Fracture toughness of weldlines in thermoplastic injection molding

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  • Fracture Toughness of Weldlines in Thermoplastic Injection Molding

    K. TOMARI and T. HARADA

    Osaka Municipal Technical Research Institute Joto-ku, Osaka, 536 Japan

    and

    2. MAEKAWA, H. HAMADA, M. IWAMOTO, &A. UKAI

    Kyoto Institute of Technology Sakyo-ku, Kyoto, 606 Japan

    Weldlines are fatal defects in many injection moldings of thermosetting resins and thermoplastics. Significant strength reduction by weldlines in thermoplastics is caused by poor adhesion, molecular orientation, and a V-notch effect. These factors have been little investigated in detail, in spite of being well known. In the present article, the V-notch effect on strength is discussed for two types of thermoplastics, polystyrene and polycarbonate. The depth of weldlines was o b tained by milling on the weldline surface, and the fracture toughness was mea- sured with a double edge notched specimen. Polystryrene, which was drastically weakened by weldlines, had relatively deeper V-notch regions and the fracture toughness was also reduced by weldlines. Although polycarbonate had the same fracture toughness as polystyrene, it had strong weldlines since the depth of weldlines was negligibly small.

    INTRODUCTION

    any have studied the weldline effect on the me- M chanical properties of injection moldings. Mal- guamera. et aL (1). discussed the effect of molding conditions on the weldline strength for polystyrene, polypropylene, etc. In another study (2), the relation- ship between the crystalline structure and strength was discussed based on scanning electron mi- croscopy (SEM), and the annealing effect was also discussed. Criens, et aL (3), clarified the relationship between part design and processing parameters for polystyrene and polyoxymethylene by examining the effects of weldlines on tensile properties of injection modlings with holes. Effects of the weldline on the strength of fiber-reinforced polycarbonate and polyamide were reported in our previous papers (4, 5). Further, there have been studies on the weldline in relation to creep and fatigue strength (6, 7) and to morphological structure (8- 10).

    It is thought that the strength reduction is caused by poor adhesion, molecular orientation perpendicu- lar to the melt flow direction, and the V-notch effect. A V-notch phenomenon has been reported to be built up on the surface of a molding by air entrapment or contamination between separate melt fronts (1 1). In spite of being well known, the V-notch effect has been little investigated in detail (1 2). The depth of a V-notch

    can be measured by the surface roughness meter, but its influence on the strength of moldings is not yet detailed. In our previous investigation (13), the depth of V-notches was determined on polystyrene injection modlings, being about 0.2 to 0.3 mm and strongly affecting the strength of molding.

    In the present study, fracture toughness of injec- tion moldings is measured using double-edge notched dumbbell specimens for polystyrene and polycar- bonate, and the V-notch effect on the strength of weldlines is discussed from the viewpoint of fracture mechanics.

    EXPERIMENTAL

    The materials used in this study were general pur- pose polystyrene (GPPS), Diarex HF-77, from Mit- subishi Monsanto Chemical Co., Japan, and polycar- bonate (PC), Iupilon S-2000, from Mitsubishi Gas Chemical Co., Japan.

    Dumbbell type tensile test specimens were pre- pared using a mold shown in Fig. 1. The specimens with a weldline at the center obtained by the double- gate molding are called weld specimens, while normal specimens without a weldline are called non-weld specimens, and were obtained by single-gate mold- ing, turning the plug P to the right-angle position to close the side gate.

    996 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1993. VOl. 33, NO. 15

  • Fracture Toughness of Weldlines in Thermoplastic

    -

    An injection molding machine, N lOOBII, manufac- tured by Japan Steel Works, was used, the perfor- mance of which was 100 tons in clamping force and 168 MPa at the maximum injection pressure. Molding conditions are listed in Table 1.

    The specimens for estimating the fracture tough- ness were double edge notched ones (DEN), shown in Fig. 2. Figures 2A and 2B show weld specimens and non-weld specimens with machines notches, respectively.

    The machined pre-notches as shown in the circle marked A were made in the following way: The U- notch with 2 or 3 mm depth and 0.6 mm width was first cut with a finer cutter at the both edges, and further sharpened at the tip by a razor blade (depth b = 1 mm). That is, the fine notch was carefully intro- duced under microscopic observations for placing it just on the weldline, thus resulting in the range of total notch length a = 0.7 to 3.2 mm for GPPS and 1.3 to 2.9 mm for PC.

    Tensile tests were conducted at a crosshead speed of 1.0 mm/min and 2.5 mm/min for GPPS and PC,

    0.50

    19.0 I 7 n l t I

    I . " I I i- ' 216 13.2 I I J

    t Fig. 1 . Layout and dimensions of mold cavity.

    Table 1. Injection Molding Conditions

    Polystyrene Polycarbonate

    Cylinder temperature PC) 180 300 Mold temperature PC) 40 80 Filling pressure* (MPa) 41.959.8 75.0 Holding pressure* (MPa) 0-89.7 75.0 Injection pressure** (MPa) 59.8 75.0 Injection time (s) 12 6 Cooling time (s) 20 20

    *for weld specimens, **for non-weld specimens.

