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PET blends
Composites Science and Technology 40 (1991) 423--435
The Effects of Injection Molding on the Mechanical Behavior of Long-Fiber Reinforced PBT/PET Blends T. Vu-Khanh, J. Denault National Research Council Canada, Industrial Materials Research Institute, 75 De Mortagne, Boucherville, Quebec, Canada J4B 6Y4
P. Habib & A. Low Baycomp, 5035 North Service Road, Bldg A-l, Burlington, Ontario, Canada L7L 4V2 (Received 12 February 1990; revised version received 11 April 1990; accepted 7 May 1990)
ABSTRACT Long-fiber reinforced thermoplastics have received much attention because of their proeessability by conventional technologies for thermoplastics. However, fiber degradation represents a major problem. This work has been undertaken with the objective of investigating the effects of injection molding parameters on fiber degradation and fracture performance in PBT-PET blend/glass fiber composites. The influences of six parameters and their interactions are analyzed, i.e. peak cavity pressure, holding pressure, back pressure, screw speed, melt temperature and barrel profile. The results show that most of the molding variables, as well as their interactions, affect the properties of the composite. Fiber-length retention is not the sole parameter to be optimized; the matrix system is also affected by molding conditions and has significant effects on composite properties. The influence of the composite microstructure, which is controlled by the molding process, on the rigidity, strength and toughness of the composite is also discussed. INTRODUCTION Whilst molding o f long-fiber thermoplastic pellets by a conventional injection method leads to composites in which fiber length is much higher 423 © Government of Canada, 1991.
424
77. Vu-Khanh, J. Denault, P Habib, A. Low
than that in current injection grades of reinforced thermoplastics, 1-3 significant fiber breakage occurs during the molding process. The numberaverage fiber length can be reduced from 12mm to about 1 mm. The mechanical behavior of short-fiber reinforced thermoplastics has been extensively investigated in the past ~-~3 and it is well known that the performance of this material depends on the retention of fiber length in the finished part. Consequently, optimization of the injection molding process is required for better mechanical performance. In this study, a statistical method was applied to optimize the injection molding conditions for a poly(butylene terephthalate)-poly(ethylene terephthalate) blend reinforced by long glass fibers. The controllable molding variables were the peak cavity pressure, the holding pressure, the back pressure, the screw speed, the melt temperature and the barrel temperature profile. The purpose of this work was to sort out the relative importance of the different molding parameters with the objective of reducing the list of potentially important factors. The mechanical properties analyzed were the elastic modulus, tensile strength and toughness. The effects of fiber length and the material microstructure on these properties were also studied in detail in order to understand the fundamental material parameters governing these major properties in most composites. By understanding the effects of molding variables on material parameters and subsequently on mechanical performance, optimum processing parameters can be defined.
EXPERIMENTAL
Materials The long-fiber composite was from Baycomp, Burlington, Ontario, Canada. The composite is in the form of pellets, about 12 mm long, containing 50% of glass fibers by weight. The matrix system is a blend of poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET). The composition of the PBT/PET blend is 70/30% by weight.
Processing Long-fiber thermoplastic pellets were molded in a standard Hengel injection molding press of 70 tons with a general-purpose screw having a compression ratio of 22:1. This screw was chosen because it has been shown that screws with high compression ratio will result in higher fiber breakage and lowerstrength moldings. ~4 To prevent fiber breakage due to the hardware system
Effects of injection molding on mechanical behavior
425
the nozzle used was 7.9mm in diameter and the mold was equipped with a runner system of 6'4 mm diameter, with large rectangular gates of 6-4mm x 3.2mm and with two thick cavities: an ASTM-D638 tensile specimen Type 1 and a rectangular plate of 6 mm x 27 mm x 124 mm. The H U N K A R CNC 1000 system provides feedback control for ram positioning, ram velocity during injection, ram hydraulic pressure during the packing and recovery phases, and barrel temperature. There is a large number of variables which might affect the fiber degradation and the mechanical performance of fiber reinforced thermoplastic. Six of them, listed in Table 1, were selected on the basis of previous experience. Six other variables including shot size, cushion, ram decompression travel, cooling time, packing time and injection flow rate were fixed at a constant level of 31.75mm, 5.6mm, 4.6mm, 20s, 9s and 19mm/s respectively throughout the study. Owing to the considerable time required to complete an experimental run, the minimum number of independent trials was defined by experimental design techniques. Response surface methodology ~5 was used to determine the factor settings to optimize the mechanical performance of the composite. Characterization Tensile tests were performed on a standard testing machine (Instron, Model 1125) with a cross-head speed of l mm/min. The deformation was measured by an extensometer, Instron Model A325-242630-004, having a gage length of 25.4mm. Notched Izod impact tests were performed according to ASTM-D256. Thermal analysis was done with a differential scanning calorimeter (DSC), SETARAM Model C D P l l l . The DSC analyses were carried out at a heating rate of 5°C/min under a nitrogen atmosphere. Fiber degradation during the injection molding process was characterized by measuring the fiber length distribution in the molded samples. Owing to the wide range of fiber lengths present in the sample (0-3-12mm), two different techniques were used to construct the complete distribution. Glass fibers were isolated from the sample by burning off the resin in an oven and thereafter separated into two portions (< 3 mm and > 3 mm) by sieving. The fibers in each fraction were weighed and analyzed to determine the number and length. The fraction of short fibers ( < 3 m m ) was analyzed with an automatic particle size analyzer, PA-720 HIAC Pacific Scientific, adapted for fiber analysis. The fraction of long fibers (> 3 mm) was analyzed with a semi-automatic image analyzer. The two fiber length distributions were combined, taking into consideration their relative weights. The complete
426
T. Vu-Khanh. J. Denault, P. Habib, A. Low
fiber length distribution was obtained after analyzing approximately 5000 fibers < 3 m m and 1000 fibers > 3 m m .
