Morphology Evolution and Elimination Mechanism of Bubble Marks on

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Morphology evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control Guiwei Dong, Guoqun Zhao, Lei Zhang, Junji Hou, Bo Li, and Guilong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04199 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Industrial & Engineering Chemistry Research

Morphology evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control Guiwei Dong, Guoqun Zhao*, Lei Zhang, Junji Hou, Bo Li, Guilong Wang

Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The morphology evolution and elimination mechanism of surface bubble marks on microcellular injection molded parts with dynamic mold temperature control were investigated. It is found that with the increase of mold temperature, the surface bubble marks with morphology of rolling ribbons and uneven gullies formed in low mold temperature gradually evolve into thin and tiny strips, and finally disappear. The evolution of surface bubble marks under high mold temperature undergoes a process from generation to disappearance, that is, the melt firstly forms a surface with originally generated marks, then the bottom melt of the generated marks is continuously pushed out and forms island protuberances, thus gradually dividing and reducing their space and volume, and finally the original marks are eliminated. The escaping from the gap between mold and melt and re-dissolving into the high temperature melt are the main ways of the gas in original surface bubble marks to disappear.

Keywords: microcellular injection molding; dynamic mold temperature control; surface bubble marks; evolution; elimination


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1. Introduction Microcellular injection molding (MIM) is an advanced foaming injection molding technology using thermoplastic as matrix and supercritical fluid (SCF, usually SCF N2 or SCF CO2) as physical blowing agent. MIM technology was first developed by Martini et al.1 and commercialized by Trexel Inc. under the Mucell name. MIM process has a series of technological advantages such as material savings, lower energy consumption, enhancement in dimensional accuracy and shortened molding cycle.2 Due to these advantages, MIM has received extensive attention of plastic processing industry and been successfully used in many areas such as packaging, automobile components, electronic advices and biological tissue engineering scaffolds in recent years.3-7 The main feature of MIM is the combination of polymer microcellular foaming technology with conventional injection molding process by utilizing the rapid and huge pressure drop in the filling stage of injection molding and inducing the gas blowing agent dissolved in polymer melt to reach super saturation and separate out to foam. Compared with the batch foaming, the forming processes of cellular structure in MIM molded parts are very complicated, because the melt foaming behaviors in MIM are not only dependent on polymer material properties, foaming agent properties and foaming agent contents, but also significantly influenced by the melt pressure and melt flow behaviors during the filling stage. In our previous studies,8-10 we have found that, due to the influence of the melt pressure and pressure distribution in the injection filling stage, the cell forming process in MIM contains “foam during filling” and “foam after filling” two processes, and moreover, the bubbles formed in “foam during filling” process near the melt flow front will be stretched, broken, pushed out and turned over to both sides of the melt flow under the action of the shear flow and fountain flow behaviors, then contact with the cold mold cavity surface and are solidified quickly, and finally form silver streaks and swirling bubble marks on the molded part. The surface bubble marks formed on the MIM molded part usually cause a rough surface and a low gloss level, which severely limits the further promotion and application of MIM technology and its products. In order to eliminate the surface bubble marks and improve the surface quality of MIM molded parts, many studies were



conducted and good results of improving the surface quality were achieved. On the basis of two-color injection molding process, Turng et al. developed a hybrid solid-micro-cellular co-injection molding process by regulating and controlling the shot size of conventional injected melt and microcellular injected melt, and a part with foamed interior and bright surface was molded.11 However, this co-injection molding process had high requirements on its molding equipment and a certain difficulty in shot size control. Yoon et al. proposed a surface treatment method by spraying a PEEK polymer thermal insulation film onto the mold to eliminate the swirling surface bubble marks, and made foamed parts with class A surface quality by using this method.12 Similar to the method of Yoon et al., Chen et al., Chen and Li et al., and Lee et al. further extended the insulation layer to composite polymer film (82%PET+18%PC),13 PET layer14 and PTFE layer,15 and also improved the surface quality of the molded parts. In addition to the above methods using thermal insulations, Chen et al. used induction heating and steam heating to heat the mold before melt injecting, and also achieved an obvious surface finish improvement of MIM molded parts, respectively.16, 17 In the above studies, whether spraying a polymer thermal insulation film onto the mold surface or heating the mold surface prior to injection, their essence is to indirectly or directly increase the mold temperature during melt filling stage, reduce the heat transfers between melt and mold surface, slow down the condensation or solidification of the polymer melt, and finally eliminate the surface bubble marks and improve the surface quality of MIM molded parts.18 And in recent years, other technologies were also proposed to solve the surface problem of MIM molded parts. For instance, Lee et al. presented a method by reducing the degree of gas supersaturation,19 Chen et al., Ruiz et al., and Li et al. respectively established a gas counter pressure system,20-23 and Hou et al. proposed a gas-assisted microcellular injection molding process.24 These above efforts also brought good effects on the surface quality of MIM molded parts. But it should be pointed out that, compared with the approach of increasing the mold temperature during the filling stage, these technologies still did not achieve obvious breakthrough development in surface quality improvement of MIM molded parts, but also needed more


