Fracture behavior of glass bead-filled poly(oxymethylene) injection moldings

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<ul><li><p>Fracture Behavior of Glass Bead-Filled Poly( Oxymethylene) Injection Moldings </p><p>S . HASHEMI, K. J. DIN, and P. LOW </p><p>University of North London London School of Polymer Technology </p><p>London N7 8DB, United Kingdom </p><p>The mechanical properties of glass bead filled poly(oxymethylene1 were investi- gated as a function of glass bead content and glass bead diameter using injection molded test pieces. Fracture toughness measurements were made using single edge-notched tension and single edge-notched bend specimens. The effect of notch orientation with respect to the mold fill direction on fracture toughness was studied using single gate and double gate moldings. Tensile strength and flexural modulus were measured using standard test pieces. </p><p>It was found that; (i) fracture toughness of the filled and unfilled polymer was relatively independent of notch orientation, (it) the presence of weldlines in the molded test pieces did not affect the fracture toughness of unfilled polymer or its composites, (iii) fracture toughness of filled polymer was always considerably lower than that of the unfilled polymer; fracture toughness decreased sharply with in- creasing bead concentration, (iv) fracture toughness was not a sensitive function of glass bead diameter; it decreased slightly as bead diameter increased, (v) strain energy release rate as measured under impact decreased with increasing bead concentration, (vi) tensile strength decreased linearly with increasing glass bead concentration and was inversely proportional to the square root of the bead diam- eter, (vii) weldlines did not affect the tensile strength of the polymer or its compos- ites, (viii) flexural modulus increased linearly with increasing glass bead concen- tration according to the Einstein equation. </p><p>INTRODUCTION In the present study, deformation and fracture of </p><p>lass bead-filled thermoplastics materials are be- G ing used in a wide variety of industrial applica- tions. While unique properties, such as dimensional stability or increased modulus, are the usual motiva- tion for exploiting glass bead-filled composites, special attention must be paid to other mechanical proper- ties, such as yield stress and fracture toughness, since they are often degraded by the presence of glass beads. Poor adhesion between bead and the matrix, is a primary cause of low strength, especially at high bead volume fractions. In addition injection molding of thermoplastics and their composites often involves circumstances leading to weldlines in the injection molded parts. In the injection molding process, weld- lines are formed by the merging or impingement of separate melt flow-fronts. They are a potential source of mechanical weakness and poor appearance. With the ever expanding use of composite materials in de- manding load-bearing applications the understanding of the weldline phenomenon and its effect on the in- use performance of fabricated parts is becoming in- creasingly important. </p><p>glass bead filled polyacetal were studied as a function of glass bead content and glass bead diameter. Defor- mation of weld and unweld samples was studied through elastic modulus, strength, and the energy to break the samples. Fracture studies involved mea- surements of the fracture toughness at a low rate using single-edge-notched specimens. These speci- mens were also used to measure strain energy release rate under an instrumented Charpy impact test. In addition effects of notch tip radius and specimen ori- entation on fracture toughness were also investigated. </p><p>EXPERIMENTAL DETAILS </p><p>Materials </p><p>Mat* </p><p>The polymer investigated in the present study is a thermoplastic copolymer of poly(oxymethy1ene) (POM) produced by Hoechst under the trade name Hos- taform C 9021. The copolymer is made from trioxane with small amounts of comonomers. It has a density of 1.41 g/cm3, the melt flow index of 9.5 g/ lO min, and </p><p>POLYMER ENGINEERING AND SCIENCE, MID-JULY 1- Vol. 38, NO. 13 1807 </p></li><li><p>the crystalline melting point in the range of 164 to 167C. </p><p>Glass Bead Content </p><p>Three composites containing different amounts of glass bead were also received under the trade name Hostaform. The glass bead-filled grades contained 10, 20, and 30% by weight glass beads. Volume fractions, 4,. of the glass beads were calculated using the re- spective densities for each material and the following relationship; </p><p>where 4ris volume fraction of the glass beads, pc is the density of composite, pr is the density of glass, and wr is the weight fraction of the glass beads in the com- posite. </p><p>The densities of the composites were obtained from the data published by Hoechst and density of glass was taken as 2.54 g/cm3. Table 1 lists the exact com- positions and material codes. </p><p>Glass Bead Diameter To study the effect of glass bead diameter on me- </p><p>chanical properties, solid glass beads of varying diam- eters were received from Potters Industries. All three grades were coated with a silane coupling agent of type CP03. The reported mean diameter