Knit Line

K.

Fractures in Injection Molded Liquid Crystalline Polymers

ENGBERG, A. KNUTSSON,* P.-E. WERNER,** and U. W. GEDDE***

Department of Polymer Technology The Royal Institute of Technology

S-1 00 44 Stockholm, Sweden

The microstructure and fracture behavior of injection molded samples of un- filled and filled grades of liquid crystalline polymers (Vectra“ and Ultrax@) con- taining; cold and warm knit lines have been studied by scanning electron micros- copy, X-ray scattering, and Instron tensile tests. Four process parameters-melt temperature, mold temperature, injection time, and holding pressure-have been varied in accordance with a 24 factorial design experiment. In cold knit lines a 200 pni thick region with transverse orientation with Hermans orientation func- tion f := 0.12-0.18 is formed on both sides on the weld area. The strength of the cold knit lines is very low, 9.0-19.8 MPa corresponding to 15-20% of the full strength of the material. The strength of the warm knit lines increases markedly with increasing distance from the insert and is generally significantly greater (40-45 MPa, 48 mm from the insert) than in the cold welds. Annealing at 260400°C of samples containing cold knit lines causes first a partial healing of the knit line and later extensive chemical degradation of the polymer.

INTRODUCTION failed at a fracture stress corresponding to 50% of

nit lines form when two or more polymer flow K fronts unify. The mechanical strength of the knit line material is generally low. This is primarily because the knit line region exhibits a disturbed and heterogeneous microlstructure with relatively few molecular connectionis bridging the weld area (1). When a knit line is formed by a co-linear impinge- ment of two melt fronts (“cold knit line”), chain ori- entation of the knit line material will be perpendic- ular to the major flow directions. This is caused by the radial flow component in the advancing flow front (2). If the knit line is formed by a splitting and rejoining of the advancing melt front when it passes a n obstacle in the cavity, a so-called “warm knit line” is formed. The material surrounding the knit line exhibits a chain orientation along the major flow direction. One important cause of mechanical weak- ness of materials containing knit lines are V-notches which form on the surface of the knit line region (3, 4). These notches are formed by compressed air which is trapped between the two approaching melt fronts and they act a s stress concentrators.

Williams and Cleeireman (5) and Criens and Moslk (1) showed that injection molded samples of brittle amorphous polymers; (PS, SAN) containing cold welds

Erlcsson Telecorn AB. P. 0. E,ox 2072. S-291 02 Krlstlanstad. Sweden. *’ Department of Structural Chernlstry. University of Stockholm. S-106 91 Stockholm. Sweden. *** To whom correspondence should be addressed.

1620 POL

the full strength of the material without knit lines. Warm welds formed in the same polymers had a corresponding fracture stress ratio of 75%. Process- ing at high melt and mold temperatures which reduce the chain orientation in the sample is favorable for the knit line properties of these polymers (1, 6). Criens and Moslk (1) showed that the knit line strength in ductile amorphous and semicrystalline polymers (ABS, PC, and POM) is about 90-100% of the full strength of the material without knit lines. They also showed that the knit line strength in these polymers was relatively unaffected by variations in process parameters. Malguarnera and Manisali (7) showed that PP exhibited knit line stress factors of about 80-90% in cold welds and that the crystalli- zation conditions play an important role for the knit line properties. The ability of a polymeric material to reduce stress concentration by yielding is thus clearly the key factor for the knit line strength. More ductile polymers exhibit a higher relative knit line strength (1).

Thermotropic liquid crystalline polymer (engineer- ing plastics presently in use are nematic main chain copolyesters) are processed by injection molding and extrusion. The low melt viscosity, good mechanical properties, and low coefficient of thermal expansion are the primary motivations for the use of these polymers. Potential problems are the unbalanced mechanical properties and the knit line properties.

.YMER ENGlNEERlNG AND SCIENCE, DECEMBER 7990, VoI. 30, No. 24

Kni t L ine Frac tures i n Injection Molded LCPs

Surprisingly no papers have been published on the knit line properties of liquid crystalline polymers. In this paper, data on the microstructure, i.e. the chain orientation distribution, and the fracture properties of both unfilled and filled liquid crystalline polymers are reported.

