Coloring Polylactide Powder for Multicolor Rapid Prototyping

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1 ing Polylactide Powder for Multicolor Rapid Prototyping Viboon Tangwarodomnukun 1, Pisut Koomsap 1 *, Pakorn Opaprakasit 2 and Thittikorn Phattanaphibul 1 1 Design and Manufacturing Engineering Asian Institute of Technology, Km. 42 Paholyothin Highway, Klong Luang Pathumthani, 1212, Thailand Corresponding author s pisut@ait.ac.th 2 Department of Common and Graduate Studies Sirindhorn International Institute of Technology, Thammasat University, Pathumthani, 12121, Thailand Abstract: Rapid prototyping process has been widely used in many industries to fabricate parts rapidly from three-dimensional CAD models. The majority of RP parts have been created with single color appearance of material used. Due to the nature of the RP techniques, limited number of them is capable of making multicolor parts even though this capability is foreseen, from industrial design point of view, to provide explicitly more product information to a development team, especially esthetic value that may be concealed in single color appearance. Toward the development of Selective Vacuum Manufacturing (SVM), a new RP technique, this paper presents a study on coloring polylactide (PLA) powder, one of applicable materials. In this study, commercial pigments, generally used to colorize the plastics for injection molding process, were employed to colorize the PLA powder. The homogeneity of colored PLA was investigated both before and after sintering process for different mixing methods. The results show that the two or more pigments should be mixed together to obtain desired color before combining with PLA powder to deliver a solid color on a prototype. Because of the provided heat, the shade of colors was slightly changed after sintering. The study on obtaining distinct boundary between different colored PLA, laid to create a part is also discussed. Keywords: ing, Polylactide, Rapid Prototyping, Selective Vacuum Manufacturing 1. INTRODUCTION Rapid prototyping (RP) is a term of technologies for producing accurate parts rapidly and directly from CAD models with little need for human intervention (Pham and Gault, 1998). Since it was initiated in the early 198s, several rapid prototyping systems have been developed through out these two and a half decades and many of them are commercially available in the market, including Stereolithography Apparatus (SLA), Selective Laser Sintering (SLS), Laminated Object Manufacturing (LOM), Fused Deposition Modeling (FDM), Solid Ground Curing (SGC), 3D Printing, etc. Even though their techniques and materials used may vary, but they all come from the same basic principle, which is material is deposited and bonded with a previous layer to form a 3D physical object layer-by-layer. From the beginning, the main focus of RP has been on making a prototype with good accuracy quickly in order to shorten product development time. RP is well accepted today in various disciplines (e.g., manufacturing, medical, architecture) because it offers faster speed, higher accuracy, and more flexibility than other conventional prototyping methods. These advantages become more trivial for fabricating object with complex geometry. The focus is boarder these days when prototypes made can be involved both directly and indirectly in rapid manufacturing, in addition to being used for visual representation and functional testing. An increasing number of organizations view RP technology as a viable method of manufacturing, and it is predicted that RP will no longer mean rapid prototyping, but rather rapid production (Drizo, 26). Some techniques have already been further researched in that direction to allow creation of finished products (Upcraft and Fletcher, 23). This success will give way to low volume complex products to be manufactured rapidly at lower cost. Visual representation of a prototype, however, remains important and also need further development, especially on coloring, in order to reduce information lost from converting a graphical model to be a physical prototype, since most available rapid prototyping techniques can produce only single colored prototypes, which can provide limited information and esthetic value to customers. 3D printing is the first commercial color RP system. Integrate coloring system into the existing 3D printing system is not too difficult because this technique has been developed based on printing technology. Similar to color inkjet printer, additive color mixture has been applied. A single nozzle, found in original 3D printing, is replaced by a set of four nozzles containing cyan, magenta, yellow, and black binders (Upcraft and Fletcher, 23). The application of printing technology has been extended to Selective Laser Sintering (SLS), which a printing mechanism of ink-jet printer is