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complicated mold structure and auxiliary equipment, even some technologies have a negative effect on the inner cell structure of MIM molded parts. Therefore, reasonably increasing the mold temperature during the filling stage becomes an effective and practical auxiliary processing method for eliminating the surface bubble marks and improving the surface quality of MIM molded parts currently. In the field of traditional injection molding, the methods of producing high quality plastic parts by changing or increasing the mold temperature during the filling stage have been a long time development. Especially in recent years, the dynamic mold temperature control technology (or rapid heat cycle molding technology) with increasing the mold temperature in the injection filling stage as the core strategy has been systematically studied and widely applied in injection molding process. For the rapid thermal cycling of injection molds, Yao et al. gave a detailed explanation and concluded that rapidly heating the mold before injection and rapidly cooling the mold after injection is a fundamental solution for many quality issues resulted by the frozen layer developed during the filling stage.25 For the application of rapid heat cycle molding, Wang et al. developed a variotherm injection molding technology and applied it on the molding of a large LCD panel.26 Xiao et al. constructed a rapid thermal cycling molding with electric heating and water impingement cooling and illustrated the feasibility of this technology in injection molding using a cover plate.27 And recently Santis et al. developed a rapid surface temperature variation system and applied it to a micro-injection molding process.28 The wide application of dynamic mold temperature control technology in injection molding process has showed various effective results in molded product performance, such as elimination of weld line,29,30 improvement of optical properties,31,32 and enhancement of dimensional accuracy.33,34 The above applications and technological advantages of dynamic mold temperature control technology have made it become an important auxiliary process for injection molding, and it has also caused a growing public concern in microcellular injection molding research field. Recently, Xiao et al. developed a rapid thermal cycling molding technology with electric heating and water cooling to eliminate the surface defects of microcellular injection molded parts, and the results showed that the microcellular POM cover



plates with glossy appearance comparable to the solid counterpart could be molded when raising the mold temperature close to or above 150°C.35 Through the above reviewing, it can be known that employing dynamic mold temperature control technology to increase the mold temperature during the filling stage has positive and significant effects on elimination of surface bubble marks and improvement of surface quality for MIM molded parts. The existing studies mainly focused on the process method construction and product quality improvement. But for the influencing relations of dynamic mold temperature control (or rapid heating and cooling) on the surface quality of MIM molded parts, it has not yet formed a deep and unified understanding due to the restriction of the research and experimental conditions. The morphological evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control are still unclearly reveled, and the control strategy of microcellular injection molding process and molded part quality under the dynamic mold temperature control condition are still not completely established. All of the above problems urgently need to be studied systematically and intensively. In this work, an experimental system of microcellular injection molding with dynamic mold temperature control by electrical heating and cooling water cooling was constructed. The morphology evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control were investigated. The influence rules of mold temperature on morphology of surface bubble marks were revealed. The elimination process and mechanism of surface bubble marks with high mold temperature during filling stage were clarified, and the main factors affecting elimination process of surface bubble marks and their affecting actions were also presented.


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2. Experimental Section 2.1 Dynamic mold temperature control system

To achieve the change and control of mold temperature during the injection filling stage, the authors of this paper developed a dynamic mold temperature control system. Figure 1 shows the comparison of mold temperature change process in dynamic mold temperature control technology and conventional injection molding. Compared with the conventional injection molding process, the most important characteristic of the dynamic mold temperature control system is the dynamic control of the mold temperature. Before the melt injection, the mold temperature is firstly heated to a pre-set upper limit. During the melt filling stage, the temperature of the mold cavity surface is kept higher than the upper limit to prevent melt solidifying prematurely. When the melt filling process is ended or in the later of packing stage, the mold temperature is cooled down quickly to a lower limit (the ejection temperature), and then the part is ejected out the mold cavity.

Figure1. Comparison of mold temperature change process in dynamic mold temperature control technology and conventional injection molding.



