Fracture toughness of injection molded glass fiber reinforced polypropylene

Fracture Toughness of Injection Molded Glass Fiber Reinforced Polypropylene

S . HASHEMI and M. KOOHGILANI

London School of Polymer Technology University of North London London N7 8DB England

The fracture behavior of polypropylene reinforced with 30% by weight of short glass fibers was studied using single and double feed plaque moldings. Plaques were injection molded using several gate types and gate positions. Fracture toughness, K,, was calculated at different positions in the plaque moldings using single edge notched tension specimens. Fracture toughness was assessed in the directions parallel and perpendicular to the mold fill direction through measurements of the load to produce complete fracture. Results indicated that the value of fracture toughness is affected by the type of gate as well by size of gate. Position of the specimen also affected fracture toughness. Generally, specimens taken from positions near cavity walls gave higher toughness values than those taken from the center of the moldings. Furthermore, fracture toughness in the transverse direction was consistently higher than in the melt flow direction. Finally, in the case of double feed moldings, a much higher fracture toughness was obtained when the initial crack was perpendicular to the weld line than when it was placed inside the weld line.

INTRODUCTION the case of a double feed moldings than single feed

lass reinforced polypropylenes are engineering G thermoplastic materials with good mechanical properties. However, as a result of the effects of molding conditions, a molded component may exhibit a very complex distribution of fiber orientations and consequently an anisotropy in mechanical properties. Many authors have studied the glass fiber orientation effects (1 -5). Folkes’s review (1) outlines many con- siderations attendant on fiber orientation within the plane and through the thickness of a molding. Other authors (e.g., 5) have recognized that fiber orientation in a molding, could not be simply represented, thus making it difficult to design efficiently with glass reinforced materials. However, what is shown by many investigators (e.g., 5) is that the thickness of the molded component consists of two skin layers and a core layer. Studies show that fibers in the skin layers are mainly oriented parallel to the melt flow direction, whereas in the core layer they are mainly oriented perpendicular to it. Moreover, orientation of the fibers in these layers seems to be affected by the thickness of the molding as well as fiber content of the material.

The work of Gennaro (6) indicates that the Young’s modulus and impact energy of single and double feed flash moldings made of glass reinforced polyamide material are affected by the type of the gating and vary from place to place within the molded compo nent. Variations observed were more significant in

- -

molding. The aim of this paper is to use linear elastic frac-

ture mechanics to study the influence of gate type, size, and position upon fracture toughness of an injection molded glass fiber reinforced polypropylene. Plaque moldings were divided into rectangular strips in order to establish the effect of specimen position on fracture toughness.

FRACTURE OF SINGLE GATE MOLDINGS: EXPERIMENTAL PROCEDURE

Processing

The material used in this study was polypropylene reinforced with 30% by weight of short glass fibers. This material, supplied by BASF under the trade name Novolen 11 11 LX GB6, had a melt flow index value of 2.5 g/lO min. The material was injection molded into a single mold cavity with dimensions 88 x 88 X 1.5 mm (thickness) on a Peco 15 Mr molding machine. Mold filling was conducted through a single-gate and by adopting gates of different shapes, sizes, and positions. These options were achieved by using insert blocks containing the gate arrangement. Table 1 summarizes shape, size, and the position of gates used in this study. I t must be noted that gate depths given in Table I are actual sizes on the gate blocks, and as indicated in some cases they are big- ger than the depth of the mold cavity. However, once

1124 P O L Y M E R ENGINEERING A N D SCIENCE, MIDJULY 1995, Vol. 35, N o . 13

Fracture Toughness of Injection Molded Glass Fiber Reinforced PP

Table 1. Gate Size. ShaDe. and Position (Dimensions Are in mm).

1 GATE I SIZE lPoSITIONl GATE

Table 2. Processing Parameters.

Injection time (s) Injection pressure (MPa) Screw speed (rpm) Cooling time (s) Mold temperature (“C) Melt temperature (“C)

zone I zone II zone 111

10 5.52-7.59 150 12 23

240 245 250

these blocks were placed in position, the actual depth of the gate was that of the mold cavity (i.e., 1.5 mm).

The processing conditions are specified in TQble 2. Apart from injection pressure, which had to be al- tered for some moldings to produce a complete molding, other processing parameters were kept constant throughout the study.