2 installed inside the SLS machine (Ming and Gibson, 1999, 26). The development of multicolor RP is not limited to powder-based RP techniques. It can also be found in LOM, solid-based RP, (Gibson and Ming, 21) and SLA, liquid-based RP (Im and et al, 22). Commodity thermoplastics have been widely used in various applications, including in RP applications, but after being used, they are difficult to destroy, and some of them also release toxics to environment. Consequently, alternative materials that are environmental friendly have been campaigned to replace the traditional plastic materials, and Polylactide or PLA has been recognized as biodegradable material. It has been used for packaging, textile, storage container products and etc (Vink and et al, 23)(Gupta, Revagade and Hilborn, 27). PLA has been researched at AIT as one of alternative materials for Selective Vacuum Manufacturing (SVM), a new RP technique currently being developed. The objective of this study was to find a suitable method for constructing a multicolor prototype from PLA powder with SVM technique. Experimental study on coloring PLA powder is presented in this paper. 2. COLORING THERMOPLASTICS Three kinds of colorant, normally used for coloring thermoplastics are dye, inorganic and organic pigments (Adams, 23). For dye, the colorants can be dissolved in plastics and provide very high color strength but they are very expensive colorant. Inorganic pigments are based on salts, oxides or metal oxides such as cadmium sulfide for yellow, lead oxide for red and cobalt oxide or aluminum oxide for blue; bright colors can be obtained easily, but they are very toxic due to metals contained in pigments. For organic pigments, they are based on carbon chemistry without any metals contained. To select a suitable type of pigment for coloring, it mainly depends on the types of plastic (Abrams, R. and et al, 21). Two general methods for designing color in various applications, including display monitor, printer, paint, and etc., are additive and subtractive methods. In additive method, all colors in the visible spectrum can be produced by regulating red, green and blue lights, called prime colors. Its application can be found in electronic display, and color printing technologies. It is quite different from subtractive method that color appeared is the color of a light that is not absorbed by the surface where the lights projected on. of an object is defined as the aspect of the appearance of an object dependent upon the spectral composition of the incident light, the spectral reflectance or transmittance of the object, and the spectral response of the observer (ASTM, 1982). The applications of subtractive methods can be found in painting and dyes technologies. In practice, three primary colors, red, yellow, and blue are generally regulated, instead of prime colors, for design color in dyes or pigments, used to create colors for thermoplastics. Secondary colors and other colors can be created by controlling the mixing ratio of primary colors as shown in Figure 1. However, the amount of added pigment is depended on the type of pigment, type of plastic and the desired weight of color (Wong and et al, 1997)(Adams, 23). Normally, solvent is added into the mixture to improve uniformity of color on the material but it also dilutes the color. Furthermore, it may change the property of material. According to Ming and Gibson (26), alcohol, a solvent, increases the penetration ability of ink into the polyamide powder when it was applied in SLS. However, the structure of material was changed when too much alcohol was added. Primary s Figure 1. Wheel

3 3. EXPERIMENTAL SETUP The main objective of this study was to determine a suitable method for coloring PLA powder that leads to a construction of a multicolor prototype with SVM technique. Experiments were conducted to find an appropriate method for mixing pigment with PLA, and for creating secondary color on PLA 3.1 SVM Technique Figure 2 illustrates the steps in SVM for creating a prototype. In this technique, a cavity profile is created on a layer of support material, and is filled with material. The material is sintered then to form a layer. This process is repeated until the vertical height of the part is completed. Several types of material can be used, including natural rubber latex but mainly in powder form (Risdiyono and et al., 26). 3.2 Materials Three-primary color pigments, red, yellow and blue, were employed for coloring PLA powder. They were commercial organic pigments generally used in injection molding process. PLA powder was prepared from its pellets. PLA pellets were commercial grade, supplied by Dow Cargill. The details for preparing PLA powder from it pellets can be found in (Phattanaphibul, and et al, 27). Figure 2. Building Schematic of SVM Technique (Risdiyono and et al., 26) 3.3 Experiment Procedures In this study, four sets of experiments, as illustrated in Figure3, were conducted in two successive steps. In the first step, the ability of coloring PLA powder was studied to find a suitable method for mixing pigment with PLA powder. The colorized powder was investigated for the homogeneity and uniformity of primary color on PLA powder both before and after sintering. The experiments were extended to study color mixing in the second step to achieve secondary colorized PLA powder. According to (Phattanaphibul, and et al, 27), PLA pellets were dissolved to be a solution before powder was produced. Therefore, color could be introduced into PLA in two states: liquid solution and powder to form colorized powder., The first step of this study was then to determine in which state of PLA that pigment should be filled. In the first method, color was filled into PLA prior to becoming powder. The primary pigment was mixed with PLA pellets in dichloromethane (CH 2 Cl 2 ). The colorized solution was sprayed into water medium mixed with poly(vinyl alcohol) surfactant that was added to facilitate the dispersion of the PLA solution droplets. The powder was filtered, dried, and disintegrated to obtain loose powder. For the second method, the pigment was mixed with loose PLA powder. A little amount of alcohol, 1 to 2% v/w, was added for better homogeneity. Similar steps were taken to evaluate color quality of PLA powder, obtained from both methods. The visual inspection was carried out first to evaluate the homogeneity and uniformity of the loose powder. A layer of PLA powder was then formed and sintered similarly to being done in SVM technique. For sintering step, the layer built was placed 1 mm underneath a halogen lamp that provided heat between 9 to 1 C for 15 seconds. A colorimeter was employed to measure color values of the powder before and after sintering process. According to CIELAB diagram shown in Figure 4, the measured values are L*, a* and b*, where L* is the darkness and lightness of color varied from to 1, a* is redness (+a*) and greenness (-a*), and b* is yellowness (+b*) and blueness (-b*).

