Weld Line Formation Research Articles

Effect of supercritical nitrogen content on the weld lines and mechanical properties of microcellular injection molded double gate PP/Al parts

AbstractInjection molding products made of aluminum flakes and polymer blends exhibit a distinctive esthetic effect. However, during the filling process, the melt flows in different directions converge and collide, resulting in the flop effect of the aluminum flake and consequent weld line formation. Herein, microcellular injection molding (MIM) was employed to fabricate polypropylene/aluminum flakes (PP/Al) composite foamed parts with distinct weld lines using supercritical nitrogen (scN2) as the physical blowing agent. The scN2 content has a significant effect on cell diameter and cell density. When the scN2 content was 0.6%, the weld line width of the foamed part was 13.03 μm, while it was 30.41 μm for the solid counterpart due to the expansion and rupture of cells in the flow front during filling. Moreover, the orientation of Al flakes was mostly along the flow direction for the foamed parts, while it was generally aligned perpendicular to the flow direction for solid parts in the weld line region. In addition, the flexural modulus of foamed parts was increased by 29% compared with the solid parts, although the tensile strength was reduced by 18% due to the alignment of Al flakes and the stress concentration on the cell walls. Therefore, this work provides insight into the improvement of flexural property and the mitigation of weld lines for injection molded composite parts using MIM.

Flow Disturbance Characterization of Highly Filled Thermoset Injection Molding Compounds behind an Obstacle and in a Spiral Flow Part.

In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. The position, length, and angle of weld lines are dependent on the gate location, injection speed, injection pressure, mold temperature, and, especially, the direction and degree of the polymer melt velocity in the mold-filling process. However, the wall surface velocity of the thermoset melt in the mold-filling process is not zero, which is not found for thermoplastic injection molding. The main reason leading to this difference is the slip phenomenon in the filling phase between the thermoset melt and the wall surface, which is directly affected by the filler content. In this study, commercial thermoset phenolic injection molding compounds with different amounts of filler were employed to investigate not only the mechanism of weld line formation and development behind an obstacle in the injection molding process but also the flow disturbance of the thermoset melt in the spiral flow part. In addition, the effect of the wall slip phenomenon on the flow disturbance characterization and the mechanism of weld lines of selected thermoset materials was carefully considered in this research. Furthermore, the generated material data sheet with the optimal developed reactive viscosity and curing kinetics model was imported into a commercial injection molding tool to predict the weld line formation as well as the mold-filling behavior of selected thermoset injection molding compounds, such as the flow length, cavity pressure profile, temperature distribution, and viscosity variation. The results obtained in this paper provide important academic knowledge about the flow disturbance behavior as well as its influence on the mechanism of weld line formation in the process of thermoset injection molding. Furthermore, the simulated results were compared with the experimental results, which helps provide an overview of the ability of computer simulation in the field of the reactive injection molding process.

Concurrent structural topology and injection gate location optimization for injection molding multi-material parts

This paper develops a novel concurrent optimization algorithm to simultaneously optimize the structural geometry and injection gate locations, dedicated to injection molding multi-material part design. Specifically, level set method is employed for structural shape and topology optimization. Different from the existing methods which count on the combination of different level set functions to represent the multi-material distribution, a novel level set-based Voronoi diagram generation algorithm is developed to simulate the weld-line formation, thus determining the material-material interfaces, since the multi-material distribution from injection molding is strongly dependent on the injection gate locations and the fluid flow condition. Then, the sensitivity of multi-material distribution against the injection gate location variables is calculated with a finite difference-based approach. Two sets of numerical examples are investigated to demonstrate the effectiveness of the proposed method. It is observed that the structural topology optimization with simultaneous injection gate location optimization for compliance minimization leads to improved multi-material design solutions than the cases with fixed injection gate locations.

Effects of Heating and Cooling of Injection Mold Cavity Surface and Melt Flow Control on Properties of Carbon Fiber Reinforced Semi-Aromatic Polyamide Molded Products.

