Knit Lines Structure in Injection Molding of Nylon 6: Relation to First Solidified Sample Cross Section

G. TITOMANLIO, S. PICCAROLO, and A. RALLIS

Istituto di Ingegneria Chirnica Viale delle Scienze

90128 Palerrno, Italy

The effect of processing variables on the knit line structure of nylon 6 samples was investigated. Either local sample thickening or grooves were observed at the knit lines for samples injected with mold conditioning temperatures of 130" and 30°C respectively. The results also indicate that shrinkage venting, either to or from the weld zone, is the dominant mechanism for knit line formation, which is thus related to the position where complete cross section solidification first takes place.

INTRODUCTION

henever separate flow fronts of polymer melt W meet, an area forms having morphology and properties different from the bulk. In the injection- molding process these areas, called "weld lines," are generated when pins are present in the mold or when molds are multigated.

Weaker mechanical properties are generally ob- served at weld lines. The effect of processing condi- tions and material properties on weld line structure and properties deserves further investigation. More- over, the results reported by different investigators do not appear to always fit a coherent picture (1 -4).

Mechanical weakening at weld lines can be related to the following factors (1-4): the molecular orienta- tion along the thickness direction due to the melt squeezing between impinging fronts, poor bonding between flow fronts, surface defects, and entrapped air between meeting flows. Furthermore, at weld zones both the amount of orientation along the sam- ple thickness direction and the lack of bonding be- tween meeting flow fronts decrease as relaxation time becomes small with respect to solidification time.

Some results on knit line structure in injection- molded samples of nylon 6 were recently reported (5, 6). Further experimental information on the struc- ture of knit lines in injection-molded samples of ny- lon 6 as related to changes of processing conditions was collected in this work in collaboration with SNIA- Tecnopolimeri.

EXPERIMENTAL

The material used was nylon 6, SNLAMID ASN 27/33 AV, manufactured by SNIA-Tecnopolimeri and having molecular weights M,, = 18,000 and M, = 37,000. Onset crystallization temperatures of 187 "C

and 164°C were measured by calorimetric tests per- formed under cooling rates of 10"C/min and 80"C/min respectively; in the same cooling rate range, the fractional crystallinity remained close to 40%. Samples were injected by a GBF G235/90 recip- rocating screw injection-molding machine operating under a microprocessor control system. The rectan- gular cavity adopted, of 3 mm thickness, was injected through two open end gates occupying opposite edges on the whole length. Weld lines formed half way from the two gates. The injection flow rate was 4.3 L/min; results on samples injected with 0.72 L/min were reported elsewhere (6).

Two values were explored for each of the process- ing variables investigated in this work. They were injection temperature T, (280" and 230°C). mold con- ditioning temperature T, ( 130" and 30"C), holding time th (1 5 and 8 s), and holding pressure Ph (80 and 30 MPa).

Temperature conditions (T( = 28OoC, T, = 13O"C),

and (Ti = 230"C, T, = 30°C) are referred to, in the following, as thermal conditions A, B, C, and D re- spectively.

Thin section specimens (0.03 mm thickness) were obtained by a Leitz 1400 Universal Microtome from the injected samples across the weld line, as shown in Fig. 1 . These specimens were analyzed by a light transmission microscope between crossed polarizers. Further experimental details are given in Re$ 6.

Micrographs of knit lines are shown in Figs . 2 through 5. Four micrographs for different values of injection temperature, T, , and mold temperature, T, , are grouped in each of Figs, 2 through 5. The micro- graphs were located as follows in each figure: high injection temperature on the left and high mold wall temperature on the top, representing from left to right and from top to bottom thermal conditions (A,

209

[Ti = 230°C. T, = 13OoC), [Ti = 280"C, T, = 30"C),

POLYMER ENGINEERING AND SCIENCE, FEBRUARY 7989, Vol. 29, No. 4

G. Titomanlio, S . Piccarolo, and A. Rallis

Weld - l ine -Plane,

Sample \Specimen

Fig. 1. Thin section specimen for transmission micros- copy cut across the weld line.

Fig. 3. Micrographs of knit line observed under polarized light for dij-ferent thermal conditions. Holding time, th = 8 s.; holding pressure, Ph = 80 MPa. Scale divisions are 0.01 mm apart.

Fig. 2. Micrographs of knit line observed under polarized light for dqferent thermal conditions. Holding time, th = 15 s.; holding pressure, Ph = 80 MPa. Scale marks are 0.01 mm apart.

