Tài liệu Mold Selection P2 docx

125 Self-degating is somewhat more expensive than edge gating and while an insert for the gate area adds some cost, it may be necessary for long running molds to avoid expensive repairs later. The somewhat higher mold cost may be well worth it in the long run. There is a number of different designs for tunnel gates [5]. The size of the tunnel gate is determined the same way as the size for any other gate; however, there is a limit to the size that the can be sheared off cleanly. If the gate is too large, hard and brittle plastics may shear poorly (very rough and uneven) and can easily damage the cavity wall where the gate is located. Submarine gating is another method of self-degating, if the piece is shallow and no gate vestige is permitted either on top or on the sides [5]. Submarine gates are more difficult to build than tunnel gates and are therefore more expensive to manufacture; the mold will also cycle slower. The vestige is similar to that of a 3-plate mold gate, but is on the underside of the product. Note: Dirt in the plastic can easily plug any cold runner gate, but such dirt is molded into the plastic as it freezes; it will be ejected with the runner so that the following cycle will see again a clean, open gate. As the runner is ground up before reusing the plastic, hopefully, the dirt will also be ground up; otherwise, sometimes later it could again plug a gate. 4.1.4.3 Inherently Self-Degating Molds 3-plate and hot runner molds are inherently self-degating, i.e., the gates break off as soon as the cavities and cores (with the products held on them) separate, provided the product stays reliably on the core as the mold opens. Gates for 3-plate molds are usually very small (“pin point gates”) and often take advantage of the fact that the plastic will heat up due to shear as it passes through the gate. Gates as small as 0.5 mm in diameter are quite common for small bottle caps, among others. There is an upper limit to the size. If too large, the vestige can become very rough and unsightly or too hot, causing stringing. The gate may even break the top of a thin product as the mold opens. 4.1.5 Hot Runner Molds Today, hot runners are a fully accepted technology and the preferred method of gating; they are replacing more and more of the older runner methods, especially the 3-plate systems. In fact, older, existing 3-plate molds can often be quite easily rebuilt into hot runner molds. In the earlier years of the hot runner technology – occasionally even today – mold makers design their own hot runner systems, either based on their own ideas or by copying other systems that gradually came on the market (see Fig. 4.30). Today, there are many well established companies specializing in hot runners, who sell either the basic hardware (manifolds, nozzles, heaters, etc.) or assembled hot runner systems, complete with all associated plates and other 4.1 Selection of an Appropriate Mold Figure 4.30 Typical hot runner, section view (Courtesy: Husky) 1281han04.pmd 28.11.2005, 11:14125 Previous Page 126 4 Mold Selection hardware ready to be joined to an otherwise complete mold built by the mold maker. All that is required for a purchase order is to specify the important interface dimensions and any production data such as plastic to be used and the mass of the product. These hot runner suppliers mass produce the hardware items and use specialized methods and equipment to produce better quality system parts at lower costs. Such hot runners are then guaran- teed to work in the new mold and eliminate the need for the mold maker to experiment and waste time and money trying to get a “home made” system to work. There are still some molds for which the advantages cannot be justified economically, especially for low production items. In these cases, the older systems, especially cold runner 2-plate molds, are still much in use. 4.1.5.1 Degradation of the Plastic in Hot Runner Systems Another important consideration is the amount (mass) of plastic injected into each cavity. Each plastic has a limited total time that it can remain exposed to high temperatures before it will start to degrade and lose at least some of its properties. A “temperature and time” graph can be obtained from the materials suppliers. Some plastics, and most of the commodity plastics, such as PS, PE, and PP, have a high tolerance for heat and can stand long exposure to high temperatures, much longer than many other so-called “heat sensitive” plastics. But even the commodity plastics will sooner or later degrade. If they are exposed too long to high temperatures, they too must be purged from the injection unit before good quality products can be produced again. When injecting into single cavity molds, there is usually no problem, because the runner system (the sprue) is relatively small compared to the mass