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PP sandwich injection moldings
Polymer Testing 24 (2005) 1062–1070 www.elsevier.com/locate/polytest
Material Properties
Effect of molding parameters on the properties of PP/PP sandwich injection moldings T. Nagaokaa, U.S. Ishiakua,*, T. Tomarib, H. Hamadaa, S. Takashimaa a
Advanced Fibro Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b Osaka Municipal Technical Research Institute, Osaka, Japan Received 25 February 2005; accepted 18 April 2005
Abstract The basic characteristics of a sandwich injection molded product depend on the properties of the respective resins that comprise the skin and core layers, and the skin/core resin volume ratio. The characteristics of the core layer resin and the skin/core ratio in particular may vary depending on the injection molding conditions. This report considers the influences that the molding conditions such as injection speed, cylinder temperature, and mold temperature confer on the mechanical properties of the sandwich moldings. The study employed, skin/core resin combinations involving similar and dissimilar materials i.e. homopolymer PP/homopolymer PP and homopolymer PP/copolymer PP, respectively. It was demonstrated that core cylinder temperature and mold temperature could be used to adjust the mechanical properties of sandwich injection moldings. In the case of single material sandwich moldings, injection speed seemed to play no significant role, even though it was clearly demonstrated that core volume increases with injection speed. However, core injection speed plays a significant role in the dual material system by lowering or increasing the mechanical strength of moldings as the case may be. Thus, the dormant or active role of injection speed depending on the material system has been highlighted. q 2005 Published by Elsevier Ltd. Keywords: Polypropylene; Sandwich injection molding; Molding parameters; Mechanical properties
1. Introduction It is known that the characteristics of a product molded by sandwich injection molding vary depending on the combination and volume ratio of the skin and core layer materials [1–11]. Watenabe et al. [12,13] investigated the flow behavior during sequential and simultaneous sandwich injection molding and established the patterns of flow as well as processing conditions that lead to breakthrough phenomena. Selective combination of properties offered by this technique range from environment friendly production
* Corresponding author. Tel.: C81 75 724 7844; fax: C81 75 724 7800. E-mail address: [email protected] (U.S. Ishiaku).
0142-9418/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2005.04.003
and cost saving to aesthetics and combinations of engineering properties. Typical material combinations and applications include; (i) Soft or unfilled skin/hard or foamed core for automotive door handles, gear lever and body panels [14], (ii) Unfilled skin/core with conductive filler used in EMI shielding for computer housings [15], (iii) Virgin skin/ recycled core used in garden furniture, automotive bumpers and fascias [16,17], (iv) In-mold paint or pigmented skin/ variable or uncolored core to eliminate finishing of product or reduce pigments costs and aesthetics in wheel trims etc. Recent studies have been exploring ways of expanding the range of application into single material co-injection molding using small injection molding machines in order to increase the volume of molded products, or single material co-injection molding for reduction of the plasticization time and recycling. There are also proposals for development of co-injection molding using similar materials for cost reduction or control of molded product characteristics.
2. Experimental
20
The scope and range of applications of sandwich injection molding would experience rapid expansion if molding condition manipulation through combination of the same or similar materials opens the way to control the characteristics of sandwich molded products. The feasibility of such expansion of the technique was in this preliminary investigation using multi-purpose resins i.e. polypropylene (PP) in order to investigate the influences that manipulation of the molding parameters such as core resin temperature, core injection speed, and mold temperature could have on the mechanical properties of sandwich molded products involving single or dual material systems. Investigation involving sandwich molding of a single material should generate interest as this scenario arises when recycled material is used as the core. It is also necessary to be able to anticipate the properties of the finished products based on processing parameters in cases involving the injection of dissimilar materials.
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T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
70 Thickness = 3 (Unit: mm)
Fig. 1. Specimen dimensions.
is injected initially, followed by a specified amount of the core material. Table 2 shows the combinations of molding conditions that were used to investigate the effect of molding parameters on mechanical properties. The skin material temperature was kept constant at 230 8C, while the effect of varying the core material temperature at three levels was investigated. The effects of mold temperature and core material injection speed were also investigated. The homopolymer grade used as skin material was clear while the copolymer was whitish.
2.1. Materials 2.3. Testing methods and apparatus The materials used in this study were homopolymer PP and copolymer PP produced by Nihon Polypropylene Co. Ltd. The two grades of polypropylene were tagged as PP1 and PP2, respectively. PP1 was used as the skin material, while the same material (PP1) and PP2 were alternated as core materials under varying molding conditions. Table 1 shows the typical properties of these materials. Based on the MFR values of the materials, it can be judged that this pair presents an ideal material combination for sandwich injection as the viscosities of these materials were close enough to yield an intermediate viscosity ratio with the lower viscosity material (PP1) being the skin material [2,9].
Tensile tests were carried out by using an Instron Universal Testing Machine (Instron Co. Ltd Type 4466). The testing speed was 10 mm/min and gauge length was 115 mm in accordance with ISO-527. Three point bending tests were also conducted on the Instron Machine at a nominal test speed of 2 mm/min, in accordance with ISO178. A polarizing microscope (OLYMPUS BX50) was used to study thin slices of cross sections of specimens. 3. Results 3.1. Properties of components
2.2. Injection molding A co-injection molding machine manufactured by Nissei Plastics Industrial Co. Ltd (FN 1000) was used for molding. This machine has a mold clamping force of 800 kN, and is equipped with a skin material injection unit having a screw diameter of 30 mm and a core material injection unit having a screw diameter of 26 mm. Fig. 1 shows the tensile testing dumbbell mold used in the experiment. For sandwich molding, the sequential injection method was employed in which the skin material Table 1 Materials specification Specimen section
Material
Melt flow rate (g/10 min)
Skin Core
PP (Homo-polymer): FC7Y PP (Block Co-polymer): BC8
2.4 1.8
Fig. 2 shows the relationship between mechanical properties (tensile and flexural) for PP1 and PP2 as a function of cylinder temperature for normal injection (i.e. single shot) with the mold temperature and the injection speed fixed at 40 8C and 70%, respectively. It can be seen Table 2 Molding parameters used Mold temperature (8C) Cylinder temperature (8C) Skin Core Injection speed Set (%) Measured (mm/s)
40, 60, 80
230 180, 230, 270 30 62
50 72
70 83
99 100
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
50
1.5
30
PP1 PP2
80 2
40
3
90
2.5
1
PP1 PP2
2.5
70 2
60 50
1.5
40
Flexural modulus (GPa)
PP1 PP2 PP1 PP2
Flexural strength (MPa)
Tensile strength (MPa)
60
Tensile modulus (GPa)
1064
1 30
20 150
170
190
210
230
250
270
20 150
0.5 290
170
190
210
230
250
270
0.5 290
Cylinder temperature (°C)
Cylinder temperature (°C)
Fig. 2. Relationship between mechanical properties and molding conditions for PP materials for normal injection.
