Plasti-Co. 3. 0 to 3. 040 and 0. As these flow fronts come together. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. . Part thickness control is through stacking of. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. We originally thought it may be caused by a silicone release agent that was occasionally sprayed on the injection mold, but testing with minimal agent and later built-in slip agent showed no change. directly! FCS Group, based in Taiwan, was established 40 years ago and is well known in Asia as a quality manufacturer of machinery, offering several lines of injection molding machines and other specialty and customized equipment. 160"). This can result in cosmetic defect or a weak point in the part. 45 in. The defect may be in part appearance or mechanical strength. gate position or runner. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares. Injection speed and pressure. Causes 1. A majority of these investigations concern amorphous polymers, whereas a number of engineering polymers are semicrystalline and also contain reinforcing fillers and fibers. 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. Use the fewest number of gates possible. Weld lines may appear like seams or scratches,. How to Eliminate Knit Lines In Injection Molding - Xcentric Mold Want to learn how to eliminate knit lines that form during the injection molding process? Then. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Injection molding machine and automation supplier Engel offers an array of technologies to simplify and speed up mold changes, including magnetic clamping platens and quick couplings; intelligent assistance systems; and the famox mold changing system for automating the set-up process. Dedicated to strict quality management and thoughtful client services, our experienced staff customers are generally available to discuss your demands and guarantee full client pleasure for Knit Line Injection Molding, Electronics. They have their origins in material choice, part design, tooling, and processingIn the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. I am working on a thin PC/ASA injection moulded component which has a tendency to snap at the Knit-point (the final point of travel at which the…Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Avoid adding threads directly on your injection molded part. 3 /sec. Inadequate injection pressure can result in knit lines and a surface that. Prevent Flow and Knit Lines Through Mold Adjustments. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Adding draft or slope to vertical walls of a part makes it easier to eject or remove the part from the mold. Knit lines, holes, part configuration and the talent of your molder all come into play. Injection molding is capable of producing a lot of aesthetic options as well. Viscosity represents a broad concept with huge effects on any efforts to standardize an injection molding process. Weld lines defects are formed when two flow fronts are merged together. 6 Monate. Extreme geometries can cause knit lines. Learn how HPP maximizes these specialized processes. 1 in. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). Resin cools as it is injected into a mold. But those key aspects can be viewed differently if you’re auditing mold changes before startup or troubleshooting changes for an already validated process. The point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. 250 in. Compression-molding is also suitable for ultra-large basic. Consider adding mold release (can be added to the polymer itself by the polymer vendor) or at machine side Knit lines Weak knit lines Increase melt temperature Mold temperature too cold Increase injection speed Avoid nucleated products if possible Inadequate ventingWhat you should know is that there are various failure modes for injection molding. Molding Troubleshooting Guide Bond Readout Bond readout is a surface distortion similar to a hump or sink that occurs over a bond line Probable Cause Material Process • Mismatch of compliance between outer panel, inner panel and adhesive • Incompatible thermal expansion coefficients between the SM and the adhesivePrevents flow and knit lines. ISSUE FACED BY THE INJECTION MOLDING COMPANIES: FLOWLINES (ALSO CALLED WELD LINES OR KNIT LINES) Definition: Streaks, orange skin, or visible flow lines in parts. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. Melt temperature does. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 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. As a result, molten plastic takes the injection mold core and cavity shape. In the world of injection molding, each weld. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. "As soon as that part or material is stressed, it can be real conducive to fracture, cracking, breaking, just because of the way it's been molded. Vibration assisted injection molding. The data suggest that either local sample thickening or grooves form at knit line when complete section solidification takes place first at weld zone or. Compression molded parts can vary in thickness without issue. Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties. These flaws can affect the aesthetic and structural integrity of the. Still, it can be seen to varied degrees depending on mold. Formation of knit line. ISO 9001:2015. The resin is transferred to the mold cavity under positive. The part is cooled in the mold until it is solid enough to be ejected. In this case it would be 0. The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. Operating Conditions • Fast injection fill speeds, supporting short injection times (1–3 s), especially in thin sectionsGate and Knit Line Locations<br />. Fewer knit lines. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all, be a cosmetic issue, or cause a potentially serious structural problem. Having uniform wall thicknesses between 0. Multiple gates will always cause knit lines. menu. The injection molding process uses a granular plastic that is grav ity fed from a hopper. ) or as thick as 25 mm (1 in. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. Poor venting. In the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. In an injection mold, the plastic fluid flows and spreads out in many directions as it comes out of the gate. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. 0 and Table 1). And the lead-time for overmolded and insert molded parts is 15 days, a bit longer than FacFox’ single-part injection molding turnaround, but still much faster than traditional insert molding. Gating. TWIM=Thin Wall Injection Molding; TSL=Toys,Sports and Leisure; NB=No Break; IMR=Inter Material Replacement. The total amount of scrap being produced during injection molding will usually be higher than that for compression molding because of the additional material loss created. สาเหตุรองของการเกิดปัญหาการเกิดประกายเงิน (silver streak) 1. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Bubbles are caused by melting flow fronts colliding in a mold cavity. They again get resolidified along the surface. Injection molding is typically a better choice for more complex parts, while compression molding is a great option for relatively simple designs, including ultra-large basic shapes that cannot be produced using extrusion techniques. Learn Injection Molding Glossary with Plastopia: Using actual molded parts (with images) as examples. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Increase the melt and/or mold temperature. Plastics Technology article (June 2018): Technical capabilities of the process have advanced to erase former cycle-time penalties, and ongoing developments address creation of a materials database and new applications in hybrid composite injection overmolding. 6. 3. Knit Lines 11: 1. Fig. Basic rule of thumb air in mold must be able to escape at theAbstract The knit line structure of injection moulded PA 6 samples was analysed by SEM and optical microscopy. More about the knit lines in injection molding. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. Also known as “stitch lines” or “weld lines”. A. The most common and one of the most challenging injection molding defects are weld and knit lines. Get a free des. Steven. Mold venting should be checked and increased if necessary, injection molding service. txt) or read online for free. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose. There are times when porous materials like Porcerax are used in problem or backfill areas. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Venting. doc), PDF File (. These knit-lines are well-known as particularly critical regions in the mold when mechanically. But applying excessive force can create problems with the part, mold, and machine. Injection moulding (U. A knit line may only be a cosmetic issue and affect. This range can be affected by the part wall thickness, melt temperature, mold temperature, injection speed, hold/pack pressures and also the time between molding and measuring. Parts as thin as 0. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. As these flow fronts come together. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares. 2. Let’s. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Knit lines. Weld Lines-3 Ways for Detecting and Correcting. Potential Knit Lines • Knit lines occur where to or mold melt fronts come together-Holes, slots, multiple gates, variations in wall thickness-Meld lines occur when the melt comes together in the same flow direction • Knit lines are areas of weakness and should be located in low stress areas . These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. (knit) line and produces a latent, or obvious, part defect. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. Polypropylene can be processed over a wide range of mold temperatures; however, for applications where aesthetics are critical, a surface temperature of 80-95˚CSink Marks Cause 1: Incorrect Melt Temperature. , Rallis, A. When setting a heat profile, consider residence time, screw RPM and filler type. When the plastic cools, it takes the shape of the mold and is then ejected and finished. Knit Lines. For. Reduce the injection pressure. Example of a gate location in a finished part Example of knit lines in a finished part. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. They appear around holes or seams of an injection molded part, and some may call them. Injection mold design and custom tooling can make or break the success of a finished plastic product. Steven. Knit line / Meld. Three terms that often get mixed up are weld, meld, and knit lines. ” What else can you do about weld lines—besides redesigning the mold or gate locations or adjust the flow pattern during fill with sequential valve gating? I recently saw a report on this issue from Moldex3D, the maker of molding simulation software. What Are Knit Lines? When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. The thickness of the part plays an important role in how difficult the molding process will be. Overall, injection molding is a relatively simple process. This course outlines mold filling and how different gating configurations affect filling. Possible causes: Low temperature or fluidity of melted plastic during injection molding. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Remedies: Raise the temperature of the mold or molten plastic. Draft. 3 /sec. Transfer molding also utilizes a press and a mold. Exploring common molding defects and. Proper venting is essential to molding a defect-free part. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them. 12515 NE 95th Street , Vancouver, WA 98682 CALL TODAY: 1-800-839-0366Contact FU CHUN SHIN MACHINERY MANUFACTURE CO. Troubleshooting Injection Molding Weld Lines. ABS is a good choice for handheld consumer devices. Disadvantage: May rust if improperly stored. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. Prototype Tool In Soon As 10 Days; Production Tool In As Soon As 3 Weeks; 10k Parts in 10 Days; GET YOUR 30 SECOND QUOTE NOW. The second mold portion being movable relative to the first mold portion and together defining a mold cavity. A knit line, also. Mold analysis tools are used by designers of molded plastic parts and by designers of the mold tools used to manufacture those parts. incomplete filling or shorts. Misra A. 030- to 0. 120 in. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. The mold’s sprue bushing and locating ring is placed in a way that makes sure the injection molding nozzle is properly aligned and centered. For this study, a special injection mold was designed and constructed (Figure 1) in order to produce specimens of various thicknesses (from 2 to 6 mm). Dabei erreichen unsere Produkte zufriedene Käufer in mehr als 30 Ländern. They occur when two resin flows meet at different. 040-in. While this process is largely efficient and reliable, it's not without its potential issues. Injection molding is the cost-effective method and is best suited for higher volumes. An edge gate introduces material at the main parting line, or where the two halves of the mold meet, with a perpendicular runner. Solution: People. As the polymer resin is injected into a mold, it flows through all parts of the mold. The mold maker may think that some minor defects are acceptable, or they may not even consider them from the beginning, which can cause problems when the injection mold is being manufactured. In the 1947 book, Injection Molding of Plastics, the late Dr. Meld and knit lines are actually both different types of weld lines. In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. land and using maximum vent depths to get them as close to self-cleaning as possible. No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. Tooling & Injection Molding Services High-Quality Complex Parts. Injection molding is a manufacturing process that involves injecting molten plastic into a mold cavity to create a specific shape or design. An injection molding machine having a first mold portion and a second mold portion. Circular Loom Machine; Raschel Knitting Machines; Shade Net Making Machinery; Inspection Machines; Non Woven Production Line. The adverse effect of knit-line formation on the mechanical properties of thermoplastics has been well documented for unfilled materials. Plastic injection force exceeding the clamping pressure of the press. Take these steps to avoid knit lines. Conclusion The formation of weld lines in micro injection molding of plastic depends on different parameters such as melt temperature, injection pressure and fabrication method. Considerations: ABS-molded parts will show. Knit / Weld Lines. Increase injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. The plastics used by injection molding processes are relatively cheap and can be used to achieve a wide variety of properties, so injection molding is popular for creating many packaging and consumer products, like LEGO!. Weld / Knit Lines; Assumptions. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. It is one of the most common blemishes for manufactured products. Alter the mold design. Excessive mould lubricant. It is also one of the most important weapons in a molder’s arsenal for making process changes. Meld and knit lines are actually both different types of weld lines. Trapped air in a mold manifests itself as a burn mark on the molded part. If you have any issues getting. A poor knit line can cause cosmetic blemishes and significantly weaken the structural integrity of a part. , LTD. Weld Lines. mold venting is at the last filling point, whereby, while the melt flows forward in the cavity, air and gases flow out through the valve. Notched weld lines then remain near the cavity walls. Different plastics make different types of knit lines. The knit line is generally opposite the gate in thermoset. Weld lines are a common imperfection to be found in most plastic injected molded parts. Als einer der führenden deutschen Hersteller von Masterbatches stellen wir nicht nur Farb-Masterbatches, Additiv-Masterbatches und funktionelle Kunststoff-Compounds her – wir leben deutsches Forscher-, Ingenieurs- und Unternehmertum. Change gate locations. Mold vents should function the same way. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rates, and holding pressures. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose. 030- to 0. 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. ”. (See Fig. Plastic injection molding is the process of injecting molten plastic material into a metal tool which then cools and ejects a plastic part from the machine. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. The last cause of flow lines may be the operators running your daily operations. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Contact Universal Plastic Mold Today. S. A structural finite element analysis (FEA) can be used to compare locations of high stress to the locations of the knit lines identified in the mold-filling analysis. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. To ensure quality parts, injection molding companies must have the best injection mold design first + (91) 96067-70777 +(971) 547-161-112. Weld lines, also known as knit lines or meld lines, occur when two or more flow fronts meet during the plastic injection molding process. As a result, prototyping is strongly recommended for parts with. A knit line is the spot where two flows meet. Mold cores can be machined from a solid piece of copper alloy or inserted to aid in construction or allow for easier replacement if a com-ponent would ever be damaged in molding. Runner Type. Introduction to Knit Line Injection Molding Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection molding. Figure 1: Knit lines in a cover plate. Weld / Knit Lines; Assumptions. 453. Too cool a melt. Injection molded plastic begins its journey through the part tooling via an entry section called a. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. Learn more about the process by reading this white paper. The comment on melt flow index is correct, but, I've molded straight PC down to . spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. 2. It is. Still, they are almost always a cosmetic issue. Thanks to the low air counter pressure in the cavity, this allows injection parameters to be managed in a larger window, optimizing injection time, part aesthetics and mechanical characteristics. Cold weld line or known as “knit line” isWeld Line defect analysis, the most common of many defects in injection moulding products. Cold Slug: First material to enter mold during injection. Weld lines, which are also referred to as knit lines, are among the most common of these blemishes. Injection Molding Guide. Thin wall injection molding can produce walls as thin as 0. Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection. Reduce the screw speed and adjust the backpressure. 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. Strength at the knit line can be as little as 20% of the. A weld line can also be called a knit line. Knit Lines. Knit lines are most common downstream of holes created in parts where the molten material meets. Multiple parts with complex design and differing wall. S. Why. In a weld line, reinforcement fibers. Draft. With plastic finishing plugs and end caps, we can help you keep your products protected while giving. 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. Figures courtesy of CAE Services. 0 mm) is crucial to ensuring that parts cool evenly. The point I am making here is, if you’re injecting 14. ) 2. Table 2. The quality of the answer is directly proportional to the information we have to work with. Insufficient injection pressure and speed can cause weld lines. Islyn Thomas summarized the need for a particular vent location better than I ever could. 2. (1. A knit line is any line, visible or not, where two resin flows meet. 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. 08 KICKOFF PRODUCTION. Meld and knit lines are actually both different types of weld lines. Brent Strong, p. 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 of the molten plastic as it flows into the injection mold tooling cavity. Parting lines lie along the edge of the molded part, between the core and the cavity. Snake Lines: The appearance of snake lines is caused by the formation of a jet effect as the melt travels through a gate and into the mold cavity. Vacuum. Once the plastic cools, the mold is removed, and the object is ready for post-processing. Injection molding is the process of injecting liquid thermoplastic into a mold. 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). POROUS MOLD MATERIALS. Weld lines are formed during cavity. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. 015in wall. Solid or semi‐solid, this material leaves a distinct border with adjacent melted material similar in. Knit lines. 17. Undercuts add cost to tooling and molding, which is recommended to eliminate or minimize as possible. Let the molten material cool to a solid-state. Meld and knit lines are actually both different types of weld lines. Know about the cause of these lines and the potential solutions. mixed manually and quickly stored in the tank of an injection machine just before the start of the resin injection or (2) the components are stored separately in different tanks of an injection machine with a mixing head. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Learn more. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Contact us. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. 156 x 1. thick. Some of the most popular brands include:Harshal Bhogesra has 9+ years of combined experience in molding, customer & supplier development. This slight depression is a tiny line called the weld line or knit line. In working with the injection molding service offered by Protolabs, you provide. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Because injection mold design and processing is complex, a set solution will not solve all problems. Solution: People. Let’s stop for a second and think about the logos we usually see on a daily basis. Learn how to troubleshoot weld lines in injection molding. In fact, knit line strength will decrease with higher filler content and with longer fibers. These materials have beenDetermining the correct tonnage provides a foundation that must be rock-solid to avoid flash and damage. Increase size of the vents. Specifically, by having a single, continuous injection outlet 140, molding material can be uniformly distributed throughout mold cavity 120 quickly and consistently without resulting in undesirable molding seams or knit lines. 1. Also discussed are weld and meld lines and their effects on molded. Learn how small changes to an injection molding design like wall thickness or material choice can lead to more durable parts that are free of cosmetic defects. It can also aid in material selection since fill. This makes the casting process as efficient and accurate as possible. Such molds or tools aren’t intended to be used in operation. Participating in DFM and mold flow analysis is a good way to achieve this. Some. Highly ScalableVacuum on the mold does help and many times eliminates the weld line. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Without adequate support pillars, flashing of the parting line faces and bowing of the plates may occur. The plastics used by injection molding processes are relatively cheap and can be used to achieve a wide variety of properties, so injection molding is popular for creating many packaging and consumer products, like LEGO! A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. That’s the air escaping out of the nozzle until the hose is full of pressurized water. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. 2. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. The fluid path has an inlet and a radially continuous, downwardly. Common causes of flashing in injection moulding. The Problems & Solutions course is designed to train injection molding personnel in. Recent developments were oriented on the analysis of the mechanical performance at local (knit lines and welds) and bulk (molded part) areas, with the influence of molding and welding conditions. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all,. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toInjection molding, producing, and forming resin materials are three core components of injection molding technology. ) are easy to produce. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. ITAR. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature. Getting started; Knit Line Injection Molding; Knit Line Injection Molding - China Manufacturers, Factory, Suppliers. This will help the mold designer to determine the allowable gate and knit line locations. Three terms that often get mixed up are weld, meld, and knit lines. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. They again get resolidified along the surface.