The dynamic mold temperature control system developed in this paper employs a control way with electric rods heating and cooling water cooling. The structural principle of the system is shown in Figure 2. It can be seen from Figure 2, the developed system consists of five main components: cooling tower, air compressor, valve exchange device, mold temperature control and monitor unit, and the electric heating mold. In the system, the cooling tower is used to supply sufficient cooling water as the cooling medium of the mold; the air compressor is used to produce compressed air with a certain pressure as the driving gas of pneumatic valves and as a medium power to exclude the residual cooling water in the mold after cooling; the valve exchange device is to switch the valves to transfer different mediums from pipelines to the mold; the electric heating mold is used to mold and shape the parts; the function of mold temperature control and monitor unit is to control the heating, cooling and excluding operations of the mold, coordinate each action of microcellular injection molding with dynamic mold temperature control by communicating with the control system of injection molding machine, and monitor or operate the whole process from a touch-screen.

Figure 2. Structural principle of the developed dynamic mold temperature control system. 2.2 Injection machine and mold

Based on the principles of dynamic mold temperature control technology and microcellular injection molding, this paper constructed a dynamic mold temperature control microcellular injection molding system, as shown in


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Figure 3. It mainly consists of injection machine, dynamic mold temperature control system, electric heating microcellular injection mold, and SCF metering and injecting system. The injection machine, modified based on Haitian MA3200, has a clamping force of 3200 kN, the maximum injection pressure of 182 MPa, the screw diameter of 60 mm, the screw length-diameter ratio of 22:1, and theoretical shot volume of 791 cm3 (based on PS). The SCF metering and injecting system, developed by research group of this paper, has a gas injecting pressure range of 10~25 MPa and a mass flow rate of 0~1.1 g/s. The injection machine equipped with a shut-off nozzle, Herzog GN2, and bottled industrial nitrogen (99.99% purity) was used as the physical blowing agent.

Figure 3. Schematic diagram of the dynamic mold temperature control microcellular injection molding system. In order to study the process mechanism of microcellular injection molding with dynamic mold temperature control and the performance of molded parts, this paper designed and manufactured an electric heating microcellular injection mold for injecting specimens and rectangular plates, as shown in Figure 4. The mold has 18 cavities, corresponding to five ASTM D638 standard tensile specimens, five ASTM D5023 standard bend specimens, four ASTM D256 standard impact specimens, and four rectangular plates with different thickness stiffeners. The developed mold assembled 18 electric rods in the stationary mold. The electric rods, MISUMI MCHPA, have a diameter of 5.9 mm and a heating watt density of 25 W/cm2. The cooling water channel has a



diameter of 8 mm and the temperature range of cooling water is 15~30°C. In addition, three pressure sensors were assembled in moveable mold for real-time collecting the melt pressure by using a mold marshalling system of Futaba EPA-002.

Figure 4. Structure of the developed electric heating microcellular injection mold. 2.3 Molded Parts and material

After injection using the above electric heating microcellular injection mold, the molded parts with the shape as shown in Figure 5 can be obtained, where the dimension of the impact specimen is 65mm×12.7mm×3.2mm, the bend specimen is 127mm×13mm×3.2mm, the tensile specimen is 165mm×12.7mm×3.2mm, the bigger


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rectangular plate is 170mm×60mm×4mm, and the smaller rectangular plate is 87mm×60mm×4mm.The material used in the experiments of this paper is acrylonitrile-butadiene-styrene copolymer (ABS), LG Chemical HF380, has a melt flow index of 43 g/min at 230°C and a density of 1.04 g/cm3 at room temperature. Before injection, the material was dried at 80°C for 4 hours to remove moisture.

Figure 5. Shape and dimensions of molded parts.

2.4 Experiments design

To investigate the morphology evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control systematically, after the molding process has run steadily, the microcellular injection molding experiments under mold temperature of 30°C were conducted firstly to obtain the molded parts in conventional low mold temperature (30°C). Then, increasing the mold temperature gradually by using the dynamic mold temperature control system and keeping the other processing conditions unchanged, the molded parts in mold temperatures of 60°C, 90°C, 120°C, 140°C and 150°C were obtained, respectively. Table 1 summarizes the processing conditions for the dynamic mold temperature control microcellular injection molding process in this study. Meanwhile, in order to investigate the effects of dynamic mold temperature control on the whole process from generation, evolution to elimination of the surface bubble marks, a particular experimental case was selected, in which the shot size is the screw injection displacement of 60mm, corresponding to the specimens filled completely and the rectangular plates filled about 50%. The molded part is shown in Figure 6(a).



