Weld line Weld line dikatakan ketika dua atau lebih aliran lelehan depan. 226-233. The impact behavior of weld-lines in injection molding. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Sequential valve gate technology. injection molding. are very minimum. Weld line location. Wavy lines are another feature of weld lines. Bociaga and Jaruga (2007) investigated flowing weld line in injection process by injection molding machine, which the part surface was observed in reflected light using the optimal microscope and. Weld line existence reduces the material strength; however, its demerit characteristics. Technol. 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. 5. Minimize the use of release agent. 3 and 53. Mold flow analysis can predict the location of weld lines. 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. A new kind of weld line was. Usually it’s a straight line, and often to be found radiating away from a hole, inclusion or other such part feature. The most simple design modification for eliminating the weakening effect of a weld line is to increase the wall thickness in the weld line zone (Figure 4). Weld lines may appear like seams or scratches, but that is not what they are. Which means engineers need to know how to account for this phenomenon in an efficient way. If the plasticizing ability of the plastics injection molding machine is insufficient, the plastic. The second technique, cascade injection moulding, can produce parts free from weld lines. Int J Adv Manuf Technol 52:131–141. (1983). (1)Injection Machine Poor plasticization and non-uniform melt temperature – appropriately extend the injection molding cycle, to ensure thorough plasticization, and change for a machine with a larger. Mechanical properties of the weld line defect in micro injection molding for various nano filled polypropylene composites. Remedies: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. In injection molding weld lines are formed whenever two flow fronts are joined. To prevent weld lines, increase the pressure and temperature. 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. 22 shows how the weld lines are formed when the melt splits around core pins during the injection molding process. 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. Voids and bubbles, 6. Slow down injection speed. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. demanding injection molding and end-use requirements. Weld lines have been one of the common defects puzzling the injection molding industry. In an injection molding process, a weld-line forms when two flow fronts meet each other. 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. Therefore, it is recommended to do a moldflow study prior to the mold being built. Equipment. 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. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. 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. Poor weld line Ⅰ. on conventional reciprocating screw injection molding machines using molding practices standard for filled engineering thermoplastics . [1] Kobayashi et al. Weld lines Weld lines are weak areas formed by converging flows of plastic that can lead to mechanical part failure if not properly accounted for in the part design. A weld line is a visible seam where two areas of the molten plastic meet. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. The Influence of Knit-lines on the Tensile Properties of Injection Molded Parts, Polymer Engineering and Science, 23(10): 591-596. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Some of the common fixes for weld lines include: Increase the resin temperature to ensure both sides of the weld line are liquid when colliding. In a meld line, the plastic comes together at an angle greater than 135 degrees. th e use of design of e xperiment techniq ue. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Alloys. Polymers 2023, 15, 2440 3 of 21. Weld lines are formed when two melt fronts come in contact with each other. The Factors that Cause Weld Line in Injection molding Pressure. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at. An injection molding gate is a critical component in mold design, acting as the passage through which molten plastic travels from the runners into the mold cavity. Bin-Xin Yang Ouyang Jie Supei Zheng Qiang Li Wen Zhou. They decrease the. Injection molding is a highly versatile manufacturing process used to produce a wide range of plastic products with precision and efficiency. Meld Lines. Amongst weld line. It occurs when melting plastic material is injected into a mold, it flows through all parts of th. Weld line defects are a common part defect that occurs when the injected plastic flows around a solid piece of the mold and does not knit back together. Various measures are taken at production sites to address these problems. Our injection molding company is explaining what these imperfections are, why they can compromise. This reduced thickness of the frozen skin further enhances the visual appeal of the. Tekanan. In the case of PC, the model predicts satisfactorily the weld-line strength as a function of injection temperature. In this blog post, we will explore the causes of weld lines in. This thermal field was transferred to an injection molding flow solver to compare simulations and experimental results from the high speed video, both with and without the effect of induction heating. Usually it’s a straight line, and often to be found radiating away from a hole, inclusion or other such part feature. This gives a clue as to how it was formed. Weld lines are a kind of Achilles heel of plastics. In injection molding, it is often important to maintain consistent color throughout the part. Increase the injection speed to reduce the amount of cooling happening in the mold. A meld line occurs when two moving melt fronts converge and flow parallel to each other. The following guidelines have. One of the things that can compromise an injection molded part is a weld line, also called a knit line. 1. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line and a comparable sample with the same geometry but without the weld line, and this measure mainly depends on the molding conditions, such as melt temperature, mold temperature, and injection pressure [8]. 0 mm, is designed to conduct the. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. For thick-walled parts, slow injection rates result in more uniform polymer distribution . Micro injection molded polymeric parts coated with functional thin films/layers show off the promising applications in microsystems area. Figure 2. Vibration assisted injection molding. For those who are new to the injection molding industry, may be confused about this product. 5. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Three terms that often get mixed up in injection molding are weld, meld and knit lines. As these flow fronts come together on the. Vertical Plastic Injection Molding Machine [10, 11, 12] 2. In the heating step, the heatingOverview of the injection molding process. Ozcelik studied the influence of the injection parameters and weld line on the mechanical properties of polypropylene (PP) during IM process via the design of experiment (DOE). Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. 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. NX Mold Flow Analysis. They can affect the strength and appearance of the part, depending on the temperature, pressure, speed, and mold design of the injection molding process. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. 943 µm to 0. Increase the mold temperature. When two or more strands of molten plastics meet in the mold, a weld. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. 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. Save. Consequently, it causes viscous dissipation which leads to the generation of heat. Problem: Weld lines are formed when plastic resin is forced around an obstruction in a mold tool and reforms on the side opposite the gate. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. The weld line refers to the process in which the liquid material is divided into two or more flows in the cavity during the injection process and merged together after a period of time [51,77,78,79]. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld. (1998) for clear photographs. 3 cc/sec, the weld lines are no longer perceivable above 130 C. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. A weld line develops when two or more flow fronts come together but do not meld completely because of partial solidification. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. Meddad and Fisa [3] found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in which the fibers are oriented almost. 1, right). Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Burn Marks. Insufficient plasticizing ability of injection molding machine. These flow. 2. Flow-induced filler orientation and weld lines are un-avoidable facets of the injection molding process. See moreIncreasing the temperature of the mold or plastic material. T. Injection pressure or. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. Kagitci YC, Tarakcioglu N (2015) The effect of weld line on tensile strength in a polymer composite part. Clamping unit berfungsi untuk memegang dan mengatur gerakan mold unit, serta gerakan ejector saat melepas benda dari molding unit, pada clamping unit lah kita bisa mengatur berapa panjang gerakan molding saat dibuka dan seberapa panjang ejector harus bergerak. 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. In this experiment, several ejector. Xie L, Zhu D, Ziegmann G, Steuernagel L (2016) Investigation on correlation between cold/hot weld line mechanical properties and micro injection molding processing parameters. One solution is to prevent the weld lines from forming in regions that are. Melt temperature does effect the weld. Weld lines can be caused by material flowing around holes or inserts in the part, multiple injection gates or variable wall thickness where hesitation or "race tracking" can occur. Weld line section presents notch or gloss difference. (1) Flash: Extra plastic material in the shape of films or burrs, which appears on the parting surface, around the runner or in the insert crevice. If so, look for an alternative resin. 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. Maintain a holding pressure that is no more than 1/2 the. In this paper, a weld line factor (W-L factor) was adopted to. • Optimize the design of the runner system. This is a video excerpt from our Problems & Solutions Course - Weldlines. This article will introduce you the usage, types, and advantages of the Sequence Injection Timer. C. 4. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. 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. This consequently produces weld lines. If the different flow fronts have cooled. Weld lines can be caused by several different problems: Low temperature of injection moulding machine barrel. Furthermore, injection molding was applied by filling these pellets into a micro-injection molding machine (C. As the unfavorable defect in micro injection molding parts, weld line brings reduced mechanical and physical properties, especially for nano filled composites. The Solutions for Poor Welding Lines. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. 2. In this paper, a kind of polypropylene (PP) foam plastic parts with special weld lines were obtained by core-back foam injection molding with chemical blowing agent. Remember, by avoiding weld lines, you can lessen operations cost. 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. Increase the injection speed. That helps. The injection molding gate generates heat by dissipation. Just check out some of it on the above lists. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. A poor knit line. Despite injection molding being recognized as a robust process for obtaining parts with high geometry accuracy, one last occurrence remains a challenge in micro-injection molding, especially when junctions are. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. 010 0. A weld line is often unavoidable. Fortunately, there are several. If the different flow fronts have cooled. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. Try to gradually raise the mold temperature and barrel temperature. The choosing optimal gate location technique has been developed to set weld lines as far as possible from the region of maximum stresses. Methodology. The main types of injection molding defects are: 1. Joe Henz is a chemical engineer with 30 years of experience in the plastics industry, primarily with GE. What Causes Weld Lines in Plastic Injection Molding Parts? I. One common issue that frequently arises during injection molding is the formation of weld lines. 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. Unfortunately, the 3D layering process creates ‘weld lines’ (an injection molding term) between layers which is essentially two layers of polymer that meet. M. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. With unchangeable and fixed requirements for the final product, their avoidance often proves difficult. After that, the compounds were injection-molded into tensile samples with and without weld lines to. In the 1947 book, Injection Molding of Plastics, the late Dr. In recent years new methods for quality prediction and optimization of the injection molding process have been proposed and applied. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. Increase the mold temperature. Plast. 020 0. Fatigue crack growth (FCG) tests were conducted on SEN specimens prepared from 100 mm. Interestingly, these materials have been shown to outperform long fiber thermoplastic (LFT) composites (in. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. The impact behavior of weld-lines in injection molding. The impact behavior of weld-lines in injection molding. 030 0. For glass-filled grades, the thickness at the weld line can be slightly larger than the nominal value due to lower shrinkage associated with vertical fiber orientation at the weld line cross. Weld lines have been one of the common defects puzzling the injection molding industry. Injection molding is a complex multi-variable, nonlinear and time-varying unsteady process, which makes it more difficult to optimize the process parameters. A weld line develops when two or more flow fronts come together but do not meld completely because of partial solidification. 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. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. 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. Simulation of polymer molding filling process with an adaptive weld line capturing algorithm. 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]. This study describes how the weld line was formed on the part. The following are the reasons. J. Melt temperature is 250°C, mold temperature is 110°C. Short Shot Short Shot adalah suatu kondisi dimana, plastik leleh yang akan diinjeksikan kedalam cavity tidak mencapai kapasitas yang ideal atau sesuai settingan mesin. DescriptionThis insertion ensures the separation of the melt flow. Flow front tracing simulation results for weld line developing process in micro injection molding with Comsol ® Multiphysics 3. Multi-Shot Molding – A molding process that uses two or more plastic materials to form a single part. Injection molding is one of the most established and well-known methods of plastic processing. As a result this. Molding with weld line model Moldex3D was used for calculate the molding of lugs (figure 1). ”Weld Lines in a plastic cover. Characteristic: Basically, there are PA6 AND PA66(Nylon 6 or Nylon 66), Nylon has tenacious and wearable, self-lubricious and smooth characteristics with non-toxic and antibiosis and high water absorbability. Weld Line Factors for Thermoplastics. Weld-line is a weak area which reduces the strength of the part locally. สาเหตุรองของการเกิดปัญหาการเกิดประกายเงิน (silver streak) 1. In this study, the effects of injection temperature, injection pressure, and fiber cont. . Fracture behavior of weld lines in injection molded components loaded under real operating conditions. A good weld typically occurs when the melt temperature where the weld line is formed is no more than 20°C below the injection temperature. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. Thus, it is important to understand (i) if the melt is able to fill the cavity up to the weld line location, (ii) where the weld line forms, and (iii) if the weld line is a cold or flowing one. 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. 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. 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. 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. Weld lines are injection molding defects that can be mitigated by a number of tactics that depend on your part design. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. Processing OptimizationReduced frozen skin thickness: High injection speeds also contribute to a thinner frozen skin at the weld line. Weld line, 5. Injection molding can be performed with only one of these materials like glass, plastics, etc. วัตถุดิบ พลาสติก PMPA ( อะคริลิก ) เกรด A. Cracking and/or Crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general. 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. Xie L, Ziegmann G (2008) A visual mold with variotherm system for weld line studying micro injection molding. 3. Moldflow is computer software that is widely used across the plastic injection molding industry to predict and optimize the injection molding process and assist with mold design. Injection pressures too high Reduce pressure . This failure causes the product to be damaged during use. Change the gate locations. So today we are going to take a look at the five most common defects in injection molding and their solutions. In order to reduce these weak points, a special ram technique for injection molding, which can be integrated into the mold, was investigated. 