that the mechanical properties of PP1 and PP2 deteriorated as the cylinder temperature increased. Cylinder temperature seems to exert a greater influence on the bending strength than on the tensile strength, as indicated by the greater rate of deterioration with increasing cylinder temperature. While Left Min
the tensile strength decreased only by approximately 10%, the bending strength decreased by as much as approximately 20%. A comparison of the experimental values with those supplied by the manufacturer (Table 1) reveals that tensile Upper
Right Max
Core
Max
Min
Skin Lower Unit: mm Left
Injection speed (%)
16-1
Right
Max
2.15
Upper
0.41
Max
1.8
Mi n
1.4
Lower
0.09
Mi n
1.8
Left
16-2
Right
Max
1.63
Upper
0.25
Max
1.80
Mi n
1.20
Lower
0.17
Mi n
1.42
Left
16-3
Right
Max
1.67
Upper
0.25
Max
1.55
Mi n
0.87
Lower
0.32
Mi n
0.93
Left
16-4
Right
Max
1.55
Upper
0.20
Max
1.5
Mi n
0.57
Lower
0.20
Mi n
0.883
Fig. 3. Cross section of core area of PP1/PP2 as a function of injection speed for mold temperature 40 8C and cylinder temperature 230 8C.
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
strength values are within the same range, while discrepancies are noted in the case of the bending test.
3.2. Effect of cross-sectional area ratio Cross sectional area ratio of skin/core was determined at the mid-point of the dumbbell specimen (Fig. 1). Fig. 3 shows micrographs of the cross sectional area as a function of injection speed with a constant 230 8C core cylinder temperatures and 40 8C mold temperature of. The dual material system was used. Fig. 4(a) elaborates the relationship between the crosssectional area ratio of the core layer (synonymous to core volume ratio) and the injection speed in a molding process, where PP1 was the skin and PP2 was the core layer, with the skin material cylinder temperature kept constant while the core material cylinder temperature was varied. At constant resin temperature, the cross-sectional area ratio of the core
1065
layer increases as the injection speed increases. This relationship between the cross-sectional area ratio of the core layer and the injection speed has been recognized in previous reports on molding processes using resins other than PP. The same holds true with molding processes using PP resins [18]. However, the effect of core cylinder temperature on the cross sectional area of the core material is variable depending on the injection speed as shown in Fig. 4(b). At low injection speed (30%) it increased slightly from 180 to 230 8C and then diminished at 270 8C. At intermediate injection speed (50%) the cross sectional area seemed not to be affected by the core cylinder temperature, while at high injection rates the (70 and 99%) the core cross sectional are decreased from 180 to 230 8C and then increased at 270 8C. Thus, it could be inferred that temperature has minimal influence while core cross sectional area is controlled by the injection rate.
(a) 42 60
40
Core cylinder temperature 20
180° C
Tensile strength (MPa)
Core cross section area (%)
(a) 80 Core cylinder temperature 180 oC 180° C
41
230° C 270 oC 270° C
40
39
230° C 270° C 0 0
20
40
60
80
38 30
100
Injection speed (%)
70
99
(b)
(b) 80
80 60
40 Injection speed 30% 20
50%
Flexural strength (MPa)
Core cross section area (%)
50
Injection speed (%)
76 Core cylinder temperature
72
180° C 230° C
68
70%
270° C
99% 0 150
64 200
250
300
Core cylinder temperature (°C) Fig. 4. Relationship between cross-section area ratio of core layer of PP1/PP2 sandwich injection molding; (a) effect of injection speed and (b) effect of cylinder temperature.
30
50
70
99
Injection speed(%) Fig. 5. PP1/PP1 sandwich injection molding at 40 8C mold temperature; (a) effect of injection speed on tensile strength and (b) relationship between flexural strength and injection speed.
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T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
Table 3 Relationship between tensile properties and core cylinder temperature Mold temperature (8C)
Core cylinder temperature (8C)
Tensile strength (MPa)
Tensile modulus (GPa)
Injection speed (%)
Injection speed (%)
30
50
70
99
30
50
70
99
60
180 230 270 180 230 270
40.54 39.33 39.16 39.06 39.88 39.27
39.81 39.83 39.15 39.38 39.76 38.9
39.65 39.77 39.35 39.01 39.98 39.23
39.79 39.87 39.43 39.33 39.52 38.96
2.33 2.18 1.95 2.28 2.38 2.07
2.31 2.18 2.04 2.27 2.24 2.07
2.3 2.35 2.03 2.25 2.28 2.03
2.28 2.34 2.14 2.36 2.22 2.11
3.3.1. Effect of injection speed of core material Fig. 5(a) shows the relationship between the tensile strength and the injection speed at mold temperature 40 8C with the skin material temperature kept constant at 230 8C, while the variations at 60 and 80 8C mold temperatures are presented in Table 3. From Fig. 5(a), and Table 3, different patterns emerged at different mold temperatures. At low mold temperature (40 8C), the core temperature of 180 8C gave moldings of lower mechanical properties than those at higher core temperatures, whereas such a clear distinction is not indicated at 60 8C mold temperature. However, at 80 8C mold temperature the intermediate core material temperature of 230 8C is favored. At all the mold temperatures, injection speed does not seem to assert much influence on mechanical properties. It is clearly demonstrated here that processing parameters play a significant role in determining the mechanical properties of sandwich injection moldings. Notably, core cylinder and mold temperatures could be synchronized to yield optimum mechanical properties even for a situation whereby the skin and core are comprised of the same material. It is well established that the temperature gradient between the skin and core plays a greater role in controlling morphology and hence mechanical properties [19–21]. Contrary to tensile strength, bending strength seemed to degenerate with increase in core cylinder temperature (Fig. 5(b)) while injection rate was not seen to play any significant role. 3.3.2. Effect of core material cylinder temperature Fig. 6(a) shows the relationship between the tensile strength and the core material cylinder temperature with core injection speed being a variable. It can be seen that an increase in the core material cylinder temperature generally results in an increase of tensile strength with the injection speed being not so significant. In contrast to
single shot molding (normal injection), the lower core cylinder temperature i.e. 180 8C molding showed superior strength with much poorer strength at 270 8C core temperature, as shown in Fig. 2. This highlights the advantages of sandwich injection at high molding temperatures to enhance mechanical properties.