range, d , are delineated in Table 2. </p><p>Composites were prepared by first mixing pre- weighed quantities of POM pellets and glass beads manually (each of which nominally contained 40% by weight glass beads) followed by compounding in a corotating twin screw extruder (Brabender 330) fitted with a rod die. The melt temperature was kept con- stant at 210 2 10C. The extrudates were then gran- ulated for injection molding of test pieces. </p><p>After compounding the exact weight fraction of glass beads was determined for each composite by burning off the polymer in a furnace and weighing the residue. The weight fractions were then calculated and found to be in the range 35 to 38% for all the composites giving volume fractions in the range of 24 to 25% (see Table 2). </p><p>Processing Each of the composites and the unfilled resin were </p><p>injection molded into tensile and flexural bars and square plaques as shown in Figs. 1 through 3. The materials were dried in an oven before molding as per </p><p>Table 1. Glass Bead Content. </p><p>S . Hashemi, K . J . Din, and P. Low </p><p>1808 POLYMER ENGINEERING AND SCIENCE, MID-JULY 1- VOI. 36, NO. 13 </p><p>PC 4, Composites Filler w,% [kg/m3] % </p><p>C 9021 GV 3/10 Glass microspheres 10 1.47 6 C 9021 GV 3/20 Glass microspheres 20 1.53 12 C 9021 GV 3/30 Glass microspheres 30 1.59 19 </p><p>(Note: mean glaaa bead diameter = 30 pm). </p><p>Table 2. Glass Bead Diameters. </p><p>Composite d (14 W,% 4t"h POM/CP03/5000 3.5-7 0.35 0.24 POM/CP03/3000 12-26 0.38 0.25 POM/CP03/2000 27-36 0.36 0.24 </p><p>Fig. 1 . Cavity and runner confguration for tensile test speci- mens. </p><p>manufacturer's recommendations. All the specimens were molded after the machine had attained steady state with respect to the preset melt and mold temper- ature. </p><p>For each material (unfilled and glass bead-filled), the tensile specimens with weldlines (double gated) and without weldlines (single gated) were molded un- der identical conditions. The mold used to prepare tensile bars consisted of two dumbbell-shaped cavi- ties each 1.7 mm thick as shown in Fig. 1 . One of which was filled with the molten material entering the mold from one end and the other was filled with mol- ten material entering from both ends. In the latter case a weldline was formed at the center of the mold- ing by two impinging flow fronts. </p><p>For each material (unfilled and glass bead-filled), the flexural specimens of dimensions 120 by 10 by 4 mm were molded using a rectangular cavity which was filled through an edge-gate located as shown in </p><p>Square plaques of dimensions 88 by 88 by 1.5 mm were also injection molded using a single mold cavity. The molten material entered the mold through a single edge-gate or a twin-gate located on one side of the square as shown in Fig. 3. In the latter case, the </p><p>Fig. 2. </p><p>Fig. 2. Cavity and runner configuration forflexural bars. </p></li><li><p>Fig. 3. Cavity and runner conflguration for plaque moldings. </p><p>molten material enters the mold cavity as two parallel flows forming a weld line in the center of the plaque. </p><p>To fabricate these test pieces for each grade of ma- terial, the processing conditions had to be altered slightly in order to produce complete moldings. The range of conditions employed to produce test speci- mens for all grades are delineated in Table 3. </p><p>Molded specimens were then used to study the de- formation and fracture behavior of polyacetal and its composites. </p><p>Mechanical Testing </p><p>Tensile and Flexural Tests </p><p>The molded tensile specimens were tested at room temperature on an Instron testing machine. The ma- jority of tensile tests were performed at a crosshead displacement rate of 5 mm/min. The nominal tensile strength value at yield, ct. was calculated using the peak load determined from the load-displacement di- agram (see Fig. 4). </p><p>The molded flexural bars were tested at room tem- perature on an Instron testing machine fitted with a three-point bend rig with a span-width of 64 mm and </p><p>Table 3. Processing Conditions. </p><p>Fracture Behavior of Glass Bead-Filled POM </p><p>POLYMER ENGINEERING AND SCIENCE, MID-JULY 1996, Vol. 36, No. 13 1809 </p><p>Conditions Resin 10% 20% 3040% </p><p>Melt temperature (C) 200 200 200 200 </p><p>Injection pressure (bar) 60 76 78 78 Mold temperature (C) 70-80 70-80 70-80 70-80 </p><p>Injection time@) 5-10 5-10 5-10 5-10 Cooling time(s) 15-20 15-20 15-20 10-20 </p><p>Rg. 4. Typical loaddisplacement diagrams for weld and un- weld specimens. </p><p>with the crosshead driven at a constant displacement rate of 5 mm/min. Initial flexural modulus, Ef, and the flexural strength, u,, were computed from the fol- lowing expressions </p><p>where P is the applied load, P, is the maximum load on the load-displacement diagram, d , is the central deflection, L is the span-width, D is the specimen depth (=4 mm), and B is the specimen thickness (= 10 mm). </p><p>Fracture Toughness, K,, Tests The fracture toughness of the materials was mea- </p><p>sured by the use of single-edge-notched tension (SENT) and single-edge-notched bend (SENB) speci- mens (see Fig. 5) . </p><p>Plaque moldings of polyacetal matrix and its com- posites with and without