Glass fiber-filled thermoplastics. II. Cavity filling and fiber orientation in injection molding

  • Published on
    06-Jul-2016

  • View
    214

  • Download
    2

Transcript

  • Glass Fiber-Filled Thermoplastics. I I . Cavity Filling and Fiber Orientation in Injection Molding

    MUNEHARU SANOUf, BIN CHUNG, and CLAUDE COHEN

    School of Chemical Engineering Cornell University

    Ithaca, New York 14853

    Mold filling of a rectangular cavity of three different thick- nesses fed from a reservoir is studied for unfilled and glass fiber-filled polypropylene and polystyrene. The shapes of flow- fronts studied by short-shots are affected predominantly by the thickness of the cavity with other parameters playing a less important role. Pressure drop versus volumetric flow rate inside the thinnest cavity is studied experimentally and predictions are made from a computer simulation of mold-filling. The orientation of fibers in the cavity is examined using a reflect- type microscope and the orientation is found to depend on cavity thickness, melt temperature, fiber content, and to a lesser extent, on volumetric flow rate. In the thinnest cavity, where the flow is quasi-unidirectional, the fibers remain in the plane of flow oriented either along the flow direction or per- pendicular to it, except in the region near the flow front, where they follow a fountain flow behavior.

    INTRODUCTION

    e use of short glass fibers as fillers in thermo- Th plastics has led to a marked improvement and easier control of certain properties of molded parts such as mold shrinkage, strength, stiffness, heat distortion, etc. The addition of glass fibers to plas- tics leads to some undesirable effects such as a higher viscosity for a given melt temperature, mak- ing the injection molding more difficult, and a bad surface finish; these problems can be circumvented or reduced by an increase in melt and cavity tem- peratures and by increasing the packing pressure of the molding cycle. The flow behavior of glass fiber-filled thermoplastics and the mechanical properties of molded parts depend not only on the properties of the polymer matrix and the glass fibers but also on the interaction between the two phases and on the orientation of the glass fibers (1).

    With the increasing use of computer design and control of the injection molding process via a sim- ulation of the flow in the moId cavity (Z), it is important to examine cavity filling with glass fiber- filled plastics and to determine how accurate cer- tain predictions can be made on the basis of the rheological and thermal properties of these mate- rials. Furthermore, a study of the orientation of glass fibers in terms of its development in the cavity and how it is affected by various process parameters should be helpful in view of optimizing these pa-

    t Permanent Address: Mitsuhishi Electric Corporation, Japan

    rameters. There have been a number of qualitative studies concerning the orientation of glass fibers and its dependence upon injection-molding condi- tions. Bright, et al. ( 3 ) studied the effect of injection speed, of four types of gate geometries, and of different polymer matrices. The influence of injec- tion pressure and rotation speed of the screw have been examined by Xavier, et al. ( 4 ) . Moskal (l), on the other hand, emphasized the necessity of accu- rate measurements of the injection-molding param- eters on orientation to obtain more quantitative results. Relationships between the orientation of glass fibers and the mechanical properties of glass/ phenolic composites have been reported by Pipes, et al. (5).

    In this paper we examine cavity filling and fiber orientation of glass fiber-filled polypropylene and polystyrene in simple rectangular cavities. Succes- sive flow-front patterns are obtained by the tech- nique of short-shots, and the cavity filling process is studied in terms of pressure drop versus volu- metric flow rate in the thinnest cavity examined. The flow in this cavity is shown by short-shots to be fairly unidirectional and thus amenable to a simple analysis (6). We also study the development of orientation in short-shot samples by observations on surfaces prepared by a metallographic polishing technique and illustrate semi-quantitatively the be- havior of orientation in full-shot samples in terms of various parameters such as melt temperature, cavity thickness, injection speed, and fiber concen- tration.

    1008 POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1985, Vol. 25, No. 16

  • Cavity Filling and Fiber Orientation in Injection Molding

    EXPERIMENTAL PROCEDURE Materials

    The materials used in this study were the three types of polypropylene and the two types of poly- styrene that have been characterized in the preced- ing paper, namely PP, PP(20), PP(40), and PS, PS(20). The glass fibers had an average length of 400 pm and a diameter of 16 pm. All resins were dried at 70C for 2 hours in a hopper dryer con- nected to the injection-molding machine before molding and were then molded at either 200 or 230C (melt temperature). PP(20) was also molded at 185C.