B) and (C, D) respectively. Scale marks 0.01 mm apart are reported in the micrograph to evaluate the mag- nification adopted. Knit lines were also observed with a Philips SEM 505 scanning electron microscope (SEM).

RESULTS

The processing variable that seems to be more relevant to knit line appearance is mold conditioning temperature, T,. Indeed, the knit line resulted in a groove when a mold conditioning temperature of

30°C was adopted, whereas local sample thickening was observed with a mold conditioning temperature of 130°C. In considering such a remarkable effect, one has to realize that crystallization temperature is low under the cooling rate actually experienced by the material; thus, the difference between crystalli- zation temperature and mold conditioning tempera- ture undergoes about a threefold change on decreas- ing T, from 130°C to 30°C.

Variations of other processing conditions are also relevant. In particular, an increase of injection tem- perature, Ti, gives rise to a significant reduction of either groove sharpness or depth when a low mold conditioning temperature is adopted, while the effect of Tt is less evident for the samples obtained with a mold conditioning temperature of 130°C. The effects of packing pressure and time are more involved and sometimes masked. A decrease of the packing time, th. from that of Figs . 2 and 4 to that of Figs. 3 and 5 gives rise to a decrease of the amount of local sample thickening and to an increase of the groove sharp- ness. Furthermore, in Fig. 5 a small groove forms on the top of the sample thickening. A decrease of the holding pressure Ph gives rise to a decrease of both local thickening and groove sharpness when holding time is large; see Figs. 2 and 4.

The effect of filling flow rate is observed by com- paring the results of Figs. 2 through 5 with those previously reported in Refs. 5 and 6, which were taken with an injection rate of 0.72 L/min. Such a comparison shows that under all operating condi-

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Knit Lines Structure in Injection Molding of Nylon 6

Fig. 4. Micrographs of knit line observed under polarized light for dgferent thermal conditions. Holding time, t h = 15 s.; holding pressure, P h = 30 M P a . Scale di,visions are 0.01 mm apart.

tions a decrease of filling flow rate reduces either the groove sharpness and depth or the amount of local sample thickening. In any case the appearance of knit line defects seems softened by a decrease of the flow rate from 4.3 to 0.72 L/min.

SEM micrographs of the weld line zone and knit line of a sample injected under conditions Ti = 230°C T, = 30°C. th = 8 s, Ph = 80 MPa, and Tt = 280"C, T, = 130°C. th = 8 s, Ph = 80 MPa are shown in Fig. 6. In this micrograph, mold machining marks are clearly evident on the sample surface and, it may be pointed out, also inside the knit line groove. This observation reveals that the polymer first comes in contact with the mold wall, solidifies on it, and only afterward, i.e., during sample cooling, the groove forms.

DISCUSSION

The hypothesis that the volume shrinkage upon solidification governs the mechanism of knit line formation has been considered in previous reports (5, 6); this hypothesis is discussed here in reference to the results presented above.

Views of knit lines showing machinin.g marks of the mold surface finish are shown in Fig. 6. In the top micrograph of Fig. 6, machining marks are clearly visible inside the groove, thus prioviding evi- dence that grooves form after solidification of the sample skin on the mold surface.

After solidification of the sample skin, if complete cross section solidification first takes place else-

where than the weld zone, holding is not effective at the weld zone and volume shrinkage can give rise to the groove. If, however, complete section solidifica- tion starts at the weld zone, volume shrinkage vents outside it, thus giving rise to a local sample thicken- ing in place of a groove.

All factors influencing the temperature field when the impinging flow fronts meet may be relevant to shrinkage localization and venting and thus deter- mine the knit line structure. These factors are ( i ) polymer cooling along the streamlines, which be- comes more relevant when the filling flow rate is small: (ii) polymer cooling along the sample thickness and formation of a frozen layer during the filling stage, this effect increasing with decreasing filling flow rate: (iii) the so-called fountain flow that gen- erates the flow front, and thus, the weld zone out of polymer coming mainly from the midline which if heat generation is negligible, is at the highest tem- perature in each cross section; (iu) the viscous gen- eration which increases with Brinkman number.

The axial position, where first complete so- lidification of the whole sample cross section takes place, was numerically calculated. The model pro- posed in Ref. 7 was adopted for the simulation of the filling stage; this model, although it does not solve volcano flow, takes at least qualitatively into account all effects mentioned in the previous paragraph. The model outlined in Refs. 8 and 9 was adopted for the numerical simulation of the holding and cooling steps. Throughout, 1 90°C was taken as solidification temperature. Both the time to first section solidifi-

Fig. 5. Micrographs of knit line observed under polarized light for dflerent thermal conditions. Holding time, t h = 8 s.; holding pressure, P h = 30 MPa. Scale divisions are 0.01 mm apart.

POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1989, Yo/. 29, No. 4 211

G. Titomanlio, S .

Fig. 6. Typical scanning electron micrographs of knit line. Operating conditions are (T, = 230°C T, = 30°C. th = 8 s, Ph = 80 MPa) in the top and (T! = 280"C, T, = 130"C, t h = 8 s, Ph = 80 MPa) in bottom micrograph.

cation, t,, and the axial distance, d,, of this section from weld line are plotted in Fig. 7 as functions of the wall temperature for both injection temperatures, Tr = 280°C and T, = 230°C and for a filling flow rate, Q = 4.3 L/min.

Figure 7 shows that the section where so- lidification takes place first moves upstream as the mold wall temperature decreases. This effect is re- lated to the thickening of the solid layer formed dur- ing the filling stage, which has indeed two parallel effects both in the same direction: One is related to the shape of the solid layer, which soon after the entrance starts to decrease in thickness (10): solidi- fication tends to move upstream when operating con- ditions are such to increase solid layer thickness during mold filling. The other is related to the viscous generation enhanced by the narrowing of the section available to the flow. When conditions are such as to give rise to this effect, the cross section where soli- dification takes place first moves upstream very quickly, by effect of a decrease in T, as Fig. 7 shows for T, = 230°C.

Piccarolo, and A. Rallis

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Fig. 7. Time to first section solidification, t , , and axial distance, d,, of this section from the weld us. mold wall temperature, T,. A, B , C, and D indicate temperature conditions as specified in the Experimental section.

It is worth pointing out that for all the cases con- sidered in Fig. 7, solidification proceeds monotoni- cally toward the weld zone and it takes place in the whole sample within a time about 10% longer than for the first section. The results of Fig. 7 have thus to be taken as indicative of what really happens, since small differences in mold wall temperature may have very large effects on the position of the first solidified section.

Inspection of the numerical results in Fig. 7 shows that under thermal condition A (T , = 280°C. T, = 130"C), solidification starts from the weld zone and solidification time, t,, is larger than both holding time values adopted for the injection. A consequence of this result is that under these conditions and for all combinations of t h and P h , shrinkage vents outside the weld zone, which is expected to be free of shrink- age grooves. Both expectations are confirmed by the data which indicate a local sample thickening at the knit line, proving that shrinkage starts to be effective and reduces sample thickness very close to welding.

The information on samples injected under ther- mal conditions D (T, = 230°C. T, = 30°C) gives rise to a consistent picture. Under these conditions, com- plete cross section solidification first takes place far from the weld zone: pressure holding is effective only upstream from the cross section where solidification first takes place, and thus shrinkage due to solidifi- cation vents in the weld zone where, indeed, grooves are observed.

For samples injected under conditions B and C, the numerical results of Fig. 7 indicate less distinct sit- uations. In fact, solidification is predicted to start very close to the weld zone and about 0.02 m away for samples injected under conditions B and C, re- spectively. Features similar to those observed in the knit line structure of samples injected under thermal conditions A and D are shown in Figs. 2 through 5

212 POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1989, Vol. 29, No. 4

Knit Lines Structure in Injection Molding of Nylon 6

with samples injected under thermal conditions B and C respectively.

CONCLUDING REMARKS

Either local sample thickening or grooves on the surface at the knit lines were observed on injected nylon 6 samples depending on mold wall tempera- ture. Evidence is given that the polymer first solidi- fies on the mold surface and only afterward does the surface defect form because of shrinkage in the zone where solidification last takes place.

These results are relevant to the mechanical weak- ening at weld lines, experimental results thereof will be reported elsewhere. Obviously, other factors may also contribute to such weakening, the relevance of all these, however, tends to vanish when relaxation time is small with respect to solidification time.

A complete understanding of mechanical proper- ties at weld zones is related to the capability of pre- dicting the evolution of temperature, frozen orienta- tion, cooling stresses, and crystallinity distributions through the whole cycle down to room temperature.

NOMENCLATURE

d,

Ph = holding pressure. Q = filling flow rate. th = holding time.

= axial distance from weld of first solidified cross section.

t,

Tt = injection temperature. T, = mold conditioning temperature.

= time to first complete cross section solidifi- cation.

ACKNOWLEDGMENT

Financial support for the research fellowship awarded to Dr. Rallis was supplied by SNIA-Tecno- polimeri, Italy. This work was carried out with finan- cial support of CNR grant CT85.01348.03.

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