Also, heating the material in the cylinder. Faster injection increases pressure in front of a flow front. Injection molding has been the most popular method for making plastic products due to. 1．Mold Temperature Too Low. Flow marks are a phenomenon in which a pattern of the flow tracks of the molten plastic remains on the surface of the molded product. V. 0010 in. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. These are typically found near the narrow sections of the mould gate and can. 5 to 2. Mold Flow Analysis. 060 and 0. Flow lines can be defined as linear grooving, or circular ripples, on the surface of a molded part that indicate the direction of material flow within the cavity of the mold. This software simulation can show how a resin will fill the mold during the injection molding process. • Reduce runner dimensions and maintain the same flow rate. Before his present assignment, Bob managed a molding operation for 25. Obviously, as the resin front moves through the mold, the fill material will always be behind the front. Defects like this make it harder to market the finished parts to the end user. On average, larger injection molding machinery can cost from $50,000-$200,000 or more. The Table 2 categorically reports Melt Temperature, Injection Pressure and Injection Speed are the main contributors in the defects such as the short-shot, weld-lines, flow marks, burn-marks etc. Turbulent Flow IS Always Better. On larger parts where knit lines or flow lines are a concern, you can use sequential valve gates to control the flow and move the knit lines. The formation process of weld lines can be described as. 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. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. These lines form as a result of material flow and solidification during the injection molding process. For optimal cooling, these waterlines are located as close as possible to the surface of the molded part. 750 in. Impurity. More about the knit lines in injection molding. Flow direction is essential for mold mapping because when water flow reverses, poor turbulence and hot spots occur. plastic mold venting. Most flow lines don’t impact the functio…Flow lines are a defect in injection molding and can be a significant issue for manufacturers. 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. 020 0. The recommended minimum production volume for injection molding is 500 units. The mold flow model is usually a single-part model, that is, the FRT part itself, but the mechanics model is often a multi-part model, an assembly of the FRT part and others. or too much shear heat burning the material prior to injection. 3. The article has been structured just for you to gain an understanding of the same so that when you plan to buy plastic injection molding, you do understand what it means to do so. Some of the most popular brands include:Fiber reinforced thermoplastics (FRTP), reinforced with glass or carbon fibers, are used to improve the mechanical strength of injection-molded products. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Well, what this simply implies is that injection polycarbonate molding makes it possible for you to create a wide range of part sizes. This. To further improve productivity, the injection mold was equipped with CCCs to shorten the cooling. Mold fill usually takes 0. Picture the flow of a liquid resin filled with, for example, glass fiber. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Learn how to determine it, and the role of SPI Mold Classes. Learn how mold flow analysis can support the development of complex and customized injection molded products by identifying defects, optimizing materials, enhancing performance, and reducing costs. NX Mold Flow Analysis. One cause of flow lines is low injection speed, which can result in the molten plastic cooling too quickly and creating a. The type of gate for manufacturing often depends on the material used and the parts’ shape. These streaks or patterns can affect a part's aesthetics, value, and. There are important differences between an injection mold sprue and its runner system. com. The mold in Figure 6 is an 8-cavity with an “H” pattern runner and a lapped edge gate. RapidDirect – Your Best Injection Molding Partner. Edge Gates. Detecting Water Flow Restrictions. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near the surface of the finished parts. As these flow fronts come together. After the plastic fills the mold, it cools and solidifies into finished part (s) which can be easily removed when the machine opens up the mold. Each gate type has specifications that significantly affect the end product’s structure and appearance. Flow marks won. 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. Burn Marks. The speed of an injection moldingRepeat the two previous steps two or three times, starving the screw periodically to allow the screw to rotate without building a shot. Flow Lines. This. This means that the skin of the part is always formed by. A knit line is when two flow fronts come together but, rather than merging, it’s like a head-on collision at a 4-way stop. Address 272 Commonwealth Drive, Carol Stream, IL 60188. Heat: Too much heat can have an appearance similar to moisture splay. In injection molding weld lines are formed whenever two flow fronts are joined. Remedies: Increasing the injection pressure will force the molten plastic against the mold cavity. Kobayashi et al. The flow fronts at different times (Fig. 080" – 0. 