[ 18 - 21 ] In addition, the effects. Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. This line does not normally impact the overall shape or dimensions of the item. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. In some cases, a weld line can also cause a loss of strength and impact resistance. 1. During injection moulding, a weld-line occurs when two or more flow fronts collide in the hollow cavity. The need to minimize fiber breakage, plus the higher viscosity of materials with. 0 mm, is designed to conduct the. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. 4. This occurs mainly in the presence of barriers that divide the melt flow in. In this mold, a visualization design and a rapid heating/cooling system were integrated, and specimens with different cross section. 3. Simulation of polymer molding filling process with an adaptive weld line capturing algorithm. Miron MC, Major Z, Adachi T. This can occur when a polymer flow splits to go around an interruption (e. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. molding Æ Thermoplastics Æ Weld line Æ Simulation Rheol Acta (2004) 43: 240–245 DOI 10. As the injection process continues, the two separated melt fronts meet again along the longitudinal section of the plate forming a weld line. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. 3. In addition a weld line can be moved to an area where it is less visible. A weld line refers to a line, notch or color change that is created on the molded part caused by the convergence of two separate flows of molten plastic. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of. As such, optimizing mold designs should always be incorporated in your production model. Silver Streak. In injection molding of thermosoftening plastics, an important aspect having a bearing on the quality of articles is the appearance of weld lines that are formed when there are inserts, barriers, molding signs, reinforcements, etc. To explore the formation of weld lines in injection molding, a. A visible weld line is observed, due to a combina-The Weld line result itself has a scale associated with the placement of the weld line that is directly relative to how severe the weld line will be based on the angle in which the two flow fronts meet. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. <Materials> Pre-dry thoroughly. Cited by (41) Effect of mould surface roughness adjustments to increase the flow path length in the injection moulding process. In this work, the weldline strength of injection molded short-glass-fiber reinforced polycarbonate was derived by an analytical method of modified rule-of-mixtures as a function of the area fraction between skin and core layers. This occurs wherever there is a hole, notch, or any other feature on a part which divides the flow of plastic into 2 separate “flow fronts. In spite of the symmetry of the molding the formed weld line was not straight, due to a slight unbalance of the two injection gates. Weld-line yield strength, yield strain, and failure strain of talc-filled polypropylene increase with increasing melt temperature and increasing hold pressure. Flow front tracing simulation results for weld line developing process in micro injection molding with Comsol ® Multiphysics 3. These flow. 2. In this experiment, several ejector. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. Below is one of the examples: !NOTE: if the weld lines is important to the cosmetic surface of your part, please bring this up to your mold maker during quoting phase. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. of micro injection molding process simulation by technology update and progress. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. This type of injection molding resolves any trouble caused by undesired welding lines. 7 of the reference. Thereby creating localized pressure within the mold. The weld lines that occur in injection mouldings are critical areas on which depends on the strength of the mouldings. Materials Science. Weld lines are a common defect generated in injection molding, which apparently affects the performance of final products, but the available reports on carbon fiber-reinforced thermoplastics are still rather few. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. Int J Adv Manuf Technol:1–11In addition to the weld line, the exposure of fibers on the molded product surface results in a rough surface, which degrades the appearance of the product. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. The most common and one of the most challenging injection molding defects are weld and knit lines. They can affect the strength and appearance of the part, depending on the temperature, pressure, speed, and mold design of the injection molding process. 943 µm to 0. Weld lines are a common occurrence in injection molding that affects the overall quality and strength of a finished product. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Compd. The injection pressure is too lowEffect of the processing conditions on the variations of the bulk temperature, T b , at the end of mold filling at the two locations: in (left) and away (right) from the weld-line (T w , T inj. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. Weld lines are a kind of Achilles heel of plastics. The formation of weld line in the injection molding method is caused by the product and mold design that is one of the common mistakes. Insufficient plasticizing ability of injection molding machine. Weld line strength is important in blow molding as well as injection molding. They decrease the. The effects of filling time, packing time, packing pressure, melt temperature, and mold temperature on the ultimate tensile strength (UTS) and the elongation value of the weld. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. In this lesson we will focus on the meld and weld lines. Learn how to avoid them and what injection molding companies do to prevent them. The weld line’s strength is reduced when. The graph shows clearly how an increasing cavity temperature correlates with a reduction in the weld lines. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. Recently hired at plastic injection molding simulation software provider Moldex3D, Nicholson was spurred to undertake the research study following a conversation with a customer experiencing weld-line woes. Which means engineers need to know how to account for this phenomenon in an efficient way. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink. This results in a region of a different level of molecular entanglements than the bulk material. Penyebab Weld Line. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. Weld line is one of the plastic injection molding defects. In recent years, researchers have. [ 18 - 21 ] In addition, the effects of flow behavior of the polymer melt due to the optimal process parameters for the minimization of weld-line effects are an important consideration. These lines can weaken the strength of the part, and they can also be unsightly. This remains as a challenge for control engineers. Technol. The thermo-rheological findings were used to investigate the sources of weld line weakness. Injection molding could be applied with many polymer materials, such as. Fisa et al. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. box, along the region of weld line formation (wc, wm, we and wl) and away from the weld line, in a radial position along the flow direction (R), as shown in Figure 2. , Ltd. of micro injection molding process simulation by technology update and progress. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. This study describes how the weld line was formed on the part. Plastic Injection Molding Process. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. – Use generous and even. Xie L. Adjust the design for the flow pattern to be a single source flow. Poor weld line Ⅰ. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. Injection molding is a complex multi-variable, nonlinear and time-varying unsteady process, which makes it more difficult to optimize the process parameters. Miron MC, Major Z, Adachi T. Weld-line is a weak area which reduces the strength of the part locally. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU. Weld Line defect analysis, the most common of many defects in injection moulding products. Microsyst Technol. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Google Scholar | Crossref | ISI. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. These potential causes include:Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. In the RHCM, the mold temperature profile plays a crucial. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. For thick-walled parts, slow injection rates result in more uniform polymer distribution . Consequently, weld regions have been observed to have inferior mechanical properties compared to the bulk. These weld lines occur usually around holes or at the material flow joint area in the injeciton molded part, some welding lins is very small, specially if. This occurs wherever there is a hole, notch, or any other feature on a part which divides the flow of plastic into 2 separate “flow fronts. Slow down injection speed. To respond to poor plasticization and uneven melt temperature, the molding cycle can be extended to make. Here we are looking at most common part defect that can come from injection molding called weld line. Weld line problem which occurs in polymer parts produced by plastic injection method and affects appearance and strength on a large scale was researched in this paper. Additionally, you can get our free Design Cube, which will provide a physical aid to help understand how knit lines form as well as other injection molding design considerations. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Increase the injection speed. Various measures are taken at production sites to address these problems. 000 0. Increase the mold temperature. Weld Lines. In the present study, Moldflow® software is applied to simulate the injection molding of automobile instrument light guide bracket and optimize the injection gate position. Weld line is formed when two different melt flows converge, as illustrated in Fig. Mechanical properties of the weld line defect in micro injection molding for various nano filled polypropylene composites. 4If the injection gate is not properly designed, it can cause the formation of defects like sink-mark, warpage, short shot, weld-line, air traps, and other faults [15]. (2) Causes & Countermeasures (See the table below) *Note: Flash also tends to occur when the injection pressure/speed are too high, but molding time is too long and mold temperature is too low. 1. Ejector System (Pötsch, 1995) . Venting at Weld Lines. The position, length, and angle of weld lines are dependent on the. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. The present work focuses on studying the effect of packing pressure on strength of stagnation weld-line. Email him at jhenz@polymer-center. une, 13: 15. Sequential valve gate technology. Flow marks in injection molding can be avoided if adequate measures are taken. The weld lines in injection moulded parts arise as a result of the collision of two fronts of the flowing material, which fills the mould cavity and are sometimes unavoidable. Weld line (also known as a knit line) is the line where two flow fronts meet when there is the inability of two or more flow fronts to “knit” together, or “weld,” during the molding process. With Ixef® PARA compounds, the strength of a weld line is typically 90 MPa (13,000 psi). Double push filling method. If the plasticizing ability of the plastics injection molding machine is insufficient, the plastic cannot be fully plasticized, resulting in weld lines when filling the mold. ac. In this paper, aiming at the problem of weld line defects in the injec- tion process of a key phone panel, the injection process is optimized by using Moldflow and drop simulation. To position weld-line to avoid critical areas for the sake of aesthetics or mechanical strength is the key to the success of bi-injection molding. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. 9 – Over Packing. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. As a defect of micro injection molding parts, weld line is unfavorable since it will influence the surface quality and mechanical properties of micro parts. Investigations on WLs in micro injection molded parts were performed in Refs. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. 2007; Lotti et al. To prevent knit lines, injection molders can increase the temperature of the molten resin to prevent solidification from starting too soon, and increasing injection speeds and pressure are also potential solutions. Fracture behavior of weld lines in injection molded components loaded under real operating conditions. Figure 1 below shows the development of a weld line. Install an ultrared light inside the hopper. To explore the formation of. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms of weld-line, etc. Weld lines are a common defect generated in injection molding, which apparently affects the performance of final products, but the available reports on carbon fiber-reinforced thermoplastics are. Miron MC, Major Z, Adachi T. Weld lines appear on the surface of a molded part where the molten material converges after splitting off into two or more directions in a mold. Fig 13:- Detailed view of weld lines. The injection molding simulation software used offers a method for the detection of weld lines that has shown to be only reliable for the detection of stagnating weld lines. Weld lines are a common issue in injection molding, and occur when two flow fronts meet and fuse together as the material cools and solidifies. Sink marks, 2. when two or more melt flow fronts contact each other immediately after the cooling process. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. It occurs when melting plastic material is injected into a mold, it flows through all parts of th. Polym Eng Sci 45(7):1021–1030. An injection molding gate is the designed opening in a mold through which molten plastic is introduced into the mold cavity, initiating the process of taking the shape of the desired part. This month, in our FREE SimTech™ series of webinars, we will explore weld lines and how they develop. 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. Weld line section presents notch or gloss difference. 226-233. Joe Henz is a chemical engineer with 30 years of experience in the plastics industry, primarily with GE. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. Try to gradually raise the mold temperature and barrel temperature. Valve gates also make weld lines less of. Meld and knit lines are actually both different types of weld lines. The. 2021年4月23日 / by admin. Abstract. The formation of weld lines and their location along the flow path is a well-studied phenomenon in conventional injection molding; however, only a few papers are devoted to micro-injection molding [11,13,14,15,16]. 21: Weld line failure caused by hoop stresses. At a low volumetric injection flow rate of 33. Bubbles are caused by melting flow fronts colliding in a mold cavity. Voids and bubbles, 6. 2-1. 1. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. replication and poor weld line strength . Consequently, it causes viscous dissipation which leads to the generation of heat. Mold. Fracture behavior of weld lines in injection molded components loaded under real operating conditions. 5. Introduction. These master batches and PP pellets were dry-blended by adjusting the ratio such that the fiber contents were 10, 20, and 30 wt%. This study focus on the. In this study, the effects of injection temperature, injection pressure, and fiber content on the mechanical properties of weld lines were studied for carbon fiber-reinforced nylon. The impact behavior of weld-lines in injection molding. Weld line existence reduces the material strength; however, its. Weld line. For those who are new to the injection molding industry, may be confused about this product. Recently, injection molding process has generated growing interest, exhibiting great potential to easily fabricate unique polymeric materials for industrial applications. 