Weld-line characteristics in short fiber reinforced thermoplastics

Fibers can greatly improve the mechanical properties of polymers but may also severely weaken molded parts at their weld-line compared to their bulk strength. The tensile properties and fiber orientation of injection and compression molded fiber reinforced Noryl and polypropylene samples with and without a weld-zone were studied. Distinct differences in structure and mechanical properties of weld-containing and weld-free samples were identified. In unfilled Noryl and unfilled polypropylene, the presence of a weld-line was found to only have a small effect on the tensile strength and modulus, while in the corresponding fiber reinforced systems, orientation of the fibrous reinforcement parallel to the weld-line caused a significant reduction of the tensile strength compared to the weld-free products. The strength ratio of welded and unwelded specimens was found to decrease with increasing fiber concentration. Quantitative determination of the glass fiber orientation distribution within the weld-line region and in the bulk was carried out by analyzing photomicrographs of polished sections at desired locations.     

Morphology and mechanical properties of injection molded articles with weld-lines

The effect of weld-lines on the morphology and mechanical properties of injection molded articles made of neat poly(butylene terephthalate) (PBT) and glass fiber-reinforced PBT was investigated. The weld-line was introduced to a molded article by using a rectangularly shaped insert inside a mold cavity, and tensile specimens were prepared at various positions through the entire molded article. The weld-line position was further checked by a short-shot experiment. Although the maximum tensile stress for specimens of neat PBT with a weld-line is almost the same as that without a weld-line, the maximum tensile stress and the elongation at break for fiber-reinforced PBT with a weld-line were found to be about half of those without the weld-line. This is attributed to the fact that the fibers near the weld-lines are oriented parallel to the weld-line direction (or perpendicular to the tensile force direction) due to stretching flow. Finally, we compared experimental results of flow pattern and fiber orientations with numerical simulations. We found that the predictions of flow fronts and fiber orientations are in good agreement with experimental results.     

Mechanical properties of injection‐molded short‐fiber thermoplastic composites. Part 1: The elastic moduli and strengths of glass‐filled poly(butylene terephthalate)

The effects of processing and part geometry on the local mechanical properties of injection‐molded, 30 wt% short‐fiber‐reinforced filled poly(butylene terephthalate) (PBT) are characterized by mechanical tests on specimens cut from rectangular plaques of different thicknesses injection molded at several different processing conditions. Stiffness data from tensile tests at 12.7‐mm intervals on 12.7‐mm‐wide strips cut from injection‐molded plaques—both along the flow and cross‐flow directions—and flexural tests on these strips show consistency of plaque‐to‐plaque local properties. Also, in addition to the well‐known anisotropic properties caused by flow‐induced fiber orientation, injection‐molded short fiber composites exhibit in‐plane and through‐thickness nonhomogeneity—as indicated by in‐plane property variations, by differences between tensile and flexural properties, and by the flexural strength being significantly higher than the tensile strength. The sensitivity of these mechanical properties to process conditions and plaque geometry have also been determined: the flow‐direction tensile modulus increases with fill time, the differences between flow and cross‐flow properties decrease with increasing thickness, and both the flow and cross‐flow flexural moduli decrease with increasing plaque thickness. While the flexural modulus is comparable to the tensile modulus, the flexural strength is significantly higher than the tensile strength. POLYM. COMPOS., 26:428–447, 2005. © 2005 Society of Plastics Engineers     

Graphene–polyamide-6 composite for additive manufacture of multifunctional electromagnetic interference shielding components

Graphene–polyamide-6 composite (GC) filament with 9%·v/v graphene concentration was applied as feedstock in filament-based material extrusion (ME) additive manufacturing. The materials are characterized by melt flow index (MFI), mechanical properties, dielectric properties, and electromagnetic interference shielding effectiveness (EMI SE) in the X-band frequency range. Despite high graphene concentration, MFI is unaffected. In ME test specimens; GC has both superior elastic modulus and tensile strength at yield when compared with the neat polymer. Enhanced mechanical properties at high graphene concentration without diminished processability highlight the suitability of polyamide-6 (PA6) as matrix material for graphene composite filaments. Scanning electron microscopy (SEM) imaging indicates graphene alignment in GC after printing. In both compression molded (CM) and ME test specimens, dielectric properties and EMI SE of PA6 are enhanced by graphene inclusion. Further analysis reveals that ME has a negative influence on absorption of electromagnetic waves, while reflection is virtually unaffected.     