EXPERIMENTAL

Three different grades of Celanese Liquid Crystal Polymer Vectra@ were used: Vectra@ A950 (referred to as V9). VectraO A515 (V5), and Vectra@ A130 (Vl) . The two latter are based on V9 and contain different additives. V 5 contains 15% (v/v) of mineral filler and V1 contains 30% (v/v) of short glass fiber. The second polymer studied was Ultrax@ 4003 (BASF; referred to as U4). This material contains no filler additive. The specimens were generated by injection molding in a Netsal Neomat 70/45 injection molding machine equipped with three different molds (Fig. 1 ).

Mold 1 generated samples containing a cold knit line (Fig. la ) . The degree of elongational flow of the two melt fronts was altered by varying the geometry (a/b ratio) of the cavity. Samples were prepared at four different a/b ratios: 0.25, 0.50, 0.75, and 1.00. Materials V 5 and U4 were used in these studies. Mold I1 generated specimens containing one warm and one cold knit line (Fig. 1 b). Mold 111 generated specimens with a warm knit line which extended 60 mm out from the insert (Fig. lc) . Materials V9 and V1 were used in the studies involving molds I1 and 111.

Four different processing parameters: melt tem- perature ( T ) , mold temperature ( M T ) , injection time (I), and holding pressure ( P ) , were varied in all the experiments (molds) according to a 24-factorial design experiment (8). The actual values of the process pa- rameters are presented in Tab le l . Two levels, one low (-) and one high (+), were fixed for each of the four variables. Each series of experiments involved all the 16 possible combinations of the variables. The main effects were assessed for each parameter ac- cording to the expression:

' t i I

Mol: 111

- 2 u(T-, M T , I , P ) MT.1.P

The factorial experiment also permitted the inter- action effects to be calculated, but no such effects were revealed. The standard errors of the main ef- fects were calculated from the 10 replicated runs obtained for each of the 16 combinations.

A number of V9 samples prepared in mold I (a/b = 1) were annealed under isothermal conditions at temperatures between 260 and 300°C for differ- High level Low level

were conducted in a specially designed mold. A Melt temperature ["C] 330 (340)'a' 290 (320)@'

Fig. 1. Molds with d imens ions in mm.

Table 1. Process Parameters Used in 24 Factorial Experiments.

ent periods of time. The annealing treatments Processing (+I (-)

small compressive load was applied over the welding Mold temperature ["C] 120 90 3.0 1 .o Injection time [s]

Holding pressure [MPa] 5.0 2.5 area during the heat treatment.

Samples of atactic polystyrene, Vesteron@ 1 14-3 1 (Hulz) were generated in all three molds (T = 250°C; (.)Values within brackets concern samples of U4.

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K. Engberg, A. K n u t s s o n , P.-E. Werner , and U. W. G e d d e

MT = 50°C; I = 2 s; P = 35 MPa) to serve as reference samples.

The fracture stress at 25°C of the different samples was obtained in an Iristron Universal Testing In- strument Model 1122 at a tensile strain rate of 1 mm/min. Specimens from mold I were tested as obtained. The specimen from mold I1 and mold 111 were divided into three separate test specimens and the narrow section of the specimens was obtained by drilling and broaching 1 Fig. 2). Samples for measure- ment of the strength of the non-knitted material were taken from molds I (applied stress parallel to the melt flow direction) and I1 (between gate and first insert; applied stress perpendicular to the melt flow direction).

The microstructure was revealed by scanning elec- tron microscopy (SEM) on samples fractured at liquid nitrogen temperature Wide-angle X-ray scattering (WAXS) was performed on 200 pm thick samples taken from the knit line region. Figure 3 shows the location of the samples studied by SEM and WAXS. A Guinier-Hagg camera with transmission geometry and strictly monochromatized CuKeI radiation with X = 0.1540598 nm was used. The single coated films were measured by a computer-controlled single beam microdensitometer (9). Diffractograms were obtained from the samples at different angular posi- tions with respect to a fixed director (flow direction) to obtain the azimuthal-angle-dependence of the scattering in order to determine the chain-axis ori- entation. Computer assisted tomography (CAT) meas- urements were carried out in a Siemens Somatom DR CT (SkellefteH Hospital, Sweden) on samples from mold I (a/b = 1).