RESULTS A N D DISCUSSION Table 1 lists the six selected process variables with their respective upper and lower limits in the experimental space. These limits were set on the basis of machine and process constraints and physical properties of materials. To explore all six variables, even at a minimum of two levels, would require 2 6, or 64, sets of different combinations. As a first screening of the effects of molding parameters, only the main effect and two-factor interaction were investigated. The minimum number of moldings is therefore reduced to 22. These experiments were planned by D-Optimal design using RS/Discover software.t 6 TABLE
i
Experimental Space for Independent Variables Variable
Low limit
High limit
Peak cavity pressure, GPa Holding pressure, GPa Back pressure, kPa Melt temperature, ~C Screw speed, rpm Barrel temperature profile
14 I 0 260 50 Decreasing
70 7 700 290 100 Increasing
The worksheet for the experiment, in which the factor settings and the resulting fiber lengths in the molding are reported, is presented in Table 2. It can be observed that, in general, these molding parameters and their interactions have significant effects on fiber attrition. Since fiber length distribution also affects the mechanical performance of the composite, the proportions of fibers longer than 1 and 3 m m are also reported for comparison purposes. As shown in Fig. 1, a decrease in the average fiber length also corresponds to a reduction in the proportions of fibers longer than 1 and 3 mm. The unique relationships also suggest that the process of fiber degradation does not change with molding variables. In order to analyze the effects of processing on mechanical properties, the elastic modulus, tensile strength and impact resistance were measured on five samples for each molding condition. The variance analyses at a confidence level of 95% revealed that, in general, the six main processing variables and
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1g 19 20 21 22
Trial number
14 70 70 14 14 70 70 14 70 14 70 70 14 70 14 70 70 14 14 70 14 70
Peak cavity pressure (GPa)
I 7 7 1 7 1 7 7 1 I 1 7 7 1 I I 7 1 7 7 7 1
Holding pressure (aPa) 0 700 0 700 0 0 700 700 0 0 700 0 700 0 0 700 0 700 0 700 700 0
Back pressure (kPa) 50 50 50 100 100 100 100 50 50 100 100 100 50 50 100 100 100 50 50 50 100 100
Screw speed (rpm)
Injection molding conditions
260 260 260 260 260 260 260 260 260 260 260 260 290 290 290 290 290 290 290 290 290 290
(°C)
Melt temperature
Decreasing Decreasing Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing Increasing Increasing Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing Increasing Increasing
Barrel temperature profile
TABLE 2 Worksheet for the Experiment and the Resulting Fiber Length
1"12 1-00 0-91 ! "03 0"71 0"89 0"97
1-45 I "23
1 "06 0-82 0"83 0"74 0-79 0'81 0-74 0"79 0-85 0.95 0-89 0"99 0"91
Average fiber length {ram)
39"8 26-5 29"9 20"9 23'2 24"3 20"1 24"4 25'! 34 "4 30"6 31"6 33"3 54"7 40"5 44-2 36"8 33'9 35"4 24-3 32"9 37"1
Percentage of fiber > lmm
4"0 0"9 0"6 0"7 1"3 1"5 0"9 0-8 2"0 2"1 1"9 3-6 2"0 10.1 6-0 4"1 3"1 1"3 4'0 0"4 1"6 1"7
Percentage of fiber > 3 mm
4~
IxJ '-..4
¢%
428
T. Vu-Khanh, J. Denault, P Habib. ,4. Low 60
5O
¢1 eoJ ¢J
4O
3O
2O J~
E
g Fig. 1. Number percentage of fibers longer than I mm (O)and 3 mm (11)as a function of average fiber length.
10
0 0.7
0.8
0.9
Average
1.0 Fiber
1.1
1.2
Length
1.3
1.4
(mm)
their interactions also have significant effects on the mechanical properties. Tables 3-5 show respectively the variables and the interactions that do not have significant effects on the tensile modulus, tensile strength and Izod impact resistance of the composite. From the D-Optimal design analysis, it is possible to define the optimum injection conditions for each property. However, surprisingly enough, these conditions do not correspond to the maximum fiber length region, which is the key parameter in long-fiber reinforced thermoplastics. In order to be able to improve the mechanical performance of the composite, the role of fiber length, in addition to that of other material parameters, must be understood. Figures 2 and 3 show the plots of the elastic modulus and strength respectively as a function of the average fiber length. In Fig. 2 it can be seen that the composite modulus is independent of fiber length. This result is in TABLE 3 Parameters and Interactions that do not Affect the Elastic Modulus First-order terms (main effect)
Screw speed Barrel temperature profile Second-order terms (interaction effect)
Peak cavity pressure/Holding pressure Peak cavity pressure/Screw speed Peak cavity pressure/Melt temperature Peak cavity pressure/Barrel temperature profile Holding pressure/Back pressure Holding pressure/Screw speed Holding pressure/Barrel temperature profile Back pressure/Screw speed Melt temperature/Barrel temperature profile
429
Effects of injection molding on mechanical behavior TABLE 4 Interactions that do not Affect the Ultimate Tensile Strength
Second-order terms (interaction effect)
Holding pressure/Melt temperature Screw speed/Barrel temperature profile
TABLE 5 Parameters and Interactions that do not Affect the lzod Impact Resistance First-order term (main effect) Screw speed Second-order terms (interaction effect)
Peak cavity pressure/Back pressure Peak cavity pressure/Melt temperature Holding pressure/Melt temperature Screw speed/Melt temperature
agreement with the postulate based on shear lag theory that the modulus of fiber composites, as measured at low strains, is close to that of continuous fiber composites and that fiber length does not affect the elastic behavior. However, the scatter of the results is much larger than the standard deviations measured on five samples for each molding condition (Table 6). This fluctuation is in fact due to the variation of other microstructural parameters such as fiber orientation. The PBT/PET long-fiber composites show layers of different fiber orientation across the thickness of the injected A
140 A
os el
120
25
100 •
2 "0 0
.