Fiber reinforced thermoplastics (FRTP), reinforced with glass or carbon fibers, are used to improve the mechanical strength of injection-molded products. However, FRTP has problems such as the formation of weld lines, the deterioration of appearance due to the exposure of fibers on the molded product surface, and the deterioration of the strength of molded products due to the fiber orientation in the molded products. We have designed and fabricated an injection mold capable of melt flow control and induction heating and cooling. This mold can both heat and cool the injection mold. It can also control the melt flow direction using a movable core pin. In this study, the above-mentioned mold was used for the molding of carbon fiber reinforced semi-aromatic polyamide. As a result, we found that increasing the heating temperature of the mold and increasing melt flow control volume contribute to the prevention of the generation of a weld line and the exposure of fibers on the molded product surface, as well as to the formation of a flat surface and increased bending strength. The relationships of these results with the carbon fiber orientation in the molded products and the crystallization of semi-aromatic polyamide were also examined in this study.

Mold-Face Heating Mechanism, Overflow-Well Design, and Their Effect on Surface Weldline and Tensile Strength of Long-Glass-Fiber-Reinforced Polypropylene Injection Molding.

Long-fiber polymers offer the advantage of a lower production cost because specific tool designs are required for conventional injection molding equipment to produce long-fiber polymer parts. The use of long fibers allows relatively high fiber aspect ratios to be obtained, thereby enhancing composite stiffness, strength, creep endurance, and fatigue endurance. However, the multigate design of the injection-molded part can result in weldline formation during the molding process, which reduces the structural strength of the molded part. Therefore, in this study, the surface quality, fiber compatibility, and structural strength of long-glass-fiber-reinforced polypropylene (PP/LGF) injection-molded samples were compared in the use versus nonuse of a mold-cavity overflow-well area and the mold-face infrared heating method. The experimental results indicate that the mold-cavity overflow-well area more greatly improved the surface roughness of the PP/LGF molded samples. Moreover, the infrared heating of the mold-face decreased the weldline depth of the samples. Optical-microscopy images and mold-cavity pressure distributions indicated that the weldline tensile strength and the interface compatibility between fibers and melts at the weldline region during the molding stage were higher in the use than in the nonuse of the mold-cavity overflow-well and mold-face infrared heating method.

Modeling and simulation of weld line location and properties during injection molding based on viscoelastic constitutive equation

Weld lines are one of the typical quality issues of injection molded parts. They are unavoidable even for moderate complex products. To explore the formation of weld lines in injection molding, a flow model based on viscoelastic constitutive equation is presented in this article to predict and evaluate of weld lines location and properties, in which Rolie-Poly constitutive equation is used to describe the viscoelastic properties of polymer melts and level set method is used to capture melt front. Furthermore, the formation and evolution processes of weld lines are investigated by co-injection molding technique in simulation, since the processes are determined by the flow front and rheological history of the melt streams. Numerical results show that weld lines can be predicted accurately, and the formation and evolution processes of weld line can be shown more clearly by co-injection molding technique. In addition, properties of polymer melt such as velocity, pressure, principal stress difference, and molecular orientation in the weld line regions are investigated and presented.

Investigation into fibre orientation and weldline reduction of injection moulded short glass-fibre/polyamide 6-6 automotive components

Due to the increasing demands on automotive components, manufacturers are relying on injection moulding components from fibre-reinforced polymers in an attempt to increase strength to weight ratio. The use of reinforcing fibres in injection moulded components has led to component failures whereby the material strength is hampered through the formation of weldlines which are also a problem for unreinforced plastics. In this study, an industrial demonstrator component has the injection locations verified through a combination of fibre orientation tensor simulation and optical microscopy analysis of key locations on the component. Furthermore, the automotive component manufactured from 30% glass fibre–reinforced polyamide 6-6 is simulated and optimized through a Taguchi parametric study. A comparison is made between the component, as it is currently manufactured, and the optimum processing parameters determined by the study. It was found that the component can be manufactured with roughly 7.5% fewer weldlines and with a mould fill time 132 ms quicker than the current manufacturing process.