RESULTS AND DISCUSSION

Figure 4 shows the stress-strain curves of a few selected specimens. Samples with cold knit lines

Gate L

a

I

111

.m,ce I I 48.0J

I

b Fig. 2. Specimens forfracture testing taken from molded parts. The narrow sections of the specimens were ob- tained by drilling arid broaching.

Fig.

Welding area

....................

Mold I

Mold 111

3.

. . . ..-+ .... * .....)......... * ...... * , I 1 9 j j j Knitline

- I i (x-y plane) . 1

48 ;

Samples for X-ray scattering.

100

C

L 1.0 1.1 1 2

L/L,

Fig. 4. Stress-strain curves from Instron experiments for samples containing a cold knit line (a]: a warm knit line (b]: without knit line with the majorflow direction parallel with the applied tensile stress (c); and without knit line with the majorflow direction perpendicular to the applied tensile stress.

(curve a) fracture in a brittle manner at low strains whereas samples containing warm welds (curve b) or without knit lines (curves c and d) exhibit some duc- tility and fracture at significantly higher stresses and strains. In specimens where the load is applied par- allel to the major melt flow direction, i.e. along the direction of chain orientation, the fracture stress is higher and the fracture strain lower than in the samples with transverse chain orientation (cf. curves c and d). Table 2 presents the average values (over all z4 factorial experiments) for the different samples (without knit lines) with both parallel and transverse chain orientation. The fracture stress for the Vectra polymers (V9, V1, and V5) was 90-95 MPa with parallel chain orientation and 63-75 MPa with trans- verse orientation. The Ultrax polymer was studied only in mold I and its fracture stress in the parallel direction was 61 MPa. Earlier data by Hedmark, e t al. (lo), on the oriented surface skin of injection molded poly(p-hydroxybenzoic acid-co-ethylene ter- ephthalate) with a molar ratio of 0.6:0.4 (“X7G) are consistent with the presented data: the fracture

1622 POLYMER ENGINEERING AND SCIENCE, DECEMBER 1990, Vol. 30, No. 24

Knit Line Fractures in Injection Molded LCPs

Table 2. Average Fracture Stress Data'") for Samples Without Knit Lines.

Fracture stress'') Material/mold Flow directionlb) ( M W

parallel 61.3 & 3.1 90.8 _t 10.7 94.3 +. 12.3 93.1 k 4.7 45.4 f 5.0

63.1 f 9.6 34.3 _t 4.1

U4/1 V9/H v1/11 v5/1 PS/I v9/111 v1/111 PS/lll

perpendicular 75.4 & 7.7

"'Data of all 2' factonal design experiments are included. ID' Flow direction with respect to direction of subsequently applied stress

Including the standard deviation for single data points.

stress and strain were respectively 160 MPa and 4.6% in the parallel direction and 22 MPa and 70% in the transverse direction. Injection molded DIN samples (DIN 53455, part 3; mold gate positioned at the end) possessed the following ultimate stress data (1 1): 120 MPa (U4). 150 MPa (VS), and 200 MPa (Vl) . The discrepancy in the fracture stress between the two sets of sample may be due to geometric factors (radius of narrowing section and surface roughness) and microstructure. The highly oriented skin mate- rial a t the "side" surface was removed by the drilling of the samples used in this study.

Figure 5 shows the main effects of the four pro- cessing parameters on the fracture stress. For the unfilled polymer (with parallel orientation) signifi- cant main effects are obtained for melt temperature and injection time. A decrease in melt temperature and injection time leads to more chain orientation and a greater strength (Fig . 5; upper diagram). Sig- nificant main effects for the mineral filled polymer were obtained for mold temperature and injection time. The lower injection time leads to more chain orientation and thus a greater strength. The reason for the positive effect of the high mold temperature on the strength is less obvious but it may be due to differences in the surface roughness in samples pro- duced under different mold temperature conditions. The bottom graph of Fig. 5 relates to the mechanical features of samples with transverse orientation, and significant and expected main effects were obtained for melt temperature and injection time, primarily due to the reduction in chain orientation caused by increased melt temperature and injection time.

