Weld Line Strength Research Articles

The mechanism of ultrasonic improvement of weld line strength of injection molded polystyrene and polystyrene/polyethylene blend parts

AbstractThe effect of ultrasonic oscillations and ultrasonic oscillation‐induced modes on weld line strength of polystyrene(PS) and polystyrene/polyethylene(PS/HDPE) blend was investigated. And the mechanism of ultrasonic improvement of weld line strength of PS and PS/HDPE blend was also studied. The presence of ultrasonic oscillations can enhance the weld line strength of PS and PS/HDPE blend. Compared with mode I(ultrasonic oscillations were induced into mold at the whole process of injection molding), the induced ultrasonic oscillations as mode II(ultrasonic oscillations were induced into mold after injection mold filling) is more effective to increase weld line strength of PS and PS/HDPE blend. The mechanism for ultrasonic improvement of weld line strength of PS and PS/HDPE blend is that the ultrasonic oscillations can improve the molecular diffusion across weld line of the melt at the core, and make against the fusion of melt at the skin. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1520–1530, 2006

Combined effect of the nature of the filler and the compatibilizer on the weld line properties of filled blends

Abstract The mutual effect of the nature of the filler and the compatibilizer on the properties of a ternary blend based on polypropylene (PP) polyamide-6 (PA) and an inorganic phase is investigated. Low aspect ratio irregular shaped CaCO3 and high aspect ratio plate-like talc are selected as inorganic phases. The effect of different filler levels on the morphology of the weld line region and the region far from the weld line are studied while special attention is paid to both uncompatibilized and compatibilized PPPA blends with maleic anhydride grafted polypropylene (MA-g-PP). It is observed from scanning electron microscopy (SEM) studies that plate-like talc reduces the elongated domain size in the weld line region much more effectively than low aspect ratio CaCO3 for uncompatibilized filled blends. Although both filler are selectively wetted by the dispersed PA phase, the morphology homogenization in the weld line region is found to be related to the ability of the filler to reduce the elongated domain size by increasing the viscosity of dispersed phase rather than the shape of the filler. The selective wetting of talc particles by the PA phase seems to be beneficial in increasing weld line strength by increasing the viscosity of the PA dispersed phase with respect to filler content much faster than CaCO3. However, the additional compatibilization effect causes much finer, homogeneous spherical PA/ CaCO3 domains dispersed within the PP matrix compared to plate-like PA/talc domains, which are highly oriented at the weld line. For that reason, the shape of the filler is of predominant importance to achieve homogeneous morphology in the weld line region. Compared to uncompatibilized filled blends, low aspect ratio irregular shaped CaCO3 yields better weld line strength with respect to all filler levels. For compatibilized PPPA filled blends, the shape of the filler is of supreme significance in order to achieve better mechanical properties especially where weld line properties are concerned.

Investigation on Weldline Strength of Short-glass-fiber Reinforced Polycarbonate Manufactured through Push-Pull-processing Technique

Push-pull-processing (PPP) is a live in-mold manipulation method of co-injection molding used for enhancing the orientation of the polymer molecules and reinforcing fillers. This technique implements an alternating shear field induced by a coordinated action of two injection units to create multiple oriented layers across the thickness of molding during the packing phase. In this investigation, the PPP is employed to enhance the weldline strength of short-glass-fiber reinforced polycarbonate (SFRPC) with respect to the fiber orientation in the weldline areas. The effects of glass-fiber concentration and processing parameters including the number of push-pull strokes and the holding pressure differences between both of the injection units have been studied. In addition the degradation of the fiber length caused by the oscillation is also investigated.