(a) 40 Tensile strength (MPa)
3.3. Sandwich molding of single material: PP1/PP1 skin/core
38
36
Injection speed 30% 50% 70% 99%
34 180
230
270
Core cylinder temperature (°C) (b) 80
Flexural strength (MPa)
80
76
72 Injection speed 30% 50% 70% 99%
68
64 180
230
270
Core cylinder temperature (°C) Fig. 6. PP1/PP1 sandwich injection molding at 40 8C mold temperature; (a) relationship between tensile strength and core cylinder temperature and (b) relationship between flexural strength and core cylinder temperature.
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
Fig. 6(b) shows the relationship between flexural strength and the core material cylinder temperature while the injection speed was varied. Bending strength decreases as the cylinder temperature rises. However, sandwich structure products show higher mechanical properties than single shot (PP1) molded products. This suggests that the internal structure of the molded product changed somehow because of the difference in injection temperature between the skin and core layer materials. Moreover, the lower the cylinder temperature, the smaller
Tensile strength (MPa)
(a) 41
Injection speed 30% 50% 70% 99%
39
37 40
60
the difference in bending strength between the single shot molded and sandwich molded products. This suggests that manipulation of the skin and core material resin temperatures can improve the bending strengths of molded products. 3.3.3. Effect of mold temperature Fig. 7 shows the relationship between tensile strength and the mold temperature. At low core material cylinder temperature (Fig. 7(a)), tensile strength generally increases with increasing mold temperature. When core material cylinder temperature is at 230 8C an increase in tensile strength with increasing mold temperature was observed. However, at 270 8C core cylinder temperature, the opposite tendency was observed whereby tensile strength decreased with increasing mold temperature (Fig. 7(c)). From the observation above, it has been demonstrated that it is possible to alter the mechanical properties of single resin sandwich moldings by manipulating the processing parameters.
(a)
80
Mold temperature (°C)
39 Injection speed
Tensile strength (MPa)
Tensile strength (MPa)
Mold temperature 34
(b) 41
40° C 60° C 80° C
32
30 Core cylinder temperature 230° C
30% 50% 28
70%
40
60
50
70
99
Injection speed (%)
80
(b)
Mold temperature (°C)
71
Flexural strength (MPa)
(c) 41
39 Injection speed 30% 50% 70% 99% 37
30
99%
37
Tensile strength (MPa)
1067
40
69
67 Core cylinder temperature 230 °C and mold temperature 40 °C
65 60
80
Mold temperature (°C) Fig. 7. Relationship between tensile strength and mold temperature of PP1/PP1 sandwich injection molding at different core cylinder temperatures; (a) 180 8C, (b) 230 8C, and (c) 270 8C.
30
50
70
99
Injection speed (%) Fig. 8. Relationship between mechanical properties and injection speed of PP1/PP2 injection molding at core cylinder temperature 230 8C; (a) tensile strength and (b) flexural strength.
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3.4. Sandwich molding of dual material: PP1/PP2 (skin/core) 3.4.1. Relationship between injection speed and mechanical strength Fig. 8(a) shows the relationship between the tensile strengths and injection speed with the mold temperatures being varied (at a skin and core cylinder temperature of 230 8C). This figure reveals that tensile strength decreases as the injection speed increases. This phenomenon is attributable to the increase in the cross-sectional area ratio of the core layer in response to the increase in the injection speed (Fig. 4(a)) as well as to the mechanical properties of the PP2 material being lower than that of the PP1 material. Fig. 8(b) shows the relationship between bending strength and injection speed at 230 8C skin and core material temperature and 40 8C mold temperature. Similar to the tensile strength, the bending strength tends to decrease as the injection speed increases.
Tensile strength (MPa)
(a) 33
31 Injection speed 29
27
30% 50% 70% 99% 180
230
270
Core cylinder temperature (°C)
Flexural strength (MPa)
(b) 71
69
Injection speed 67
30% 50% 70% 99%
65 180
230
270
Core cylinder temperature (°C) Fig. 9. Relationship between mechanical properties and core cylinder temperature of PP1/PP2 sandwich injection molding at 40 8C mold temperature; (a) tensile strength and (b) flexural strength.
