Ghost marks--gloss-related defects in injection-molded plastics.
The perceived quality relates to the impression that a customer gains in the sensory interaction with a product and is of great concern especially in the development and production of high-quality and so-called premium products. Apart from properties such as functionality and reliability typically connected to the quality of a product, the perceived quality impression is to a significant extent determined by the appearance of its surface.
In the automotive industry, polymeric materials are frequently used especially in interior trim applications and often in the form of injection-molded components. In the strive for a high quality impression these surfaces typically have to meet very strict requirements of a certain color or a specific gloss level. In addition to the desired color and gloss a particular texture is imposed into the surface in order to provide a more sophisticated impression. This texture is often a leather-like pattern or, when a more discrete impression is desired, more randomized patterns are usually chosen. Occasionally, so-called technical patterns, taking numerous forms and shapes are used. It is furthermore of great importance that the surface is free from aesthetic defects such as weld lines, flow marks, sink marks, or other gloss - and color variations. The production of injection-molded products free from such appearance defects places high demands on the design engineer as well as the injection molder and may be a complicated matter depending largely on the complexity and shape of the object to be produced. Optimizing the design of the component, the mould and the manufacturing process is essential in order to reduce the amount of defects.
The surface appearance of the injection-molded components in the automotive interior that are visible to the customer is carefully assessed during product development before receiving approval for mass production. In this process a particular surface defect has been observed on components manufactured from an acrylonitrile-butadiene-styrene copolymer (ABS). The defect is characterized by a local change in gloss or lightness which is only visually noticeable in certain viewing angles and conditions of illumination. In other viewing angles the defect may be hardly noticeable or even completely undetectable. Due to the nature of the defect it has informally been called ghost mark. On the opposite side of the ghost mark there is always a corresponding structure or feature in the mould. This structure or feature may for instance be a reinforcement, a clip fixing, or a production date marking. In the present study, the deviation in gloss in the defect region could usually not be detected with a conventional glossmeter as the illumination and detection angles of such instruments are limited and fixed.
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The aim of this article is to describe and characterize the aesthetic defect called ghost mark and discuss possible formation mechanisms during the injection molding process.
The scientific literature on injection-molding defects related to surface appearance is fairly limited although some references may be found to appearance-related surface defects such as weld lines and flow marks. One example is the review on some surface defects related to gloss given in Ref. 1. In general, very little is reported on gloss-related defects other than flow marks such as the striped patterns, also known as tiger stripes, formed by alternating dull and glossy regions approximately perpendicular to the flow direction (2-4).
Other literature dealing with surface defects in injection-molded components is in the form of troubleshooting guides where gloss-related defects are only very briefly commented on. In such guides gloss differences appearing due to wall thickness variations are mentioned. These gloss variations are associated with differences in cooling conditions and shrinkage variations for which an optimized holding pressure or adapting the injection profile are suggested as possible solutions (5), (6).
The polymer used for injection molding the components included in the study was a commercial acrylonitrile-butadiene-styrene copolymer (ABS) grade for injection molding which has a melt mass-flow rate at 220 [degrees]C and 10.0 kg in the order of 20 g/10 min (ISO 1133). The grade chosen is a typical grade used for automotive interior trim applications.
The study included several injection-molded components in the production development phase. The components were trim panels intended for the interior of an automobile and were imposed with three different textures shown in Fig. 1. The textures labeled (a) fine and (b) leather-like were fairly isotropic textures typical for automotive interior applications whereas (c) had a directional pattern made up of elliptical grains. The fine texture had an approximate RMS (root-mean-square) -roughness value, [S.sub.q] = 5 [micro]m whereas the RMS-roughness of the leather-like and the directional texture was approximately [S.sub.q] = 30 [micro]m.
Visual assessments of the surface appearance of the components were performed in a light cabinet in accordance with ASTM D 1729 with a CIE daylight illuminant D65 in order to simulate normal daylight viewing conditions.