Weldline strength in injection molded HDPE/PA6 blends: Influence of interfacial modification

Most injection molded objects contain defects known as weldlines. This defect may introduce an element of weakness affecting the object's performance. Weldlines are particularly problematic in multiphase materials where the situation may be exaggerated by component mismatch on the two sides of the interface that results in additional weakening when the two components do not adhere well to each other. In addition, weldline behavior is influenced by orientation and morphological effects. This paper deals with relationships between the structure and the mechanical properties in injection molded high density polyethylene polyamide-6 blends. The weldline effect is investigated in detail. Two molds were used to generate weldlines: a double-gated tensile bar cavity in which the weldline results from the meeting of two melt fronts flowing into each other from opposite directions, and a film-gated rectangular plaque mold with a circular insert that divides the melt front in two. Following the recombination of the fronts, there is additional flow as the melt fills the mold cavity. Two preparations containing 75 vol % of polyamide-6 and 25 vol % of polyethylene with and without compatibilizer were studied. In the first case, a compatibilizer was incorporated into the polyethylene prior to compounding with the polyamide-6. In the directly molded tensile bar the minor phase is strongly oriented parallel to flow. Only in the core, which represents about 10% of the sample thickness, do the dispersed phase particles assume spherical shape. The morphology of the weldline is closely related to that of the skin: the elongated structures are oriented parallel to the weldline plane. The effect of the compatibilizer on the mechanical properties (without the weldline) of the directly molded tensile bars is minor: It is overshadowed by the flow-induced morphology. The weldline strength loss is about 40% in the noncompatibilized blend. The introduction of the compatibilizer has restored the material's ability to yield and the properties are close to those measured without the weldline. For the second type mold, the effect of the weldline is less pronounced and the effect of the distance from the insert is negligible. The anisotropy is quite pronounced in the noncompatibilized blend. In compatibilized blends, all tensile properties are unaffected by the presence of weldline, except for the 2-mm-thick plaque in the position close to the insert. The properties in the direction parallel to flow are similar to the type I mold and not affected by the increase of plaque thickness. Consequently one may question the utility of the directly molded tensile specimens in studying various aspects of the mechanical behavior of multiphase materials where the flow-generated structure is very different from that found in “real” injection molded parts. © 1995 John Wiley & Sons, Inc.     

Effects of Processing Parameters on Tensile Strength of Thin-Wall HDPE Parts Injection Molded by Ultra High-Speed Process

This study explores how ultra high-speed processing parameters affect the melt flow length and tensile strength of thin-wall injection molded parts. A spiral shaped mold with a specimen thickness of 0.4 mm and a width of 6 mm was first constructed to test the melt flow length as an index of process capability for ultra high-speed injection molding. It was observed that the flow length increases with increasing injection speed. High-density polyethylene (HDPE) tensile test specimens with different thicknesses (0.6 mm and 2 mm) were also molded for tensile tests. Both single gate and double gates were used to form parts without and with weldlines. Injection molding trials were executed by systematically adjusting related parameters setting including mold temperature, melt temperature, and injection speed. The parts’ tensile strengths were measured experimentally. It was found that tensile strengths of 0.6 mm thick parts both with and without weldlines were higher than those of 2 mm thick parts. The tensile strength of 0.6 mm thick specimens increases with increasing mold temperature, melt temperature and injection speed, whereas tensile strength in 2 mm thick specimens was only weakly dependent on the corresponding processing parameters. Furthermore, 0.6 mm thick specimens with weldlines had tensile strengths lowered about 9.6% compared to parts without weldlines. For 2 mm thick part the corresponding reduction is 4.3%.     

