Vibration assisted injection molding. ac. Mold vents should function the same way. injection molding is followed by the welding. A quantitative evaluation of the appearance quality of weld lines in RHCM parts. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Polyamide-6 (PA-66) was used as. Introduction. The effects of fiber concentration has been studied in detail. Effects of mold temperature and pressure on shrinkage 0. In an injection molding process, a weld-line forms when two flow fronts meet each other. 2) Vents in an injection mold should let the air out, but not the liquid plastic. In this study, polypropylene (PP) was compounded respectively with carbon nano fibers (CNFs) and TiO 2 nano particles at various weight fractions (10, 20, 30, 35. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Redesign the mold to prevent the weld line. In this experiment, several ejector. Silver Streak. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. In many cases, you will also see that the part is hanging up on the core during ejection. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. Gate Placement: Properly positioning the gate can direct the melt flow to minimize weld lines or relocate the weld line to non-visible areas of the part. 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. 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. when two or more melt flow fronts contact each other immediately after the cooling process. 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. of micro injection molding process simulation by technology update and progress. If so, look for an alternative resin. Melt temperature does effect the weld. One common issue that frequently arises during injection molding is the formation of weld lines. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. Causes and Remedies of Sink Marks In Injection Molding. Sink marks, 2. 010 0. This weld line can act as a weak spot in the material. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. 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. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. Sink Marks Cause 3: Improper Pack or Hold Pressure. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Three terms that often get mixed up are weld, meld, and knit lines. A weld line is a mark left on the surface of a plastic injected molded part. Injection molding weld lines where two or more flow fronts come together in the mold. Weld-line is a weak area which reduces the strength of the part locally. Figure 2. A flat product case was considered as an example of a typical aircraft structural element. 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. A visual mold with variotherm system for weld line study in micro injection molding. These potential causes include: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. Therefore, it is recommended to do a moldflow study prior to the mold being built. weld lines in infill analysis of injection molding simulation was observed [5]. The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. As the unfavorable defect in micro injection molding parts, weld line brings reduced mechanical and physical properties, especially for nano filled composites. Weld line, 5. Specifically, injection-mold lines are often associated with poor mechanical strength. A weld line is a mark left on the surface of a plastic injected molded part. Try to gradually raise the mold temperature and barrel temperature. 2. This occurs mainly in the presence of barriers that divide the melt flow in. Alter the mold design. Knit lines are hard to eliminate for injection molded plastic parts but. There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. According to the characteristics. CrossRef View in Scopus Google Scholar [17] G. Weld-line is a weak area which reduces the strength of the part locally. Edge Gates. If 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]. In this paper, a weld line computer determination method based on filling simulation with surface model is proposed, from which the positions and lengths of the weld lines can be predicted. 48 applied high-frequency induction heating to eliminate weld lines in an injection-molded plastic part, and experimental observation indicated that weld depth was reduced from 2. The most common and one of the most challenging injection molding defects are weld and knit lines. 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. 2(a, b). Usually it’s a straight line, and often to be found radiating. Key words: weld line strength, thermoplastic composites, injection molding, wall thickness, skin and core layers Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand. If you have critical requirements for strength of knit lines, please call a Protolabs applications engineer at 877-479-3680 to discuss, or email at [email protected]. 13. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. When an external load is applied to a weld line region, the part’s potential for mechanical failure is increased. 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. Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. This is particularly true if the wall thickness in the weld is thin – i. Heating and cooling mould systems, which cost extremely high, are a typical technology to prevent the weld-line. Kobayashi et al. Weld lines may occur on a molded part’s surface where, after breaking off into two or more directions in a mold, the molten material has converged. การแก้ไข ชิ้นงานจุดที่เกิดความอ่อนแอบริเวณ weld line คือต้องใช้ความเร็วในการฉีดต่ำในจุด. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Raise the injection speed. 2021年4月23日 / by admin. It occurs when melting plastic material is injected into a mold, it flows through all parts of th. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold. 2020-01-1306. Weld line is formed when two different melt flows converge, as illustrated in Fig. + Increase mold temp, or materials’ temp, or holding pressure. Plastic Rapid Injection Molding is popular not only in low volume manufacturing but also in large quantities molding, it is the most common way to manufacture plastic parts. Wavy lines are another feature of weld lines. In this paper, a weld line factor (W-L factor) was adopted to. Weld lines are one of the typical quality issues of injection molded parts. This is expected to achieve maximum results while the pressure holding effect will also increase. Polym. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. Investigations on WLs in micro injection molded parts were performed in Refs. 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. 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. Weld lines are visible to varying degrees on any molded part depending on gate location, flow length, melt/mold temperature, cavityA weld line occurs when two or more molten plastic flows meet and fuse during the injection molding process, resulting in a visible line on the surface of the molded part. This process makes possible to produce a wide range of products with different sizes, shapes, and for varying applications. Figure 1a illustrates how the weld line is produced when. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Injection molding could be applied with many polymer materials, such as. + Improve vent inside the mold. Polymers 2023, 15, 2440 3 of 21. 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. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. The spreading mold release agent on the part cavity surface might eventually cumulate at the V-notch groove of the weld line after each injection molding shot. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. When it comes to injection molding, the chances you might end up making a costly mistake is too high. Typical injection molding processing temperatures of TPU materials are as follows: IM ZONE 60A TO 75A 80A TO 90A 95A TO 75D REAR 315 – 340 °F 345 – 370 °F 375 – 400 °FTo study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. e. (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. You can expect a mold shrinkage percentage of. In the present work, the micro specimens were investigated. Weld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. Azuddin / Procedia Engineering 184 ( 2017 ) 663 – 672 665 2. Many studies were conducted to explain the weakness at the. You will have to confirm them before tooling. 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. Ghazanfarpour. Flash. . gasket seal leakage where the weld line crosses a gasket. The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come. Insufficient plasticizing ability of injection molding machine. Optimizing the injection parameters through combination of adjusting the molding temperature, varying the gate pressure, and increasing the dwell time. Sink Marks Cause 2: Low Pack and Hold time. 8. 1. Its appearance can be improved by adjusting parameters. They found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in whichEffects of mold temperature and pressure on shrinkage 0. Weld line existence reduces the material strength; however, its demerit characteristics. 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. The Solutions for Poor Welding Lines. The weld line is then created with a higher flow front temperature and is packed with more pressure. If you are familiar with any form of scientific injection molding, regardless of the name, then you know one of the first steps is performing a viscosity curve study. The resulting defects don’t stop at flash only. 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. Sink Marks 2. The most common method for the weld-line characterization is mechanical testing by means of a tensile or bend stage [6,7]. The thermo-rheological findings were used to investigate the sources of weld line weakness. Flow-induced filler orientation and weld lines are un-avoidable facets of the injection molding process. The Difference Between Knit and Meld Lines–and Why It Matters. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. Figure 2. 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. Meanwhile, Using more advanced cooling method such as cyclic cooling or conformal cooling. They decrease the. This can occur when a polymer flow splits to go around an interruption (e. injection molding, (b) weld line and meld line, and (c) injection sample size and shape. 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. To explore the formation of weld lines in injection molding, a. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Weld lines in injection molding are frequently unavoidable. 1. Weld lines. The main types of injection molding defects are: 1. M. Better weld line quality was obtained by Huang. One of the things that can compromise an injection molded part is a weld line, also called a knit line. 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. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Valve. 1. (1983). 005 0. This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. Weld lines are a common imperfection to be found in most plastic injected molded parts. Gate location should set to avoid weld lines, keep clean of the important position of the products,. Effect of process parameters on the relative weld line strength From injection molding practice and literature, 1, 3,4 it is generally accepted that: if a weld line forms before the filling is. The literature on the formation of weld lines, and their consequences, is more abundant on the area of injection molding [5-12] than on extrusion [13]. Adjust the design for the flow pattern to be a single source flow. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Download : Download high-res image (856KB) Download : Download full-size image; Fig. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the. Redesign the mold to prevent the weld line. Insufficient plasticizing ability of injection molding machine. In a meld line, the plastic comes together at an angle greater than 135 degrees. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. This leads to a better molecular chain entanglement, which in turn creates a better weld. Ejector System (Pötsch, 1995) . Fig 13:- Detailed view of weld lines. Weld line defects in injection molding is a line or lines on the part surface that caused the part looking ugly or cuased part lifetime, or even broken easily. They are unavoidable even for moderate complex products. The injection molding gate generates heat by dissipation. The simulated cold weld line length is consistent with the experimental one when a 115 °C mold temperature is adopted; with lower mold temperatures, the simulated lengths are smaller than the experimental ones—in accordance with the literature on the conventional injection molding process [26,27]—probably because of the definition adopted. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. In this lesson we will focus on the meld and weld lines. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. 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. If the different flow fronts have cooled. 010 0. Int. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. Weld-line is a weak area. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. 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. One solution is to prevent the weld lines from forming in regions that are. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. The utility model relates to a mold structure capable of eliminating the appearance of weld lines of a hole-shaped injection molded product. 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. Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off in a mold. Weld Lines-3 Ways for Detecting and Correcting. These lines. In the sense that the weld line is better formed before the resin. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPThree terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. 4 at different time steps (legend scale is volume fraction of. 1. Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. Oil lines and reservoirs are typical examples for hollow composite parts which are conventionally produced using a multi-stage process. Features such as sharp corners or undercuts can. After that, the compounds were injection-molded into tensile samples with and without weld lines to. Ni. As the injection process continues, the two separated melt fronts meet again along the longitudinal section of the plate forming a weld line. Weld Line Factors for Thermoplastics. The more gate points there are, the more weld lines will occur. Mérillou, J. The hair-like weld line is the. Weld line and warping are two critical defects for injection-moulded part. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. 4. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. This failure causes the product to be damaged during use. The weld lines in blow molding occur wherever the pari son knits together in the head, mold tacking areas and mold parting pinch-off region. Flow LinesThe 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). In narrow fields, a limited amount of molten liquid can solidify and block the flow. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. the three-plate mold often adopt this kind of gate. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. This article will review how and why weld lines occur, how mold gates effects weld line location and strength, the importance of material selection and Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. When two or more streams of plastic meet and fuse together, a visible line or weakness is formed on the surface of the finished product. Therefore, more studies covering this subject are required in order to improve the insight regarding the mechanism involved in the formation of weld lines in extrusion and,From the so-produced materials, specimens with weld lines were produced according to the first test series, but varying the injection molding parameters only twice, between the best and worst settings found in the previous study with the polypropylene types with MFR 8 g/10 min, i. 3 the path lines of selected material elements can be observed which are. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. (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. Weld lines appear as a result of the flow of molten plastic during the injection molding process. For example, filled resins and ABS are more likely to. Meld Lines. 2. , 2 (3–4) (1988. Wavy lines are another feature of weld lines. Weld lines: Cold feedstock in the die: Increase injection speed, mold temperature, and feedstock temperature, enlarge gate opening, add venting channels or overflow wells near weld line locations, move gate location, redesign parts to avoid stream partition: Flow mark: Cold feedstock in the dieWeld lines are a common imperfection to be found in most plastic injected molded parts. 18/06/2021. Participating in DFM and mold flow analysis is a good way to achieve this. In conclusion, reducing weld lines in injection molding requires a combination of factors, including adjusting the melt temperature, injection speed, pressure, and gate size, as well as using a hot runner system, modifying the part design, using a different material, reducing the wall thickness, using a mold release agent, and working. If the large molded product is. 