2007; Lotti et al. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. Insufficient injection pressure and speed can cause weld lines. Weld lines can appear on the components when the molten plastics meet each other as they are flowing from two different parts of the mold. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPAbstract. [8,9] It is capable of. To explore the formation of weld lines in injection molding, a. 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. Sink marks are a common type of injection molding defect. Weld lines represent the region where two separated melt fronts recombine. 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. Weld-line is a weak area which reduces the strength of the part locally. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. 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. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). 2. One of the factors that determines the quality of injection molded parts is the weld line [50,75,76]. In this mold, a visualization design and a rapid heating/cooling system were integrated, and specimens with different cross section. This paper presents the. 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. As a result this weld line will be a weak point in this part. In this study, one injection mold. CrossRef View in Scopus Google Scholar [17] G. The molded plastic part may also appear to be rust-colored. The most common and one of the most challenging injection molding defects are weld and knit lines. This can occur when a polymer flow splits to go around an interruption (e. Examples of such failure are shown in Figure 1. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. 015 0. Weld lines appear as a result of the flow of molten plastic during the injection molding process. In this experiment, several ejector. In this lesson we will focus on the meld and weld lines. Knit lines in injection molding are undesirable, especially when surface appearance and part strength are significant concerns. This type of injection molding resolves any trouble caused by undesired welding lines. The position, length, and angle of weld lines are dependent on the. Sink Marks Cause 1: Incorrect Melt Temperature. Five industry pros with more than 200 years of combined molding experience provide step-by-step best practices on mounting a mold in a horizontal injection molding machine. Injection molding. Remedies: Raise the temperature of the mold or molten plastic. % glass fibers – PA6 Aramid SV 30-1ETM (figure 1b). Change the gate locations. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Munich, Germany: Carl Hanser Verlag, 1994. Weld lines appear as a result of the flow of molten plastic during the injection molding process. Powder injection molding comprises several processing steps, and defects may occur in each step if care is not taken. This method was adopted. The injection molding process can be optimized to maximize weld line strength by increasing. With Ixef® PARA compounds, the strength of a weld line is typically 90 MPa (13,000 psi). The terahertz time-domain spectroscopy (THz-TDS) imaging technique was used to detect the weld line defect of an injection-moulded short glass fiber-reinforced composite. These master batches and PP pellets were dry-blended by adjusting the ratio such that the fiber contents were 10, 20, and 30 wt%. Weld lines can impact the strength and appearance of the final part, and are often considered a defect. 1. When two or more streams of plastic meet and fuse. 2-1. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. 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. C. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. 10 in Nguyen-Chung et al. Weld line strength is influenced by the temperature at which the weld line is formed and the pressure exerted on the weld until the part freezes, (pressure is 0 at the weld line). 3 cc/sec, the weld lines decrease over temperatures up to 150 C, while at higher volumetric injection flow rates of 43. The resulting defects don’t stop at flash only. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. From Understanding Weld-line Integrity by Sabic Innovative Plastics, p. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. Cited by (41) Effect of mould surface roughness adjustments to increase the flow path length in the injection moulding process. examined the relationship between the structural and mechanical qualities in HDPE/PA6 mixture which is molded with weld line and without weld line in plastic injections [6]. Weld lines often generate mechanically weak areas, because of the unfavorable perpendicular orientation of the macromolecules, fibers and/or other fillers to the direction of the main flow. Reduce the amount of release agents. From Understanding Weld-line Integrity by Sabic Innovative Plastics, p. The gate is the critical bridge between the runners (the channels through which the molten plastic travels) and the actual mold. Causes. Weld lines are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. Venting at Weld Lines. 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. This occurs mainly in the presence of barriers that divide the melt flow in. Ejector System (Pötsch, 1995) . . Controlling the mold’s temperature could also minimize the weld line’s effects. Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. A meld line is defined as the re-merging of two flow fronts. 2. It is important to investigate the weld line because the weld line area can induce potential failure of structural application. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Wavy lines are another feature of weld lines. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. 22: Weld line formation when melt splits around core pins. The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. 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. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of. Table 2-Glass Fiber Glass Fiber Glass Fiber Glass Fiber 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.