Evaluation of mechanical properties of adjacent flow weldline

AbstractWeldlines occur at the interface of two adjacent flows of material behind an obstructive pin in a cavity in injection molding (meldline or hot weldline). Tensile strength of such “adjacent flow weldline” in injection molded polystyrene plates was evaluated by a mechanical step‐by‐step milling technique. The strength when the milling depth was 1/5 of the thickness from each surface was about the same and independent of the distance from the pin. In contrast, the strength without milling decreased once and then increased along the flow direction. This demonstrates that the strength of a weldline is predominantly dependent on the properties of the surface layer of the weldline. The depth of the surface layer was defined as the depth of the weld, Dw. Dw reduced monotonously along the flow direction and faded away with the V‐notch, resulting in an increase of strength along the direction. On the other hand, it was considered that the farther from the pin, the flow‐induced molecular orientation in the surface layer is greater. It caused a decrease of the strength along the flow direction. The sequence of decrease and increase in tensile strength of adjacent flow weldline is due to the complex effect of these two contradictory factors. POLYM. ENG. SCI., 45:1180–1186, 2005. © 2005 Society of Plastics Engineers

Ultrasonic improvement of weld line strength of injection-molded polystyrene and polystyrene/polyethylene blend parts

The effect of melt temperature, ultrasonic oscillations, and induced ultrasonic oscillations modes on weld line strength of polystyrene (PS) and polystyrene/polyethylene (PS/HDPE) (90/10) blend was investigated. The results show that the increase of melt temperature is beneficial to the increase of weld line strength of PS and PS/HDPE blend. Compared with PS, the increase of melt temperature can greatly enhance the strength of PS/HDPE blends. For PS, the presence of ultrasonic oscillations can enhance the weld line strength of PS at different melt temperatures. But for PS/HDPE blends, the presence of ultrasonic oscillations can improve the weld line strength when the melt temperature is 230°C, but when the melt temperature is 195°C, the induced ultrasonic oscillations hardly enhance the weld line strength. Compared with Mode I (ultrasonic oscillations were induced into the mold at the whole process of injection molding), the induced ultrasonic oscillations as Mode II (ultrasonic oscillations were induced into the mold after injection mold filling) is more effective at increasing the weld line strength of PS and PS/HDPE blends. The mechanism for ultrasonic improvement of weld line strength was also studied. POLYM. ENG. SCI., 45:1666–1672, 2005. © 2005 Society of Plastics Engineers

Effects of geometry and injection-molding parameters on weld-line strength

Polymeric flows in microchannels are found to differ significantly from those in macrogeometries. Increasing the mechanical properties of microstructures is one of the most important issues in injection-molding processes. Weld-line characteristics of structures with different cross-sections are investigated in this study. The effects of process parameters and cross-sectional dimensions on the tensile strength of a weld line are discussed. A mold was designed in such a way that specimens with and without weld lines can be developed separately. Five specimens, with different cross-sections, are injection-molded simultaneously. Both polypropylene (PP) and high density polyethylene (HDPE) are used in this study. With the Taguchi method, four process variables: melt temperature, mold temperature, injection speed, and packing pressure were found to be the most influential. Experimental results show that the weld-line strength from a standard test is not applicable in microinjection molding. The microstructure of weld lines is clearly observed from the micrographs. POLYM. ENG. SCI., 45:1021–1030, 2005. © 2005 Society of Plastics Engineers

Application of Statistic Estimation to Weldline Strength of Injectionmolded Thermoplastic Composites

The injection molding of thermoplastic composite materials has become an important process in industry, due to the possibility of producing near shape article having exceptional physical and mechanical properties. However there are still some problems that confound the overall success of this technology. The forming of weldlines wherever polymer flow fronts meet is one of them. In this report, an L’18 experimental matrix design based on the Taguchi method was conducted to study the effect of different processing parameters on the weldline strength of injection-molded thermoplastic composites. Experiments were carried out on a 4.1 oz reciprocating injection molding machine. The materials used were acrylonitrile–butadiene–styrene, and 20 and 30% glass-fiber-filled ABS composites. A plate cavity with an obstacle in the center was used to create a weldline. After molding, the weldline strength of the composites was determined by a tensile tester. Various processing variables were studied in terms of their influence on the weldline strength of injectionmolded composites: melt temperature, mold temperature, melt filling speed, melt filling pressure, packing pressure, and size of the obstacle. For the factors selected in the experiments, size of obstacle and melt temperature were found to be the important parameters affecting the weldline strengths of injection-molded composites. The weldline strength of virgin material increased with the distance from the obstacle, while the weldline strength of composites decreased with the distance.