4 After the suitable method was identified for achieving primary colorized PLA powder in the first step, it was utilized in the second step to achieve secondary colorized PLA powder. In this study, the selected secondary colors were orange, violet, and green that could be obtained from mixing two primary colors referring to the color wheel shown in Figure 1. Two mixing options were experimented. The first option was two pigments were mixed together before combining with PLA, and the second one was two different primary colorized powders were mixed together. Similar steps were carried out to evaluate the color quality of the secondary colorized powder. ing Primary colors Secondary colors Pigment + PLA in CH 2 Cl 2 Pigment + PLA powder Mixing pigments before combining with PLA Mixing different primary colorized PLA powders Evaluate the homogeneity and the uniformity of powder before sintering Sintering process Evaluate the color quality of sintered part Figure 3. Mixing Methods and Steps of Investigation Figure 4. CIELAB Diagram 4. RESULTS AND DISCUSSIONS Four sets of experiments were conducted in two steps to identify appropriate method for mixing pigment with PLA, and for obtaining secondary color on PLA powder. 4.1 Experimental results on coloring PLA powder The first set of experiments was to find a suitable method for coloring PLA powder with three primary pigments: red, yellow and blue. Pigments were investigated with PLA solution, and PLA powder. In the first method, each primary

5 pigment was mixed with PLA pellets in dichloromethane to form a colorized PLA solution. The solution was sprayed into the water medium. The powder was filtered out, dried, and disintegrated. It was found that the powder stuck together in the medium after spraying, and very difficult to disintegrated after drying. The color obtained on the dried PLA patch was lighter than the pigment. The color was non-uniform but homogenous within each color shade. From observation during the process, the dispersion of pigment in solution was good, but the color was diluted. It was suspected that some pigment particles dispersed further into the medium because the organic pigment is not completely dissolved in polymers (Abrams and et al, 21). The problem on non-uniformity of color on powder may be caused by precipitation of pigments in the PLA solution. It was observable that the color of the sprayed solution became lighter and lighter comparing to when it was first sprayed, and the precipitation of pigment was found in the spray gun. It seems that this approach was not appropriate for coloring PLA powder; therefore, this set of experiments was stopped at preparing colorized PLA powder. Another issue that must be addressed if this approach would be applied is contamination in the medium may affect the color of powder. In the second method, each primary pigment was mixed with loose PLA powder. It was found that the homogeneity of color and the uniformity of powder before sintering were good, and the mixed powder remained loose. A layer was then constructed and sintered. The sintering condition was under solid state sintering where PLA powder was observed to be partially melted and adhered to each other to retain the layer s shape. Figure 5 shows the colorized PLA powder, obtained before and after sintering. From the observation, the colors were slightly changed after sintering. The colorimeter was employed to measure each color, both before and after sintering. The results are showed in Table 1. According to the measurement, the red specimen, for instance, became lighter with more greenness and blueness after sintering. Student s t-test, a statistical method, was then used to analyze the color variation for all three primary colors, and the results confirmed with 95% confidence interval that color changes were caused by sintering process. The effect of the three primary color pigments on measured values was analyzed by ANOVA. The results are shown in Figure 6. Clearly, the degree of change of L*, a* and b* values is depended on color used. The negative value of dl*, da* and db* means it is turned to be lighter, more red and more yellow respectively. It was found also that when too much pigment was added, the sintered PLA powder became more brittle and the bonding between powder particles was poor. a. ized Powder before Sintering b. ized Powder after Sintering Figure 5. Comparison of ized PLA Powder (Red, Yellow and Blue) between before and after Sintering Table 1. Averaged L*, a* and b* Values of Primary s both Before and After Sintering Process Before sintering process After sintering process Difference Change of color shade* L* a* b* L* a* b* dl* da* db* Red Lighter Yellow Darker Blue Lighter *The change of color shade after sintering was observed by visual inspection.