The cold knit lines generated in Mold I with differ- ent geometries (a/b ratio) had indeed a low strength which was essentially unaffected by the geometry of the mold (Fig. 6). Data for the average fracture stress of the samples containing cold welds are presented in Table 3. The strength of the filled polymers, par- ticularly the fiber-filled grade (Vl) was significantly greater than that of the unfilled LC grades (U4 and V9). Significant positive main effects on the strength were obtained, especially for melt temperature and to a lesser extent for mold temperature (Fig. 7). The fiber-filled polymer (Vl) again exhibited the strongest increase in knit line strength (3-4 MPa) for each of

men Mold InJnlectlon Holding temp. temp. t h e pressure

Fig. 5. Main effects offracture stress of samples without knit lines. The upper figure concerns samples prepared in mold I . The tensile stress is applied parallel to the melt

flow direction. The lower figure relates to samples pre- pared in mold 11. The tensile stress was applied perpen- dicular to the meltflow direction.

FIQ 5 Engberg et al

0 t i 4 0 v5

P i

I I I 1 0.25 0.50 0.75 1 00

a h

Fig. 6. Fracture stress of cold knit lines [mold I ) plotted as a function of mold geometry [a/b), i.e. degree of elon- gationalflow in the cauity.

Table 3. Average Fracture Stress Data'") for Samples With Cold Knit Lines.

~ ~~ ~ ~ ~ ~ ~ ~~~ ~~~~~~~~~~

Fracture stress'b) Material/mold Mold ( M W

u4 I 9.0 k 1.4 v9 I1 10.8 _t 1.6 v5 I 14.7 _t 1 .O v1 I I 19.8 & 3.0 PS I 24.2 k 0.7

I I 22.9 k 1.4 PS "'Data of all factorial design experiments are included.

Including the standard deviation for single data points.

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K . Engberg , A. K n u t s s o n , P.-E. Werner , a n d U. W. G e d d e

- h

Melt Mold Injnctlon Holdlng temp. temp. t h e pressUXe

Fig. 7. Estimated main elfects of processing parameters on the cold knit line strength. The bars indicate the standard deuiation of a single data point.

these processing variables. The injection time was a less important factor except in the case of the fiber- filled grade (Vl). The maximum strength of the V1 samples (cold welds) obtained at the higher melt tem- perature and longer injection time was 23.6 MPa whereas the recorded minimum value obtained at low melt temperature and short injection time was 16 MPa. The knit line factor (uknltIlne/uo) ranged from 10 to 16% for the unfilled polymers (U4 and V9). The filled grades (V1 and V5) exhibited somewhat higher values, 14-21%. The reference material, PS, exhib- ited a cold knit line fracture stress of 24.2 MPa for samples generated in mold I (a/b = 1.00) and 22.9 MPa for samples generated in mold I1 which is close to the earlier reported (1, 5) 50% of the strength (45.4 MPa) of the non-knitted material (cf. Tables 2 and 3) .

The test results for sample B (mold 11) were associ- ated with a significant scatter. Early crack formation of the material in the region behind the insert was evident (see the early drop in the stress-strain curve in curve b of Fig. 4). The extreme brittleness of this material may be due to V-notches and to the “cold character of the weld just behind the insert. The average knit line fracture stress data of these speci- mens and the main eflfects discerned were marred by the large standard deviations and no further conclu- sions could be drawn based on these data.

Figure 8 presents fracture stress data on samples containing warm knit lines produced in mold 111. The general trend is that the strength increases as a function of the distance from the insert. The speci- mens located 7.5 mm from the insert displayed frac- ture stress values similar to those obtained in sam- ples with cold welds. ‘The knit line factor values were however greater, 15% and 35%. respectively, for V9 and V1. The unfilledt polymer (V9) exhibited a frac- ture stress of about 45 MPa (knit line factor = 60%) for the specimens located further away from the insert. The glass-fiber-filled grade exhibited a contin- uous increase in strength with increasing distance from the insert, finatlly reaching a value of 39 MPa (knit line factor = 62%). The reference polymer (PS) exhibited only a minor increase from 23 to 30 MPa

(68 to 87% in knit line factor) in strength as a func- tion of distance from the insert.