Table 1. Processing conditions for dynamic mold temperature control microcellular injection molding process. Injection speed (mm/s)

44

Melt temperature (°C)

240

Shot size (mm)

60

SCF percentage (%, weight)

0.4

Cooling time (s)

40

Packing time (s)

0

Mold temperature (°C)

30

60 90

120 140 150

2.5 Characterization and measurement

For the microcellular injection molded parts under dynamic mold temperature control and conventional low mold temperature in this study, the tensile specimens completely filling the same cavity were chosen as the first group of investigation objects. The observation samples with a dimension of 10mm×10mm were cut out in their clamping sections far from gate, as shown in Figure 6(b). The samples were characterized and measured by a Gloss meter (JFL-BZ60), a Scanning electron microscope (JOEL JSM-6610LV), and an Optical profiler (Veeco NT9300). In this way, the changes of surface quality of molded parts and the morphology of bubble marks under different mold temperatures were obtained. Then, the smaller rectangular plates filling the same cavity half were chosen as the second group of investigation objects. The observation samples with a length of 10 mm were cut out in the flow front of the chosen plates, as shown in Figure 6(c). The samples were characterized by a Scanning electron microscope (JOEL JSM-6610LV). The changes of morphology of bubbles on flow front of molded parts with different mold temperatures were obtained. Finally, the bigger rectangular plates filling the mold cavity half under the mold temperature of 140°C were chosen as the third group of investigation objects. The observation samples with a dimension of 10mm×5mmwere cut out from the flow front to the gate with an equal interval of about 6mm, as shown in Figure 6(d). The samples were characterized by a Scanning electron microscope (JOEL


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JSM-6610LV), and the changes of morphology of surface bubble marks and fractured surfaces under mold temperature of 140°C were obtained. All of the SEM samples were fractured in liquid nitrogen, then the fractured surface of every samples was coated with an approximately 10 nm thick layer of gold, and observed with a voltage of 15 kV.



Figure 6. Molded parts and sampling positions.


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3. Results and discussion 3.1 Morphology evolution of surface bubble marks for molded parts with different mold temperatures

The gloss values of the same position in microcellular injection molded parts as shown in Figure 6(b) under different mold temperatures were measured by using the gloss meter. Three measurements were taken for each mold temperature and the average value was taken as the final result. Figure 7 gives the gloss value and changes of microcellular injection molded parts under different mold temperatures. It can be found that, for the molded part in conventional low mold temperature, the gloss value is 0, while the gloss value of molded parts increases gradually with the increase of the mold temperature. When the mold temperature increases to 140°C and above, the gloss values are tending to be stable at 72.7. This indicates that the increase of the mold temperature plays an important role in improving the quality of microcellular injection molded parts. And as we knew that the surface bubble marks are the major cause of low gloss defect for microcellular injection molded parts. While the gradual increase of mold temperature changes the morphology of bubble marks on surface of the microcellular injection molded parts and thus improves the surface quality. Therefore, in the following sections, this paper will address the detailed discussions about the morphology evolution of surface bubble marks under different mold temperatures.

Figure 7. Gloss value changes of microcellular injection molded parts under different mold temperatures.



By taking the samples of microcellular injection molded parts under different mold temperatures in the same position as shown in Figure 6(b) for SEM observation, the morphology changes of surface bubble marks under different mold temperatures were obtained, as shown in Figure 8. It can be seen that, in conventional low mold temperature condition, the microcellular injection molded parts has obvious surface bubble marks, and the surface bubble marks show a thick and big rolling ribbons appearance with a stacked and clustered state. With the increase of mold temperature, these rolling ribbons appearance gradually alleviates. When the mold temperature rises to 90°C, the thick and big ribbons on microcellular injection molded parts have evolved into thin and tiny strips, but their marks on the molded part are still clearly visible. With the further increase of the mold temperature, the thin strips surface bubble marks become smaller and thinner. When the mold temperature reaches to 120°C, a large area of smooth surface begins to appear on the molded part surface, and the surface bubble marks remaining on the molded parts are no longer thin strips, but change to some gas drops with a small length-diameter ratio or small shallow dents. Finally, when the mold temperature increases to 140°C and 150°C, all of the surface bubble marks on microcellular injection molded parts are eliminated and the surface inside the observed area has completely changed to a smooth surface.

Figure 8. Morphology changes of surface bubble marks on microcellular injection molded parts under different mold temperatures.