Scanning Electron Microscopy (SEM) Characterization
The scanning electron micrographs were obtained using a Digital Scanning Electron Microscope Zeiss 940A (Carl Zeiss, Germany). The analyzed surfaces were initially coated with an approximately 5 nm thick layer of gold using with a Sputter Coaler SI50B (BOC Edwards, UK).
Multiangle Surface Reflection Characterization
The reflectance measurements were performed with a MA68II multiangle spectrophotometer from X-rite (USA) which conforms to ISO 7724. The instrument is equipped with a gas-filled tungsten lamp giving an incident light angle of 45 [degrees]. The spectral range is 400 to 700 nm and the spectral interval is 10 nm. The instrument simultaneously measures the reflectance properties at live different angles relative to the specular angle, 15[degrees], 25[degrees], 45[degrees], 75[degrees], and 110[degrees]. The instrument has a repeatability with a CIELAB total color difference of [DELTA]E* = 0.02.
[FIGURE 2 OMITTED]
RESULTS AND DISCUSSION
Visual Assessments and Photographs
Characteristic for the ghost mark is that it is visually apparent only in certain illumination conditions and in particular viewing angles. The photographs in Fig. 2 show the appearance of a typical ghost mark on a component manufactured with the directional texture. The photographs were taken in the light cabinet described above. Figure 2a shows the defect in the most obvious viewing angle where the defect appears as a significantly glossier region than the surrounding area. In Fig. 2b, the same location on the component is shown, however, in a viewing angle approximately opposite to the viewing angle in Fig. 2a. When viewed from this direction, the defect is not visible at all. In Fig. 2c, the structure (a clip fixing) on the backside of the component corresponding to the defect on the front side is also shown.
Figure 3 shows the appearance of another typical ghost mark on a component manufactured with the leather-like texture. Figure 3a and b show the defect in two of the most obvious viewing angles. In Fig. 3a, the defects appeared as duller and lighter regions compared with the surrounding defect-free areas. When slightly altering the viewing angle the appearance of the defects altered and they appeared as significantly more glossy regions than the surrounding area as shown in Fig. 3b. In Fig. 3c, the same location on the component is shown, however, in a viewing angle approximately opposite to the viewing angles described before. When viewed from this direction, the defects were not noticeable at all. Figure 3d shows the structures on the backside of the component corresponding to the defects on the front side. In this case, the defects were related to a production date marking, a material marking and a reinforcing element.
The surface defects were also studied by means of scanning electron microscopy. Figure 4a and b shows the difference between a defect-free region and an area where there was a defect, respectively, on a component manufactured with the directional texture. In areas where the texture was defect-free, the grains of the texture were intact and damages of the surface could not be detected. However, in the region of the defects, deformations of the texture were apparent. Most evident were the deformations on the tips of the grains in the texture pattern. The position of these flat deformations was consistently on the same end of the grains throughout the whole area of the defect. However, toward the edges of the defects the deformed and flattened areas became smaller and more uneven, i.e., less perfect in their mirror-like appearance. The intact grains of a defect free area which are shown at a higher magnification in Fig. 5a and b should be compared with the flattened or mirror-like deformations shown in Fig. 5c. The texture is, however, not only deformed at the tip of the grains. Similar but smaller deformations can been seen throughout the whole grain surface, though mainly occurring on one side of the grain as shown in Fig. 5d.
From the scanning electron micrographs it was confirmed that the deformations of the grains were facing the viewing angle in which the defect was visually most apparent. It seems like the deformations act as small mirrors altering the reflectance properties of the surface rendering a more glossy appearance in certain viewing angles. The angle of the deformation, in relation to the mean plane of the surface, seemed constant throughout the whole area of the defect which most likely amplifies the angle dependency of the visual appearance of the defect.
[FIGURE 3 OMITTED]
Scanning electron micrographs of another component manufactured with the leather-like texture is shown in Fig. 6. The micrograph to the left (a) was taken from a defect-free area and the texture in this area was intact. In the micrograph taken from an area of the defect, the texture is deformed as shown in Fig. 6b. The upper part of the texture grains appeared to have been shifted upwards. The movement of the upper part of the grain has resulted in an elongation/stretching of the material between the upper and the lower part of the texture. It was confirmed that the defects were most obvious in a viewing direction perpendicular to the elongated material which is from below in Fig. 6b.