Figures 9 and 10 present data of the main effects of the process parameters on the knit line strength. For the unfilled polymer, melt temperature and

t 4

0 1 ” s I ’ I ’ I ’ I 0 10 20 30 40 50

Distance from insert lmml

Fig. 8. Knit line strength of warm welds as a function of location. i.e. distance from the insert (mold 111). The bars indicate the standard deuiation of a single data point.

Melt Mold Injection Holdlng temp. temp. tlme pressure

Fig. 9. Estimated main effects of processing parameters on the warm knit line strength of unfilled grade V9. The notations A, B. and C refer to the specimen position in molded samples (mold 111). The bars indicate the standard deuiation of a single data point.

Melt Mold Injection Holdlng temp. temp. tlme pressure

Fig. 10. Estimated main effects of processing parameters on the warm knit line strength of fiber-reinforced grade V I . The notations A , 8. and C refer to the specimen position in molded samples (mold Ill) . The bars indicate the standard deuiation of a single data point.

1624 POLYMER ENGlNEERlNG AND SCIENCE, DECEMBER 1990, Vol. 30, No. 24

Kni t L ine Frac tures i n Injection Molded LCPs

injection time seem to be the important parameters. The increase of 4 k 1 MPa in knit line strength inde- pendent of distance from insert accompanying the increase in injection time from 1.0 s to 3.0 s is anomalous. The effect of melt temperature is also surprising. It should be noted however that these main effects are relatively small, only 5- 12% of the average values, for samples B and C. The processing conditions seem to have only a minor influence on the warm welds of fiber-filled polymer V1 (Fig. 10). All effects are insignificant except for those of the melt temperature on sample B for which a 20% in- crease in strength was obtained when the melt tem- perature was decreased from 330 to 290°C.

The tomography measurements performed on sam- ples with knit lines clearly shows that no internal voids were present in these samples. Thus, the ma- terial on both sides of the weld area has been brought to contact. A scanning electron micrograph of a sam- ple with a cold knit line obtained by freeze fracturing at liquid nitrogen temperature is presented in Fig. 11. A distinct 200 pm thick zone of fibrous material is observed beside the knit line. A 300 pm thick zone displaying some transverse flow features is observed beneath the fibrous zone. Figure 12 pre- sents the X-ray diffraction patterns of these zones and it can be concluded that the fibrous material exhibits some chain orientation. For each sample 10 different diffractograms were obtained, the scatter- ing being measured at different azimuthal angles, 4 = 0, 10, 20, . . . go", where 4 = 0" is parallel with the director which is parallel with the knit line. The scattering peak appearing at 8 z 21" corresponding to the d-spacing 0.2 10 nm were selected for the meas- urement of chain axis orientation. This is in accord- ance with earlier X-ray data by Blackwell, et al. (1 2). The Hermans orientation function f was obtained by

Fig. 11. Scanning electron micrograph of fracture sur- face displaying the orientation distribution in the area near the cold knit line. Note the transverse orientation of the adjacent material. A: Knit line. B: End of fibrous zone. C: End of zone exhibiting transverseflow features.

treating the X-ray data as follows:

(3)

where I+ is the intensity of scattering corresponding to the 0.21 0 nm spacing at the azimuthal angle 4. It is here assumed that the scattering is symmetric around 4 = go", i.e. 190-x = 190+x. Figure 13 presents data on chain orientation for both samples obtained from cold and warm welds. The oriented surface layer just beside the cold knit line is only 200 pm thick, the same as the thickness of the fibrous zone and extends over the entire length of the knit line (Fig. 11 ). The material is non-oriented beneath this zone. The chain orientation distribution is essentially the same in both the pure grades (V9 and U4). The transverse chain orientation of the material near the warm knit line of mold I11 decreases weakly with increasing distance from the insert. In the center of the welding area (xz plane) the material is almost isotropic 48 mm from the insert.

