Surface Roughness at the Melt/Gas Transition Sites of Gas-Assist Injection Molded Thermoplastic Composites

Transcription

1 Surface Roughness at the Melt/Gas Transition Sites of Gas-Assist Injection Molded Thermoplastic Composites SHIH-JUNG LIU 1 AND I-TA CHANG Department of Mechanical Engineering Chang Gung University Tao-Yuan 333, Taiwan ABSTRACT: Gas assist injection molding has proven itself a breakthrough technology in molding thermoplastic composites. However, there are still unsolved problems that confound the overall success of this technique. This report was to study the surface roughness phenomenon occurring at the melt/gas transition sites of gas assist injection molded composite parts. The material used was 35% glass-fiber filled Nylon-6 composite. Experiments were carried out on an 80-ton injection molding machine equipped with a pressure-controlled nitrogen-gas injection unit. A spiral mold was used for all experiments. After molding, a roughness meter was used to measure the surface quality at the melt/gas transition sites of the parts. Various processing variables were studied in terms of their influence on the surface roughness of molded composites: melt temperature, mold temperature, melt filling speed, short-shot size, gas pressure, and gas injection delay time. A scanning electronic microscope was also used to characterize the surface roughness phenomenon of molded parts. It was found that the roughness at the transition sites was mainly caused by the fiber exposure of molded composites. Experimental investigation of a gas assist injection molding problem can help our understanding of the formation mechanism of surface roughness at the melt/gas transition sites, so that steps can be taken to improve the surface quality of molded parts. INTRODUCTION GAS ASSIST INJECTION molding [1] has increasingly become an important process in industry, due to its flexibility in design and manufacturing of plastic parts. In gas assist injection molding, the mold cavity is partially filled with the polymer melt followed by the injection of inert gas into the core of the polymer melt. A schematic diagram of gas assist injection molding is illustrated in Figure 1. Gas assist injection molding can produce parts incorporating both thick and thin 1 Author to whom correspondence should be addressed. Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 21, No. 03/ /02/ $10.00/0 DOI / Sage Publications 217

2 218 SHIH-JUNG LIU AND I-TA CHANG Figure 1. Schematic of gas assist injection molding process. sections with less residual stress and warpage and better surface finish. It requires lower clamping force than the conventional injection molding process. Typical applications include (1) tube and rod-like parts, such as clothes hangars, hammer handles and chair armrests, (2) large sheet-like structural parts with a built-in gas channel network, such as automotive panels and business machine housings, and (3) complex parts consisting of both thick and thin sections, such as automotive window guidance channels [2]. The gas assist injection molding process can also enable even more design freedom, material savings, weight production, and cost savings in tooling and press capacity requirements. Despite the advantages associated with the process, the molding window and process control become more critical and difficult since additional processing parameters are involved. These new gas-related processing parameters include the amount of melt injection, gas pressure, delay time of gas injection, and gas injection time. The gas assist injection molding process involves dynamic interaction of two totally different materials (polymer melt and filling nitrogen gas), the product, tooling and mold process design are quite complicated. Previous experience with the conventional injection molding process is no longer sufficient to deal with this process [3 15]. Today, short glass-fiber reinforced thermoplastic composites have the fastest rate of development and represent 30% of the composite market [16 20]. Gas assist injection molding of glass-fiber reinforced composites [21] has the capability of producing parts having thick and thin sections with a good structured rigidity. However, there are still some unsolved problems that confound the overall success of this technology. Parts roughness occurring at the melt/gas transition sites of molded composite surfaces is one of them. Liu and Chang [22] proposed, in their study of float-shaped cavities, that the surface roughness of molded composites is induced by the jetting and irregular flows of the polymer melt during the filling process. The purpose of this report was to study the surface roughness phenomenon occurring at the melt/gas transition sites of molded composite parts. The materials