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Figure 8 illustrates a two-dimensional spatial evolution process of the surface bubble marks on microcellular injection molded parts with the increase of mold temperature. In order to give a comprehensive analysis about the morphology evolution of surface bubble marks with the mold temperature increase, the samples under different mold temperatures, as shown in Figure 6(b), were observed using an Optical profiler, and the changes of 3D topography of surface bubble marks and roughness values of molded parts with different mold temperatures are shown in Figure 9. The results show that the surface bubble marks on microcellular injection molded parts under low mold temperature (30°C) present a topography of uneven gullies in three-dimensional space, and the peaks and valleys undulate terribly. Within the scope of the sampled area, the arithmetical mean deviation of the profile (Ra) and the maximum peak to valley height (Rt) are 994.75 nm and 14.22 µm, respectively. With the increase of mold temperature, the topography of uneven gullies on the molded parts surfaces gradually become flat and smooth. When the mold temperature increases to 120°C, there have been almost no visible uneven gullies on the molded part surface and the Ra is decreased to 258.67 nm. As mold temperature is successively increased to 140°C, the topography of uneven gullies on the molded parts disappears completely and the Ra reaches to the lowest value of 116.04 nm. This indicates that the surface roughness of microcellular injection molded parts Ra is decreased by 88.3% when the mold temperature increases from 30°C to 140°C.

Figure 9. 3D topography evolution of surface bubble marks on microcellular injection molded parts under different mold temperatures.



3.2 Influence of mold temperature on the formation process of surface bubble marks

In our previous studies,8 we have found that the surface bubble marks on microcellular injection molded parts are generated by bubbles formed in “foam during filling” process near the flow front, which are stretched, broken and pushed out by melt shear flow and fountain flow behaviors, contact with the cold mold cavity surface, and finally remain on the product surface. To investigate the influence of mold temperature on the forming process of surface bubble marks, the bubble morphology on melt flow front with different mold temperatures as shown in Figure 6(c) was observed. The results are shown in Figure 10. From the SEM pictures in Figure 10, it can be seen that with the mold temperature increasing from 30°C to 150°C, the bubble morphologies on melt flow front have no obvious change, and all bubbles are stretched and broken in the studied six mold temperature levels. This indicates that increasing the mold temperature during the filling stage almost has no influence on the forming process of surface bubble marks. Similar to the low mold temperature condition, high mold temperature still does not eliminate the surface bubble marks on melt flow front. There are still the obviously stretched and broken bubbles on the melt flow front.

Figure 10. Bubble morphology on melt flow front with different mold temperatures. In addition, it can also be found from Figure 10, that the statuses of the stretched and broken bubbles on the


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melt flow front under different mold temperatures have a slight difference. The higher the mold temperature is, the bigger the broken cells are. In particular, in high mold temperature, there are smaller broken cells appearing at the bottom of the bigger broken cells, which makes the bubble broken pattern more complicated. Analyzing the reasons of the above phenomenon, one is because the high mold temperature slows down the cooling speed of the front melt and thereby accelerates the bubble growing, another is because the increase of mold temperature enhances the front melt flow ability, aggravates the shear flow and fountain flow behaviors, and enlarges the bubble distortion. The comprehensive effect of these two aspects leads to the above phenomenon. Combining the above analyses of the morphology evolution of surface bubble marks under different mold temperatures shown in Figure 8 and 9, and the influence of mold temperature on the formation process of surface bubble marks shown in Figure 10, it can be concluded that the increase of mold temperature can eliminate the surface bubble marks of microcellular injection molded parts, but this elimination process does not mean that the high mold temperature avoids the generation of the stretched and broken bubbles on melt flow front during the filling, conversely, the stretched and broken bubbles on melt flow front are still generated under high mold temperature, and the bubble broken pattern is more complicated than that under low mold temperature condition. Therefore, this paper proposes that the elimination of surface bubble marks of microcellular injection molded parts under high mold temperature during the filling stage undergoes a process from generation to disappearance. In this process, under high mold temperature, the molded microcellular part first generates surface bubble marks similar to the low mold temperature condition, then as the melt filling process continues, the generated surface bubble marks gradually disappear under the influence of high mold temperature, and finally a microcellular part with smooth and glossy surface is molded.

3.3 Generation and elimination of surface bubble marks under high mold temperature

In order to further reveal the elimination process from generation to disappearance of surface bubble marks under high mold temperature, the morphologies of surface bubble marks in the different positions from the flow



front to the gate in the same molded part with the mold temperature of 140°C were examined. The results are shown in Figure 11.