150 in. 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. Some of these factors could be the flow rate, the pressure, or the temperature settings of plastic injection molding. Injection molding tool life is the number of production cycles that an injection mold can complete before it needs to be replaced. A poorly designed or maintained water-cooling system can have a serious impact on production efficiency and cause many maintenance issues. Flow lines are a defect in injection molding and can be a significant issue for manufacturers. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. Here are the top 5 injection molding defects that can occur, and how to solve them: Flow / Weld Lines. Ensure sufficient cooling time to avoid locking residual stresses into the part. Knit and Meld lines are inherent to plastic injection. 3 the path lines of selected material elements can be observed which are. Note: It is applicable to thermoplastic materials only. The weld line and the flow pattern of co-injection molding can be seen from Fig. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. Molders should make sure the weld line is formed during the first-stage filling of the part, if possible. ”Poor venting, along with the diesel effect, can also cause the mold to wear on the mold parting lines, resulting in flash (excess plastic) on the injected part. mold and the blow molding process. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and. Troubleshooting Injection Molding Sink Marks. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. A knit line may be unnoticeable or may look like a crack. Midland, MI. Using a thermoplastic that stays in a. Sprues are usually tapered, and they come with ends that are bigger at the top. There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. 1 – In plastic injection molding, the flow ratio is the distance molten plastic must travel, divided by the wall thickness of the part. Once you have a cool shell formed the rest of the mold entering absorbs heat from the layer next to the wall. Certain injection molding processes may not be effective for the production of some parts. Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks. 16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. Short shots refer to incomplete filling of a mold cavity. Vacuum on the mold does help and many times eliminates the weld line. This is achieved by using the needle valve nozzles to control mould filling so that only one flow front exists at a time. Flow lines are linear marks on the surface of a molded product, which indicate the flow direction of the molten plastic. Strengthening. The linear mold shrinkage of injection molded parts will vary from 0. 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. 2) show that the weld line is formed as expected from the middle of the cavity towards the wall. Mold is not vented properly causing slow flow at knit lines. The production cost is closely related to productivity. (3) Improving the surface gloss of the product. Flow Rate. Phone (630) 665-8590. These benzene rings also increase the viscosity, reduce shrink rate, and increase strength with the exception of knit and/or meld lines. They may also take the form of ring-shaped patterns when staying close to the mold's entry point, which is the plastic injection nozzle. On a small four-cavity tool, the part had a U-shaped notch in the center. Let’s review some methods to prevent flow and knit lines in plastic injection molding. Flow rates and a fluid are input to the cooling channels,. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. (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. sufficiently melted to a molten state in the barrel of the machine before the injection. tall, and 0. Flow marks are caused by a variety of factors in the injection molding making technology, and they happen at the gate. mold temperature con-trol fluid flow in a mold, either laminar or turbulent, is by a dimensionless number. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the weld line can be seen clearly from the Fig. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. The Analysis and countermeasures of flow lines in injection molded products Read more. In the injection molding process of plastic parts, flow mark is a common defect due to the defects of parts design and injection molding process [4]. The size, shape, and location of the gate have a significant impact on the finished product. ABS resin has low thermal deformation temperature, but poor heat resistance. However, surface quality is difficult to predict due to many influencing factors and their complex interactions. Look for signs of stickiness or burning. Definition: Off-tone streaks, patterns or lines that appear on parts. Increase the mold temperature. 5. 1. In Fig. Accept All. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. 5-3. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Edge Gates. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. Flow marks, also known as flow lines or weld lines, are common defects that can occur in plastic molded parts. In injection molding, it is often important to maintain consistent color throughout the part. 005 0. Warping. 