6 dl* da* Blue Red Yellow Blue Red Yellow 1 5 db* -5 Blue Red Yellow Figure 6. The Change of L*, a* and b* to Primary s 4.2 Experimental results on obtaining PLA powder with secondary colors From the first step, it was concluded that pigment should be mixed with PLA powder. However, only primary colors were tested. Therefore, further experiments were conducted in this step to determine a suitable method for producing secondary color PLA powder. In the first method, two primary pigments were mixed to yield a homogeneous secondary color, before mixing with PLA powder. The results showed that the dispersion of pigments was good, and the mixed color was homogeneous and followed the color wheel. The recommended mixing method from the first step was followed. The prepared color was mixed with PLA powder and evaluated. The result showed that the uniformity of colorized powder was also good. It can be seen that the result was similar to when the primary color was experimented. The sintered layer of orange color is compared with the sintered layers of its two primary colors in Figure 7. The measured color values before and after sintering are shown in Table 2. Another method was to mix two different primary colorized powders together instead of mixing two color pigments. The obtained homogeneity of color was not good because pigment had already coated on powders for each color then the mixing of two colored powders could not yield the homogeneity of new color, even though, the uniformity of colorized powder was good. Sintering process could not improve homogeneity since it did not increase the dispensability of pigments at all. Therefore, it was conclude that pigment should be mixed prior to combining with PLA powder. This holds true also for obtaining PLA powder with tertiary colors or others. Two or more colors can be applied in the same layer as shown in Figure 9 shows the possibility of making multicolor layer. The quality of distinct boundary can be obtained when the feed rate of powder and travel speed have to be maintained properly.

7 Figure 7. Red, Orange (5% of Red and 5% of Yellow) and Yellow Sintered Layers Table 2. Averaged L*, a* and b* Values of Secondary s (Pigments were Mixed Before) Before sintering process After sintering process Difference Change of color shade* L* a* b* L* a* b* dl* da* db* Red+Yellow Darker Yellow+Blue Lighter Blue+Red Darker *The change of color shade after sintering was observed by visual inspection dl* 3 2 da* Blue+Red Red+Yellow Yellow+Blue -3 Blue+Red Red+Yellow Yellow+Blue db* 2.5. Blue+Red Red+Yellow Yellow+Blue Figure 8. The Change of L*, a* and b* to Secondary s

8 a. Two ized Powder in the Same Layer b. The Boundary between Two ized Powders Figure 9. Two s in the Same Layer 5. CONCLUSION Two sets of experiments were carried out in this study. The experimental results lead to the following conclusions. First, the pigment should be mixed with loose PLA powder to obtain good color quality on a prototype. In case that the prototype requires colors that are not available. Those colors should be prepared from available color pigments before being mixed with PLA powder. There is a potential for creating a multicolor prototype from colorized PLA powder using SVM technique. However, many more researches need to be done to achieve multicolor layer for accurate representation of color on the prototype. Feeder nozzle should be studied to handle various colorized powders, prepared in advance. If one nozzle will be used, the color contamination must be solved. The contamination will be solved with one nozzle for one color but the number of colors that can be handled for each layer will be limited by the number of nozzle available. Powder flow and heat distribution should be well controlled. ACKNOWLEDGMENT This research is supported by the Royal Thai Government (RTG) research grant under the contract number RTG57. PLA pellets were commercial grade, supplied by Dow Cargill. REFERENCES 1. Abrams, R., Ali, M., Denton, P., Igualada, J.A., Groen, M. and Gschwind, E. (21). Colouring Plastics: Fundamentals and Trends. Plastics Additives and Compounding, July-August: Adams, R. (23). Mastering the Challenges of Colouring Plastics. Focus on Pigments, 7: ASTM. (1982) Annual Book of ASTM Standards Parts 28: Paint Pigments, Resins and Polymers. American Society for Testing and Materials. USA. 4. Drizo, A. (26). Environmental impact of rapid prototyping: an overview of research to date. Rapid Prototyping Journal, 12: Gibson, I. and Ming, L. W. (21). Colour RP. Rapid Prototyping Journal, 7: Gupta, B., Revagade, N. and Hilborn, J. (27). Poly(lactic acid) Fiber: An Overview. Progress in Polymer Science, 32: Im, Y.G., Chung, S.I., Son, J.H., Jung, Y.D., Jo, J.G. and Jeong, H.D. (22). Functional Prototype Development: Inner Visible Multi- Prototype Fabrication Process Using Stereo Lithography. Journal of Materials Processing Technology, :

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