3.4.2. Relation between cylinder temperature and strength properties Fig. 9(a) shows the relationship between the tensile strength and core cylinder temperature at different injection speeds. It is clear from this figure that the rise in cylinder temperature results in lower tensile strength. This is attributable to the similar tendencies that PP1 and PP2 resins have as single materials, as shown in Fig. 2. Fig. 9(b) shows the relationship between bending strength and cylinder temperature at different injection speeds. The variations of bending strengths are large at lower cylinder temperature, but less significant at higher cylinder temperature. Bending strength decreases as cylinder temperature rises at lower injection speeds. However, the strength increases as the cylinder temperature rises at higher injection speeds. The sequential sandwich injection molding process involves the initial injection of the skin material followed by the injection of the core material. In the previous studies by the authors, the sequence of events that lead to the formation of suitable sandwich moldings were discussed [12,13]. Here, it was noticed that differences in mechanical properties were brought about by differences in core material temperature and mold temperature. This could be attributed to the temperature gradient between the skin material of the molding that is in contact with the mold wall and the core material. Other factors that are expected to play a significant role are those that traditionally impose skin-core morphologies in single shot moldings, such as thickness and injection pressure [19–21]. In an attempt to understand the dynamics involved, mechanical properties of moldings were studied along the thickness direction ranging from the surface towards the center. The parallel portion of the dumbbell shaped specimen was cut and sliced as shown in Fig. 10. Dumbbells shaped samples were die-cut from the thin slices and mechanical testing was done. From Fig. 11(a), for the single shot molding, it can be seen that the mechanical properties near the surface are well defined with high values that increased from the surface to a peak and then levelled off towards the center. This is a case of typical conventional injection molding in which skin-core morphology is imposed by rapid rate of skin cooling, shear flow induced orientation along the flow direction with a core facilitated by the lower cooling rate that enhanced crystallinity [12]. On the contrary, the profile for the two-shot sandwich molding (for a single material) is different (Fig. 11(b)). The difficulty in obtaining slices from the surface of the sandwich moldings is also worth noting, while the corresponding values are less than those of single shot conventional molding. This could be attributed to the sharper temperature gradient between the skin and the core, thus giving rise to a segmented morphology in contrast to that of the conventional molding in which the temperature gradient is more gradual. The skin layer is therefore not so
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
1069
(a) 50 40 30 20 0
10
20
30
40
50
40
50
40
50
Depth from the surface [%] Tensile strength [MPa]
tightly bonded to the core in the case of sandwich moldings and this would explain the difficulty in slicing off uniform layers from the surface with a microtome. The differences are even more pronounced and mechanical properties values deteriorate further for the mixed material system (Fig. 11(c)). To further investigate morphology, cross-sections of specimens were microtomed and viewed with a polarizing microscope (Fig. 12). The micrograph at the top left hand corner covers the entire thickness of the sample while the top right hand corner is magnified to highlight the features of the sandwich specimen. The skin is clearly indicated on top with a clear boundary as depicted in a magnified section (a) It can be seen that spherulite sizes increase from (a) through (b) and (c) as indicated by the magnified slides below.
Tensile strength [MPa]
Fig. 10. Slicing apparatus and procedure for determining mechanical properties along the thickness direction.
(b) 50 40 30 20
0
10
20
30
4. Conclusions In normal single shot injection moldings, the strengths of the specimens decreased as the molding temperature increased particularly in terms of bending strength. In the case of single material sandwich moldings, tensile strength increased with increase in core material temperature while bending strength showed the opposite tendency. In sequential sandwich molding, the cross-sectional area ratio of the core layer increases in proportion to the increase in the injection speed. This tendency remains constant irrespective of resin combinations.
Tensile strength [MPa]
Depth from the surface [%]
(c) 50 40 30 20
0
10
20
30
Depth from the surface [%]
Fig. 11. Variation of tensile strength along the thickness direction at injection speed 70% and Mold temperature 40 8C; (a) normal injection, (b) single material sandwich injection and (c) dual material sandwich injection.
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T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
500µm
200µm
a b
c
(a) 50µm
(b) (c)
Fig. 12. Polarization micrographs of the cross section of dual material sandwich injection. Top left showing entire thickness, top right highlighting variation of properties and bottom (a), (b) and (c) are magnifications from the top.
Injection speed, core cylinder temperature and mold temperature are factors that can influence the mechanical strength of sandwich injection moldings. The influence of injection speed, however, depends on the skin/core material combination. It seems to be less influential in single material sandwich moldings, whereas it plays a dominant role in dual material sandwich moldings.
References [1] J.L. White, R.C. Ufford, K.C. Dharod, R.L. Price, J. Appl. Polym. Sci. 16 (1972) 1313. [2] I.H. Southern, R.L. Ballman, ACS Polym. Preprints, 13 (1972) 106; D.C. Bogue, M. Yamamoto, J.L. White (Eds.), US-Japan Seminar on Polymer Processing and Rheology Appl. Polym. Symp. 20 (1978), p. 175. [3] T.C. Yu, C.D. Han, J. Appl. Polym. Sci. (1983) 1203 (see also, C.D. Han, J. Appl. Polym. Sci. 17 (1973) p. 1289). [4] E.A. Everage, Trans. Soc. Rheol. 17 (1973) 629. [5] W.J. Schrenk, T. Alfrey, SPE J. 29 (1973) 38. [6] B.L. Lee, J.L. White, Trans. Soc. Rheol. 18 (1974) 467. [7] J.L. White, B.L. Lee, Polym. Eng. Sci. 15 (1975) 481. [8] D.H. Chang, J. Appl. Polym. Sci. 17 (1973) 1289. [9] S.S. Young, J.L. White, E.S. Clark, Y. Oyanagi, Polym. Eng. Sci. 20 (1980) 98.
[10] C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 1976. [11] U.S. Ishiaku, H. Hamada, M. Mizoguchi, W.S. Chow, Z.A. Mohd Ishak, Polym. Comp. 26 (2005) 52–59. [12] D. Watanabe, U.S. Ishiaku, T. Nagaoka, K. Tomari, H. Hamada, Int. Polym. Process. XVIII/4 (2003) 398–404. [13] D. Watanabe, U.S. Ishiaku, T. Nagaoka, K. Tomari, H Hamada, Int. Polym. Process. XVIII/4 (2003) 405–411. [14] P. Atkinson, I.P. Bagdatlionglu, International Polypropylene Conference. Conference Proceedings 42C12, October (1994) 156. [15] Plastics and Rubber Weekly, No. 1109 (1985) p.8 October. [16] H. Eckardt, Plaste und Kautschuk 34 (1987) 267. [17] M. Smith, R. Valentage, SPE Automotive TPO Global Conference 2000. Conference Proceedings, October (2000) p. 275. [18] H. Hamada, H. Inoya, N. Kunimune, S. Nagata, S. Takashima, M. Mizoguchi, S. Kuriyama, The structure and the mechanical properties of an ultra high-speed sandwich injection-molded product Proceedings of the Japan Society of Plastics Processing, Kanazawa (2003) pp. 77–78. [19] Z.A. Mohd Ishak, U.S. Ishiaku, J. Karger-Kocsis, J. mater. Sci. 33 (1998) 3377. [20] J. Karger-Kocsis, Composites 21 (1990) 243. [21] J. Karger-Kocsis, K. Friedrich, Plast. Rubb. Process. Appl. 12 (1989) 63.