Injection Molding and Appearance of Cellulose-Reinforced Composites

Composite materials based on an ethylene-acrylic acid (EAA) copolymer and 20 wt% cellulose fibers were compounded by two runs in a twin-screw extruder. The composite material with cellulose fibers (CF) and a reference of unfilled EAA were injection molded into plaques using three different temperature profiles with end zone temperatures of 170°C, 200°C, and 230°C. The injection molded samples were then characterized in terms of their mechanical properties, thermal properties, appearance (color and gloss), and surface topography. The higher processing temperatures resulted in a clear discoloration of the composites, but there was no deterioration in the mechanical performance. The addition of cellulose typically gave a tensile modulus three times higher than that of the unfilled EAA, but the strength and strain at rupture were reduced when fibers were added. The processing temperature had no significant influence on the mechanical properties of the composites. Gloss measurements revealed negligible differences between the samples molded at the different melt temperatures but the surface smoothness was somewhat higher when the melt temperature was increased. In general, addition of the cellulose to the EAA reduced the gloss level and the surface smoothness. POLYM. ENG. SCI., 60:5–12, 2020. © 2019 Society of Plastics Engineers     

Two-dimensional mapping of tensile strength for injection molded composites of glass fiber reinforced polypropylene

An array of polypropylene composites were injection molded into rectangular plaques having a single side gate. As a consequence of non-symmetrical flow patterns, giving complex distribution of fiber orientations, measured tensile strength of test specimens cut along X-Y directions produced nonlinear and anisotropic trendlines. Chemical coupling, glass fiber content, and the choice of short or long glass fiber reinforcement are shown to be material variables influencing the magnitude of the tensile strength at any given localized position. These data verify the notion that tensile strength evaluation along just the flow direction is insufficient for anticipating end use performance.     

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

The mechanical properties of glass bead filled poly(oxymethylene) were investigated 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. It was found that; (i) fracture toughness of the filled and unfilled polymer was relatively independent of notch orientation, (ii) 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 increasing 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 diameter, (vii) weldlines did not affect the tensile strength of the polymer or its composities, (viii) flexural modulus increased linearly with increasing glass bead concentration according to the Einstein equation.     

Coupling of flow simulation and structural analysis for glass-filled thermoplastics

In the automotive industry, glass-filled thermoplastics are used in air intake manifolds, radiator tanks, and many other parts. Plastic parts have many advantages over metal parts including mass, design flexibility and parts consolidation. However, widespread application of glass-filled thermoplastic materials has been limited in many cases by the inability to accurately predict performance and durability. Fiber-filled injection-molded parts contain complex fiber orientation patterns. This fiber orientation state affects material properties including elastic modulus and strength and part properties including shrinkage and warpage. Tremendous amounts of time and money can be saved if one can predict the moldability and mechanical properties of a part at the design stage. In this work, we present a method where commercially available injection molding and structural analysis software may be coupled together with appropriate material property data to improve prediction of structural performance for parts molded from short glass fiber-filled plastics. In addition, we compare the experimental and predicted performance based on simple equations and complex finite element calculations. Two major conclusions may be drawn from this work. In general, the assumption of geometric nonlinearity must be made. Also, an orthotropic material model is generally more robust and accurate than an isotropic analysis. Polym. Compos. 25:343–354, 2004. © 2004 Society of Plastics Engineers.     

Fatigue fracture of reaction injection molded (RIM) nylon composites

This study was undertaken to determine how milled glass fibers affect the fatigue resistance of reaction injection molded (RIM) nylon 6. Specifically the effects of glass content, fiber length, orientation, and surface treatment were investigated. The fatigue crack growth rates for unfilled and glass-filled samples were observed to follow the well-known Paris equation in terms of dependence on cyclic stress intensity factor. For the unfilled nylon a line shaped zone was observed in advance of the crack tip. Fractography results suggest that the zone was the projection of the actual crack tip profile through the thickness of the sample rather than a distinct plastic or deformation zone. The fatigue fracture surface exhibited a patchy type structure with features 50–150 μm in size, suggesting a void coalescence type of mechanism as has been reported for injection molded nylons. A diffuse damage zone, several millimeters in size, was observed at the crack tip for the glass-filled RIM nylon 6. The zone was observed to pulsate with the applied oscillating load. The growth of the damage zone volume with increasing crack length (and thus increasing stress intensity factor range) followed the Paris law, as did the crack growth rate data. The damage mechanism is attributed to void formation and microcracking at the fiber–matrix interface. The results of this study show that, for milled glass-reinforced RIM nylon 6, the crack growth rates were much more rapid than observed for injection-molded nylon 6 containing chopped glass fibers. This difference is attributed to the greatly reduced glass fiber lengths for the milled glasses.     