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. (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. investigated the impact of injection molding parameters on weld line formation and strength reduction. Gates should be designed larger than normal and located to minimize weld line formation. The weld lines that occur in injection mouldings are critical areas on which depends on the strength of the mouldings. 020 0. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. Place a vent in the weld line area to remove the entrapped air. They appear on the surface of injection molded parts like 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 still rather few. A weld line is often unavoidable. What are Weld Line Defects? Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. With tighter heat controls, you can keep the material at the proper temperature to make sure the plastic keeps flowing and fills the mold cavity completely. Weld line formations can be weak if they fall in areas that will bear stress or impact. Weld Lines 5. In some cases, a weld line can also cause a loss of strength and impact resistance. The impact behavior of weld-lines in injection molding. 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. Jiquan Li, Taidong Li, +1 author. 13 shows the best gate location for which the weld lines . Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. 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. (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. That helps. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPAccording to the weld line distribution map of six gate schemes shown in . A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. 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. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of (V R) in locations close to and locations distant from the weldAmong defects of injection molding parts, weld lines are the most common defect. By the reflection method and a high speed camera, the visual system can be used to record and observe the forming process of micro weld lines during the. In order to increase the quality of the products manufactured by injection molding, RTC technology can be used to achieve higher mold temperatures. Weld lines are also known as knit lines. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. In the first stage, the two part halves are injection molded and then, in a second process step outside of the molding machine, they are welded together. Weld lines represent the region where two separated melt fronts recombine. Abstract. 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. 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. Thereby creating localized pressure within the mold. When the two such melt flows. The structure is more complex with longer molding cycle . Weld lines. Founder, Injection Molding Solutions. 15 shows the frozen layer and weld lines at the end of mold filling. 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. 030 0. Weld line. Introduction. 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. Injection Molding Know How. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Emerging Materials Research. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. It is possible to pre. Injection molds: (a) mold type A, (b) fixed plate of the mold type B, and (c) moving plate of mold type B The second mold (mo ld type B) was used for the ISO D2 specimen, as shown in Figure 2b,c . You can locate the gate at the thick part of the injection molding to prevent air pockets. It is normally at the meeting points of two flow fonts. A meld line occurs when two moving melt fronts converge and flow parallel to each other. 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. Its properties make it possible to create thin-walled parts having close tolerances in fast cycles over long. Injection-molding machine is the Negri Bossi VE210-1700 with screw diameter 60 mm. The wheel has to be gated at three places on the hub and. These are caused by the shot of molten plastic moving at different speeds throughout the injection. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. Some of these are listed below: Plastic warpage. Injection pressures too high Reduce pressure . Vents in the mold allow the mold to fill. In addition a weld line can be moved to an area where it is less visible. Provide a gas vent. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Knit and meld lines are inherent to plastic. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. These lines are formed when two or more flowing molten plastic streams meet and fuse together, creating a visible line on the surface of the molded part. This month, in our FREE SimTech™ series of webinars, we will explore weld lines and how they develop. Failure may result due to the assembly torque, or end-use application loads (temperature, creep). In other words, weld lines reduce the mechanical properties of the product. 2. 943 µm to 0. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. Bubbles are caused by melting flow fronts colliding in a mold cavity. If you turn on the spigot to a garden hose, you will hear a hissing sound at the nozzle end for the first few seconds. If the melt temperature is low or the injection speed and pressure are low, you may end up with a more pronounced weld line. The four sets of plaques are produced by changing the magnitude of packing pressure equal to 60% of filling pressure, with increment of 20% up to 120% of filling. Meld lines are formed when two separate flow fronts meets. 1. 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. You can choose material of low viscosity and can increase the injection pressure. 18/06/2021. This may require higher melt temperature or lower injection speed . Design. These lines occur when two or more flow fronts meet and fuse together, resulting in a visible seam on the molded part.