Material Properties
Effect of molding parameters on the properties of PP/PP sandwich injection moldings T. Nagaokaa, U.S. Ishiakua,*, T. Tomarib, H. Hamadaa, S. Takashimaa a
Advanced Fibro Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b Osaka Municipal Technical Research Institute, Osaka, Japan Received 25 February 2005; accepted 18 April 2005
Abstract The basic characteristics of a sandwich injection molded product depend on the properties of the respective resins that comprise the skin and core layers, and the skin/core resin volume ratio. The characteristics of the core layer resin and the skin/core ratio in particular may vary depending on the injection molding conditions. This report considers the influences that the molding conditions such as injection speed, cylinder temperature, and mold temperature confer on the mechanical properties of the sandwich moldings. The study employed, skin/core resin combinations involving similar and dissimilar materials i.e. homopolymer PP/homopolymer PP and homopolymer PP/copolymer PP, respectively. It was demonstrated that core cylinder temperature and mold temperature could be used to adjust the mechanical properties of sandwich injection moldings. In the case of single material sandwich moldings, injection speed seemed to play no significant role, even though it was clearly demonstrated that core volume increases with injection speed. However, core injection speed plays a significant role in the dual material system by lowering or increasing the mechanical strength of moldings as the case may be. Thus, the dormant or active role of injection speed depending on the material system has been highlighted. q 2005 Published by Elsevier Ltd. Keywords: Polypropylene; Sandwich injection molding; Molding parameters; Mechanical properties
1. Introduction It is known that the characteristics of a product molded by sandwich injection molding vary depending on the combination and volume ratio of the skin and core layer materials [1–11]. Watenabe et al. [12,13] investigated the flow behavior during sequential and simultaneous sandwich injection molding and established the patterns of flow as well as processing conditions that lead to breakthrough phenomena. Selective combination of properties offered by this technique range from environment friendly production
* Corresponding author. Tel.: C81 75 724 7844; fax: C81 75 724 7800. E-mail address: [email protected] (U.S. Ishiaku).
0142-9418/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2005.04.003
and cost saving to aesthetics and combinations of engineering properties. Typical material combinations and applications include; (i) Soft or unfilled skin/hard or foamed core for automotive door handles, gear lever and body panels [14], (ii) Unfilled skin/core with conductive filler used in EMI shielding for computer housings [15], (iii) Virgin skin/ recycled core used in garden furniture, automotive bumpers and fascias [16,17], (iv) In-mold paint or pigmented skin/ variable or uncolored core to eliminate finishing of product or reduce pigments costs and aesthetics in wheel trims etc. Recent studies have been exploring ways of expanding the range of application into single material co-injection molding using small injection molding machines in order to increase the volume of molded products, or single material co-injection molding for reduction of the plasticization time and recycling. There are also proposals for development of co-injection molding using similar materials for cost reduction or control of molded product characteristics.
2. Experimental
20
The scope and range of applications of sandwich injection molding would experience rapid expansion if molding condition manipulation through combination of the same or similar materials opens the way to control the characteristics of sandwich molded products. The feasibility of such expansion of the technique was in this preliminary investigation using multi-purpose resins i.e. polypropylene (PP) in order to investigate the influences that manipulation of the molding parameters such as core resin temperature, core injection speed, and mold temperature could have on the mechanical properties of sandwich molded products involving single or dual material systems. Investigation involving sandwich molding of a single material should generate interest as this scenario arises when recycled material is used as the core. It is also necessary to be able to anticipate the properties of the finished products based on processing parameters in cases involving the injection of dissimilar materials.
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T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
70 Thickness = 3 (Unit: mm)
Fig. 1. Specimen dimensions.
is injected initially, followed by a specified amount of the core material. Table 2 shows the combinations of molding conditions that were used to investigate the effect of molding parameters on mechanical properties. The skin material temperature was kept constant at 230 8C, while the effect of varying the core material temperature at three levels was investigated. The effects of mold temperature and core material injection speed were also investigated. The homopolymer grade used as skin material was clear while the copolymer was whitish.
2.1. Materials 2.3. Testing methods and apparatus The materials used in this study were homopolymer PP and copolymer PP produced by Nihon Polypropylene Co. Ltd. The two grades of polypropylene were tagged as PP1 and PP2, respectively. PP1 was used as the skin material, while the same material (PP1) and PP2 were alternated as core materials under varying molding conditions. Table 1 shows the typical properties of these materials. Based on the MFR values of the materials, it can be judged that this pair presents an ideal material combination for sandwich injection as the viscosities of these materials were close enough to yield an intermediate viscosity ratio with the lower viscosity material (PP1) being the skin material [2,9].
