same sample on the measured values of ABS resin color were investigated.
Acrylonitrile-butadiene-styrene (ABS) resin is an opaque, amorphous thermoplastic engineering plastic with a complex two-phase structure. it began to be recognized and used by the general public in the 1970s, and the market demand developed rapidly in the 1990s. Currently, ABS is widely used in domestic and international markets, especially in the construction, home appliance and automotive industries.
The chromaticity index of ABS resin is as important as the mechanical properties in the market to judge the quality of its products. The national standard GB/T 12672-2009 stipulates that ABS resin is tested using SH/T 1541-2006 test method for appearance of thermoplastic granules, but in practical application, the method uses visual inspection and other means, which can no longer meet normal production needs due to differences in human eyesight. At present, the test method of measuring the tri-stimulus value, chromaticity and color difference with spectrophotometer by the calibration color plate and sample color plate has been widely used in the manufacture and application of resin.
The color of an object can be represented by the Lab color model, which consists of three elements, one element is the luminance L, and the other a and b are two color channels. a includes colors from dark green (low luminance value) to gray (medium luminance value) to bright pink (high luminance value); b is from bright blue (low luminance value) to gray (medium luminance value) to yellow (high luminance value). Therefore, the change of resin chromaticity can be studied by the change of L, a and b values. Except for the errors brought about by injection molding, etc., the detection factors affecting the color and chromaticity of ABS resin are mainly focused on the use of testing instruments. In this paper, the influencing factors of ABS resin chromaticity detection are studied by measuring the resin chromaticity under various conditions.
1. Experimental part
1.1 Raw material
ABS resin: Grade 0215A, China Petroleum Jilin Petrochemical Corporation.
1.2 Apparatus and equipment
Injection molding machine: EC-100C-2A, Toshiba Machine Group; spectrophotometer: Ultra ScanPro, HunterLab, USA; spectrophotometer: CM-3700A, Konica Minolta Co. Ltd. (Japan); mold for injection molding: family type mold, Dalian Hualu Co.
1.3 Experimental method
ABS resin analysis samples were prepared by injection molding machine according to GB/T12672-2009. The sample was stored in a constant temperature and humidity chamber for 8h, and then the L, a and b values of the sample and the standard plate were measured by spectrophotometer respectively. The color space was selected as Hunter’s Lab color space, and the light source was selected as C2°, and the test environment was 25℃ and 50% RH.
2. Results and Discussion
2.1 Background plate factor
The background plates of Ultra ScanPro from HunterLab, CM-3700 and CR-300A spectrophotometers from Konica, Japan were used as the background plates for testing, and the same ABS sample was placed afterwards.
Fig.1 Effect of different background plates on the L value of the same sample
Figure.2 Effect of different background plates on the a-value of the same sample
Figure.3 Effect of different background plates on the b-value of the same sample
Among the self-contained standard plates, the standard plate values of UltrascanPro were XYZ (83.23,98.44,105.53), CM-3700A were XYZ (93.45,86.65,106.08), and CR-300 were XYZ (92.87,94.87,111.79). The data show that there is a large difference in the chromaticity of these three standard plates (the above data are obtained retroactively under C2° conditions, and XYZ is a common color space of the International Lighting Association, which is convertible with Lab color space). From Figure 1 – Figure 3 can be seen, different background plate on the test results have different effects, which is related to the characteristics of ABS. ABS resin sample transmittance of about 40%, when the background plate is white, spectrophotometer light source, due to reflection, diffuse reflection, secondary reflection and other factors, coupled with the background plate itself, the difference in light absorption capacity, different types of light sources (such as sampling halogen lamp, xenon lamp), etc., these factors will affect the colorimetric results. All these factors will affect the chromaticity results.
2.2 The effect of sample thickness on the test results
The results are shown in Figure 4 and Figure 6.
Figure.4 Effect of sample thickness on brightness L value of ABS resin
Figure.5 Effect of sample thickness on chromaticity a value of ABS resin
Fig.6 Effect of sample thickness on chromaticity b value of ABS resin
From Figure 4-Figure 6, it can be seen that the luminance L-value of the same samples starts to decrease gradually after the thickness increases. This is because the effect of the high gloss white background plate used for the measurement gradually diminishes as the sample thickness increases. The chromaticity a value gradually increases and the chromaticity b value gradually decreases, mainly because at the beginning, as the sample thickness increases, the tri-stimulus value of the relevant hue changes, leading to a decrease in the chromaticity b value and an increase in the a value in the color coordinate system, while the influence of the background plate becomes weaker and weaker and turns from bright to dark. The color difference ΔE value (also known as chromatic aberration, can be simply understood as color difference, some chromatic aberration can be distinguished intuitively with the naked eye, while some chromatic aberration can only be distinguished with the help of professional instruments). ΔE usually indicates the color difference between the sample and the standard, ΔE = [(ΔL)2 + (Δa)2 + (Δb)2]1/2) gradually increases, which varies with the change of L, a and b values, to be sample The thickness of the sample increases to 16.4mm (no longer transmitting light), L, a and b values are no longer changing, then the measurement results really reflect the color of the sample itself, thus illustrating the influence of the background plate on the brightness L and chromaticity a and b measurement results.
2.3 The effect of sample test position on the test results
The Ultra ScanPro spectrophotometer was used to take the same batch of ABS resin standard samples placed behind the background plate, and four sets of data were tested, and the results are shown in Figures 7 and 9.
Figure.7 Effect of test position on the brightness L value of ABS resin
Figure.8 Effect of test position on the chromaticity a value of ABS resin
Fig.9 Effect of test position on b value of chromaticity of ABS resin
From Figure 7-Figure 9, it can be seen that the results of measuring the L, a and b values at different locations of the sample exhibit certain differences, especially the differences between different measurement surfaces, mainly due to the different finish of the mold cavity mirror and the fixed mold mirror at each location, and the different fluid temperatures at various points in the mold cavity leading to the differences in the final test results.
2.4 Influence of other conditions on the chromaticity test
In practice, in addition to the above conditions, other factors will also have an impact on the colorimetric analysis, such as light path, color space, spectrophotometer integrating sphere material, light source, color decay, temperature, injection conditions, mold surface finish (roughness), etc. will have an impact on the colorimetric analysis. The influence caused by optical path, color space, spectrophotometer integrating sphere material, light source, color decay is a hardware system problem, which is not covered in this paper. When the conditions are mature, this topic will analyze the influence of temperature, injection conditions, mold surface finish (roughness) and other factors on the colorimetric test.
- (1) The measurement conditions such as thickness, background plate and other factors have a great influence on the measurement results of product chromaticity.
- (2) Under the same conditions, the different test positions also have some influence on the results, so the test should be kept as consistent as possible.
- (3) The test results under different test conditions are not comparable, and the fundamental solution to such problems is to establish standard test conditions.
Source: China Manufacturer of Spectrophotometer – www.spectrumgfa.com