Data from the annealing experiments are pre- sented in Fig. 14. The trend is quite clear despite the small number of data points taken. There is a pronounced tendency for healing (i.e. increased strength) at short annealing times at the tempera- tures used (260 to 300°C) whereas at longer times the strength of the knit lines decreases. The cause of this reduction in strength is chemical degradation, the latter being noticed also in discoloration and extensive void formation.

Knit lines form when two melt fronts meet. The strength of the weld depends on the extent to which the healing process has propagated. When two

I I I I I I 10 15 20 2.5 30 35

0 Fig. 12. WAXS ray diJfraction pattern for samples taken from the adjacent area of the cold weld lines. The diJfer- ent dvfractograms are associated with diJferent azi- muthal angles as shown in the figure.

POLYMER ENGlNEERlNG AND SCIENCE, DECEMBER 1990, Vol. 30, No. 24 1625

K. Engberg , A. K n u t s s o n , P.-E. Werner , and U. W. G e d d e

c( 0.10

0.05

0 00 0 200 400 600 ROO

Distance from knit line lpml

Distance from insert (mml

Fig. 13. Hermans orientation function [ f : based on WAXS data) plotted versus the distance from the knit line (mold I ; upper diagram) for different distances [y) from the side surface: and versus the distance from the insert (mold I l l : lower diagram). The sampling is described in Fig. 3.

260%

0 280°C

m 2wc 0 300°C

0

1000 10000 10 L . . . . 9 . . . . . . J

10 100

Annealing time [PI

Fig. 14. Knit line strength as a function of annealing time for samples of V 9 originar'ly prepared in mold I [a/b = 1 ) and later annealed at dgferent temperatures as shown in thefigure.

materials are brought into contact, three processes may occur (1 3):

(i] Establishment of contact by visco-elastic de- formation of surface irregularities and adhe- sion between the surfaces.

(ii) Inter-diffusion of molecular segments across the interphase.

(iii) Formation of new entanglements in the inter- phase region.

The low strength of the liquid crystalline polymers in a comparison with flexible-chain polymers seems to indicate that either of the three processes (i-iii) are retarded in liquid crystalline polymers. One im- portant implication of the local orientational order in a nematic melt is that self diffusion becomes highly anisotropic. The molecules are transported parallel

to the domain director. In the case of the cold knit line with the 200 pm thick zone with transverse orientation the molecules are more mobile in the plane of the weld rather than across. To accomplish a more complete healing by processes ii and iii ran- domization of the oriented zone must occur. This is a very slow process for nematics (1 4) which explains the low strength of the cold knit lines. This was further supported by the isothermal annealing ex- periments which were conducted a t 260-300°C of the nematic liquid. The healing process is indeed very slow under the quoted conditions. The maximum attained strength was 34.9 MPa (after 5 min at 300°C) which is only 30% of the full strength.

The influence of the five processing variables (including the a/b ratio) is limited for the pure LCP's and for the grade with the nonfibrous filler V 5 (Fig. 7). The effect value was always less than 1.5 MPa. Generally significant effects were only obtained for the melt temperature which at its high level value caused an increased strength. The fiber-filled grade ( V l ) on the other hand was more responsive to the processing parameters. Strong and significant ef- fects were obtained for melt temperature, mold tem- perature, and injection time (Fig. 7). Thus, any at- tempt towards increased knit line strength should for this class of welds be directed towards minimi- zation of chain orientation. The warm welds dis- played a strong increase in the strength with increas- ing distance from the insert. This finding indicates that the combination contact time and flow is the important factor. The healing process is clearly as- sisted by the sliding flow of the two melt streams. I t is reasonable to assume that processes i and ii occur to a greater extent in a flowing system than in a static system as in case of the cold knit lines.

ACKNOWLEDGMENT

The reported study has been supported by the Swedish Board for Technical Development (STU), grant 87-00959. The injection molding performed at Ericsson Telecom AB, Kristianstad, is gratefully acknowledged.

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Knit Line Fractures in Injection Molded LCPs

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