Figure 11. Morphologies of surface bubble marks in the different positions from flow front to gate in the same molded part with the mold temperature of 140°C. By comparing the morphologies of surface bubble marks in different positions of the molded part shown in Figure 11, it can be seen that, under the mold temperature of 140°C, there are a lot of surface bubble marks in position 1 near the flow front, and the morphology of the surface bubble marks is basically the same as that of the conventional molded microcellular parts with low mold temperature. Both of them have thick and big rolling ribbons appearance. While, with the melt filling process continues, the surface bubble marks in the positions away from the flow front gradually become flattened, more and more island protuberances are ejected to contact with the mold cavity surface at the bottom of the originally generated big stretched and broken bubbles, the big bubble


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marks are divided into the smaller ones. Meanwhile, the volumes of surface bubble marks remaining on the surface become smaller and smaller, and their distribution also become more and more uniform, as shown in position 4 and 5. Finally, in the position 7 near the gate, the originally generated surface bubble marks almost disappear except only few wispy marks remaining on the surface, and a large area of smooth surface has already formed. Therefore, it can be believed that the surface bubble marks are eliminated in this position. From Figure 11(i, ii), it can also be seen that the gradually ejected island protuberances of bottom melt in the surface bubble marks, as well as the continuous reduction and homogenization of remained surface bubble marks’ volume are the major ways of the elimination of surface bubble marks. The authors consider that the achievement and implementation of the above elimination ways need two supporting conditions. One is that the skin melt with surface bubble marks has good flow ability. Another is that the pressure of internal melt where surface bubble marks locate is big enough to eject the bottom melt to laminate the mold cavity surface. And here the employment of a high mold temperature during the filling stage just makes the two supporting conditions possible. Under high mold temperature condition, especially for the mold temperature higher than the glass transition temperature of amorphous polymers or the crystallization temperature of crystalline polymer, when polymer melt contacts with the mold cavity surface, the thermal transmission between them are relatively slow, and the temperature of skin melt is usually higher than the above two temperatures. So the skin melt does not solidify and has favorable flow ability during the filling stage. In order to study the melt pressure variation in microcellular injection molding with high mold temperature, Figure 12 gives the melt pressure distribution in melt filling stage and the melt pressure history recorded by pressure sensors in microcellular injection molding under mold temperature of 120°C. It can be found that, the melt pressure during the filling stage has an approximately linear relationship with the distance away from melt flow front. The farther away from flow front, the bigger the melt pressure is, as shown in Figure 12(a). In addition, the melt pressures after melt filling in microcellular injection molding are not zero. This indicates that the melt foaming and the bubble growing have a pressurized function to



melt self, as shown in Figure 12(b). Based on the existing researches, it is known that the melt foaming occurs along with the melt filing process. Therefore, during the melt filling stage, when the melt pressure and the bubble growing pressure accumulate to a certain extent, they will eject the bottom melt of surface bubble marks to contact with the mold cavity surface and eliminate the bubble marks.

Figure 12. Melt pressure distribution in melt filling stage and the melt pressure history in the positions of pressure sensors with mold temperature of 120°C: (a) melt pressure distribution; (b) melt pressure history.

3.4 Elimination mechanism of surface bubble marks under high mold temperature condition

Based on the above analysis and discussion, the authors proposed the elimination mechanism of bubble marks on surface of microcellular injection molded parts under high mold temperature condition, as shown in Figure 13. And it can be summarized as following: At first, in the melt flow front during the melt filling stage of microcellular injection molding, subject to the influence of the melt shear flow and fountain flow behaviors, the


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bubbles formed in “foam during filling” process are stretched, broken, and turned into long fractured bubbles out of the surface of melt flow front. The long fractured bubbles contact with the mold cavity surface, and a surface of molded part with original surface bubble marks is formed, as shown in Figure 13(a). Then, due to the high mold temperature, the formed surface with the original surface bubble marks will not solidify immediately, and the melt of surface skin still has good flow ability. With the melt filling process continuing and the melt flow front moving forward, the bottom melt of the original surface bubble marks is gradually ejected out to contact with the mold surface under the combined effect of increasing melt pressure and bubble growing pressure and form the island protuberances which continuously divide and reduce the space and volume of original surface bubble marks, as shown in Figure 13(b). Finally, the formed island protuberances increase more and more, the space and volume of the original surface bubble marks become less and less, and at the end the original surface bubble marks are eliminated and the part surface becomes a flat and smooth one, as shown in Figure 13(c).

Figure 13. Schematic diagram of elimination process of surface bubble marks: (a) originally generated surface bubble marks; (b) island protuberances; (c) original surface bubble marks are eliminated.