Dynamic-mechanical and transient testing of practical plastic specimens using a new servo-hydraulic testing system

A servo-hydraulic testing system which can simultaneously produce high loads and rapid response is described. This system allows the testing of relatively large plastic specimens, which makes it possible to perform dynamic-mechanical and transient experiments on high modulus materials, specimens routinely molded for tensile testing, and specimens removed from actual processed articles. Such large specimens also permit the use of extensometers directly attached on the gage section, providing important data for the accurate calculations of material properties. Dynamic-mechanical data on poly(methyl methacrylate), glass-filled nylon, and talc filled polypropylene specimens removed from an injection-molded dishwasher tub were obtained to demonstrate the capability of the system. Stress relaxation experiments in the nonlinear range were performed on polycarbonate to illustrate the transient testing capability of the system.     

Modification of polymers using multilayered “smart pellet” additives: Part II

A magnesium-based inorganic whisker was compounded with polypropylene and with polysulfone (FP/PP and FPSF/PP^*, respectively) and then multilayered into alternating structures with unfilled polypropylene (PP). These multilayered materials were cut into FP/PP and FPSF/PP^* “smart pellets”, which were then added to polypropylene matrix polymer as masterbatches to deliver potential reinforcement to injection molded parts. The morphologies of both the smart pellets and the composites produced with them were studied by scanning electron microscopy (SEM). The inorganic whiskers were found to be aligned in the machine direction in the smart pellets. Mechanical properties of the composites were investigated by performing tensile, flexural, and impact strength tests. Inorganic whiskers combined with PSF offered higher flexural modulus in comparison to those via conventional blending; no significant improvement was observed in tensile modulus or impact strength of these composites.     

Effect of process conditions on the weld‐line strength and microstructure of microcellular injection molded parts

This study was aimed at understanding how the process conditions affect the weld‐line strength and microstructure of injection molded microcellular parts. A design of experiments (DOE) was performed and polycarbonate tensile test specimens were produced for tensile tests and microscopic analysis. Injection molding trials were performed by systematically adjusting four process parameters (i.e., melt temperature, shot size, supercritical fluid (SCF) level, and injection speed). For comparison, conventional solid specimens were also produced. The tensile strength was measured at the weld line and away from the weld line. The weld‐line strength of injecton molded microcellular parts was lower than that of its solid counterparts. It increased with increasing shot size, melt temperature, and injection speed, and was weakly dependent on the supercritical fluid level. The microstructure of the molded specimens at various cross sections were examined using scanning electron microscope (SEM) and a light microscope to study the variation of cell size and density with different process conditions.     

振动技术在注射成型中的应用

介绍了振动技术在汪射成型中的应用,描述了模腔内振动剪切流动作用机理。实验研究表明,通过振动的作秀能提高注射制品的拉伸强度,利用螺杆振动技术而制备的ＰＰ,ＨＤＰＥ和ＰＳ注射制品的拉伸强度分别提高１８．３％２２％和１６％。     

A modified model predictions and experimental results of weld-line strength in injection molded PS/PMMA blends

In this paper, the model based on melt diffusion and Flory-Huggins free energy theory for predicting the weld-line strength of injection molded amorphous polymers and polymer blends parts were modified by considering the diffusion thickness in the interface as a function of contact time. The modified model for weld-line strength prediction of homopolymers and polymer blends were, respectively, used to evaluate the weld-line strength of Polystyrene (PS) and Poly(methylmethacrylate) (PMMA), and that of PS/PMMA blends. The model predictions show that the theoretic predictions as a function of temperature and contact time for PS, PMMA and PS/PMMA (80/20, 70/30) are in good agreements with corresponding experimental results. However, the model predictions for PS/PMMA (20/80, 30/70) blends are much higher than experimental results. The morphology in weld-line regions for PS/PMMA (20/80, 30/70) shows lack of dispersed PS phase. Near the weld-line regions, dispersed PS phase is highly oriented along the weld-line. In theoretic prediction for polymer blends, three kinds of diffusion: Polymer A-Polymer A and Polymer B-Polymer B self-diffusions and Polymer A-Polymer B mutual diffusion were considered. This is why model predictions for PS/PMMA (20/80, 30/70) blends are higher than experimental results.     