Investigations on the Weldline Tensile Strength of Thin-wall Injection Molded Parts

It is known that the weldline reduces the mechanical strength of the conventional injection molded parts. Systematic studies on the weldline strength of thin-wall molded parts were not yet reported. This study investigates the effect of processing conditions including melt temperature, mold temperature, injection speed and packing pressure on the weldline strength of thin-wall parts. Tensile test specimen of 2.5, 1 and 0.8 mm thick were injection molded under specified conditions. Both single gate and double gates were used to form parts with and without weldlines. Part tensile strengths were measured experimentally. From the experimental results, it was found that higher melt temperature and mold temperature as well as faster injection speed will increase weldline strength whereas higher packing pressure would decrease weldline strength. Melt temperature and mold temperature are two parameters that affect weldline strength most significantly within the current molding window. Higher melt and mold temperatures not only lower the residual stress but also help the diffusion of molecular chains leading to a higher degree of bonding at the weldline interface. On the other hand, high packing pressure leads to higher residual stress formation and reduces the molecular bonding rate. Part thickness also exhibits significant effect on weldline strength reduction. Thinner parts resulting in higher percentage of frozen layer in filling process thus limit the bonding rate at weldline interface. From the regression analysis, the KIM model with corrected terms in the fitting coefficient was found to correlate process conditions and weldline strength reduction quite well.

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.

A new approach to increase weld line strength of incompatible polymer blend composites: selective filler addition

A ternary blend composite composed of two immiscible organic phases, polypropylene (PP) and polyamide-6 (PA), and talc as inorganic filler is studied in terms of weld line properties and morphology. Effects of different filler levels as well as compatibilizer on weld line strength under tensile loading condition of each polymeric phase are investigated. Special attention is paid to relate the nature of dispersed domains especially in weld line regions with final performance. It is observed from scanning electron microscopy (SEM) studies that addition of talc filler which is selectively wetted by dispersed PA phase dramatically reduces the elongated domain size in the weld line region and causes to much more homogeneous microstructure. The selective wetting of talc particles by PA phase, therefore, seems to be beneficial in increasing weld line strength via increasing the viscosity of the PA dispersed phase and consequently decreasing the elongated domains at weld line region. Compared to uncompatibilized blend composites, compatibilized ones represent much higher weld line strength for all levels of talc loading.

Flow analysis of the weld line formation during injection mold filling of thermoplastics

To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. The thermo-rheological findings were used to investigate the sources of weld line weakness. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near the surface of the finished parts. The main source for the weld line weakness seems to be the V-notch that arises due to the poorly bonded region near the surface in combination with the large shrinkage as a result of extremely high molecular orientations induced at the end of the filling. Furthermore, the empirical knowledge was confirmed that weld lines are rather more sensitive to the local flow situation than the global processing conditions. Melt and mold temperatures can be considered to be the most important factors which influence the weld line strength.

Influence of temperature on weldline strength of injection moulded short glass fibre styrene maleic anhydride polymer composites

The effect of weldlines on the tensile strength of injection moulded styrene maleic anhydride (SMA) reinforced with 20, 30, and 40% weight fractions &phis;f of short glass fibres was investigated between 25 and 120°C. The weldline was formed in the moulded specimens by direct impingement of two opposing melt fronts. In the absence of weldlines, the tensile strength increased linearly with &phis;f and decreased linearly with temperature. In the presence of weldlines, the tensile strength of the matrix and its composites was greatly reduced. Weldline strength had a maximum at &phis;f≈10%, beyond which it decreased with further increase in &phis;f. A significant drop in weldline strength was noted with increase in temperature. The loss in matrix strength was attributed to the orientation of the polymer chains perpendicular to flow in the weldline region, while the loss in composite strength was attributed to the orientation of the fibres perpendicular to flow in the weldline region. The weldline integrity factor increased with increasing temperature but decreased with increasing &phis;f.