With regard to the disappearing ways of gas inside the original surface bubble marks, this paper considers that the main way is that the gas escapes to the air from the gap between mold and melt surface. This is because there inevitably exist more or less gaps between mold and melt surface and the diffusion coefficient of the gas itself is also very big, so it is easy for the gas escaping to the air in the process of island protuberances forming and dividing the original surface bubble marks. As for few of gas which cannot escape to the air due to no gaps existing around them, under the circumstance of surface bubble marks gradually reducing and the gas pressure gradually increasing, it re-dissolves into the high temperature melt again. In order to further verify the proposed elimination mechanism and the gas disappearing ways, the cross-section morphologies of samples, as shown in Figure 6(d) 1-7, in the molded part with the mold temperature of 140°C were examined again, and the SEM photos near the skin layer are shown in Figure 14. At the beginning of the melt contacting the mold surface, the surface has obvious bubble marks originally generated. They have of clear and visible uneven gullies morphology, and the unfoamed skin layer is not formed at this time, as shown in Figure 14 (1-3). As the melt filling process continuing, the uneven gullies of the original surface bubble marks are quickly filled, and most of the surface contacts with the mold surface. Meanwhile, the bubbles near the skin are gradually compressed to smaller ones and the skin layer is formed partly, as shown in Figure 14(4-6). In the position 7 near the gate, it can be found that there are no fluctuations on the surface of molded part, and it evolved into a completely flat surface. This indicates that the surface bubble marks are eliminated totally, there are no cells existing in the skin region, and a distinct unfoamed skin layer is formed. The above results not only verify the elimination mechanism proposed in this paper being in good agreement with practical situation, but also demonstrate the bubbles re-dissolving into melt again under the pressure function.


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Figure 14. SEM photos near the skin layer of cross-section surface morphologies of samples (as shown in Fig. 6(d) 1-7) of the molded part with the mold temperature of 140°C. On the basis of the above study about the effect of dynamic mold temperature control on surface quality of microcellular molded parts, the influence of mold temperature on the internal cellular structure of molded parts was also examined in this work. Figure 15 shows the morphology changes of cellular structure under mold temperature of 60℃, 90℃ and 140℃. It can be seen that with the increase of mold temperature, the cell size increases, especially in high mold temperature of 140℃, there are many large cells and the diameters of them increase to more than 200 µm. Meanwhile, the cell density shows an opposite pattern with the increase of mold temperature, the higher the mold temperature is, the smaller the cell density is. And more notably, when the mold temperature increases, the big cells and small cells gradually gather up and lead to an increase of cell diameter dispersion.



Figure 15. Morphology changes of internal cellular structure of microcellular injection molded parts under different mold temperatures And finally, the effect of mold temperature via dynamic mold temperature control on mechanical properties of molded parts was studied. The results showed that the tensile strength, flexural strength of molded specimens without weld line changes a little with the increase of mold temperature, while the tensile strength, flexural strength of molded specimens with weld line increases obviously with the increase of mold temperature. It was found that in the mold temperature of 60℃, the tensile strength and flexural strength of molded specimens with weld line are 14.29 MPa and 14.95 MPa, and when the mold temperature increases to 140℃, the tensile strength and flexural strength of the same specimens with weld line increase to 26.27 MPa and 56.44 MPa, respectively. And for the notched and unnotched impact strength of molded specimens, they both show a slightly decrease trend with the mold temperature increases.

4. Conclusions This paper systematically investigated the morphology evolution and elimination mechanism of bubble marks on surface of microcellular injection molded parts with dynamic mold temperature control. The following conclusions were drawn:


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(1) Increasing mold temperature can effectively improve the surface gloss and eliminate the surface bubble marks of microcellular injection molded parts. With the mold temperature increasing, the surface bubble marks with morphology of rolling ribbons and uneven gullies formed in conventional low mold temperature condition gradually evolve into thin and tiny strips, and eventually disappear. Under the processing conditions studied in this paper, when the mold temperature increases to 140°C, the surface of microcellular injection molded part has changed into a smooth surface basically. (2) The microcellular injection molded parts under high mold temperature have a similar surface bubble marks formation process to the conventional molded ones under low mold temperature. But the elimination of surface bubble marks on microcellular injection molded parts with high mold temperature during the filling stage undergoes a process from generation to disappearance. The molded microcellular part first generates surface bubble marks on melt flow front, and then the generated surface bubble marks disappear at the position away from melt flow front. (3) The elimination mechanism of surface bubble marks on microcellular injection molded parts under high mold temperature condition is as follow: Firstly, on the melt flow front during the melt filling, a part surface with surface bubble marks originally generated is formed. Then, as the melt filling process continues, the bottom melt of the original surface bubble marks is gradually ejected out due to the good flow ability of polymer melt under high mold temperature and the increasing melt pressure and bubble growing pressure, and contacts with the mold cavity surface to form island protuberances. The protuberances continuously divide and reduce the space and volume of original surface bubble marks. Finally the original surface bubble marks are eliminated, and the surface of molded part becomes a flat and smooth one. The escaping from the gaps between mold and melt surface and re-dissolving into the high temperature melt are the main ways of the gas in original surface bubble marks to disappear. (4)In practical production, through dynamically controlling the mold temperature during the microcellular



injection, enhancing the flow ability of the melt, and reasonably regulating the melt pressure distribution, the surface bubble marks of the microcellular injection molded parts can be eliminated effectively.