[FIGURE 4 OMITTED]
Also in the case of components manufactured with the fine texture a deformation of the texture could be detected in the defect region. Scanning electron micrographs of the surface of such a textured component are shown in Fig. 7. The micrographs shown are representative for the observed topography in the different areas. The micrograph to the left (a) was taken from a defect-free area and the texture in this area was intact. However, in the micrograph taken from an area with a defect (b), the texture was deformed. The deformed structure is more evident in Fig. 7c which is an enlargement of the area in Fig. 7b. As in the case of the leather-like texture, a movement of the texture has resulted in a elongation/stretching of the material which can be identified as a slight directional pattern of the texture shown in Fig. 7b and c (vertically in the image). Also in this case the defects were most obvious in a viewing direction perpendicular to the elongated material which is from below in Fig. 7b.
Multiangle Reflectance Measurements
As mentioned earlier the diverging gloss of the defect region, compared with the surrounding area, was not possible to detect with a conventional glossmeter. In order to characterize the differences in light reflectance, the reflectance of the defect-free as well as that of the area with a surface defect was measured by means of a multiangle spectrophotometer. Measurements were done in several different directions in order to simulate various viewing angles and directions of illumination and the light reflected from the surface was delected. Five repeated measurements were taken and the standard deviation was typically less than 1.5% of the measured reflectance. Examples of the results obtained are shown in Figs. 8 and 9 in order to demonstrate the difference in reflectance properties between a defect-free area to that of a surface defect.
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In Fig. 8 the angle-resolved light reflectance from an injection-molded component having the directional textured is shown. The area having a surface defect exhibited a somewhat higher reflectance (than the defect-free region) at angles of detection close to the specular angle (15[degrees] and 25[degrees] relative to the specular angle) and very slightly less reflectance at the angle of detection furthest away from the specular angle. The higher visually perceived gloss on areas having the ghost mark defect is likely connected to their higher reflectance in angles close to the specular angle. In this Fig. 8 the measurement was performed with the incident light perpendicular to the elliptical grains in the pattern. Similar results were obtained regardless of the measurement direction.
The ghost marks on surfaces imposed with a leather-like texture exhibited both higher as well as lower perceived gloss depending on the angle of viewing as mentioned above and shown in Fig. 3. In Fig. 9, the results from the multiangle reflectance measurements on a component with the leather-like texture are shown. In this case the reflectance from the area with a ghost mark was lower than the reflectance from the defect-free region at angles near the specular angle. However, the situation was reversed at the 45[degrees] detection angle which was the same as the normal of the surface. In this case the measurement was performed in such a manner that the incident light was in the same direction from where the defect was visually most apparent (see Fig. 3a and b).
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
When multiangle reflectance measurements were performed with the incident light in the opposite angle to the most apparent viewing direction (which is the direction in which the defect was visually not detectable, see Fig. 3c), the difference in reflectance from an area with and without a ghost mark defect was less evident which is shown in Fig. 10.
These measurements thus complement the visual observations in the sense that they show that the flow marks reflect the light in a manner different from the ideal surface and that their visibility depends on the illumination conditions and the viewing angle.
Relations to Processing Conditions and Possible Mechanisms of Formation
The SEM studies indicate that it is likely that the ghost marks are associated with small-scale deformations of the surface texture. It is suggested that this deformation takes place when the surface regions are in a semisolid state. Small scale refers to that the deformations were smaller than a characteristic dimension of the imposed texture. A rather extensive study on the influence of the processing conditions on the formation of the ghost marks was undertaken. The pressure during the injection molding process was here noted to affect the occurrence of the defects. Particularly significant was that when the holding pressure was increased, the ghost marks appeared and they became more pronounced with increasing holding pressure. Trials have been done where the mould cavity only partially was filled and in this case no ghost marks appeared. The ghost marks were not visible as the degree of filling was increased and did not appear until the cavity was exactly filled. Furthermore, it was noted that when the holding pressure was substantially reduced, the ghost marks were transformed into or replaced by sink marks.