Specimen Preparation Ten plaques were molded for each gate: five were

used to study fracture in Y-direction, and the remain- ing five were used to study fracture in the X-direction, as shown in FYg. 1. Each plaque was then divided into five rectangular strips of dimensions 88 X 15 x 1.5 mm. Specimens with their length parallel to the X-direction were labeled X1 to X5, and specimens with their length parallel to the Y-direction were labeled Y1 to Y5, as shown in Fig. 1. Each specimen was then notched to produce a single edge notched test (SENT) specimen. Edge notches were inserted midway along the length of the specimens in two steps: first a saw cut was made, which was then sharpened with a razor blade. I t must be noted that for center edge gated plaques, the edge notch was placed either in the direction of possible melt flow (MFD) or perpendicular to the melt flow direction. For the comer gate and angle gate moldings, the initial notch was not in either of the two directions. How- ever, ten specimens were prepared from these two moldings in which the initial notch was introduced in the flow and in the transverse directions, as shown in Fig. 2 (note that only one specimen could be prepared per molding).

1.64 a R

I I L

i, k

(Injection point t (In~ection .) point (Jnjection point for

for edge gated for angle gated comer gated plaques) plaques) plaques)

.m------4

i, Fg. 1 . Single feed moldings.

Testing Conditions

Fracture toughness measurements, K,, were car- ried out on the single edge notched tension specimens with gauge length (Z) of 45 mm, as shown in Fig. 3. This parameter was then examined as a func-

POLYMER ENGINEERING AND SCIENCE, MIDJULY 7995, Vol. 35, No. 13 1125

I Fig. 2. ingS.

S. Hashemi and M. Koohgilani

Calculation of Fracture Toughness, K, The critical stress intensity factor, K,, the so-called

fracture toughness, was determined using Linear Elastic Fracture Mechanics (LEFM). According to LEFM, K , can be calculated by means of the following equation;

Ic-------SS------+

K , = a ,Yh (1)

where 0;l is the gross fracture stress, a is the pre- notch length, and Y is a geometrical factor. Since the specimen ends in this study were clamped, the calculation of the geometry factor, Y, as given by Brown and Strawley (7). was inappropriate. They assume that the tensile force is uniformly distributed across the width of the specimen. This assumption is consis- tent with pin-loading, provided that the distance b e tween the pins is not less than three times that of the specimen width. Moreover, when SENT specimens are pin-loaded, the crack tip experiences a bending effect, which would be absent when the specimen ends are fuced. Harris (8) obtained an expression for Y for the case in which the specimen ends are clamped and the distance between the clamped ends is three times the specimen width, W. He defined Y as

Flow and transverse specimens for comer feed mold-

T-T Z=-45mm

I

Fig. 3. Single edge notched geometry.

(2) 5 G

2 1/2 Y =

[20 - 13(a/W) - 7 ( a / W ) 1 The Y function given by Eq 2 was used in Eq I for calculating K , in this study.

RESULTS AND DISCUSSION

Fracture Behavior

One of the advantages of fracturing test specimens at low speed is that the time scale over which fracture processes take place is often long enough to allow various stages of crack propagation to be detected. The main features of the fracture process in a polypropylene composite are shown in Fig. 4. As shown in the Figure, the load-displacement diagram exhibit.ed several features that are identified on the diagram. When the applied load reached a critical value, a tiny narrow stress whitened region containing some damaged material was noted at the tip of the crack (initiation). The load at which this damage zone was first observed (with the aid of an optical microscope) is referred to here as the initiation load, Pinit. As loading increased, the damage zone extended in the width direction of the test specimen under

. .~ .- - - . - rising load. Eventually, the damage zone extended across full width of the test specimen as soon as the applied stress reached a maximum value, P,. At the maximum load the initial crack began propagating unstably through the ligament area, which by now was completely damaged and weakened. Thus the maximum load on the load-displacement diagrams also represents the load at which an unstable fracture takes over. In some test specimens, the final stage of the fracture process was accompanied by

tion of specimen position and notch orientation. All tests were performed in an Instron testing machine at a crosshead speed of 5 mm/min using pneumatic clamps. The load-displacement trace for each test specimen was monitored on the Instron chart recorder. All tests were performed at room temperature.

1126 POLYMER ENGINEERING AND SCIENCE, MID-JULY 1995, Vol. 35, No. 13


DAD

fully damaged

within the damaged zone 1 Unstable fracture due initiated at the tip of A damaged the crack zA-[ to a crack pmpagating

[Matrix fibrillation I

.)