•
.
•
.
.
8O 1S
6O ¢ 0 >"
40 5
0 0.7
2O •
O.IJ
0.9
1.0
1.1
1.2
1.3
1.4
u.7
Average Fiber Length (ram)
Fig. 2.
Young's modulus as a function of average fiber length.
,
0.8
-
,
0,9
•
,
1.0
-
,
1.1
-
i
1.2
,
i
1.3
,
1.4
Average Fiber Length (turn)
Fig. 3.
Ultimate tensile strength as a function of average fiber length.
430
77. Vu-Khanh, J. Denault, P. Habib, A. Low TABLE 6 Elastic Modulus and Izod Impact Resistance in Different Molding Conditions
Trial number
Elastic modulus (G Pa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2l 22
19 20 21 20 18 19 19 21 19 20 19 21 24 17 18 20 21 20 19 20 22 20
Standard deviation {G Pa)
l=od impact (J,;m)
Standard deviation (J/m)
213 165 144 155 149 t7l 139 160 240 171 160 123 139 320 245 208 t92 187 213 160 t65 197
9 24 12 17 27 8 10 3 2l 10 9 6 21 43 38 t4 26 29 4 16 20 11
samples. Previous work 3 has shown that the thickness of each layer is greatly dependent on the fiber concentration in the composite. Moreover, in the present work scanning electron microscope observations showed that fiber alignment is also dependent on the injection molding conditions. As opposed to the elastic modulus, the plot of the composite strength as a function of the average fiber length (Fig. 3) clearly demonstrates two distinct groups of data. The first group shows a relatively constant strength versus fiber length, whereas the second group shows a decrease in the composite strength with increasing average fiber length. These results are in contradiction with previous investigations, 3'1v in which it has been demonstrated that fiber length is a major parameter in the strength of longfiber reinforced thermoplastics. In fact it has been found that the strength of discontinuous fiber composites can be adequately predicted by the modified shear-lag theory. Figure 4 shows the comparison between the measured strength of PBT/glass composites and the calculation. From the postulated
Effects of injection molding on mechanical behavior
431
140 v .C
•
120
•
•
100
60
40' •
20
q
E ~. •.~
, 0
t
.
10
J
20
.
,
.
3O
,
.
50
40
Fiber Volume Fraction
[%]
Fig. 4. Calculated and measured tensile strength of long-fiber reinforced PBT at various fiber orientations: I1, well oriented in the loading direction; 0 , randomly oriented; A, orientation perpendicular to the loading direction.
stress built up in the fiber by shear stresses, the composite strength can be described by t s o'~ = ~/o
-~--r + r/o li < Ic
a*Vj 1
1~
lj > 1¢
in which ~ is the interfacial shear strength, r is the fiber radius, l¢ is the critical fiber length, l~ and lj are the actual fiber lengths below and above l~ respectively, V~and Vj are the corresponding fiber volume fractions, o'~' is the fiber strength, o"m is the stress in the matrix at fracture, Vf is the total fiber volume fraction, and % is the apparent orientation factor in the sample. In the above equation, % was determined from the elastic modulus of the composite using the modified rule of mixtures: t9 E¢ -- ~/or/,EfVf+ Em(l - V~)
(2)
where ~/oand 7, are associated respectivelywith the fiber alignment and the fiber length in the tested specimen. In addition, when fibers are discontinuous, the fiber may carry stress only by a shear transfer process at the interface. On the basis of the concept of Kelly and Tyson2° for linear stress transfer, Bowyer and Bader4 have pointed out that, for composites with varying fiber lengths, at any value of composite strain, e¢, there is a critical fiber length given by 1~¢ = Era¢r/T
(3)
where r is the fiber radius and z denotes the interfacial shear stress. Consequently, at very low strain, the effects of stress build-up from the ends may be neglected, and the stiffness is given by the modified rule of mixtures which only includes the fiber alignment factor, r/o:
E~ = rloefVt + E(I -- Vt)
(4)
432
T. Vu-Khanh, J. Denault, P. Habib, .4, Low
The interracial shear strength, r, was determined from eqn (3) by measuring the fiber pull-out length. Measurements on various long-fiber reinforced thermoplastics containing different fiber orientations at7 have given enough evidence to suggest that the above approach is a good description of the strength of fibrous composites, and that the longer are fibers, the stronger the composite (see Fig. 4). The results in Fig. 3 suggest that fiber attrition is not the sole important factor for the mechanical performance of the composite. The analysis of these results reveals that the second group of data (lower-strength region) belong to the melt temperature of 290°C. It is therefore possible that high temperature can lead to matrix degradation. Figure 5 shows the DSC spectra of three selected samples from both groups and of the pellet before molding. It can be seen that molding conditions strongly affect the microstructure and morphology of the matrix. In some cases the melting peak of PET disappears, suggesting its degradation. The result also shows that the melting peak associated with the PBT matrix shifts toward a lower temperature at certain molding conditions, indicating the presence of smaller crystalline entities. In the case of a melt temperature of 260:C
(a)
--------~BT ~
,,n
c~ 2
fF o
_.J
u,.I :E (3 t-r" iaj
© 0 Z
,;o ,;o ,~o 21o 2;0 2;0 TEMPERATURE
(~C)
Fig. 5. Differentialscanning calorimetric thermograms of (a) PBT/PET long glass fiber pellets, (b) and (c) PBT/PET long-fibercomposites molded at 260°C. and (d) PBT PET longfiber comoosites molded at 290~C.