3 Surface Roughness at Melt/Gas Transition Sites 219 used were short glass-fiber filled Nylon-6 composites. Experiments were carried out on an 80-ton injection molding machine equipped with a high pressure nitrogen-gas injection unit. Various processing variables were studied in terms of their influence on parts surface quality. The final goal of this research is to better understand the formation mechanism of surface roughness at the melt/gas transition sites, so that steps can be taken to irmprove the surface quality of molded parts. EXPERIMENTAL The resins used in this study were commercially available grade 35% E-glass-fiber filled Nylon-6 composites (Ginar Chem., Taiwan) [23]. The fiber in the composites has a diameter of 10 µm and an approximate aspect ratio of 10, as measured by the resin supplier [23]. Table 1 lists the characteristics of the composite material. Gas assist injection molding experiments were conducted on an 80-ton injection molding machine (Taichung Mach. VS-80, Taiwan). A pressure-controlled nitrogen gas injection unit (Gas Injection model PPC-1000, U.K.) was attached to the machine [14]. A spiral cavity with a square cross section was used in this study. Figure 2 shows the dimensions of the cavity. The molding experiments were performed using a modular mold which enables varying the radius of curvature of the part from R =0 to R = 42.5 mm using different mold inserts (Figure 2). The temperature of the mold was regulated by an oil-circulating mold temperature control unit. After molding, the surface roughness at the melt/gas transition sites of the molded composites was measured. A roughness meter (Hommelwerke model T2000, Germany) with a measurable range of ±200 µm was used. The arithmetic average value Ra of filtered roughness profile defined by the following equation is used (ISO 4287/1): R a 1 x= lm = y dx l x = 0 m (1) Table 1. Characteristics of the glass-fiber reinforced nylon composite [23]. Property ASTM 35% G.F. Nylon Density (g/cm 3 ) D Water absorption rate (% within 24 hrs) D Hardness (R-scale) D Flexural strength (MPa) D Flexural modulus (GPa) D Impact strength (J) D Elongation rate (%) D Shrinkage rate (%) D

4 220 SHIH-JUNG LIU AND I-TA CHANG Figure 2. Dimensions of (a) the mold cavity, and (b) the exchangeable inserts (unit: mm).

5 Surface Roughness at Melt/Gas Transition Sites 221 which was determined from deviation about the center line within the evaluation length l m. Various processing variables were studied in terms of their influence on parts surface quality: melt temperature, mold temperature, melt filling speed, short-shot size, gas pressure, and gas injection delay time. Table 2 lists these processing variables as well as the values used in the experiments. RESULTS AND DISCUSSION Gas assist injection molding experiments were conducted with an 80-ton injection molding machine equipped with a high-pressure nitrogen gas injection unit [14]. All molded composites exhibited obvious surface roughness at the melt/gas transition sites of the parts. Various processing variables were studied in terms of their influence on surface roughness of the molded parts. Table 2 lists these processing variables as well as the values used in the experiments. To mold the parts, one arbitrary processing condition was chosen as a reference (the shaded one in Table 2). By changing one of the parameters in each test, we were able to find out the effect of every factor on Processing Parameters Table 2. The processing variables as well as the values used in the experiments. a b c d e f g h Melt Temp. ( C) Mold Temp. ( C) Melt Filling Speed (%) Melt Filling Pressure (MPa) Short- Shot Size (mm) Gas Pressure (bar) Gas Injection Delay Time (sec) Radius of Curvature (mm)

6 222 SHIH-JUNG LIU AND I-TA CHANG the surface roughness of gas assist injection molded composites. Melt Temperature and Mold Temperature The effect of melt temperature on parts surface roughness was investigate. Five temperatures of the polymer melt were set in the gas assist injection molding experiments: 260 C, 265 C, 270 C, 275 C and 280 C. The measured results in Figure 3 shows that part surface roughness decreased with melt temperature. The effect of mold temperature was also studied. Five different mold temperatures were selected from 55 C to 95 C (Table 2). The experimental result in Figure 4 suggests that surface roughness at the transition sites of gas assist injection molded nylon composites decreased with mold temperature. Short-Shot Size The gas assist injection molded composites in the experiments were subjected to different short-shot size melt fillings (screw displacement ranges from 65 to 73 mm). The measured result in Figure 5 does not show an obvious effect of short-shot size on the surface roughness of gas assist injection molded composites. Figure 3. Effect of melt temperature on the surface roughness of gas assist injection molded composites.

7 Surface Roughness at Melt/Gas Transition Sites 223 Figure 4. Effect of mold temperature on the surface roughness of gas assist injection molded composites. Melt Filling Pressure and Melt Filling Speed The effect of melt filling pressure on the parts surface quality was investigated. Five different filling pressures, ranging from 140 MPa to 180 MPa, were set for the experiments. The measured result in Figure 6, like that of short-shot size, did not show any obvious effect on the surface roughness of molded composites. The effect of melt filling speed on the surface quality of molded composites was also studied. Different filling speeds, from 45% to 85% of the maximum available speed of the injection molding machine, were selected. The experimental result in Figure 7 shows that the surface roughness of the parts increased with the melt filling speed up to 75%, and decreased with increased melt filling speed after that. Gas Injection Pressure and Gas Injection Delay Time Parts were molded with different gas injection pressures. Five different gas pressures (40 to 80 bars) were selected to mold the composites. Figure 8 shows the measured relation between the parts quality and the gas injection pressure. The result suggests that one can decrease the surface roughness by increasing the gas injection pressure. The effect of gas injection delay time on the surface roughness was also studied.