Tensile tests were carried out by using an Instron Universal Testing Machine (Instron Co. Ltd Type 4466). The testing speed was 10 mm/min and gauge length was 115 mm in accordance with ISO-527. Three point bending tests were also conducted on the Instron Machine at a nominal test speed of 2 mm/min, in accordance with ISO178. A polarizing microscope (OLYMPUS BX50) was used to study thin slices of cross sections of specimens. 3. Results 3.1. Properties of components
2.2. Injection molding A co-injection molding machine manufactured by Nissei Plastics Industrial Co. Ltd (FN 1000) was used for molding. This machine has a mold clamping force of 800 kN, and is equipped with a skin material injection unit having a screw diameter of 30 mm and a core material injection unit having a screw diameter of 26 mm. Fig. 1 shows the tensile testing dumbbell mold used in the experiment. For sandwich molding, the sequential injection method was employed in which the skin material Table 1 Materials specification Specimen section
Material
Melt flow rate (g/10 min)
Skin Core
PP (Homo-polymer): FC7Y PP (Block Co-polymer): BC8
2.4 1.8
Fig. 2 shows the relationship between mechanical properties (tensile and flexural) for PP1 and PP2 as a function of cylinder temperature for normal injection (i.e. single shot) with the mold temperature and the injection speed fixed at 40 8C and 70%, respectively. It can be seen Table 2 Molding parameters used Mold temperature (8C) Cylinder temperature (8C) Skin Core Injection speed Set (%) Measured (mm/s)
40, 60, 80
230 180, 230, 270 30 62
50 72
70 83
99 100
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
50
1.5
30
PP1 PP2
80 2
40
3
90
2.5
1
PP1 PP2
2.5
70 2
60 50
1.5
40
Flexural modulus (GPa)
PP1 PP2 PP1 PP2
Flexural strength (MPa)
Tensile strength (MPa)
60
Tensile modulus (GPa)
1064
1 30
20 150
170
190
210
230
250
270
20 150
0.5 290
170
190
210
230
250
270
0.5 290
Cylinder temperature (°C)
Cylinder temperature (°C)
Fig. 2. Relationship between mechanical properties and molding conditions for PP materials for normal injection.
that the mechanical properties of PP1 and PP2 deteriorated as the cylinder temperature increased. Cylinder temperature seems to exert a greater influence on the bending strength than on the tensile strength, as indicated by the greater rate of deterioration with increasing cylinder temperature. While Left Min
the tensile strength decreased only by approximately 10%, the bending strength decreased by as much as approximately 20%. A comparison of the experimental values with those supplied by the manufacturer (Table 1) reveals that tensile Upper
Right Max
Core
Max
Min
Skin Lower Unit: mm Left
Injection speed (%)
16-1
Right
Max
2.15
Upper
0.41
Max
1.8
Mi n
1.4
Lower
0.09
Mi n
1.8
Left
16-2
Right
Max
1.63
Upper
0.25
Max
1.80
Mi n
1.20
Lower
0.17
Mi n
1.42
Left
16-3
Right
Max
1.67
Upper
0.25
Max
1.55
Mi n
0.87
Lower
0.32
Mi n
0.93
Left
16-4
Right
Max
1.55
Upper
0.20
Max
1.5
Mi n
0.57
Lower
0.20
Mi n
0.883
Fig. 3. Cross section of core area of PP1/PP2 as a function of injection speed for mold temperature 40 8C and cylinder temperature 230 8C.
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
strength values are within the same range, while discrepancies are noted in the case of the bending test.
3.2. Effect of cross-sectional area ratio Cross sectional area ratio of skin/core was determined at the mid-point of the dumbbell specimen (Fig. 1). Fig. 3 shows micrographs of the cross sectional area as a function of injection speed with a constant 230 8C core cylinder temperatures and 40 8C mold temperature of. The dual material system was used. Fig. 4(a) elaborates the relationship between the crosssectional area ratio of the core layer (synonymous to core volume ratio) and the injection speed in a molding process, where PP1 was the skin and PP2 was the core layer, with the skin material cylinder temperature kept constant while the core material cylinder temperature was varied. At constant resin temperature, the cross-sectional area ratio of the core
1065
layer increases as the injection speed increases. This relationship between the cross-sectional area ratio of the core layer and the injection speed has been recognized in previous reports on molding processes using resins other than PP. The same holds true with molding processes using PP resins [18]. However, the effect of core cylinder temperature on the cross sectional area of the core material is variable depending on the injection speed as shown in Fig. 4(b). At low injection speed (30%) it increased slightly from 180 to 230 8C and then diminished at 270 8C. At intermediate injection speed (50%) the cross sectional area seemed not to be affected by the core cylinder temperature, while at high injection rates the (70 and 99%) the core cross sectional are decreased from 180 to 230 8C and then increased at 270 8C. Thus, it could be inferred that temperature has minimal influence while core cross sectional area is controlled by the injection rate.
(a) 42 60
40
Core cylinder temperature 20
180° C
Tensile strength (MPa)
Core cross section area (%)
(a) 80 Core cylinder temperature 180 oC 180° C
41
230° C 270 oC 270° C
40
39
230° C 270° C 0 0
20
40
60
80
38 30
100
Injection speed (%)
70
99
(b)
(b) 80
80 60
40 Injection speed 30% 20
50%
Flexural strength (MPa)
Core cross section area (%)
50
Injection speed (%)
76 Core cylinder temperature
72
180° C 230° C
68
70%
270° C
99% 0 150
64 200
250
300
Core cylinder temperature (°C) Fig. 4. Relationship between cross-section area ratio of core layer of PP1/PP2 sandwich injection molding; (a) effect of injection speed and (b) effect of cylinder temperature.
30
50
70
99
Injection speed(%) Fig. 5. PP1/PP1 sandwich injection molding at 40 8C mold temperature; (a) effect of injection speed on tensile strength and (b) relationship between flexural strength and injection speed.