Characterization of injection-molded high-strength/high-stiffness thermoplastic hybrid materials containing thermotropic liquid crystal polymer (LCP), polyphenylene sulfide (PPS) with carbon fibers

In this study, thermotropic liquid crystal polymer (LCP)-based hybrid materials with various carbon fiber (CF) and polyphenylene sulfide (PPS) contents were developed to replace the heavy metal brackets used for liquid crystal displays (LCDs) in thin, light mobile devices. To determine the physical properties of the composite, several characterization methods, including mechanical, thermal, and morphological tests, were used. As the CF content increases, the tensile and flexural moduli also increase because of the high modulus values of CF. However, some strength-related properties, such as the tensile, flexural, and impact strength, decrease because of the lack of interfacial compatibility between the CF and matrix polymers. Additionally, as the thickness of the LCP-based hybrid material decreases, the flexural strength and flexural modulus increase because of the highly oriented characteristics of LCPs at the surface. To improve the interfacial strength between the CF and LCP, epoxy is a good compatibilizer. In addition, PPS can be used to reduce the weld line formation of LCP and improve the processability of LCP-based hybrid materials with high CF contents.

Study of the Effects of Injection Molding Parameter on Weld Line Formation

Micro weld lines are the common problem that occurs for plastic molded parts through injection molding process. This study focus on the formation of weld lines of molten polypropylene and polypropylene-glass-fiber reinforced composite under different parameters (melt temperature, material & mold design). Three different mold with different design were fabricated and injected with molten polypropylene and polypropylene-glass-fiber reinforced. The experimental result of the length, angle and location of the weld line were compared to the result from Autodesk Simulation Moldflow Adviser software. The result show a good resemblance with polypropylene with melt temperature 240oC and polypropylene-glass-fiber reinforced (10, 20, 25%) but not with polypropylene with melt temperature 250oC and 260oC. It is also concluded that the melt temperature, type of material and mold design affects the formation of weld lines.

소형 공기청정기 하우징의 성형성 향상을 위한 런너시스템 설계 및 스크류 전진 거리 예측

The runner system of the injection mould and the injection volume of the injection molding process greatly affect the quality of the produced part. The goal of this paper is the design of the runner system and the prediction of the injection volume for the injection moulding of a housing part of small-size air cleaner to improve the formability through the three-dimensional injection moulding analysis. The effects of the runner system of the mould on the injection moulding characteristics are investigated. From the results of the investigation, a proper design of the runner system with uniform filling characteristics and the minimized defect formation is obtained. In addition, the influence of the moving distance of the screw on filling characteristics, weldline formation and deformation characteristics is examined. From the results of the examination, an appropriate moving distance of the screw for the housing part of small-size air cleaner is estimated.

Prediction of Weld Line Strength in Injection-Moulded Parts Made of Unreinforced Amorphous Thermoplastics

In the production of injection-moulded parts, the formation of weld lines is usually unavoidable. These often represent mechanical weaknesses. Currently, however, there are no methods available to parts designers that allow a reliable design of plastics parts while taking account of the mechanical influence of the weld line. A calculation routine for the prediction of weld line strength in unreinforced thermoplastic parts therefore has been developed at the Institute of Plastics Processing (IKV) in Industry and the Skilled Crafts at RWTH Aachen University, Germany. In this routine, a reduction factor for the material strength of the weld lines is calculated based on process data generated in injection-moulding simulations for use in structural simulations. The calculation is based on a physical model of molecular interdiffusion, which has been empirically expanded and calibrated on the basis of experimentally determined data.

Prototype System to Study the Effect of Weld Lines on the Performance of Extruded Profiles

Abstract In several thermoplastic extrusion based processes the melt flow has to be divided into independent streams that are merged before the flow channel outlet. This promotes the formation of weld lines, usually related to mechanical weak zones and aesthetic imperfections. There is a large amount of scientific publications related to weld lines formed in other processing techniques, e. g. injection molding. However, studies related to extrusion are scarce. This work presents the conception of a new prototype system that allows performing systematic studies aiming a better understanding of the effect of weld lines in extrusion. A case study illustrates the employment of the developed device to study the effect of several process parameters (e. g. spider leg location, processing temperature and flow rate) on the mechanical performance of the extruded profiles.