Effects of injection molding on mechanical beha~'ior
433
(higher-strength group of data), although the strength of the composite is significantly higher, it does not increase with increasing fiber length as expected. Fiber length retention is therefore not the dominant parameter in the performance of the composite. The effects of processing variables on other microstructural parameters of the composite must also be thoroughly understood in order properly to optimize the molding parameters and the mechanical performance. This aspect is being investigated and will be reported in the near future. In terms of toughness, although the composite exhibits a purely brittle behavior without any noticeable permanent deformation before fracture in the tensile test, in the presence of a crack, local damage around the crack tip results in a ductile type of fracture. The measurement of the intrinsic fracture toughness of the composite is therefore not possible with the sizes of the molded samples. Figure 6 shows a plot of the recorded maximum load as a function of the ratio of crack length to specimen width for three-point bend samples, on a given molding. In order to verify the extent of local damage in the sample, the maximum load corresponding to the case of gross damage of the net section of the sample has also been calculated and is shown by the continuous line. The calculation gives = ---g--
i-
(5)
where W is the specimen width, B the thickness, S the span, a the crack length, and cr¢ is taken as the measured strength of the composite. It is clear that the maximum recorded loads in the fracture tests correspond to the gross yielding of the net section and the fracture tests are no longer meaningful. However, observation of the surface area under the loaddeflection diagram revealed that there is a significant difference from one molding condition to another, even though the tensile strength is the same. This indicates that the absorbed energy in the fracture process varies with the molding condition. In order to characterize the composite's toughness qualitatively, Izod impact tests were measured on the moldings. The results indeed suggest that the molding parameters have strong effects on the Izod impact resistance of the composite (see Table 6). Figure 7 shows the plot of Izod energy as a function of average fiber length in the molding. As opposed to the strength property, there is a unique relationship between the Izod energy and fiber length, suggesting therefore that the impact fracture process is mainly governed by the reinforcement. The matrix plays a lesser role in terms of fracture energy, and optimization of the impact resistance of the composite could be based on reduction of fiber attrition.
434
T. Vu-Khanh, J. Denault, P. Habib, A. Low
10oo 900 000 Z '1o
a0o
700 t~ "N
600
O S00
E E
400
x
1 200~0,0,
,
~" _
,
o N .
0.0
0:,
0:,
0:,
0.,
a/W
Recorded m a x i m u m load o f P B T / PET long-fiber composites as a function of the ratio of crack length to specimen width for three-point bend samples. Fig. 6.
0.7
0.9
.
.
.
1.1
.
1.3
Average Fiber Length (mm)
Fig. 7. Izod impact resistance of PBT/PET long-fiber composites as a function of average fiber length.
SUMMARY AND CONCLUSION In this preliminary work on the effects of injection molding parameters on the mechanical performance of PBT-PET blend/long glass fiber composites, it has been shown that most of the molding variables and their interactions have significant effects on the properties. The results show that fiber-length retention is not the sole parameter to be optimized but that the matrix system is also affected by the molding conditions and has significant effects on composite properties. The process of fiber attrition does not change from one molding parameter to another. Molding conditions leading to maximum fiber length in the composite do not result in maximum strength. The composite elastic modulus remains constant with fiber length but is affected by other microstructural parameters. The result is in agreement with shear lag theory regarding the stress transfer between the matrix and the fiber in fibrous composite. In terms of toughness, although the composite is brittle on the macroscopic scale, the damage zone in front of the crack tip leads to a non-linear type of fracture. The measurement of the intrinsic fracture toughness therefore requires very large samples. The fracture process (after crack initiation) has also been found to be governed by molding parameters. As opposed to the strength property, the Izod impact energy of the composite is mainly controlled by fiber-length retention. This work is being pursued with a detailed investigation on the effect of molding conditions on the microstructure and morphology of the matrix, in connection with fiber attrition and orientation in the molded parts.
Effects of injection molding on mechanical behavior
435
REFERENCES 1. Gibson, A. G., McClelland, A. N. R., Cuff, G. & Gore, C. R., Processing of long fiber reinforced thermoplastics. Proc. ECCM Conf. Bordeaux, 1985, pp. 511-19. 2. Cianelli, D. A., Travis, J. E. & Bailey, R. S., How to process long fiber reinforced thermoplastics. Plast. Technol. (1988) 83-9. 3. Denault, J., Vu-Khanh, T. & Foster, B., Tensile properties of injection molded long fiber thermoplastic composites. Polym. Comp., 10 (1989) 313-21. 4. Bowyer, W. H. & Bader, M. G., On the reinforcement of thermoplastics by imperfectly aligned discontinuous fibres. J. Mat. Sci., 7 (1972) 1315-21. 5. Bader, M. G. & Collins, J. E, Progress in Science and Engineering of Composites, Vol. 1, ed. T. Hayashi, K. Kawata & S. Umekawa. Tokyo, ICCM-IV, 1982, p. 1067. 6. Curtis, P. T., Bader, M. B. & Bailey, J. E., The stiffness and strength of a polyamide thermoplastic reinforced with glass and carbon fibers. J. Mat. Sci., 13 (1978) 377-90. 