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The thickness of the component also seemed to influence the formation of the defects. When the thickness was reduced to below 3 mm the marks were in several cases more apparent. However, further trials have shown that increasing the thickness alone does not remove the ghost marks completely.
Surface defects resembling ghost marks have been reported in Ref. 7. In that case, the deformation of the surface texture was related to insufficient draft causing mechanical damage during the ejection phase of the molding cycle. Despite the similarity of the deformations, the defects described in the present case, was quite frequently found on surfaces which, in the mould, were more or less perpendicular to the angle of ejection. Thus, it is not very likely that the ghost marks would develop during the ejection phase due to a poor draft.
A possible explanation for the occurrence of the defects is that they are caused by forces imposed by the holding pressure. When the holding pressure is applied, the material is not fully solidified permitting a deformation of the surface texture as evident from the SEM micrographs. Such deformations of the material would be more pronounced near structures such as clip fixings and reinforcements since the larger amount of material delays both the cooling and the solidification of the corresponding region (in relation to the surrounding areas). Additionally, effective cooling generally is not feasible in these areas of the mould core. However, ghost marks may also occur corresponding to a marking on the back side, such as a production date marking, which cannot be associated with significant increase in wall thickness or to an insert in the mould core that is not cooled. In the latter case, the slower cooling in the insert region again can be associated with deformations imposed by the holding pressure.
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It may also be that the ghost marks appears or are enhanced due to a nonuniform thermal surface shrinkage during the cooling of the product in the mould.
In this study, a specific grade of ABS was used which raises the question of the effect of the selection of the material. Similar defects have been observed in components manufactured in polycarbonate/ABS blends. However, the importance of the selection could obviously be a subject of a more detailed study.
A further study on the formation of ghost marks is in progress employing two mould cavities. This study will aim to clarify the influence of different types of texture, texture differences on a microscale, and texture orientation relative to the flow direction on the formation of the ghost marks as well as to achieve a more complete understanding of the underlying mechanisms. Also the influence of varying the processing conditions will be studied. The results will be reported in a coming article.
The ghost mark defect is characterized by a local change in gloss of textured injection-molded ABS components which is only visually detectable under certain viewing conditions. The altering of the gloss was related to small-scale deformations of the surface texture possibly caused by a shear force imposed by the holding pressure or by nonuniform shrinkage. The change in the light reflectance properties caused by the ghost mark defect could be characterized by means of multiangle reflectance measurements.
Despite that the visual appearance of the defects on all three textures was fairly similar it may not be presumed that it is the same mechanism of formation of the ghost marks on all three textures. Judging from the scanning electron micrographs there were similarities between the deformations in the texture of the leather-like and the fine texture. The appearance of the deformations of the directional textured differed, however, significantly from those in the other textures.
Retor Group, Sweden, is gratefully acknowledged for technical support and for the performance of the test trials.
NOMENCLATURE ABS Acrylonitrile-butadiene-styrene copolymer ASTM American Society for Testina and Materials CIE Commission Internationale de VEckunige CIELAB CIE color coordinates ISO International Organization for Standardization SEM Scanning electron microscope D65 CIE standard illuminant, average daylight with a color temperature of 6500 K [DELTA]E* CIELAB total color difference
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Sofie Ignell, (1), (2) Peter Porsgaard, (3) Mikael Rigdahl(2)
(1) Department of Perceived Quality, Volvo Car Corporation, SE-405 31 Goteborg, Sweden
(2) Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-412 96 Goteborg, Sweden
(3) Department of Strategy and Concept, Volvo Car Corporation, SE-405 31 Goteborg, Sweden
Correspondence to: Sofie Ignell; e-mail: firstname.lastname@example.org
Contract grant sponsors: The Swedish Agency for Innovation Systems (Vinnova), Volvo Car Corporation, Chalmers University of Technology (the Area of Advance Production).
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2011 Society of Plastics Engineers
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|Author:||Ignell, Sofie; Porsgaard, Peter; Rigdahl, Mikael|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2012|
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