Effect of Weld Line Formation on Morphology and Mechanical Properties for 3D-MID Technology

The topic of this research work was to demonstrate the feasibility of a 3D-MID concept using injection molding technique and investigate the effects of two weld line types on the structure and mechanical properties such as tensile, flexural strength and morphology. In order to obtain more understanding of the bonds between polymer and metals, two different polymer bases of polyphthalamide (PPA) with the same type and amount of filler content were produced by injection molding at the different processing conditions. A mold was designed in such a way that weld and meld line can be produced with different angles by changing as insert inside of the mold. The mechanical properties such as stiffness, tensile strength and flexural strength were determined in tensile and flexural tests, respectively. The results showed in line with the expectation of high reduction on mechanical properties in area where weld/meld lines occurred. The result of tensile test was clearly seen that weld and meld line showed a considerable influence on mechanical properties. The reduction in tensile strength was approximately 58% according to weld line types, whereas in flexural strength was approximately 62%. On the other hand, the effect of the injection times and mold temperatures on the tensile strength were marginal.

Effect of Weldline on Fatigue Life of Natural Rubber Parts

The presence of weldline in rubber products is regarded as a one of the most undesirable phenomena, since it result in poor mechanical properties. Designers typically minimize the influence of weldlines by their placement in non-critical areas and the application of a factor of safety based on design specifications and anticipated failure modes. Such practice results in over-design of parts, since the factor of safety is generally independent of the material. Truly cost-effective part design requires a better understanding of the behavior of polymers under different loads and failure conditions. A large amount of literatures have been published in regard to the effects of weldlines on the static mechanical properties of thermoplastics and thermoplastic composites. In the present study, the effect of weldline on the fatigue life of natural rubber was first investigated with regarding to the filler types and contents. The use of design data which takes into account the weldline formation as well as the filler loading will enable design engineers to more accurately predict the performance of rubber molded part under fluctuating load.

고무사출성형의 적정설계

K사의 고무 사출성형에 있어서 애로사항인 등속조인트 부트(boots)의 크 (crack) 발생 등의 문제점을 해결하기 위하여, 상용 CAE 프로그램인 MOLDFLOW(Ver. 5.2)를 이용한 전산모사를 수행하여 적정금형설계를 도출하고 적정작업조건을 구축하였다. 그 결과 크 의 발생 원인은 크 이 발생하는 위치에 형성되는 weld 및 meld line의 형성 때문이고, 또한 크 이 발생하는 위치에서의 가류(curing)가 불완전한 것이 확인되었다. 이와 같은 weld 및 meld line의 형성을 방지하기 위해서 게이트(gate)의 위치를 변경하고 최적위치에 설계함으로써, 유동선단(melt front)의 충돌 또는 수지흐름의 만남을 최소화하는 충전패턴(fill pattern)을 형성하고 부트 안쪽 하단의 크 발생을 방지하였다. Weld 및 meld line과 에어트랩(air trap) 불량이 가장 큰 게이트 위치는 각각 최적 게이트위치를 기준으로 서로 정반대 방향임이 관찰 되었다. 한편 몰드(mold)의 온도를 <TEX>$170^{\circ}C$</TEX>로 유지하게 함으로써 크 이 발생했던 위치에 가류조건을 만족시켰다. The optimum mold design and the optimum process condition were constructed upon executing process simulation of rubber injection molding with the commercial CAE program of MOLDFLOW (Ver. 5.2) in order to solve the process-problems of K company relating to cracks, which occurs at the inner cavity wall of C. V. joint boots. As a result it was confirmed that the real cracks occurs at the exactly same position of the cavity as exhibits the defects of weld and meld line and unsatisfactory curing according to the result of simulation. In order to prevent the occurrence of weld and meld line at the defect-position, the location of gate was altered to the optimum position of the cavity. Consequently the filling pattern was established to minimize the degree of the melt-fronts confronting or the melt-flows melding to prevent the occurrence of weld and meld line at the defect-position. It was observed that both gate-positions to maximize the degree of the formation of weld and meld line and air traps are located, respectively, in opposite direction each other with reference to the optimum gate position. In addition, the temperature of mold was raised by <TEX>$10^{\circ}C$</TEX> and maintained at <TEX>$170^{\circ}C$</TEX> for satisfactory curing.