8 224 SHIH-JUNG LIU AND I-TA CHANG Figure 5. Effect of short-shot size on the surface roughness of gas assist injection molded composites. Figure 6. Effect of melt filling pressure on the surface roughness of gas assist injection molded composites.

9 Surface Roughness at Melt/Gas Transition Sites 225 Figure 7. Effect of melt filling speed on the surface roughness of gas assist injection molded composites. Figure 8. Effect of gas pressure on the surface roughness of gas assist injection molded composites.

10 226 SHIH-JUNG LIU AND I-TA CHANG The gas delay times selected for the experiments were between 0 to 4 seconds. The measured surface roughness of the molded parts in Figure 9 suggests that one can improve the surface quality of the parts by adopting a short gas injection delay time. Radius of Curvature Parts were molded with different radii (from 0 to 42.5 mm as listed in Table 2) of curvature of the cavity. Figure 10 shows the measured parts quality molded by different radii of curvature. The result suggests that one can decrease the surface roughness by using a mold of larger curvature radius. Table 3 lists the effect of various processing parameters on the surface roughness of gas assist injection molded composite parts, based on the above experimental results. To better understand the roughness phenomenon, molded composites were observed under a scanning electronic microscope (Jeol Model 5410, Japan) [25]. Figure 11 shows the photograph of the composite parts with and without surface roughness. At the melt/gas transition areas with surface roughness (left hand side area in the figure), fibers are clearly seen on the parts surface, while for the areas without roughness (right hand side area in the figure), no fibers are observed on the part surface. It was also observed that the nylon material shrinks more in the rough Figure 9. Effect of gas injection delay time on the surface roughness of gas assist injection molded composites.

11 Surface Roughness at Melt/Gas Transition Sites 227 Figure 10. Effect of the radius of curvature on the surface roughness of gas assist injection molded composites. transition area than in the smooth surface area. In gas assist injection molding, the mold cavity is partially filled with the polymer melt followed by the injection of inert gas into the core of the polymer melt. Due to the gas injection delay time, the material at the flow front cools down somewhat. As the gas is injected, the material at the flow front of the polymer melt is pushed against the mold wall due to a fountain flow effect [2]. As soon as the melt contacts the mold wall it tends to cool. The polymer shrinks much more than the glass fiber [22] and may thus leave the fiber exposed to the parts surface. Parts roughness may thus form at the melt/gas transition surfaces, as can be seen in Fig- Table 3. Effect of processing parameters on the surface roughness of gas assist injection molded composite parts. Processing Parameter Melt temperature ( ) Mold temperature ( ) Gas pressure ( ) Gas injection delay time ( ) Radius of curvature ( ) Surface Roughness

12 228 SHIH-JUNG LIU AND I-TA CHANG Figure 11. Microscopic photograph of part surface with roughness (left hand side area), and without roughness (right hand side area). ure 11. The injected high-pressure gas also pushes the composite against the mold wall [1]. This will minimize the shrinkage of the materials and the consequent parts roughness, except at the melt/gas transition areas which have been cooled and which leave the fiber exposed to the surface of the composites. For the factors selected in the experiments, a higher melt temperature was found to improve the surface quality of the molded composites (Figure 3). Increasing the melt temperature keeps the material hot for a longer time for the gas pressure packing. This minimizes the shrinkage of the polymer [26] as well as the surface roughness. As soon as the melt begins to enter the cavity it starts to cool. In order to minimize the surface roughness, the temperature of the melt must remain high enough for a period sufficient for the gas to pack the composite [14]. This is aided by having a high mold temperature so that the melt will not cool too rapidly (Figure 4). In gas assist injection molding, the gas pressure acts as a holding pressure just as in that of conventional injection molding [2]. One can decrease the surface roughness by increasing the gas pressure, as shown in Figure 8. It is mainly due to the fact that increasing the gas pressure decreases the shrinkage of the nylon matrix [26] as well as the possible exposure of the glass fibers. This will make the final part surface smoother.