    Weld line

    (B)Non-weld

    Detail of A Fig. 2. Specimens forfracture toughness test, double edge notched (DEN) specimens.

    respectively, at 20 f 2C and 60 k 5%RH with a uni- versal test machine, an Autograph IS-lOT, from Shi- madzu Co., Japan.

    CALCULATION OF FRACTURE TOUGHNESS

    The critical stress intensity factor, Kc, the so-called fracture toughness, of double edge notched (DEN) specimens with pre-notch length, a, was calculated at the maximum load, P,,. by the following equation (14).

    D n 1 / 2

    F l m a x U BW

    KC =

    F = 1.98 + 0.36(2a/W) - 2.12(2a /~) '+ 3 . 4 2 ( 2 a / ~ ) ~ (1)

    where B is the thickness of the specimen, W is the width, and F is the correction factor depending on geometry of DEN specimen. If the notches made in the specimen are regarded as V-grooves, the equation below can be given:

    (2) where 3' is the corrected thickness and Bn is the true thickness. In this study, Bn is expressed by the following relation:

    where V is the depth of V-notch generated during molding ( V = 0.1 in this case).

    Bn=(B-2V) (3)

    RESULTS AND DISCUSSION Polystyrene

    For discussion purposes, the results of our ear- lier experiments are cited in the following two subsections ( 13).

    Weld Strength of Polystyrene Fig. 3 shows the relationship between holding pres-

    sure and weld strength ULU for polystyrene. At the filling pressure of 41.9 MPa, u w has the low value of

    30 ' L 6 2 5 A-A-A-~- - I

    /

    150 20 40 60 80 100 Holding Pressure(MPa)

    Fg. 3. Relationship between tensile strength and hold- pressure:_filling pressure, 0 41.9, A 59.8 MPa.

    997 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1993, Vol. 33, No. 15

  • K. Tomari, T. Harada, 2. Maekawa, H. Hamada, M. Iwamoto, and A. ukai

    about 20 MPa at the holding pressure 0 MPa, and increased with increasing holding pressure up to 25 MPa. Since the non-weld strength u n was 54 MPa, the reduction of strength in the weld specimens was between 54 and 63%. At the filling pressure of 59.8 MPa, u w was about 25 MPa independently of the holding pressure. The strength reduction was about 54%.

    I t is considered that such a remarkable strength reduction is caused by the V-notch at the weld re- gion, and that the change of more than 20% in the strength depends on the depth of the V-notch. re- lated to the holding pressure.

    Depth of Weldline According to the microscopic observation of the

    fracture surface of the weld, the fracture surface was confirmed to have a smooth circumference area 0.2 mm wide, the inside of which was rough. After as- suming the V-notch to exist in the smooth area, the depth of V-notch was determined accurately in the previous paper by measuring the strength variation of the specimen with the stepwise elimination of the weldline using a milling machine.

    Fig. 4 shows the relationship between the depth of cut at the weld region, Dc, and tensile strength. The Dc at saturation, 0.2 mm in this Figure, could be defined as a depth of weldline, Dw, because of disappearance of the V-notch effect beyond the Dc value. Dw was between 0.2 and 0.3 mm for polystyrene. Fg. 5 shows the relationship between Dw and the

    weld strength, uw. I t was found that u w was markedly dependent on Dw. The deeper the Dw, the lower the weld strength.

    According to the surface roughness test, the depth of V-notches was reported to be several micrometers (15). It was

    5 0

    40 - B -30 5

    v, 20 22

    ?

    m c W L +

    v) c

    10

    0

    hundreds of micrometers. Thus the fracture tough- ness test was conducted using DEN specimens to evaluate the depth of weldlines in more detail.

    Fracture Toughness Test With DEN Specimen

    hgure 6 shows the typical load-displacement curves, for both non-weld and weld specimens, ob- tained from the tensile test with DEN specimens of GPPS. Brittle failure was exhibited. Since the load increases linearly with increasing displacement be- fore breakage, the concept of the linear fracture me- chanics can be applicable.

    Figure 7 shows the relationship between the frac- ture toughness, Kc, calculated by Eq 1 and the rela- tive notch depth, 2a/ W. For each specimen, Kc was almost constant independent of the notch length in the range between 0.1 and 0.5 times 2a/W. Thus the fracture toughness was the plane strain fracture toughness. From now, Kc obtained with Eq 1 is re- garded as plane strain fracture toughness, KIc. The average value of K,, for non-weld specimens was 3.1 MPa.mI2 and agreed with Marshalls study (161, in which K,, was reported to be about 1 to 4.5 MPa.m/ for non-weld specimens.

    30

    25

    20

    15 0.2 0.3 0.4 quite- different from our results-

    Depth of weld line, Dw (mm)

    Fg. 5. Relationship between depth of weldline (Dw). and weld strength (u w) for polystyrene.