Experimental validation of simulated weld line formation in injection moulded parts

The interest in weld line analysis of injection-moulded parts has increased in the past few years, mainly because of the ever-increasing requirements for the performance of injection-moulded items. Weld lines are formed when two melt fronts come in contact with each other. Whereas the total elimination of weld lines is not always possible without modifying the part geometry, their negative influence on part performance and appearance can be minimized. This can be done by trial and error experiments or by model prediction. The cost and time efficiency of the latter makes it a preferred route for weld lines analysis. Computer simulation packages of injection moulding are capable of accurately predicting the weld line location, but none of the current ones can predict the weld line contact angle or mechanical properties quantitatively. This paper focuses on the analysis of weld line formation and suggests ways to modify the finite element mesh to get better results.

Dispersion, mechanical and thermal properties of nano graphite platelets reinforced flouroelastomer composites

AbstractThe potential of exfoliated graphite nano platelets (xGnP™) as reinforcing fillers in flouroelastomer has been investigated. The dispersion of the nano graphite platelets in the polymer matrix has been investigated by WAXD, SEM, TEM, EPMA and AFM. WAXD studies indicated that the processing of composites did not change the inter-gallery distance (d-spacing) of the graphite platelets. The effect of increasing nano graphite loadings on mechanical properties like tensile strength, modulus and tear resistance has been studied. Formation of weld lines on the fracture surface of the composite has been observed by SEM. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with nonreinforced polymer.

Film insert molding as a novel weld‐line inhibition and strengthening technique

AbstractSpecimens with weld lines were produced via conventional and film insert molding techniques using two types of materials as the substrate resin, i.e. a polycarbonate/acrylonitrile‐butadiene‐styrene (PC/ABS) blend and glass fiber‐filled polycarbonate (PC‐gf). The formation and morphology of the weld line region was assessed with and without the presence of 0.5‐mm‐thick PC film inserts. The weld line formation and characteristics were found to be dependent on the extent of interaction between the injected resin and the mold surface or the film insert. Better interfacial interaction between the substrate and film led to the distortion of the weld line orientation, which significantly enhanced the mechanical properties of the weld line. The incorporation of glass fibers into the substrate resin would usually reduce the resistance of the weld line towards tensile, flexural and impact loadings. However, with the attachment of film inserts, the mechanical properties of the weld line region have significantly improved, even with the presence of rigid fibers. Upon examination of tensile and impact fracture surfaces of film insert specimens, a unique orientation of fibers across the weld line (parallel to the flow direction and perpendicular to the weld line) could be observed at regions directly under the film. The combination of favorable properties from the unique fiber orientation and distortion of the weld line, as well as the ability of the film to effectively dissipate forces towards a larger area, have synergistically contributed towards the mechanical property enhancement of the weld line region in film insert moldings. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Visualization Analysis of Flow Behavior During Weld-line Formation in Injection Molding Process

This paper presents a quantitative visualization analysis of flow behavior that was carried out to study flow behavior during weld-line formation. A visual mold with an obstacle pin in a plate-shaped cavity was designed, and weld-line formation was recorded for post-analysis. The main focus was on the motion behavior of the two adjacent flows originated by the obstacle pin. A particle tracking technique was used to track the melt and quantify its motion at the merging front to investigate flow behavior in orientation point of view. A micro-photography study on the cross-section of the weld-line was also carried out.

Computer Aided Engineering Design of Powder Injection Molding Process for a Dental Scaler Tip Mold Design

Powder Injection Molding (PIM) has recently been recognized as an advanced manufacturing technology for low-cost mass production of metal or ceramic parts of complicated geometry. With this regards, design technology of dental scaler tip PIM mold, which has complex shape and a slim core pin of 0.6 mm diameter, with the help of computer-aided analysis for powder injection molding process was developed. Computer-aided analysis for dental scaler tip mold was implemented by finite element method with non-Newtonian fluid, modified Cross model viscosity, PvT data of powder/binder mixture. The core deflection analysis of dental scaler tip PIM mold during PIM filling process was also investigated. Compter-aided analysis results, such as filling pattern, weldline formation, and air vent position prediction were investigated and eventually showed good agreements with experimental results.