7. Ramsteiner, F. & Teyson, R., Tensile and impact strength of unidirectional, short fiber reinforced thermoplastics. Composites, 10 (1979) 111-17. 8. Darlington, M. W., Gladwell, B. K. & Smith, G. R., Structure and mechanical properties in injection moulded discs of glass fiber reinforced polypropylene. Polymer, 18 (1977) 1269-74. 9. Darlington, M. W., McGinley, P. L. & Smith, G. R., Structure and anisotropy of stiffness in glass fiber-reinforced thermoplastics, d. Mat. Sci., 11 (1976) 877-81. 10. Halpin, J. C., Stiffness and expansion estimates for oriented short fiber composites. J. Comp. Mat., 3 (1969) 732--4. 11. Dingle, L. E., Aligned discontinuous carbon fiber composites. In Proc. Fourth International Conference on Carbon Fibres and their Composites and Applications. Plastics Institute, London, 1974, paper 11. 12. Darlington, M. W. & Smith, G. R., Mechanical properties of glass fibre reinforced and mineral reinforced polyamides 6 and 66. Plast. Rubber, 3 (1978) 97-108. 13. Karger-Kocsis, J. & Friedrich, K., Fracture behavior of injection-molded short and long glass fiber--Polyamide 6.6 composite. Comp. Sci. Tech., 32 (1988) 293325. 14. Schmid, B. S., Injection moulding of long fiber reinforced thermoplastics. Kunststoffe, 79 (1989) 624-30. 15. Box, G. E. E & Wilson, K. B., On the experimental attainment of optimum conditions. J. Royal Stat. Sot., Series B, 13 (1951) 1-38. 16. 'R.S. Discover', Version 1.0 (1986), BBN Software Products Corporation, 10 Fawcett Street, Cambridge, MA 02238, USA. 17. Vu-Khanh, T. & Denault, J., Report on Microstructural Analysis and Quantitative Characterization of the Tensile Behavior of Long Fiber Reinforced Thermoplastics. IUPAC Working Party 4.2.1, Characterization of Finite Length Composites, 1989. 18. Kelly, A. & MacMillan, N. H., Strong Solids. Clarendon Press, Oxford, 1986. 19. Cox, H. L., The elasticity and strength of paper and other fibrous materials. Brit. J. Appl. Phys., 3 (1952) 72-9. 20. Kelly, A. & Tyson, W. R., Tensile properties of fiber-reinforced metals: coppertungsten and copper-molybdenum. J. Mech. Phys. Solids, 13 (1965) 329-50.
The Effects of Injection Molding on the Mechanical Behavior of Long-Fiber Reinforced PBT/PET Blends T. Vu-Khanh, J. Denault National Research Council Canada, Industrial Materials Research Institute, 75 De Mortagne, Boucherville, Quebec, Canada J4B 6Y4
P. Habib & A. Low Baycomp, 5035 North Service Road, Bldg A-l, Burlington, Ontario, Canada L7L 4V2 (Received 12 February 1990; revised version received 11 April 1990; accepted 7 May 1990)
ABSTRACT Long-fiber reinforced thermoplastics have received much attention because of their proeessability by conventional technologies for thermoplastics. However, fiber degradation represents a major problem. This work has been undertaken with the objective of investigating the effects of injection molding parameters on fiber degradation and fracture performance in PBT-PET blend/glass fiber composites. The influences of six parameters and their interactions are analyzed, i.e. peak cavity pressure, holding pressure, back pressure, screw speed, melt temperature and barrel profile. The results show that most of the molding variables, as well as their interactions, affect the properties of the composite. Fiber-length retention is not the sole parameter to be optimized; the matrix system is also affected by molding conditions and has significant effects on composite properties. The influence of the composite microstructure, which is controlled by the molding process, on the rigidity, strength and toughness of the composite is also discussed. INTRODUCTION Whilst molding o f long-fiber thermoplastic pellets by a conventional injection method leads to composites in which fiber length is much higher 423 © Government of Canada, 1991.
424
77. Vu-Khanh, J. Denault, P Habib, A. Low
than that in current injection grades of reinforced thermoplastics, 1-3 significant fiber breakage occurs during the molding process. The numberaverage fiber length can be reduced from 12mm to about 1 mm. The mechanical behavior of short-fiber reinforced thermoplastics has been extensively investigated in the past ~-~3 and it is well known that the performance of this material depends on the retention of fiber length in the finished part. Consequently, optimization of the injection molding process is required for better mechanical performance. In this study, a statistical method was applied to optimize the injection molding conditions for a poly(butylene terephthalate)-poly(ethylene terephthalate) blend reinforced by long glass fibers. The controllable molding variables were the peak cavity pressure, the holding pressure, the back pressure, the screw speed, the melt temperature and the barrel temperature profile. The purpose of this work was to sort out the relative importance of the different molding parameters with the objective of reducing the list of potentially important factors. The mechanical properties analyzed were the elastic modulus, tensile strength and toughness. The effects of fiber length and the material microstructure on these properties were also studied in detail in order to understand the fundamental material parameters governing these major properties in most composites. By understanding the effects of molding variables on material parameters and subsequently on mechanical performance, optimum processing parameters can be defined.
EXPERIMENTAL
Materials The long-fiber composite was from Baycomp, Burlington, Ontario, Canada. The composite is in the form of pellets, about 12 mm long, containing 50% of glass fibers by weight. The matrix system is a blend of poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET). The composition of the PBT/PET blend is 70/30% by weight.