DISPLACEMENT

Fig. 4. ll~pical load-displacement trace.

matrix fibrillation. Since fibrils spanned the two fracture surfaces, the applied load was transmitted from one half of the specimen to the other half through these fibrils. In these specimens, the complete sepa- ration of the two fracture surfaces occurred when fibrils reached their breaking strain. Thus, the sud- den drop in load was often followed by a slow decay- ing one.

Fracture toughness, K,, in this study was assessed through measurements of the load to produced complete fracture, 1.e.. through P,. According to Eq 1, for K, to be a characteristic parameter, its value should be independent of the crack length, a Since our aim here is to compare K , values of a wide range of plaque moldings with different gatings, it is essential to eliminate any variations in K , that may arise as a result of variations in crack length. To ensure this, several SENT specimens were cut from position X3 on several plaques that were molded using a semicircular gate. These specimens were notched to various a/ W ratios and subsequently fractured at a constant crosshead speed of 5 mm/min. K , values obtained from these tests are presented in Table 3. It can be seen that K , values vary in a systematic manner with a/W. K , increases with increasing a/W but reaches a constant value of 3 M P a G when a / W r 0.30. Based on these results, all the SENT specimens were notched to a/ W ratio of 0.5 to avoid any variations in K , due to crack length. Also given in Table 3 are ratios of net-section stress, a,. to tensile yield stress, cry (measured as 76 MPa at 5 mm/min under uniaxial tensile test). Net-section stresses were calculated using the following equation:

a, 1 - a / W

a, = (3)

Table 3. Effect of Crack Length on Fracture Toughness. ~

U C Kc aIW (MPa) ( M P a f i ) ( m I U Y

0.09 33.79 2.46 0.49 0.16 24.64 2.54 0.39 0.20 23.13 2.67 0.38 0.25 20.45 2.73 0.36 0.30 21.98 2.95 0.41 0.35 17.83 3.00 0.36 0.42 16.69 3.20 0.38 0.50 13.38 2.93 0.35 0.51 13.19 3.00 0.35 0.62 1 1.03 3.08 0.38 0.64 10.30 3.00 0.38

Results indicated that the ratio of an/cry is below 0.5 and therefore use of LEFM was fully justified.

I t must be noted that the a/ W dependence of K , in Table 3 suggests that the R-curve effect is occur- ring during fracture. The toughness values reported here must therefore be regarded as apparent toughness rather than the true fracture toughness. Never- theless, since the aim of this study is to use K , for comparative purposes, the apparent toughness should suffice.

Fracture Toughness in the Flow Direction

Figure 5 shows mean fracture toughness (K, ) values of the material at different specimen location in the X-direction using several types of gate. Figure 5 shows clearly that fracture toughness within the fiber reinforced plaques depends on specimen location as well as gate type. The highest K , for specimens with their length perpendicular to the melt flow direction (notch in the flow direction) is obtained for the specimens at position X1 for all types of gate. This is due to the presence of a higher percentage of fibers aligned normal to the fill direction at position X1 near the cavity wall and is dictated by fiber orientation with respect to direction of loading. For a given type of the gate, variation in fracture toughness from position X3 to position X5 is practically insignificant, indicating that the microstructures of the moldings at the notch tip in these locations are more or the less the same, i.e., the material has the same state of orientation with respect to the notch width.

The type of gate also affected the value of fracture toughness. It can be seen that among edge gated plaques, semicircular edge gated plaques gave the highest values of fracture toughness and those man- ufactured by a rectangular edge gate gave the lowest values. However, from results presented in Fig. 6 one may conclude that the microstructure of the molding is affected by gate dimensions as well as gate geometry.

Another feature of the data in Fig. 5 is that position of the gate also affects the microstructure of the molding and hence the value of fracture toughness. It can be seen that the comer gate and off-center angle gate moldings give higher toughness values than cen-


S. Hashemi and M. Koohgiiani

4

--o- Vedgegate

5

- E 3.5 k

2.5 I 1 x1 x2 x 3 x 4 x 5

SPECIMEN POSITION Fig. 5. Fracture toughness us. specimen position in the X-direction.

4 I I I

................................................................................................................................ --%--.

- -* - Circular gate (r -3.87 mm)

Rectangular gate 18.08 x 3.68 mm) x 3.97 mm) .....................................................................................................................................................