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Table 3 Relationship between tensile properties and core cylinder temperature Mold temperature (8C)
Core cylinder temperature (8C)
Tensile strength (MPa)
Tensile modulus (GPa)
Injection speed (%)
Injection speed (%)
30
50
70
99
30
50
70
99
60
180 230 270 180 230 270
40.54 39.33 39.16 39.06 39.88 39.27
39.81 39.83 39.15 39.38 39.76 38.9
39.65 39.77 39.35 39.01 39.98 39.23
39.79 39.87 39.43 39.33 39.52 38.96
2.33 2.18 1.95 2.28 2.38 2.07
2.31 2.18 2.04 2.27 2.24 2.07
2.3 2.35 2.03 2.25 2.28 2.03
2.28 2.34 2.14 2.36 2.22 2.11
3.3.1. Effect of injection speed of core material Fig. 5(a) shows the relationship between the tensile strength and the injection speed at mold temperature 40 8C with the skin material temperature kept constant at 230 8C, while the variations at 60 and 80 8C mold temperatures are presented in Table 3. From Fig. 5(a), and Table 3, different patterns emerged at different mold temperatures. At low mold temperature (40 8C), the core temperature of 180 8C gave moldings of lower mechanical properties than those at higher core temperatures, whereas such a clear distinction is not indicated at 60 8C mold temperature. However, at 80 8C mold temperature the intermediate core material temperature of 230 8C is favored. At all the mold temperatures, injection speed does not seem to assert much influence on mechanical properties. It is clearly demonstrated here that processing parameters play a significant role in determining the mechanical properties of sandwich injection moldings. Notably, core cylinder and mold temperatures could be synchronized to yield optimum mechanical properties even for a situation whereby the skin and core are comprised of the same material. It is well established that the temperature gradient between the skin and core plays a greater role in controlling morphology and hence mechanical properties [19–21]. Contrary to tensile strength, bending strength seemed to degenerate with increase in core cylinder temperature (Fig. 5(b)) while injection rate was not seen to play any significant role. 3.3.2. Effect of core material cylinder temperature Fig. 6(a) shows the relationship between the tensile strength and the core material cylinder temperature with core injection speed being a variable. It can be seen that an increase in the core material cylinder temperature generally results in an increase of tensile strength with the injection speed being not so significant. In contrast to
single shot molding (normal injection), the lower core cylinder temperature i.e. 180 8C molding showed superior strength with much poorer strength at 270 8C core temperature, as shown in Fig. 2. This highlights the advantages of sandwich injection at high molding temperatures to enhance mechanical properties.
(a) 40 Tensile strength (MPa)
3.3. Sandwich molding of single material: PP1/PP1 skin/core
38
36
Injection speed 30% 50% 70% 99%
34 180
230
270
Core cylinder temperature (°C) (b) 80
Flexural strength (MPa)
80
76
72 Injection speed 30% 50% 70% 99%
68
64 180
230
270
Core cylinder temperature (°C) Fig. 6. PP1/PP1 sandwich injection molding at 40 8C mold temperature; (a) relationship between tensile strength and core cylinder temperature and (b) relationship between flexural strength and core cylinder temperature.
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
Fig. 6(b) shows the relationship between flexural strength and the core material cylinder temperature while the injection speed was varied. Bending strength decreases as the cylinder temperature rises. However, sandwich structure products show higher mechanical properties than single shot (PP1) molded products. This suggests that the internal structure of the molded product changed somehow because of the difference in injection temperature between the skin and core layer materials. Moreover, the lower the cylinder temperature, the smaller
Tensile strength (MPa)
(a) 41
Injection speed 30% 50% 70% 99%
39
37 40
60
the difference in bending strength between the single shot molded and sandwich molded products. This suggests that manipulation of the skin and core material resin temperatures can improve the bending strengths of molded products. 3.3.3. Effect of mold temperature Fig. 7 shows the relationship between tensile strength and the mold temperature. At low core material cylinder temperature (Fig. 7(a)), tensile strength generally increases with increasing mold temperature. When core material cylinder temperature is at 230 8C an increase in tensile strength with increasing mold temperature was observed. However, at 270 8C core cylinder temperature, the opposite tendency was observed whereby tensile strength decreased with increasing mold temperature (Fig. 7(c)). From the observation above, it has been demonstrated that it is possible to alter the mechanical properties of single resin sandwich moldings by manipulating the processing parameters.
(a)
80
Mold temperature (°C)
39 Injection speed
Tensile strength (MPa)
Tensile strength (MPa)
Mold temperature 34
(b) 41
40° C 60° C 80° C
32
30 Core cylinder temperature 230° C
30% 50% 28
70%
40
60
50
70
99
Injection speed (%)
80
(b)
Mold temperature (°C)
71
Flexural strength (MPa)
(c) 41
39 Injection speed 30% 50% 70% 99% 37
30
99%
37
Tensile strength (MPa)
1067
40
69
67 Core cylinder temperature 230 °C and mold temperature 40 °C
65 60
80
Mold temperature (°C) Fig. 7. Relationship between tensile strength and mold temperature of PP1/PP1 sandwich injection molding at different core cylinder temperatures; (a) 180 8C, (b) 230 8C, and (c) 270 8C.
30
50
70
99
Injection speed (%) Fig. 8. Relationship between mechanical properties and injection speed of PP1/PP2 injection molding at core cylinder temperature 230 8C; (a) tensile strength and (b) flexural strength.
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3.4. Sandwich molding of dual material: PP1/PP2 (skin/core) 3.4.1. Relationship between injection speed and mechanical strength Fig. 8(a) shows the relationship between the tensile strengths and injection speed with the mold temperatures being varied (at a skin and core cylinder temperature of 230 8C). This figure reveals that tensile strength decreases as the injection speed increases. This phenomenon is attributable to the increase in the cross-sectional area ratio of the core layer in response to the increase in the injection speed (Fig. 4(a)) as well as to the mechanical properties of the PP2 material being lower than that of the PP1 material. Fig. 8(b) shows the relationship between bending strength and injection speed at 230 8C skin and core material temperature and 40 8C mold temperature. Similar to the tensile strength, the bending strength tends to decrease as the injection speed increases.
Tensile strength (MPa)
(a) 33
31 Injection speed 29
27
30% 50% 70% 99% 180
230
270
Core cylinder temperature (°C)
Flexural strength (MPa)
(b) 71
69
Injection speed 67
30% 50% 70% 99%
65 180
230
270
Core cylinder temperature (°C) Fig. 9. Relationship between mechanical properties and core cylinder temperature of PP1/PP2 sandwich injection molding at 40 8C mold temperature; (a) tensile strength and (b) flexural strength.