Author Information Corresponding author * E-mail address: [email protected] (Guoqun Zhao). Notes The authors declare no competing financial interest.

Acknowledgements The authors would like to acknowledge the financial support from the Climbing Program for Taishan Scholars of Shandong Province of China (TSPD20110804), Research Award Fund for Shandong Province Excellent Innovation Team (No. 2012-136), National Natural Science Foundation of China (No.51675308), and the project was supported by State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology(No. P2018-002).

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List of figures Figure1. Comparison of mold temperature change process in dynamic mold temperature control technology and conventional injection molding. Figure 2. Structural principle of the developed dynamic mold temperature control system. Figure 3. Schematic diagram of the dynamic mold temperature control microcellular injection molding system. Figure 4. Structure of the developed electric heating microcellular injection mold. Figure 5. Shape and dimensions of molded parts. Figure 6. Molded parts and sampling positions. Figure 7. Gloss value changes of microcellular injection molded parts under different mold temperatures. Figure 8. Morphology changes of surface bubble marks on microcellular injection molded parts under different mold temperatures. Figure 9. 3D topography evolution of surface bubble marks on microcellular injection molded parts under different mold temperatures. Figure 10. Bubble morphology on melt flow front with different mold temperatures. Figure 11. Morphologies of surface bubble marks in the different positions from flow front to gate in the same molded part with the mold temperature of 140°C. Figure 12. Melt pressure distribution in melt filling stage and the melt pressure history in the positions of pressure sensors with mold temperature of 120°C: (a) melt pressure distribution; (b) melt pressure history. Figure 13. Schematic diagram of elimination process of surface bubble marks: (a) originally generated surface bubble marks; (b) island protuberances; (c) original surface bubble marks are eliminated. Figure 14. SEM photos near the skin layer of cross-section surface morphologies of samples (as shown in Fig. 6(d) 1-7) of the molded part with the mold temperature of 140°C. Figure 15. Morphology changes of internal cellular structure of microcellular injection molded parts under different mold temperatures.


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Figure1. Comparison of mold temperature change process in dynamic mold temperature control technology and conventional injection molding.



Figure 2. Structural principle of the developed dynamic mold temperature control system.


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Figure 3. Schematic diagram of the dynamic mold temperature control microcellular injection molding system.



Figure 4. Structure of the developed electric heating microcellular injection mold.


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Figure 5. Shape and dimensions of molded parts.



Figure 6. Molded parts and sampling positions.


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Figure 7. Gloss value changes of microcellular injection molded parts under different mold temperatures.



Figure 8. Morphology changes of surface bubble marks on microcellular injection molded parts under different mold temperatures.


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Figure 9. 3D topography evolution of surface bubble marks on microcellular injection molded parts under different mold temperatures.



Figure 10. Bubble morphology on melt flow front with different mold temperatures.


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Figure 11. Morphologies of surface bubble marks in the different positions from flow front to gate in the same molded part with the mold temperature of 140°C.



Figure 12. Melt pressure distribution in melt filling stage and the melt pressure history in the positions of pressure sensors with mold temperature of 120°C: (a) melt pressure distribution; (b) melt pressure history.


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Figure 13. Schematic diagram of elimination process of surface bubble marks: (a) originally generated surface bubble marks; (b) island protuberances; (c) original surface bubble marks are eliminated.



Figure 14. SEM photos near the skin layer of cross-section surface morphologies of samples (as shown in Fig. 6(d) 1-7) of the molded part with the mold temperature of 140°C.


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Figure 15. Morphology changes of internal cellular structure of microcellular injection molded parts under different mold temperatures.



List of tables Table 1. Processing conditions for dynamic mold temperature control microcellular injection molding process. Injection speed (mm/s)

44

Melt temperature (°C)

240

Shot size (mm)

60

SCF percentage (%, weight)

0.4

Cooling time (s)

40

Packing time (s)

0

Mold temperature (°C)

30

60 90

120 140 150


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84x47mm (300 x 300 DPI)


Morphology Evolution and Elimination Mechanism of Bubble Marks on

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