    Injection Molding The injection molding was carried out on a Boy

    50-T machine with a 3.8 cm (1.5 in.) diameter screw and maximum values of 50 ton, 115 cm3 (7.0 in.7, 1.17 X 10 dyne/cm2 (1.69 X lo4 psi), and 136 cm3/sec (8.3 in.3/sec) for the clamping force, shot size, injection oil pressure, and volumetric flow rate, respectively. The cavity dimensions were 7.62 cm (3 in.) in length, 3.8 cm (1.5 in.) in width, and one of three different thicknesses: 0.1 cm (0.04 in.), 0.254 cm (0.1 in.), and 0.508 cm (0.2 in.). These cavities were fed from a reservoir, as illustrated in Fig. 1 . The screw movement was monitored with a linear-variable-differential transformer (LVDT) and the pressure drop between two points, PI and Pz, shown in Fig. 1 , was measured with two pressure transducers in the case of the cavity of 0.1 cm thickness. These measurements were recorded on a visi-corder. The mold temperature around the cavity was controlled between 30 and 35C by circulating water. The injection speed setting (Q) was varied from 10 to 70 percent of the maximum value. The applied injection oil-pressure setting ( P o ) in terms of percentage of the maximum oil pressure was set as follows: the minimum oil pres- sure ( Pmi,,) for filling a cavity under a given setting of Q was determined by increasing the oil pressure from a low value until the cavity was just filled; and Po was then set by increasing Pmin by a certain percentage of the maximum oil pressure (e.g. 0, 10, 30 percent, etc.). The short-shots were carried out by decreasing the amount of the molten polymer stored in front of the screw.

    Fiber Orientation Observation of the glass fiber orientation on a

    reflect-type microscope with an adapter and a cam- era was performed on surfaces of the molded sam- ples prepared with a metallographic polishing tech- nique using Alumina powder having a grain diam- eter of about 1 pm (7).

    The coordinate system used in describing the orientation angles of fibers is shown in Fig. 2. The axis ratios, b/a, ( i = x, y, z ) , where h is the fiber diameter and ai is the long axis of an ellipse of a fiber cut perpendicular to the i-axis, can be pains- takingly measured from photographs of the spe- cially polished surfaces. These ratios are related to the orientation angles & (i = x, y, z ) by:

    6 . 0 7

    10.0

    I r l

    10.oP. w * 6.0r

    Fig. 1 . Diagram of cavity {all dimensions are given in mm). PI and Pz denote pressure transducers and El and Ez indicate ejector- pin locations.

    i k - - - - - - - -A

    I

    Y

    r / X

    c

    Y

    =Y

    X J

    Fig. 2. Coordinate system used in describing thefiber orientation {upper figure) and a fiber cut parallel to the z-x plane (lower figure),

    cos 4i = b/ai [I1 The orientation angle (&) of the major fiber axis with the flow direction (y) could, in principle, be directly obtained from a picture of a cross-sectional cut parallel to the z-x plane, i.e. perpendicular to the flow direction. This is inconvenient and leads to large errors whenever C#J~ is less than 45 degrees because of the low eccentricity of the ellipse ob- tained (a,/b < 1.41) which makes a calculation of $y difficult and inaccurate. In our samples, we find that 4y is generally less than 45 degrees and we therefore devote our major attention to observa-

    POLYMER ENGlNEERlNG AND SCIENCE, NOVEMBER, 1985, Yo/. 25, No. 76 1009

  • Muneharu Sanou, Bin Chung, and Claude Cohen

    tions of the y-z plane, where the following relation is used for an indirect calculation of $y:

    cos $y = 41 - (b/a,)sin 8, PI The distribution of orientation angles is calculated by dividing the thickness of the cross-section into ten layers and using the Krenchel equation for the average orientation angle in each layer [ 11:

    131

    [41

    - $y = COS-~F

    where F is given by I71

    F = 1 ~k C O S ~ $y k= 1

    with uk being the fraction of fibers with orientation $y and m being the total number of fibers in the examined layer. The layers were taken long enough such that the effect of length on qY was negligible. This occurred for 10 < m < 2 5 .

    RESULTS AND DISCUSSION

    Flow-Front Patterns The cavity thickness has the most evident effect

    on the flow-front shapes obtained by the short-shot technique. In Fig. 3 it is seen that only the thinnest cavity ( H = 1 .0 mm) leads to an approximately uni- directional flow. The PP(40) showed almost parallel flow fronts beyond the location of pressure trans- ducer P I in the cavities of thickness 1.0 mm and 2.!54 mm, whereas PS(20) showed such a behavior only in the thinnest cavity. The pattern of the flow- front, especially in the early phase of cavity filling, strongly depends on the relative friction in the reservoir and the cavity. For the thinnest cavity, the reservoir is filled first and then the cavity is filled. However, for the thicker, less resistive cavi- ties, some flow into the cavity occurs simultane- ously near the gate as the reservoir is still filling. This results in short-shots represented by the dash and dash-dot curves of Fig. 3. The peculiar flow- front shape of the first curve of PS(20) in the thickest cavity is due to a jetting flow that occurs in the reservoir. Chan, White, and Oyanagi also observed some instances of jetting with glass fiber- filled plastics (8). We found that jetting occurred in the reservoir of all our samples, but affected only the flow front shapes of the early phase of cavity filling of PS(20) in the thickest cavity. PS(2O) has the highest viscosity of all the materials studied here and the pressure was probably not high enough to fill the reservoir before flow into the relatively low resistance cavity occurred. Other factors affecting the flow-front pattern were studied (9), e.g. injection speed, melt temperature, fiber content, but their effect is negligible in comparison to the effect of the thickness of the cavity.