3.4.2. Relation between cylinder temperature and strength properties Fig. 9(a) shows the relationship between the tensile strength and core cylinder temperature at different injection speeds. It is clear from this figure that the rise in cylinder temperature results in lower tensile strength. This is attributable to the similar tendencies that PP1 and PP2 resins have as single materials, as shown in Fig. 2. Fig. 9(b) shows the relationship between bending strength and cylinder temperature at different injection speeds. The variations of bending strengths are large at lower cylinder temperature, but less significant at higher cylinder temperature. Bending strength decreases as cylinder temperature rises at lower injection speeds. However, the strength increases as the cylinder temperature rises at higher injection speeds. The sequential sandwich injection molding process involves the initial injection of the skin material followed by the injection of the core material. In the previous studies by the authors, the sequence of events that lead to the formation of suitable sandwich moldings were discussed [12,13]. Here, it was noticed that differences in mechanical properties were brought about by differences in core material temperature and mold temperature. This could be attributed to the temperature gradient between the skin material of the molding that is in contact with the mold wall and the core material. Other factors that are expected to play a significant role are those that traditionally impose skin-core morphologies in single shot moldings, such as thickness and injection pressure [19–21]. In an attempt to understand the dynamics involved, mechanical properties of moldings were studied along the thickness direction ranging from the surface towards the center. The parallel portion of the dumbbell shaped specimen was cut and sliced as shown in Fig. 10. Dumbbells shaped samples were die-cut from the thin slices and mechanical testing was done. From Fig. 11(a), for the single shot molding, it can be seen that the mechanical properties near the surface are well defined with high values that increased from the surface to a peak and then levelled off towards the center. This is a case of typical conventional injection molding in which skin-core morphology is imposed by rapid rate of skin cooling, shear flow induced orientation along the flow direction with a core facilitated by the lower cooling rate that enhanced crystallinity [12]. On the contrary, the profile for the two-shot sandwich molding (for a single material) is different (Fig. 11(b)). The difficulty in obtaining slices from the surface of the sandwich moldings is also worth noting, while the corresponding values are less than those of single shot conventional molding. This could be attributed to the sharper temperature gradient between the skin and the core, thus giving rise to a segmented morphology in contrast to that of the conventional molding in which the temperature gradient is more gradual. The skin layer is therefore not so
T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
1069
(a) 50 40 30 20 0
10
20
30
40
50
40
50
40
50
Depth from the surface [%] Tensile strength [MPa]
tightly bonded to the core in the case of sandwich moldings and this would explain the difficulty in slicing off uniform layers from the surface with a microtome. The differences are even more pronounced and mechanical properties values deteriorate further for the mixed material system (Fig. 11(c)). To further investigate morphology, cross-sections of specimens were microtomed and viewed with a polarizing microscope (Fig. 12). The micrograph at the top left hand corner covers the entire thickness of the sample while the top right hand corner is magnified to highlight the features of the sandwich specimen. The skin is clearly indicated on top with a clear boundary as depicted in a magnified section (a) It can be seen that spherulite sizes increase from (a) through (b) and (c) as indicated by the magnified slides below.
Tensile strength [MPa]
Fig. 10. Slicing apparatus and procedure for determining mechanical properties along the thickness direction.
(b) 50 40 30 20
0
10
20
30
4. Conclusions In normal single shot injection moldings, the strengths of the specimens decreased as the molding temperature increased particularly in terms of bending strength. In the case of single material sandwich moldings, tensile strength increased with increase in core material temperature while bending strength showed the opposite tendency. In sequential sandwich molding, the cross-sectional area ratio of the core layer increases in proportion to the increase in the injection speed. This tendency remains constant irrespective of resin combinations.
Tensile strength [MPa]
Depth from the surface [%]
(c) 50 40 30 20
0
10
20
30
Depth from the surface [%]
Fig. 11. Variation of tensile strength along the thickness direction at injection speed 70% and Mold temperature 40 8C; (a) normal injection, (b) single material sandwich injection and (c) dual material sandwich injection.
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T. Nagaoka et al. / Polymer Testing 24 (2005) 1062–1070
500µm
200µm
a b
c
(a) 50µm
(b) (c)
Fig. 12. Polarization micrographs of the cross section of dual material sandwich injection. Top left showing entire thickness, top right highlighting variation of properties and bottom (a), (b) and (c) are magnifications from the top.
Injection speed, core cylinder temperature and mold temperature are factors that can influence the mechanical strength of sandwich injection moldings. The influence of injection speed, however, depends on the skin/core material combination. It seems to be less influential in single material sandwich moldings, whereas it plays a dominant role in dual material sandwich moldings.
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[10] C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 1976. [11] U.S. Ishiaku, H. Hamada, M. Mizoguchi, W.S. Chow, Z.A. Mohd Ishak, Polym. Comp. 26 (2005) 52–59. [12] D. Watanabe, U.S. Ishiaku, T. Nagaoka, K. Tomari, H. Hamada, Int. Polym. Process. XVIII/4 (2003) 398–404. [13] D. Watanabe, U.S. Ishiaku, T. Nagaoka, K. Tomari, H Hamada, Int. Polym. Process. XVIII/4 (2003) 405–411. [14] P. Atkinson, I.P. Bagdatlionglu, International Polypropylene Conference. Conference Proceedings 42C12, October (1994) 156. [15] Plastics and Rubber Weekly, No. 1109 (1985) p.8 October. [16] H. Eckardt, Plaste und Kautschuk 34 (1987) 267. [17] M. Smith, R. Valentage, SPE Automotive TPO Global Conference 2000. Conference Proceedings, October (2000) p. 275. [18] H. Hamada, H. Inoya, N. Kunimune, S. Nagata, S. Takashima, M. Mizoguchi, S. Kuriyama, The structure and the mechanical properties of an ultra high-speed sandwich injection-molded product Proceedings of the Japan Society of Plastics Processing, Kanazawa (2003) pp. 77–78. [19] Z.A. Mohd Ishak, U.S. Ishiaku, J. Karger-Kocsis, J. mater. Sci. 33 (1998) 3377. [20] J. Karger-Kocsis, Composites 21 (1990) 243. [21] J. Karger-Kocsis, K. Friedrich, Plast. Rubb. Process. Appl. 12 (1989) 63.