Processing Long-fiber thermoplastic pellets were molded in a standard Hengel injection molding press of 70 tons with a general-purpose screw having a compression ratio of 22:1. This screw was chosen because it has been shown that screws with high compression ratio will result in higher fiber breakage and lowerstrength moldings. ~4 To prevent fiber breakage due to the hardware system
Effects of injection molding on mechanical behavior
425
the nozzle used was 7.9mm in diameter and the mold was equipped with a runner system of 6'4 mm diameter, with large rectangular gates of 6-4mm x 3.2mm and with two thick cavities: an ASTM-D638 tensile specimen Type 1 and a rectangular plate of 6 mm x 27 mm x 124 mm. The H U N K A R CNC 1000 system provides feedback control for ram positioning, ram velocity during injection, ram hydraulic pressure during the packing and recovery phases, and barrel temperature. There is a large number of variables which might affect the fiber degradation and the mechanical performance of fiber reinforced thermoplastic. Six of them, listed in Table 1, were selected on the basis of previous experience. Six other variables including shot size, cushion, ram decompression travel, cooling time, packing time and injection flow rate were fixed at a constant level of 31.75mm, 5.6mm, 4.6mm, 20s, 9s and 19mm/s respectively throughout the study. Owing to the considerable time required to complete an experimental run, the minimum number of independent trials was defined by experimental design techniques. Response surface methodology ~5 was used to determine the factor settings to optimize the mechanical performance of the composite. Characterization Tensile tests were performed on a standard testing machine (Instron, Model 1125) with a cross-head speed of l mm/min. The deformation was measured by an extensometer, Instron Model A325-242630-004, having a gage length of 25.4mm. Notched Izod impact tests were performed according to ASTM-D256. Thermal analysis was done with a differential scanning calorimeter (DSC), SETARAM Model C D P l l l . The DSC analyses were carried out at a heating rate of 5°C/min under a nitrogen atmosphere. Fiber degradation during the injection molding process was characterized by measuring the fiber length distribution in the molded samples. Owing to the wide range of fiber lengths present in the sample (0-3-12mm), two different techniques were used to construct the complete distribution. Glass fibers were isolated from the sample by burning off the resin in an oven and thereafter separated into two portions (< 3 mm and > 3 mm) by sieving. The fibers in each fraction were weighed and analyzed to determine the number and length. The fraction of short fibers ( < 3 m m ) was analyzed with an automatic particle size analyzer, PA-720 HIAC Pacific Scientific, adapted for fiber analysis. The fraction of long fibers (> 3 mm) was analyzed with a semi-automatic image analyzer. The two fiber length distributions were combined, taking into consideration their relative weights. The complete
426
T. Vu-Khanh. J. Denault, P. Habib, A. Low
fiber length distribution was obtained after analyzing approximately 5000 fibers < 3 m m and 1000 fibers > 3 m m .
RESULTS A N D DISCUSSION Table 1 lists the six selected process variables with their respective upper and lower limits in the experimental space. These limits were set on the basis of machine and process constraints and physical properties of materials. To explore all six variables, even at a minimum of two levels, would require 2 6, or 64, sets of different combinations. As a first screening of the effects of molding parameters, only the main effect and two-factor interaction were investigated. The minimum number of moldings is therefore reduced to 22. These experiments were planned by D-Optimal design using RS/Discover software.t 6 TABLE
i
Experimental Space for Independent Variables Variable
Low limit
High limit
Peak cavity pressure, GPa Holding pressure, GPa Back pressure, kPa Melt temperature, ~C Screw speed, rpm Barrel temperature profile
14 I 0 260 50 Decreasing
70 7 700 290 100 Increasing
The worksheet for the experiment, in which the factor settings and the resulting fiber lengths in the molding are reported, is presented in Table 2. It can be observed that, in general, these molding parameters and their interactions have significant effects on fiber attrition. Since fiber length distribution also affects the mechanical performance of the composite, the proportions of fibers longer than 1 and 3 m m are also reported for comparison purposes. As shown in Fig. 1, a decrease in the average fiber length also corresponds to a reduction in the proportions of fibers longer than 1 and 3 mm. The unique relationships also suggest that the process of fiber degradation does not change with molding variables. In order to analyze the effects of processing on mechanical properties, the elastic modulus, tensile strength and impact resistance were measured on five samples for each molding condition. The variance analyses at a confidence level of 95% revealed that, in general, the six main processing variables and
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1g 19 20 21 22
Trial number
14 70 70 14 14 70 70 14 70 14 70 70 14 70 14 70 70 14 14 70 14 70
Peak cavity pressure (GPa)
I 7 7 1 7 1 7 7 1 I 1 7 7 1 I I 7 1 7 7 7 1
Holding pressure (aPa) 0 700 0 700 0 0 700 700 0 0 700 0 700 0 0 700 0 700 0 700 700 0
Back pressure (kPa) 50 50 50 100 100 100 100 50 50 100 100 100 50 50 100 100 100 50 50 50 100 100
Screw speed (rpm)
Injection molding conditions
260 260 260 260 260 260 260 260 260 260 260 260 290 290 290 290 290 290 290 290 290 290
(°C)
Melt temperature
Decreasing Decreasing Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing Increasing Increasing Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing Increasing Increasing
Barrel temperature profile
TABLE 2 Worksheet for the Experiment and the Resulting Fiber Length
1"12 1-00 0-91 ! "03 0"71 0"89 0"97
1-45 I "23
1 "06 0-82 0"83 0"74 0-79 0'81 0-74 0"79 0-85 0.95 0-89 0"99 0"91
Average fiber length {ram)
39"8 26-5 29"9 20"9 23'2 24"3 20"1 24"4 25'! 34 "4 30"6 31"6 33"3 54"7 40"5 44-2 36"8 33'9 35"4 24-3 32"9 37"1
Percentage of fiber > lmm
4"0 0"9 0"6 0"7 1"3 1"5 0"9 0-8 2"0 2"1 1"9 3-6 2"0 10.1 6-0 4"1 3"1 1"3 4'0 0"4 1"6 1"7
Percentage of fiber > 3 mm
4~
IxJ '-..4
¢%
428
T. Vu-Khanh, J. Denault, P Habib. ,4. Low 60
5O
¢1 eoJ ¢J
4O
3O
2O J~
E
g Fig. 1. Number percentage of fibers longer than I mm (O)and 3 mm (11)as a function of average fiber length.