2.5 , I 0 I x1 x2 x 3 x 4 x 5

SPECIMEN POSITION

Ftg. 6. Fracture toughness us. specimenposition for rectangular and circular edge gated moldings in theX-direction for two daerent gate sues.

ter edge gated moldings. Recall that for these two moldings the initial notch was at 45" to the probable direction of melt flow. The crack in these specimens crosses the flow lines as it propagates across the sample width, and because of this the material offers a greater resistance to crack propagation. However, when the initial notch was placed in the direction of melt flow (see Fig. 2). fracture toughness was in the range of 2.85 0.08 M P a G , which is roughly 12% lower than the value obtained when the initial notch was at 45" to the melt flow direction.

Fracture Toughness Transverse to the Melt Flow Direction

Figure 7 shows the fracture toughness (K,) of the material at different specimen locations and for several gate types transverse to the flow direction.

Clearly, fracture toughness in the transverse direction depends on the specimen location and the type of gate used in molding of the test pieces. One may observe that specimens taken from the positions closer to the cavity walls (Y1 and Y5) give a much

1128 POLYMER ENGINEERING AND SCIENCE, MIDJULY 1995, Vol. 35, No. 13


a E 2 3.5

\

- v) VJ

i 2

! 2 3 :

ck

2.5

higher K , than those taken from the center of the plaque. The real cause of this was the existence of a fiber orientation in the vicinity of the walls where most fibers were aligned parallel to the walls, leaving the molding strong to the load parallel to the direction of fiber alignment.

Generally, there is more variation in fracture toughness with specimen position in the transverse direction than was noted in the flow direction. I t is noteworthy that the effect of gate shape and size on fracture toughness in the transverse direction was similar to that observed in the flow direction. This means that in both directions the circular gate gave the highest and the rectangular gate gave the lowest

/..-..... ................................. "" .................................... ~ ...... " .................................... "

- --a,-- C i d a r edge gate --%I--. Rectangular e a e ..................... " ..................... .... .

---e-- Flash gab --e- vgate

----*..- Angle gate ! .......................................................................................................................... -

toughness values. However, as illustrated in Ftg. 8, dimensions of the gate also affect values of fracture toughness.

The position of the gate also affected the value of fracture toughness in the transverse direction. Cor- ner gate and off-center angle gate moldings consistently gave lower toughness values than centrally edge gated moldings. For the case in which the initial notch was perpendicular to the melt flow direction, the comer gate moldings gave a mean K, value of 3.4 f 0.12 M P a 6 .

I t is noteworthy that fracture toughness values in the transverse direction were consistently higher than in the flow direction, indicating that the complex fiber

SPECIMEN POSITION FUJ. 7 . Fracture toughness us. specimen position in Y-direction.

4.5 ..................... ".I .................. --%I--- drcdar gate (r I 567 mm)

1 1 t

2.5 I 1 I I Y1 Y2 Y3 Y4 Y5

SPECIMEN POSITION

FYg. 8. Fracture toughness us. specimen position in the X-direction for two dtfferent gate sues.


S. Hasherni and M. Koohgilani

orientation distribution in injection moldings causes anisotropy in fracture toughness. Ratios of K , in the Y direction to K , in the X direction were calculated for two specimen positions-one at the center of the molding (Y3 and X3) and the other at the cavity wall (Y1 and X1). Results shown in Fig. 9, indicate that the toughness ratio for all edge gated moldings is greater than 1. Specimens taken from the center of the moldings (X3, Y3) gave a toughness ratio of about 1.08 and those taken from the positions X1 and Y1 gave a toughness ratio of 1.2, both ratios being more or less independent of the gate type. It is noteworthy that for the comer gate and the off-center angle gate moldings, toughness ratios were slightly lower than 1 , indicating these moldings are less anisotropic than the edge gated moldings.

DOUBLE-GATE MOLDINGS

Moldings were prepared in the form of square plaque of dimensions 88 x 15 x .5 mm with two edge gates (semicircular in shape and each with diameter of 3.21 mm) located on the same side of the square, as shown in Fig. 10. The molten material enters the mold cavity as two “parallel flows,” forming a weld line in the plaque. The injection molding machine was set at 5.52 MPa injection pressure, 150 rev/min screw speed, 10 s injection time, 12 s cooling time, and a barrel temperature profile of 240, 245 and 250 “C. The mold temperature was 20°C.