A study on weld line morphology and mechanical strength of injection molded polystyrene/poly(methyl methacrylate) blends

AbstractIn this work, the mechanical strength and weld line morphology of injection molded polystyrene/poly(methyl methacrylate) (PS/PMMA) blends were investigated by scanning electron microscopy (SEM) and mechanical property test. The experimental results show that the tensile strength of PS/PMMA blends get greatly decreased due to the presence of the weld line. Although the tensile strength without the weld line of PS/PMMA (70/30) is much higher than that of the PS/PMMA (30/70) blend, their tensile strength with weld line shows reversed change. The viscosity ratio of dispersed phase over matrix is a very important parameter for control of weld‐line morphology of the immiscible polymer blend. In PS/PMMA (70/30) blend, the PMMA dispersed domains at the core of the weld line are spherically shaped, which is the same as bulk. While in the PS/PMMA (30/70) blend, the viscosity of the dispersed PS phase is lower than that of the PMMA matrix, the PS phase is absent at the weld line, and PS particles are highly oriented parallel to the weld line, which is a stress concentrator. This is why weld line strength of PS/PMMA (30/70) is lower than that of PS/PMMA (70/30) blend. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1856–1865, 2002; DOI 10.1002/app.10450

Weld-Line Strength of Rubber in Injection Molding: Effect of Injection Factors and Compound Characteristics

Abstract The effect of injection molding system to weld-line strength in rubber O-rings was studied using a V-shape two-stage REP injection machine. Two types of injection molds were designed and built, a standard dumbbell mold with double gates and a circular cross-section O-ring mold. Several formulations of carbon black filled NR and SBR compounds were used and vulcanization temperature was either 180 or 200 °C throughout. The results show that mold cavity pressure, compound viscosity and compound scorch time are important variables for the weld-line strength of the products. The shot volume change had no direct effect on strength, but mold cavity pressure was an important factor; unfilled shot volume gave low cavity pressure thus lowering the weld-line strength of the O-rings. The compounds having 45 or lower Mooney viscosity, ML(1 + 4)120 °C, had the same weld area strength as that of the other regions of the O-ring, but the high viscosity compounds produced low weld-line strength. Only the compounds with Mooney scorch time shorter than 10 minutes gave low weld-line strength.

Co-continuous polycarbonate/ABS blends

Co-continuous PC/ABS (50/50) blends were studied with a variable polybutadiene (PB) content (0–40%) in ABS. Polycarbonate (PC), styrene-acrylonitrile (SAN) and PB were blended in two steps using a twin screw extruder. Rectangular bars were injection moulded and notched Izod impact tested at different temperatures and in single edge notch tensile tests at 1m/s and different temperatures. Co-continuous PC/ABS gave a brittle-to-ductile transition temperature lower than expected based on notched Izod results for dispersed ABS in PC. The brittle-to-ductile transition temperature, in the co-continuous PC/ABS blends, decreased with increasing rubber content in SAN. The fracture energies showed an optimum at 15% PB in SAN while at the same time a delamination was seen on the ductile fracture surface, due to failure of the PC/SAN interface. Delamination disappeared when the rubber content in SAN or the temperature was increased. Specimens containing a welding were injection moulded to study the influence of rubber and AN content in the SAN on the interface. Weldline strength of the blends was very poor compared to PC, but improved with increasing rubber content in SAN.

Optimizing the Weldline Strength of Injection Molded Liquid Crystal Polymers

The injection molding of liquid crystalline polymers has become an important process in industry, due to its capability of producing near net-shape articles having exceptional physical and mechanical properties. There are still, however, some problems that confound the overall success of this technology. The forming of weldlines wherever polymer flow fronts meet is one of them, In this study, an L'18 experimental matrix design based on the Taguchi method was conducted to optimize the weldline strength of injection-molded liquid crystal polymers. Experiments were carried out on a 4.1 oz injection-molding machine. A plate cavity with an obstacle at the center was used to create a weldline. After molding, the weldline strength of the parts was determined by a tensile tester. Of the parameters selected in the main experiments, geometry of obstacle and polymer melt temperature were found to be the principal parameters affecting the weldline property of injection-molded thermotropics. The weldline strength of injection-molded parts decreased with the size of the obstacles. It is shown that by setting the optimum processing parameters, one will be able to mold LCP parts of the highest weldline strengths.