10
0 0.7
0.8
0.9
Average
1.0 Fiber
1.1
1.2
Length
1.3
1.4
(mm)
their interactions also have significant effects on the mechanical properties. Tables 3-5 show respectively the variables and the interactions that do not have significant effects on the tensile modulus, tensile strength and Izod impact resistance of the composite. From the D-Optimal design analysis, it is possible to define the optimum injection conditions for each property. However, surprisingly enough, these conditions do not correspond to the maximum fiber length region, which is the key parameter in long-fiber reinforced thermoplastics. In order to be able to improve the mechanical performance of the composite, the role of fiber length, in addition to that of other material parameters, must be understood. Figures 2 and 3 show the plots of the elastic modulus and strength respectively as a function of the average fiber length. In Fig. 2 it can be seen that the composite modulus is independent of fiber length. This result is in TABLE 3 Parameters and Interactions that do not Affect the Elastic Modulus First-order terms (main effect)
Screw speed Barrel temperature profile Second-order terms (interaction effect)
Peak cavity pressure/Holding pressure Peak cavity pressure/Screw speed Peak cavity pressure/Melt temperature Peak cavity pressure/Barrel temperature profile Holding pressure/Back pressure Holding pressure/Screw speed Holding pressure/Barrel temperature profile Back pressure/Screw speed Melt temperature/Barrel temperature profile
429
Effects of injection molding on mechanical behavior TABLE 4 Interactions that do not Affect the Ultimate Tensile Strength
Second-order terms (interaction effect)
Holding pressure/Melt temperature Screw speed/Barrel temperature profile
TABLE 5 Parameters and Interactions that do not Affect the lzod Impact Resistance First-order term (main effect) Screw speed Second-order terms (interaction effect)
Peak cavity pressure/Back pressure Peak cavity pressure/Melt temperature Holding pressure/Melt temperature Screw speed/Melt temperature
agreement with the postulate based on shear lag theory that the modulus of fiber composites, as measured at low strains, is close to that of continuous fiber composites and that fiber length does not affect the elastic behavior. However, the scatter of the results is much larger than the standard deviations measured on five samples for each molding condition (Table 6). This fluctuation is in fact due to the variation of other microstructural parameters such as fiber orientation. The PBT/PET long-fiber composites show layers of different fiber orientation across the thickness of the injected A
140 A
os el
120
25
100 •
2 "0 0
.
•
.
•
.
.
8O 1S
6O ¢ 0 >"
40 5
0 0.7
2O •
O.IJ
0.9
1.0
1.1
1.2
1.3
1.4
u.7
Average Fiber Length (ram)
Fig. 2.
Young's modulus as a function of average fiber length.
,
0.8
-
,
0,9
•
,
1.0
-
,
1.1
-
i
1.2
,
i
1.3
,
1.4
Average Fiber Length (turn)
Fig. 3.
Ultimate tensile strength as a function of average fiber length.
430
77. Vu-Khanh, J. Denault, P. Habib, A. Low TABLE 6 Elastic Modulus and Izod Impact Resistance in Different Molding Conditions
Trial number
Elastic modulus (G Pa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2l 22
19 20 21 20 18 19 19 21 19 20 19 21 24 17 18 20 21 20 19 20 22 20
Standard deviation {G Pa)
l=od impact (J,;m)
Standard deviation (J/m)
213 165 144 155 149 t7l 139 160 240 171 160 123 139 320 245 208 t92 187 213 160 t65 197
9 24 12 17 27 8 10 3 2l 10 9 6 21 43 38 t4 26 29 4 16 20 11
samples. Previous work 3 has shown that the thickness of each layer is greatly dependent on the fiber concentration in the composite. Moreover, in the present work scanning electron microscope observations showed that fiber alignment is also dependent on the injection molding conditions. As opposed to the elastic modulus, the plot of the composite strength as a function of the average fiber length (Fig. 3) clearly demonstrates two distinct groups of data. The first group shows a relatively constant strength versus fiber length, whereas the second group shows a decrease in the composite strength with increasing average fiber length. These results are in contradiction with previous investigations, 3'1v in which it has been demonstrated that fiber length is a major parameter in the strength of longfiber reinforced thermoplastics. In fact it has been found that the strength of discontinuous fiber composites can be adequately predicted by the modified shear-lag theory. Figure 4 shows the comparison between the measured strength of PBT/glass composites and the calculation. From the postulated
Effects of injection molding on mechanical behavior
431
140 v .C
•
120
•
•
100
60
40' •
20
q
E ~. •.~
, 0
t
.
10
J
20
.
,
.
3O
,
.
50
40
Fiber Volume Fraction
[%]
Fig. 4. Calculated and measured tensile strength of long-fiber reinforced PBT at various fiber orientations: I1, well oriented in the loading direction; 0 , randomly oriented; A, orientation perpendicular to the loading direction.