    Analysis of Pressure Drop Versus Volumetric Flow Rate

    The experimental measurements of the pressure drop AP12 between the two flush-mounted trans- ducers at points 1 and 2 (see Fig. 1 ) along the flow

    :p I :02 ;

    I ; I ,* .

    Fig. 3. Effect of the cavity thickness ( H ) on theflow-front patterns obtained with short-shots. Topfigure: PP(40); T = 200C; Q = 50 percent setting. Bottomfigure: PS(20); T = ZOOC; Q = 50 percent setting. Solid curves are for H = 1.0 mm, dash-dot curves are for H = 2.54 mm, and dash curves are for H = 5.08 mm.

    direction in the thinnest cavity are presented in Figs. 4 and 5. The solid lines (a, b, or c) through the data points are drawn to help visualize the experi- mental results. In this cavity, the assumption of unidirectional flow can be made on the basis of the flow-front observations made above. The predic- tions of APlz versus the volumetric flow rate Q can be made with a relatively simple flow simulation model (6, 10) once the appropriate viscosity, ther- mal conductivity and heat capacity of the material are determined.

    The flow curves of a thermoplastic obtained at different temperatures can in general be super- posed into a master curve by plotting 7/70 versus qoy where 70 is the zero shear rate viscosity. As previously reported by Czarnecki and White (1 l), we find that this superposition of flow curves can be applied equally well to the results of fiber-filled polymers. An example is shown in Fig. 6, where results from the preceding paper for PP(40) are presented in the form of a master curve. As previ- ously noted (1 l), master curves of the other mate- rials fell within a narrow band around the curve shown in Fig. 6. For each material, all the experi- mental data can be fitted reasonably well with the following equation:

    with vo = exp ( T d T )

    [51

    1010 POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1985, Vol. 25, No. 16

  • r

    3.0

    N i E s

    = 2.0- * OD 0

    v v-

    1. a

    -

    c

    \

    0

    Ex~*rimont Simuirtion PS With 2 0 Yo GF 8 t 2OO0C : C c c

    I 1 1 I I I 1 1

    PS With 2 9 % OF 8 t 2 3 O o C : b bb

    PS With 0 90 OF 8 t 2 3OoC : 8 a.

    Fig. 4 Pressure drop versus volumetricflow rate in the thinnest cavity jhr unfilled and 20 percent glass fiber filled polystyrene. Curves a, h, and c are drawn through the experimental points.

    where the four parameters B, Tb, C, and n have been determined on the basis of minimizing the root-mean-square deviation with respect to the data. Table 1 lists the values of these parameters for the different materials and also for filled mate- rials that have been subjected to the injection- molding process. As discussed in the preceding paper, the latter were found to have different flow curves due to the breakage of glass-fibers in the process. In the simple mold-flow analysis program used here (lo), only the asymptotic power-law form of the generalized equation, [Eq 51, is needed and can be written as

    17 = A exp (T, /T) in-' 161 where A = (B"/C) and T, = nTh.

    The specific heat (C,) of each material has been measured with a differential scanning calorimeter (DSC) under a heating rate of 10"K/min. In the analysis of the injection-molding flow analysis, con- stant average C, values in the temperature range of the flow process have been used. These values are listed in Table 2.

    Curves aa, hh, and cc are the corresponding predictions fromflow simulations.

    To estimate the thermal conductivity of the glass- filled materials, we adopted the Lewis-and-Nielsen semi-empirical model (1 2) which calculates the ef- fective conductivity ( K , ) of a composite material as

    [71

    @ = Volume fraction of dispersed phase. K , = Thermal conductivity of continuous phase. K,, = Thermal conductivity of dispersed phase. A = Constant = 0.5 for uniaxially-oriented fibers

    an, = Constant = 0.82 for rods or fibers in uniaxial With glass fibers as the dispersed phase, K d = 24 x

    cal/(cm sec "K). The resulting values of the thermal conductivity for our composite materials

    with heat flow perpendicular to the fibers.

    random-packing pattern.

    POLYMER ENGlNEERlNG AND SCIENCE, NOVEMBER, 1985, VoI. 25, No. 16 1011

  • 3.0

    cc

    Exg.rlmont Simul8tion

    PP Wlth 4 0 % O F at 200OC : b b b , CC.

    - PP with 0% O F a t 2000C:...

Recommended

View more >