Each plaque was then divided into five rectangular strips of dimensions 88 X 15 X 1.5 mm, as shown in Fig. 10. As before, specimens with their length perpendicular to the melt flow direction were labeled Xl-X5. The initial notch in these specimens was placed inside the weld line, as shown in Fig. 9. Speci- mens with their length parallel to the melt flow direction were also notched and labeled Yl-Y5. All test specimens were tested to failure at a crosshead speed of 5 mm/min, as before. Results obtained from these tests are shown in Figs. 11 and 12 as K, versus specimen position and compared with the results

Circular Rectangular Flash V Corner Angle

GATE

Flg. 9. h-acture toughness ratio ( Y 3 / X 3 and Yl / X l ) .

-t

88

Y (MFD)

I t, t t (INJECTION POINTS)

t (INJECTION POINTS)

Q. 10. Doublefeed molding.

t

Y (MFD)

t, obtained by way of moldings having a single circular gate of the same size.

Clearly, fracture toughness values for double gate moldings in the flow direction are considerably smaller than those of a single gate molding, as shown in Fig. 1 1. This is dictated by the fiber orientation at the weld line with respect to the direction of loading and the crack. The existence of a unique fiber orientation distribution in the vicinity of weld lines, where most fibers are aligned along the weld, leaves the molding weak to loads normal to the weld line. As a result, fracture toughness was reduced by weld lines.

In the transverse direction, fracture toughness showed a clear maximum ( K , = 4.3 M P a 6 ) at specimen position Y3 (see Fig. 12). This is also dictated by the fiber orientation at the weld line with respect to

1130 POLYMER ENGINEERING AND SCIENCE, MID-JULY 1995, YO/. 35, NO. 13

3 -

- x1 x2 x3 x4 x5

,...........................................,,....,..,,,.......,.. .................................. I --Q-. Single gate (r = 3 . ~ 7 mm)

/ --*-- Doublegate(r=S.S7mm) ....................... ~

D-----------.._q--------------- Q ---__ ---------- -- ---_ --- --. 13%.

SPECIMEN POSITION

Fig. I I . Fracture toughness us. specimen position in the X-direction for single gate and double gate moldings.

SPECIMEN POSITION

Fig. 12. Fracture toughness us. specimen position in the Y-direction for single gate and double gate moldings.

the direction of loading and the crack. In this case, the alignment of the fibers along the weld leaves the molding strong to loads in the direction along the weld line. The initial notch in specimens taken from position Y3 is perpendicular to the weld line, and therefore it encounters a higher resistance to crack propagation than any other position inside the molding-thus indicating that the complex fiber orientation distribution in injection moldings causes anisotropy in fracture toughness of welded components.

CONCLUSIONS

Results presented in this paper indicate that fracture toughness varies from position to position within the injection molded plaque. In single gate moldings, specimens taken from positions near the mold walls gave consistently higher toughness values compared with the specimens taken from the positions near the center of the molding. Results also indicate that the toughness of the material is affected by the type of gate as well as its dimensions. Furthermore, in all

POLYMER ENGINEERING AND SCIENCE, MIDJULY 1995, VoI. 35, No. 13 1131

S . Hashemi and M. Koohgilani

cases, fracture toughness in the transverse direction was always higher than that in the flow direction. In the case of double-feed moldings, with the initial notch placed inside the weld line, a considerable re- duction in fracture toughness was ascertained. On the other hand, higher toughness values were measured when the initial notch was perpendicular to the weld line.

ACKNOWLEDGMENT The authors thank the BASF Plastics Division for

providing the material.

1.

2.

3.

4. 5. 6. 7. 8.

REFERENCES M. J. Folkes, Short Fiber Reinforced Thermoplastics, R e search Studies Press, Chichester, U.K. (1982). C. M. R. Dunn and S. Turner. Composites-Standards, Testing, and Design, IPC Science and Technology Press, Guildford. U.K. (1974). P. F. Bright and M. W. Darlington, Plast Rubb. Roc. Appl, 1. 139 (1981). D. McNally, Polyrn Plast Technol Eng., 8. 55 (1977). K. Friedrich, Compos. Sci Technol, 22. 42 (1985). A. Gennaro. Plast Rubb. Process. Appl, 9, 241 (1988). W. F. Brown and J. E. Strawley, ASTM STP, 410 (1966). D. 0. Harris, I. Bas. Eng., 49, 89 (1967).

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Fracture toughness of injection molded glass fiber reinforced polypropylene