13 Surface Roughness at Melt/Gas Transition Sites 229 Figure 12. Gas flow channels of different radii of curvatures. Increasing the gas injection delay time increases the cooling time [14] of the polymer melt (Figure 9). It becomes more difficult for the composites to be packed by the subsequent nitrogen gas. Filling the mold cavity as rapidly as possible [22] should minimize the surface roughness and obtain parts with the best surface quality, as shown in Figure 7. Finally, the surface roughness of molded parts increases as the radius of the curvature of the flow channel is decreased. This is due to the fact that in gas assist injection molding the gas always takes the route of least resistance as shown in Figure 12. As the radius of curvature of the flow channel is decreased, the gas goes along the inner side of the channel. More polymer melt thus accumulates on the outer side of the channel. The shrinkage of the nylon matrix increases due to the melt accumulation. It also becomes more difficult for the material to be packed by the high pressure nitrogen. The surface roughness increases accordingly, as shown in Figure 10. CONCLUSIONS This report has examined the effect of different processing factors on surface roughness at the melt/gas transition sites of gas assist injection molded composites. The following conclusions can be drawn based on the current study. 1. The occurrence of surface roughness at the melt/gas transition sites mainly

14 230 SHIH-JUNG LIU AND I-TA CHANG resulted from the exposure of glass fiber on the surface of gas assist injection molded nylon composites. 2. One can improve the surface quality of a gas assist injection molded composite by increasing the melt and mold temperature, increasing the gas pressures, or shortening the gas injection delay time. 3. One can also decrease the transition roughness by adopting flow channels of a larger radius of curvature. In this study, the mechanism of surface roughness formation at the melt/gas transition sites has been explained and steps can thus be taken to ensure that the roughness can be minimized. This provides significant advantages in improving product quality of gas assist injection molded composites. ACKNOWLEDGEMENT The authors would like to express their gratitude to the National Science Council of Taiwan, R.O.C. for their funding suppport under grant NSC E REFERENCES 1. S. Shah, SPE-ANTEC Tech. Paper, 37, 1494 (1991). 2. L.S. Turng, Adv. Polym. Tech., 14, 1 (1995). 3. S.Y. Yang, F.Z. Huang and W.N. Liau, Polym. Eng. Sci., 36, 2824 (1996). 4. S.Y. Yang, S.J. Liou and W.N. Liou, Adv. Polym. Tech., 16, 175 (1997). 5. A.J. Poslinski, P.R. Oehler and V.K. Stokes, Polym. Eng. Sci., 35, 877 (1995). 6. R.E. Khayat, A. Derduri and L.P. Hebert, J. Non-Newtonian Fluid Mech., 57, 253 (1995). 7. S.C. Chen, K.F. Hsu and K.S. Hsu, Num. Heat Trans., 28, 121 (1995). 8. S.H. Parng and S.Y. Yang, Inter. Polym. Proc., 13, 318 (1998). 9. S.C. Chen, N.T. Cheng and S.Y. Hu, J. Appl. Polym. Sci., 67, 1553 (1998). 10. Y.S. Soh and C.H. Chung, J. Reinf. Plast. Comp., 17, 935 (1998). 11. Y.Y. Nie, L.S. Turng and K.K. Wang, Adv. Polym. Tech., 16, 159 (1997). 12. X. Lu, H.H. Chiang, J. Zhao and S.C. Chen, Polym. Eng. Sci., 39, 62 (1999). 13. G.A.A.V. Haagh, H. Zuidema, F.N. van de Vosse, G.W.M. Peters and H.E.H. Meijer, Inter. Polym. Proc., 12, 207 (1997). 14. S.J. Liu, J.H. Chang, C.Y. Ho and S.W. Hung, Int. Polym. Proc., 14, 191 (1999). 15. S.J. Liu and Y.C. Wu, Int. Polym. Proc., 15, 297 (2000). 16. D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd Ed., Cambridge Solid State Science Series (1996). 17. F.R. Jones, Handbook of Polymer-Fiber Composites, Longman, England (1994). 18. P. Gerald, J. Raine and J. Pabiot, J. Reinf. Plast. Comp., 17, 922 (1998). 19. J. Ko and J.R. Youn, Polymer Composites, 16, 114 (1995). 20. J.P. Greene and J.O. Wilkes, Polym. Eng. Sci., 37, 1019 (1997). 21. L. Averous, J.C. Quantin, A. Crespy and D. Lafon, Polym. Eng. Sci., 37, 329 (1997).

15 Surface Roughness at Melt/Gas Transition Sites S.J. Liu and J.H. Chang, Polym. Comp., 21, 322 (2000). 23. Mapex, Ginar Chem. Co., Taiwan. 24. O. Becker, D. Karsono, K. Koelling and T. Atlan, SPE-ANTEC Tech. Paper, (1997). 25. L.C. Sawyer and D.T. Grubb, Polymer Microscopy, Chapman & Hall, York (1994). 26. I.I. Rubin, Injection Molding Theory and Practice, Wiley, New York (1972).