1. From a technical point of view, valve gate technology enables the production of low-stress injection molding parts, which almost always meet the requirements of a very low vestige. The accompanying table lists the flow rates for various barrel diameters at various injection velocities. Here are some of the most common scenarios for cracks in injection molded parts: Cracks in the Direction of Flow – If the crack develops in the direction of the plastic’s flow into the mold cavity, the cause is molecular orientation. Increase molten plastic temperature during injection molding. Several factors contribute to the formation of flow lines, including material properties, mold design, and process parameters. The mold material 6 can have several effects on. We heat the plastic to a molten phase, inject it into the mold and cool it into a preferred shape. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. Non-uniform cooling of mold causes warping, creating internal stresses within the part and resulting in distortions. Typically, flow lines are caused by the liquid plastic cooling at different rates. During plastic injection molding, molten material flows through an opening called a gate and fills a mold cavity. When a poor-quality plastic that has impurities in it is used for injection molding, it is less likely to flow smoothly through the mold. Reducing injection speed will allow more time for the air to escape from the cavity through the vents, but this leads to other problems, such as short shots (incomplete parts). Polymer Injection Molding: Gas-assisted Molding. FLOW – A qualitative description of the fluidity of a plastic material during the process of molding. The strong molecular. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. The recommended minimum production volume for injection molding is 500 units. 3). 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) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld-line behavior. Solution: Utilize a material that has the stiffest flow possible without causing non-fill. 015 0. (1. Circuit Identification. Make sure the venting is not too large. There are three stages of flow – laminar, transient and turbulent. Flow marks, also known as flow lines, are molding defects that can occur in the manufacturing process of injection molding. This can prevent the opportunity for trapping air. Flow lines are surface irregularities, wavy patterns, or a rippled effect, perhaps of a slightly different color, that can appear in injection molded parts. You can avoid having any weld lines with cascade injection moulding. Flow marks: Flow lines Directionally "off tone" wavy lines or patterns Injection speeds too slow (the plastic has cooled down. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. Higher numbers assure that water flow is adequate, and that turbulent flow is at an acceptable level. Introduce automation into the injection molding process for improved efficiency. Laminar flow in a plastic mold, described by. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. In these cases each circuit might remove a different. The proposed method is first applied to the simulation of HDPE melt in a classic Poiseuille flow case, and then two. Polycarbonate is a tough, optically clear or opaque, biocompatible plastic with good thermal resistance. Energy use by thermoplastics injection molding machines is estimated to result in global CO 2 emissions in the order of 80 million metric tons annually. Cascade Moulding Avoids Weld Lines. Flow Lines: Flow lines on thinner areas of molded components appear as a wavy pattern, often a different color than the surrounding area. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. They also can appear as ring-shaped patterns near the entry points of the mold. Features such as sharp corners or undercuts can. Mold Flow - Position ribs such that the plastic has an optimal flow path. 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. Aberdeen Technolgies makes the mold manufacturing process for injection molding. In the process of injection molding, the quality of the mold plays a decisive role in the quality of the finished product. vent the ejector pins and vent the lifters and the parting lines. If the cornflake pigment contains a large amount of fine particles, this will result in a higher definition of flow and weld lines. Prior to this late stage in production, however, 3D printing is much more affordable and flexible for products in the early stages of design. Shear; There are two causes of splay brought on by shear. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. The gate vestige for injection molding is usually narrow. Flow lines and weld lines are lines or streaks in the final molded part. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. This allows the melt to keep enough velocity to ensure proper filing. Heat: Too much heat can have an appearance similar to moisture splay. The good news is that flow lines are only an aesthetic concern and do not affect the strength or structural integrity of a part, which will perform as intended. Figures 3A, B and C depict different examples of “spot” vents. 2. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. A. Inadequate cooling time. Avoiding Voids and Sinks. ” Flow marks do not affect component integrity. PartWeld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. A void occurs during cooling while the part is inside or outside the mold. If the mold itself has cracks or defects, the finished product will also have various problems. Reduce the temperature of the mold and plastic being injected. What Are Flow and Knit Lines? Flow lines are defects that appear as wavy. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. If the cornflake pigment contains a large amount of fine particles, this will result in a higher definition of flow and weld lines. Flow lines are aesthetic defects that occur on plastic products. Flow lines are linear grooving or circular ripples on the surface of an injection molded part that indicate the direction of material flow within the cavity. G. These are caused by the shot of molten plastic moving at different speeds throughout the injection mold, which ultimately causes the resin to solidify at different rates. In injection molding, it is often important to maintain consistent color throughout the part. ³/sec. The injection molding gate generates heat by dissipation. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. Basic Repairs for Warping. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. reduce the ability to positively influence the strength. It is a type of distortion caused by this premature solidification. and most commonly, thermoplastic polymers are used. Locally black discolorations of the molded part occur in the area of the weld lines, at the end of the flow path or of individual areas of the molded part (for example, a rib). There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. 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. flow rate, in turn, causes the heat transfer effi ciency of the cooling cha nnels to vary from one. However, you can easily prevent such defects by simply adjusting the temperature, flow rate, or other mold parameters. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. 6T) as thick as the nominal wall. Plastic Injection Molding Process. The material is kept poured into the mold for some time. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Flow lines, also referred to as flow marks or weld lines, are visible lines or patterns on the surface of a plastic component. Flow lines can occur if any of the following affect the mold: A runner, gate, or sprue that is too small for purpose creates more flow resistance. Utilize hot. (All Illustrations: S. In this study, a new specimen design is introduced for the tensile testing of micro parts. The chart below shows recommended wall thicknesses for common injection molding resins. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. Poor mold surface n Refinish a poor or worn mold surface. ³/sec. injection molding. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Otherwise, the heat transfer of. In this guide, we cover evaluation and also the remedy of flow lines in. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. A look at how taking the time and data to analyze a mold problem will help you repair it right—and permanently—the first time. Effects of mold temperature and pressure on shrinkage 0. 0 seconds, depending on part volume, runnergate style and size, cavity location and injection pressure. The CoolingCare system tests cooling channels for leaks and blockages. 31″) will require more. In this regard, the gate form with less shunt should be used, and the gate position should be selected reasonably. This design is deliberate in order to help the mold. These defects include the following: 1. Since these materials behave and respond differently, it is crucial. The injection molding process involves the injection of a polymer melt into a mold where the melt cools and solidifies to form a plastic product. This depends on a. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. This requires skilled labor to mold new parts that have less molding defects. Manifesting as wavy lines or streaks on the surface of the molding, flow lines generally occur when molten material flows. 10(a) (please see Fig. 5. In the development of our calculator. Short Shots. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. The Priamus Fill system is designed for automatic closed-loop control of multi-cavity and family molds, multi-material injection, sequential valve-gated molds, as well as LSR molds. , 1 IN, 1 OUT, 2 IN, 2 OUT). The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0. More Control. Possible Causes & Remedies: Machine Cause. wide. This can be achieved by connecting ribs to each other. After exhaustive research, testing and pilot-scale demonstration, the team developed a breakthrough new technology utilizing low constant injection pressure. The edge gate is the most popular and straightforward injection mold gate design. Source: Google. Investigate the mold for signs that a water leak is present, and correct the condition. Melt delivery management has a major impact on final part quality and the ultimate success of the production line. Possible Sources of Weld/Knit Lines: Temperature from the injection molding machine, runners, and/or the mold itself in order to ensure the plastic resin flows properly; Insufficient pressure resulting in the flow fronts not being merged back together with strong enough; Mold design, especially location of gates and runners impacting the resin flow Knit lines are formed in injection molding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). When modeling injection molding, the global approach is necessary to take into account the solid polymer transport, polymer melting and the polymer melt flow. This action could indicate that one portion of the plastic flow will move more quickly than the other. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. The barrel temperature is too high. With a minimum of 1. On that note, we. Flow lines – A common plastic injection molding defect in which discolored ring-shaped streaks, patterns, or wavy lines appear around gates or narrow sections of the finished product. As a result this weld line will be a weak point in this part. 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. 6 T) as thick as the nominal wall. Weld lines are formed during cavity filling whenever two separated melt flow font recombine. In addition to the recommended wall thickness, it’s important to consider a material’s flow characteristics. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. GPM: Measuring the amount of water passing through the mold every minute is a key measurable in determining turbulent flow. They are also known as flow lines. Effects of mold temperature and pressure on shrinkage 0. Description: Flow lines are streaks, patterns, or lines - commonly off-toned in color - that show up on the prototype part as a consequence of the physical path and cooling profile. If the mold temperature is too low, the material temperature will drop very fast, so the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction. The moldability validation and flow analysis tools enable you to iden-tify and correct potential injection molding problems as you design the product. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. To explore the formation of weld lines in injection molding, a flow model based on viscoelastic constitutive equation is presented in this article to predict and evaluate of weld lines location. Taking proper care to design the mold properly, avoiding bottlenecks, and changing the number or location of gates can improve performance. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Mold flow marks, also known as flow lines or weld lines, are often caused by variations in the flow of molten plastic material during the injection molding process. Sink marks are like little craters formed on an injection-molded product. This can be achieved by reducing the flow. Meld and knit lines are actually both different types of weld lines. low of a melting time will cause flow lines to occur. Thickness –Make the bottom of the boss 60% (0. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. Machine. Thin walls of greater length require greater pressure, which requires larger injection molding machines and. Flow Lines Or Flow Marks. The molding process is the leading cause of defects in injection molding parts. Listed below are some factors to consider when designing ribs for injection-molded parts. There are different levels of program available on the market. [1]#TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines : lines occur when two flow fronts meet and fuse together in the injection molding process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Learn about prevention methods and references. On the surface of a molded object, flow lines (also called flow marks) refer to the phenomenon that linear grooves or circular ripples showing the direction of material flow inside the mold cavity. Weld line. The part surface was observed in reflected light using optical microscope and the pictures of. Each material has a maximum flow length for a given injection pressure and wall thickness — this is known as the flow ratio. This can be caused by a poor mold design, or plastic temperatures, mold temperatures, injection speeds, or. If performing a resin change from one polymer to another a difference in viscosity can cause flow lines or gels. The 7/16-inch size is a happy medium between many small lines and the fact that larger lines don’t particularly increase the heat exchange benefit. One of the things that can compromise an injection molded part is a weld line, also called a knit line. These streaks or patterns can affect a part's aesthetics, value, and overall quality. While the sprue is milled through the entire thickness of the injection mold, passing from the outer surface to the parting line (where the two mold halves meet), the runners are comparatively shallow channels milled into the face of one or two mold halves. After discussing the injection molding learning process and the underpinning philosophy in the first two parts of this series, Bill Tobin gets to the heart of the matter—the mold—and shares a few tricks of the trade. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. A meld line occurs when two moving melt fronts converge and flow parallel to each other. It is crucial to manage flow lines to ensure. Optimize the runner system and locate gates near the thin walls. Although they typically do not impact the functionality of a product, flow lines—off-color streaks or patterns across a product’s surface—may look unprofessional when visible to consumers. to 0. Flow Lines. Mold makers should also pay attention to sprue, runner, or gate design. The injection speed should remain consistent as the plastic enters the mold, otherwise, it will slow down, cool, and cause flow marks. Incorrect gate positions cause multiple flow fronts that struggle to merge seamlessly. Injection molding (or injection moulding) is a widely used manufacturing process in many different industries such as aerospace, automotive, medical, and even. One way to reduce the changes in speed of the flow front is to tailor the injection-speed profile.