Determination of stresses in formed plastics parts

To determine the optimum conditions for vacuum forming quality parts, a precise method of determining internal stress required development. The approach investigated was based on the shrinkage of a heated stressed part. Small stress should be proportioned to strain. By exposing loaded tensile specimens simultaneously with the formed part, the material modulus is determined. Results indicated creep which was minimized by using the correct modulus to calculate internal stress. Stresses in a normally formed part and in a part formed cool were compared at two temperatures. Values were higher in most areas of the cool-formed part and consistent at the two temperatures. This method probably can be used on other materials and possibly on injection molded parts.     

Self‐reinforced polypropylene/LCP extruded strands and their moldings

Polypropylenes (PP) of various molecular weights were mixed with a thermotropic liquid crystal polymer (LCP) and strands were prepared by extrusion and stretching. The strands were subsequently pelletized and then injection molded at temperatures below the melting point of LCP. The mechanical properties and the morphology of the strands and injection‐molded specimens were investigated as a function of draw ratio, LCP concentration, and PP molecular weight. The results for strands show that an increase in the draw ratio, LCP concentration and matrix molecular weight in general enhance the modulus and tensile strength. However, the tensile properties of injection‐molded specimens are found to be reduced compared with those of the original strands, in particular at high LCP concentration. The morphology of LCP changes from spherical or ellipsoidal droplets to elongated fibrils in the strands as the draw ratio increases, but this aligned LCP fibrillar morphology was not transferred to the injection‐molded specimens because of the disorientation of fibrils during injection molding. Compatibilization of PP/LCP blends was also studied by using various polymers. Maleic anhydride and acrylic acid modified PPs improved the tensile properties modestly, but maleic anhydride modified EPDM reduced the tensile properties.     

Effect of sample thickness on the mechanical properties of injection-molded polyamide-6 and polyamide-6 clay nanocomposites

The effect of the thickness on the mechanical properties of injection-molded specimens of pure polyamide-6 (PA6) and polyamide-6 clay nanocomposites (PA6-NC) with 5 wt% of layered silicates was investigated. Plates of 0.5, 0.75, 1 and 2 mm thickness were characterized in the injection direction using Dynamic Mechanical Analysis under torsion and tension respectively, and tensile tests. The fracture surfaces were analyzed by Scanning Electron Microscopy. In contrast with PA6, PA6-NC showed thickness effect and clear differences in the mechanical and thermomechanical properties between skin and core, especially in the 2 mm thick samples. Increasing thickness in PA6-NC led to a reduction of tensile modulus and yield stress. In the fracture surface of the thicker tensile specimens the formation of a sheet-like structure was observed. Multiple voiding in the core causing initial failure in this region and a stiffer skin with a better orientation of the layered silicates in the injection direction are two important elements of a micromechanical model proposed in this paper to explain the fracture mechanism in PA6-NC.     

Vibration welding of glass-filled poly(styrene-co-maleic anhydride)

Vibration welding is used to assess the weldability of 16 wt% glass-filled poly(styrene-comaleic anhydride) (16-GF-SMA). Data are presented on the strengths of butt welds for two specimen thicknesses and T-welds for one specimen thickness. The maximum weld strength of butt joints is shown to be only 35% of the tensile strength of the material. T-joints are shown to have only 61% of the strength of butt joints. The relative butt-weld strengths of 16-GF-SMA are much lower than those measured in other glass filled resins: 71% in a 20-wt% glass-filled modified poly(phenylene oxide); 68 and 60%, respectively, in 15- and 30-wt% glass-filled grades of poly(butylene terephthalate); and 58% in a 40-wt% glass-filled polyamide 6,6.