weldlines were cut into a rectangular strip of dimensions 88 by 10 by 1.5 mm in order to produce a series of SENT specimens. The length of each rectangular strip was either parallel to the melt flow direction (MFD) or perpendicular to it (see Fig. 6). The initial notch ran either perpendicular or parallel to the MFD. These different directions of crack propagation relative to MFD are referred to as transverse direction (TD) and the flow direction (FD) in the following text. For double-feed moldings, the ini- tial notch in the FD specimens was always inside the weld line. The edge notches were introduced mid-way along the length of the specimens by first forming a saw cut which was then sharpened using a razor blade with a tip of radius of approximately 6 pm. The notch-to-depth ratio, a / D , ranged from 0.1 to 0.6. The tests were performed on an Instron testing machine at a crosshead displacement rate of 5 mm/min. All the </p></li><li><p>S. Hashemi, K. J. Din, and P. Low </p><p>a +MFD </p><p>t' </p><p>D=lOmm L </p><p>t P </p><p>(SENT) GATE </p><p>t - L = 4 0 U U l l - t </p><p>(SENB) Fig. 5. Specimen confluration for fracture toughness rnea- surernents. </p><p>specimens were tested using pneumatic grips with sample gauge length of 30 mm. </p><p>The critical stress intensity factor, K,, the so-called fracture toughness for the SENT specimens was calculated at the peak load using the following equation ( 1 1; </p><p>(3 ) </p><p>where P, is the peak load on the load-displacement trace, B is the thickness of the specimen (= 1.5 mm), D is the width of the specimen (= 10 mm), and Y is the geometrical factor introduced to account for the finite width effect. It must be noted that since SENT speci- mens were tested with their ends clamped, the calcu- lation of the geometry factor, Y, as given by Brown and Srawley ( 1 ) was inappropriate. They assume that the tensile force is uniformly distributed across the sam- ple width which is consistent with pin-loading config- </p><p>GATE </p><p>WELDLINE </p><p>J </p><p>MFD </p><p>A GATE </p><p>Fig. 6. Specimen orientations for SENT specimens. </p><p>uration provided that the distance between the pins is not less than three times that of the specimen width. Furthermore, when SENT specimens are pin-loaded, the crack tip experiences a bending effect which would be absent when the specimen ends are fixed. Harris (2) obtained the following expression for Y for the case in which the specimen ends are clamped; </p><p>Y = 5 6 J20 - 13(a/D) - 7 ( a / D I 2 (4) </p><p>The Y function given by Eq 4 does not vary as much with a/D as the Brown and Srawley's function (see </p><p>1810 POLYMER ENGINEERING AND SCIENCE, MIDJULY 7996, Wol. 3s, No. i3 </p></li><li><p>Fracture Behavior of Glass Bead-Filled POM </p><p>where @ is a known function of crack length given by (3); </p><p>t o ~ " " ' ' " " " " " ' ' . ' . ' . ' . . . </p><p>o 0.1 0.2 0.3 0.4 0.6 0.8 0.7 </p><p>am </p><p>Fig. 7. Geometrical factor, Y, us. a1 W. </p><p>Fig. 71 and was used throughout this study for ana- lyzing the SENT data. </p><p>Series of single-edge notched bend specimens (SENBI having thickness, B, and depth, D, of 4 and 10 mm respectively (see Fig. 5) were also prepared using the flexural bars notched at the center by a sharp razor blade. The notch-to-depth ratio, a / D , ranged from 0 . 2 to 0.6. The specimen support span was ad- justed to give a span-to-specimen depth ratio, L / D , of 4 . Specimens were placed on the support with the notch edge on the lower side and directly in line with the point of loading. In addition, sets of SENB speci- mens with notch-to-depth ratio, a / D , ranging from 0 . 2 to 0.6 were prepared using two V-shaped cutters; one having a tip radius of 0.25 mm and the other 1 mm. All SENB specimens were tested on an Instron testing machine at a crosshead displacement rate of 5 mm / min. </p><p>Fracture toughness in bending mode was calcu- lated at the peak load, P,. using the following equation; </p><p>3P,L K , = - 2BD2 y@ ( 5 ) </p><p>where Y for the SENB specimen with L / D of 4 is given by ( 1 ) ; </p><p>(6) Y = 1.93 - 3 . 0 7 ( a / D ) + 1 4 . 5 3 ( ~ / 0 ) ~ </p><p>- 2 5 . 1 ~ ( u / D ) ~ + 2 5 . 8 ( a / D ) 4 Impact Strain Energy Release Rate, G,, Measurements </p><p>One approach in assessing toughness is to use an impact pendulum and measure the energy, U, ab- sorbed in fracturing a series of notched Charpy bars. For each bar, the depth D, width or thickness, B, and crack length a, (as in Fig. 5) is recorded. A fracture toughness also known as the critical strain energy release rate, G,, is then calculated from a straight line graph according to the equation (3): </p><p>U = BD@G, ( 7 ) </p><p>j- Y 2 ( x ) . x . dx Y 2(x). x (8) </p><p>- L + </p><p>18DY 2(x) . x @ = </p><p>where x = a / D and Y(x) is the geometrical factor which for SENB geometry with LID of 4 is that given by Eq 6. </p><p>To determine G,, of the unfilled resin and its com- posites, flexural bars having depth, D, and thickness, B, of 10 and 4 mm respectively were notched at the center by a V-shaped cutter of tip radius 0.25 mm to obtain notch-to-depth ratio, a / D , ranging from 0.1 to 0.6. The specimen support sy...</p></li></ul>


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