    I

    1 1 1 1 I l

    0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Dc (mm)

    Fig. 4. Relationship between tensile strength and depth of cut (Dc): j?Uing pressure 4 1.9, holding pressure 41.9 MPa.

    1.2 z v u 0.8 a 0 J

    0.4

    0

    Non - weld -

    0 0.5 1.0 1.5 20

    Displacement (mm) Fig. 6. lljpical load-displacement curue in fracture tough ness test for GPPS.

    998 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1993, VOl. 33, NO. 75

  • Fracture Toughness of Weldlines in Thermoplastic

    On the other hand, K,, of weld specimens had a quite low value of 0.71 MPa.m'/2, about 80% lower than that of non-weld specimens.

    Consequently, V-notch propagation was facilitated by the lower fracture toughness because the molecu- lar bonding across the weldline interface is poorer than in the bulk material.

    Observation of the Fracture Surface

    Figures 8 and 9 show the micrographs of the frac- ture surfaces near the V-notch tip of weld specimens without machined notches and near machined-notch tips of DEN specimens, respectively. The arrow mark in the Figure indicates the direction of crack propaga- tion. The rib or the ripple pattern, an undulation usually taking place with polystyrene, could be o b served on each surface. For both weld and DEN speci- mens similar fracture surfaces were exhibited, in which crack propagation along the width direction could be recognized. Therefore, a V-notch on the weldline surface could act as the initiator of crack propagation.

    Fracture Toughness Estimated From the Depth of Weldlines

    The weld strength is relevant to the depth of the V-notch which exists on the weldline, as shown in

    0 0.1 0.2 0.3 0.4 0.5

    2al W Rg. 7. Relationship fracture toughness (Kc). and 2 a / W (polystyrene).

    Fg. 8. Micrograph offracture surface near the V-notch in the weld specimen.

    Fig. 5. By substituting (T w for P,,/BW in Eq 1 and Dw for a, the fracture toughness of unnotched weld specimens (K,,)* could be calculated. It is not the true fracture toughness, but can be regarded as the apparent fracture toughness. ( K,,)* is constant inde- pendently of the molding conditions as shown in Fig. 10. The average (K,,)* is about 0.71 MPa.m'/' and entirely agrees with the K,, of the DEN specimen.

    I t is confirmed that the depth of weldline, Dw, 0.2 to 0.3 mm, which was obtained by the milling method as mentioned in the previous report, is equal to that of the pre-crack that initiates brittle fracture.

    Polycarbonate

    Depth of Weldlines

    The depth of weldlines of PC was also measured by the milling method mentioned under Polystyrene. Figure 11 shows the relationship between the depth of cut, Dc, and the tensile strength of PC weld speci- mens. Since PC still has substantial ductility, the strength at break was adopted for estimation. The deviation was relatively larger than that of GPPS (see Flg. 4). The strength was low just at Dc = 0 and was about 60 MPa in almost the whole range of Dc. I t meant that the depth of weldline was negligible small for PC. I t is concluded that the weldline surface of PC is smooth and strong.

    -~

    Fig. 9. Micrograph of fracture surface near machined pre crack in the weld specimen.

    1.5 h

    N -E 1.2 ti3

    0.9 Y

    A 59.8

    0 ' J 0 x) LO a 80 100

    Holding Pressure (MPa) Fig. 10. Effect of holding pressure on apparent fracture toughness.

    999 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1993, VOI. 33, NO. 15

  • K. Tomari, T. H a r a d a 2. Maekawa, H . Hamada, M. Iwamoto, andA. Ukai

    Fracture Behavior

    Figure 12 shows the typical load-displacement curve of PC. PC samples always exhibited necking after the yield point, breaking after deformation. The non-weld sample broke after the center of the test bar was maximally stretched, while the weld specimen broke simultaneously as necking reached the weld- line, at relatively low strain. Thus, the non-weld sam- ple elongated more than the weld specimen. The strength reduction of PC due to the weldline was about 27%. which was not as remarkable as with GPPS.

    40 0.0 0.1 0.2 0.3 0.4 0.5

    Dc (mm) Flg. 11. Relationship between tensile strength and Dc; cylin- der temperature: 280C.

    4

    3 2 Y W

    - 0 2 a 0 J

    1

    0 0 50 100 150

    Displacement (mm) FLg. 12. mpical load-displacement curve for polycarbonate.

    Fracture Toughness of Weldline

    Although PC is a ductile material, the DEN speci- mens invite brittle fracture. The concept of linear fracture mechanics, therefore, can be applicable to PC as well as GPPS.

    The relationship between 2 a/ W and Kc is shown in Fig. 13. In the range of 2 a / W between 0.2 and 0.5, Kc was constant and, therefore, was regarded as K,,, although the deviation was greater than that for GPPS. K,, was 3.3 MPa.m'/2 for both the non-weld and the weld samples, that is, the fracture toughness was not weakened by the weldline.

    For PC, the chemical bonding at the weldline inter- face is as strong as that of the bulk material. I t might be considered that pronounced molecular orientation resulting from necking led to the debonding of molec- ular...

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