010 0. Injection pressures too high Reduce pressure . Additionally, you can get our free Design Cube, which will provide a physical aid to help understand how knit lines form as well as other injection molding design considerations. <Mold> Increase (or decrease) the size of the runners and gates. Compd. As compared to a weld line, the flow fronts come together at less than 135 degrees. This paper presents the. 3. Weld lines are one of the typical quality issues of injection molded parts. Weld Lines. Poor weld line Ⅰ. Criens, R. The weld lines are an integral part of plastic injection molding and can seldom be prevented, even by specifically planning the mold with inserts for holes or the like. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. This paper evaluates the shrinkage and strength of weld line using Design of Experiment and Response Surface Methodology in multi-objective optimisation utilising the injection moulding parameters. With Ixef® PARA compounds, the strength of a weld line is typically 90 MPa (13,000 psi). Park et al. DescriptionThis insertion ensures the separation of the melt flow. A simulation of mold filling with multi-inlets or an insert in the cavity is pursued with an improved Level Set Method. They can affect the strength and. injection molded pipe-fittings with and without weld line. They. Wang, G. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different. The occurrence of a weld line means a significant problem both aesthetically and mechanically in the design of injection moulded parts. com1. These are caused by the shot of molten plastic moving at different speeds throughout the injection. Proper venting is also required to ensure good weld-line quality. Usually it’s a straight line, and often to be found radiating away from a hole, inclusion or other such part feature. Polyamide 6 (Lanxess, Durethan B 30 S 000000, non-reinforced, ISO 1874-PA 6, GR, 14-030, Cologne, Germany) was used for this experiment. The possibility of the functional thin film for. After that, the compounds were injection-molded into tensile samples with and without weld lines to. Inadequate back pressure. Since it is often impossible to avoid weld lines as well as a limited possibility to increase the weld line strength by adjusting the injection molding parameters, a method to predict the weld line strength in the product development process is shown below. This study focuses on the effect of melt temperature on. Make the size of runner channel wider and/or shorter. Its properties make it possible to create thin-walled parts having close tolerances in fast cycles over long. What Causes Weld Lines in Plastic Injection Molding Parts? I. Mold flow analysis can predict the location of weld lines. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Star Rapid offers tips and tricks to improve your prototypes and production parts. They occur when melt flow fronts collide in a mold cavity. Consultant @ Effinno Technologies, BSME, MBA - Bridge between Plastics Injection Molding Knowledge and Practice. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPEffects of mold temperature and pressure on shrinkage 0. ). Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. The formation of weld line in the injection molding method is caused by the product and mold design that is one of the common mistakes. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. 1, right). 5. Optimization of Injection Molding Process to Minimize Weld-line and Sink. In narrow fields, a limited amount of molten liquid can solidify and block the flow. Plast. ABS is a commonly used plastic in injection molding due to its excellent mechanical properties and chemical resistance. The resulting defects don’t stop at flash only. This consequently produces weld lines. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. The injection molding gate generates heat by dissipation. This failure causes the product to be damaged during use. Ada berbagai penyebab mengapa weld line bisa muncul pada permukaan produk plastik. This study examines the impact of injection parameters on the weld line strength of the polyamide 6 and 30% fiberglass (PA6 + 30% FG) composite samples. 5 mm, 1. They found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in whichTo investigate the effect of hot gas heating in injection molding on the weld line appearance, an experiment was conducted with the molding parameters listed in Table 2. technical ulletin SPECIALTY POLYME RS Design and Production of Injection Molding ToolsWeld lines have been one of the common defects puzzling the injection molding industry. C. One solution is to prevent the weld lines from forming in regions that are. Thus, it is important to understand (i). Figure 2. G. (3-1) Increasing the Resin Temperature at the Weld Line Section. Reduce the amount of release agents. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. Re-balancing entire manufacturing lines may be required to effectively deliver and sustain SMED excellence. 2. In an injection molding process, a weld-line forms when two flow fronts meet each other. Improper fusion of two fluid flows, a state before weld line. Weld line, 5. NX Mold Flow Analysis. 2) Vents in an injection mold should let the air out, but not the liquid plastic. Procedia Mater Sci 2014; 3: 2048–2053. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Injection Molding Glossary; Weld Lines; Weld Lines. Weld-line is a weak area which reduces the strength of the part locally. This may require higher melt temperature or lower injection speed . In this study the polycarbonate (PC) and carbon fiber (CF) are used as the experimental materials to investigate the impact of injection molding process parameters on tensile strength of weld line. 1. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. We have various types and models available. , 47 (2008. Again, consult with professionals as to how you could improve mold designs. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. They occur when melt flow fronts collide in a mold cavity. Yes, it does help, for two reasons: Often the weld line traps air and the ‘dump’ acts as a vent to allow the air to escape. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. A poor knit line. Interestingly, these materials have been shown to outperform long fiber thermoplastic (LFT) composites (in. Many works deal with weld lines and their dependence on processing parameters for conventional injection molding, but only a few works focus on the weld line in µIM. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. In this study, one injection mold. Weld line Weld line dikatakan ketika dua atau lebih aliran lelehan depan. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. The thermo. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. The impact behavior of weld-lines in injection molding. Flow analysis of the weld line formation during injection mold filling of thermoplastics. Minimize the use of release agent. 4. A weld line refers to a line, notch or color change that is created on the molded part caused by the convergence of two separate flows of molten plastic. A quantitative evaluation of the appearance quality of weld lines in RHCM parts. The tensile stresses of the weld lines with ultrasonic functions are 8. Remember, by avoiding weld lines, you can lessen operations cost. If so, look for an alternative resin. This article will introduce you the usage, types, and advantages of the Sequence Injection Timer. The formation of weld lines and their location along the flow path is a well-studied phenomenon in conventional injection molding; however, only a few papers are devoted to micro-injection molding [11,13,14,15,16]. As a result this. Meld lines are formed when two separate flow fronts meets. An evaluation model based on artificial neural network method. + Adjust injection speed. One common issue that frequently arises during injection molding is the formation of weld lines. The flow disruptor is a small insert introduced inside the injection molding cavity, in the weld line area, and its objective is to mix the melt and disrupt fountain flow in order to blur and. Fellahi et al. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. [ 18 - 21 ] In addition, the effects of flow behavior of the polymer melt due to the optimal process parameters for the minimization of weld-line effects are an important consideration. To prevent weld lines, increase the pressure and temperature. A weld line is often unavoidable. and most commonly, thermoplastic polymers are used. Vertical Plastic Injection Molding Machine [10, 11, 12] 2. Since weldline in plastic injection molding (PIM) has an influence on not only the strength but also the surface appearance of a plastic product, it is preferable to reduce it as much as possible for high product quality. A knit line, also called a weld line, occurs when the liquid plastic moves through the mold and hits more melted plastic and they don’t flow or fuse together. Siemens PLM Software. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld. (1) make the mold vent larger at location with weld line; (2) Check if the mold vent is blocked or not or use vacuumized forming. <1> As illustrated in Figure 4, the existence of. The weld line’s strength is reduced when. To resolve this issue, rapid heat cycle molding (RHCM) has attracted attention as an innovative PIM technology. industry, to deliver intuitive injection mold-ing simulations embedded in NX. Increase the melting temperature. • Optimize the design of the runner system. + Increase mold temp, or materials’ temp, or holding pressure. 6. In this study, epoxy molding compounds (EMC) filled with glass beads were fabricated with a twin-screw-extruder (TSE) and tempered before further processing to generate different degrees of cross-linking (0–15%) in the compounds. This happens when the molten plastic is injected into the mold cavity and flows around an obstacle such as a pin, hole, or protrusion, and then comes back together. Injection molding weld lines where two or more flow fronts come together in the mold. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. As a defect of micro injection molding parts, weld line is unfavorable since it will influence the surface quality and mechanical properties of micro parts. That’s the air escaping out of the nozzle until the hose is full of pressurized water. ResourcesTo study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. As a result this weld line will be a weak point in this part. Processing Optimization Reduced frozen skin thickness: High injection speeds also contribute to a thinner frozen skin at the weld line. The. 2. ”. Injection molding of large/complicated products are usually prepared by multiple compounds, which produces weldlines once the melt fronts are joined by impingement flow. Wolfgang Homes, head of the applications department of Eurotool, a manufacturer of hot runner systems, clarified the two methods for IMI, noting that. Weld line additionally called as knit line, is a defect that occurs when the injected plastic meets within a. In the present work, the micro specimens were investigated.