stress built up in the fiber by shear stresses, the composite strength can be described by t s o'~ = ~/o
-~--r + r/o li < Ic
a*Vj 1
1~
lj > 1¢
in which ~ is the interfacial shear strength, r is the fiber radius, l¢ is the critical fiber length, l~ and lj are the actual fiber lengths below and above l~ respectively, V~and Vj are the corresponding fiber volume fractions, o'~' is the fiber strength, o"m is the stress in the matrix at fracture, Vf is the total fiber volume fraction, and % is the apparent orientation factor in the sample. In the above equation, % was determined from the elastic modulus of the composite using the modified rule of mixtures: t9 E¢ -- ~/or/,EfVf+ Em(l - V~)
(2)
where ~/oand 7, are associated respectivelywith the fiber alignment and the fiber length in the tested specimen. In addition, when fibers are discontinuous, the fiber may carry stress only by a shear transfer process at the interface. On the basis of the concept of Kelly and Tyson2° for linear stress transfer, Bowyer and Bader4 have pointed out that, for composites with varying fiber lengths, at any value of composite strain, e¢, there is a critical fiber length given by 1~¢ = Era¢r/T
(3)
where r is the fiber radius and z denotes the interfacial shear stress. Consequently, at very low strain, the effects of stress build-up from the ends may be neglected, and the stiffness is given by the modified rule of mixtures which only includes the fiber alignment factor, r/o:
E~ = rloefVt + E(I -- Vt)
(4)
432
T. Vu-Khanh, J. Denault, P. Habib, .4, Low
The interracial shear strength, r, was determined from eqn (3) by measuring the fiber pull-out length. Measurements on various long-fiber reinforced thermoplastics containing different fiber orientations at7 have given enough evidence to suggest that the above approach is a good description of the strength of fibrous composites, and that the longer are fibers, the stronger the composite (see Fig. 4). The results in Fig. 3 suggest that fiber attrition is not the sole important factor for the mechanical performance of the composite. The analysis of these results reveals that the second group of data (lower-strength region) belong to the melt temperature of 290°C. It is therefore possible that high temperature can lead to matrix degradation. Figure 5 shows the DSC spectra of three selected samples from both groups and of the pellet before molding. It can be seen that molding conditions strongly affect the microstructure and morphology of the matrix. In some cases the melting peak of PET disappears, suggesting its degradation. The result also shows that the melting peak associated with the PBT matrix shifts toward a lower temperature at certain molding conditions, indicating the presence of smaller crystalline entities. In the case of a melt temperature of 260:C
(a)
--------~BT ~
,,n
c~ 2
fF o
_.J
u,.I :E (3 t-r" iaj
© 0 Z
,;o ,;o ,~o 21o 2;0 2;0 TEMPERATURE
(~C)
Fig. 5. Differentialscanning calorimetric thermograms of (a) PBT/PET long glass fiber pellets, (b) and (c) PBT/PET long-fibercomposites molded at 260°C. and (d) PBT PET longfiber comoosites molded at 290~C.
Effects of injection molding on mechanical beha~'ior
433
(higher-strength group of data), although the strength of the composite is significantly higher, it does not increase with increasing fiber length as expected. Fiber length retention is therefore not the dominant parameter in the performance of the composite. The effects of processing variables on other microstructural parameters of the composite must also be thoroughly understood in order properly to optimize the molding parameters and the mechanical performance. This aspect is being investigated and will be reported in the near future. In terms of toughness, although the composite exhibits a purely brittle behavior without any noticeable permanent deformation before fracture in the tensile test, in the presence of a crack, local damage around the crack tip results in a ductile type of fracture. The measurement of the intrinsic fracture toughness of the composite is therefore not possible with the sizes of the molded samples. Figure 6 shows a plot of the recorded maximum load as a function of the ratio of crack length to specimen width for three-point bend samples, on a given molding. In order to verify the extent of local damage in the sample, the maximum load corresponding to the case of gross damage of the net section of the sample has also been calculated and is shown by the continuous line. The calculation gives = ---g--
i-
(5)
where W is the specimen width, B the thickness, S the span, a the crack length, and cr¢ is taken as the measured strength of the composite. It is clear that the maximum recorded loads in the fracture tests correspond to the gross yielding of the net section and the fracture tests are no longer meaningful. However, observation of the surface area under the loaddeflection diagram revealed that there is a significant difference from one molding condition to another, even though the tensile strength is the same. This indicates that the absorbed energy in the fracture process varies with the molding condition. In order to characterize the composite's toughness qualitatively, Izod impact tests were measured on the moldings. The results indeed suggest that the molding parameters have strong effects on the Izod impact resistance of the composite (see Table 6). Figure 7 shows the plot of Izod energy as a function of average fiber length in the molding. As opposed to the strength property, there is a unique relationship between the Izod energy and fiber length, suggesting therefore that the impact fracture process is mainly governed by the reinforcement. The matrix plays a lesser role in terms of fracture energy, and optimization of the impact resistance of the composite could be based on reduction of fiber attrition.
434
T. Vu-Khanh, J. Denault, P. Habib, A. Low
10oo 900 000 Z '1o
a0o
700 t~ "N
600
O S00
E E
400
x
1 200~0,0,
,
~" _
,
o N .
0.0
0:,
0:,
0:,
0.,
a/W
Recorded m a x i m u m load o f P B T / PET long-fiber composites as a function of the ratio of crack length to specimen width for three-point bend samples. Fig. 6.
0.7
0.9
.
.
.
1.1
.
1.3
Average Fiber Length (mm)
Fig. 7. Izod impact resistance of PBT/PET long-fiber composites as a function of average fiber length.
SUMMARY AND CONCLUSION In this preliminary work on the effects of injection molding parameters on the mechanical performance of PBT-PET blend/long glass fiber composites, it has been shown that most of the molding variables and their interactions have significant effects on the properties. The results show that fiber-length retention is not the sole parameter to be optimized but that the matrix system is also affected by the molding conditions and has significant effects on composite properties. The process of fiber attrition does not change from one molding parameter to another. Molding conditions leading to maximum fiber length in the composite do not result in maximum strength. The composite elastic modulus remains constant with fiber length but is affected by other microstructural parameters. The result is in agreement with shear lag theory regarding the stress transfer between the matrix and the fiber in fibrous composite. In terms of toughness, although the composite is brittle on the macroscopic scale, the damage zone in front of the crack tip leads to a non-linear type of fracture. The measurement of the intrinsic fracture toughness therefore requires very large samples. The fracture process (after crack initiation) has also been found to be governed by molding parameters. As opposed to the strength property, the Izod impact energy of the composite is mainly controlled by fiber-length retention. This work is being pursued with a detailed investigation on the effect of molding conditions on the microstructure and morphology of the matrix, in connection with fiber attrition and orientation in the molded parts.
Effects of injection molding on mechanical behavior
435
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