The effect of weld-lines on the morphology and mechanical properties of amorphous polyamide/poly(ethylene- ran-propylene) blend with various amounts of an in situ compatibilizer

The effect of weld-lines on the morphology and mechanical properties of 80/20 (wt/wt) amorphous polyamide and poly(ethylene- ran-propylene) (EPR) blend with various amounts of maleic anhydride grafted EPR (EPR-M) as an in situ compatibilizer was investigated. In this blend, the effect of crystallization on weld-line strength was completely excluded; thus mechanical properties depend upon the blend morphology and interfacial adhesion strength. As the amount of EPR-M was increased, the dispersed domain sizes were reduced, whereas the shear modulus and the viscosity of the blend increased. We have shown that the injection-molded blend without weld-lines exhibited traditional skin–core morphology. Namely, the dispersed domains at sub-skin layers are strongly elongated toward the flow direction, while those at core regions are not deformed. The sub-skin layer of specimens without weld-lines was significantly reduced with increasing amount of EPR-M. For specimens with weld-lines, the dispersed domains of the blend without EPR-M at sub-skin layer are elongated perpendicularly to the flow direction. However, with increasing EPR-M content, very fine and isotropic morphology is observed. Both the elongation at the break and Izod impact strength of the weld-specimen increased steadily with increasing amount of EPR-M. But these values are still lower than those of non-weld specimens, even for blends with large amounts of EPR-M. This is because some dispersed domains in weld specimens are elongated perpendicularly to the tensile (or impact) direction. However, the ‘V-notch’ found at the weld-line of specimens does not affect the mechanical properties.

An experimental matrix design to optimize the weldline strength in injection molded parts

AbstractThe forming of weldlines wherever polymer flow fronts meet is one of the problems that confound the overall success of injection molding technology. In this study, an L'18 experimental matrix design based on the Taguchi method was conducted to optimize the weldline strength of injection molded thermoplastics. Experiments were carried out on a 4.1 oz injection molding machine. A plate cavity with an obstacle at the center was used to create a weldline. After molding, the weldline strength of the parts was measured with a tensile tester. Of the factors selected in the main experiments, melt temperature and mold temperature were found to be the principal factors affecting the weldline property of injection molded thermoplastics. Weldline strengths of injection molded parts increase with the size of the obstacles.

Weld line strength of poly(vinyl chloride)/polyethylene blends

Stress–strain behavior coupled with fractography was used to investigate the weld line strength of 30/70 w/w poly(vinyl chloride)/high density polyethylene (PVC/HDPE) blends. The weld line strength determined in uniaxial tension depended upon the domain shape of the PVC phase at the weld line, with elongated domains causing weld line weakness. To alter the PVC domain shape, the viscosity ratio ( η PVC/ η HDPE) of the blend was varied by changing the PVC molecular weight. The domain shape at the fracture initiation site was used in conjunction with a modified Nielsen approach to predict the ductile to brittle transition at the weld line. For the composition studied, a critical aspect ratio of the PVC phase of 1.24 was determined. Elongation of the PVC domains above this critical value led to brittle weld lines. The domain shape at the weld line was modeled by a Taylor analysis of droplet deformation in an elongational flow field. The calculations predicted that a viscosity ratio of 21 would produce a particle with an aspect ratio of 1.24. The observed weld line strength confirmed this prediction: blends with a viscosity ratio below 21 were brittle, and those with a viscosity ratio above 21 had ductile weld lines.

Weld Line Strength in PC/ABS Injection Moldings

Abstract Polycarbonate(PC)/ABS is a very common immiscible polymer blend. The internal structure such as distribution of each component in the injection moldings of PC/ABS would be strongly affected by the processing parameters. Especially, the weld line formed by meeting two melt flows and the internal structure at the weld area would be a very complex form. By observing the etched samples of PC/ABS injection moldings using scanning electron microscopy, a weld part with fine dispersion of ABS was found in the area below V-notch. When tensile load was applied perpendicular to the weld line, the fracture initiated from the V-notch, but the crack propagated along the interface between the weld part with fine dispersion and the part behind the weld part. This leads to unique characteristics of the weld strength.