of the product; the plastic inside the hot machine nozzle and inside the sprue are replaced at every shot. With multi-cavity molds, where we need a heated distribution system – the hot runner manifold and the drops through which the plastic must flow to the cavities – the plastic can reside there for a considerable time. If the products are large and the channels in the manifold are relatively small, there is little concern, because the plastic residing in the manifold (the “inventory”) is replaced quickly (after one or a few shots), especially if the molding cycles are short. But small channels in the manifold cause a large pressure drop, especially if high pressure is required to fill the cavities. A compromise – an optimal condition – must be reached. The hot runner manufacturer uses computer calculations, based on the information on the product, its mass and shape (wall thickness), the cavity spacing, the plastic, etc., so that a manifold with the optimal channel sizes can be supplied. Manifolds for multi-cavity molds for very small products with little mass are more difficult, because the amount of plastic required at every shot is small and the plastic advances only slowly through the manifold channels, especially with longer molding cycles. This means that the plastic is exposed to the melt temperature within the hot runner system for a long time. Any, even minor, stoppage can bring the plastic close to or over the permissible Make sure to use clean plastic when operating hot runner systems with open gates A good rule of thumb is to have less than three shots of plastic in a hot runner 1281han04.pmd 28.11.2005, 11:14126 127 temperature/time limit at which it starts to degrade. If this is expected to be a problem, a cold runner mold should be selected or a better hot runner system may have to be developed, maybe together with a specialist in this field. 4.1.5.2 Open Hot Runner Gates There are two distinct styles of open gates, the circular pinpoint and the annular ring gate. Both gates function on the same principle, by (1) freezing off at the end of the injection cycle to avoid drooling while the mold is open for ejection, and by (2) opening up again, triggered by the pressure of the hot plastic as it is injected during the next cycle. The functioning of both styles of these gates depends entirely on (a) the operating conditions such as melt temperature, the nozzle temperature, the injection pressure, the timing, and (b) on the design characteristics of the system used, such as the cavity cooling, the size and shape of the gate, and the design of the hot runner system. Because of the small size of the gate (a small round hole in one case, a very narrow, circular gap in the other case), a serious drawback of the open gate hot runner system is its sensitivity to “dirt” (paper, wood, tobacco, metal chips, etc.) in the plastic. Unlike with the cold runner gates, any dirt fully or even only partially blocking the small passages will cause the cavity not to be fully filled. Additional dirt will remain there until the mold is stopped and the dirt removed by opening and cleaning the hot runner system. With good mold designs, the cavity plate can be pulled while the mold is in the machine and the obstructed gate or gates can be cleaned. There will be an interruption in molding, resulting in lost production of maybe one hour or maybe a whole day, which may nullify any savings from utilizing the system. Note that molding at lower melt temperatures is of greater advantage even though it requires higher injection pressures to fill the cavities Figure 4.32 Schematic of circular open gate. The gate opens and closes based on thermal cycling and control of temperature and pressure in the gate (Courtesy: Husky) Figure 4.33 Schematic of annular open gate. This gate also opens and closes based on thermal cycling and control of temperature and pressure in the gate (Courtesy: Husky) 4.1 Selection of an Appropriate Mold A hot runner should not consume any more than 25% of the available injection pressure Figure 4.31 Hot runner maintenance while in the press is an important feature 1281han04.pmd 28.11.2005, 11:14127 128 4 Mold Selection 4.1.5.3 Valve Gates (Hot Runner) This is a method to control the opening and closing of the gate, either by mechanical means or by electrically controlling the melt temperature within the gate. The mechanically controlled gates use a moving pin that opens and closes the gate as required during the cycle. Today, the valve pin is actuated mostly by compressed air and occasionally by hydraulic pistons. Figure 4.34 shows a schematic of one of several typical valve gates. The principle here is that a pin (A) is mechanical moved into and out of the gate (B) on every cycle to open and close the gate. It allows for faster cycles and higher quality gates. The main advantages of valve gates are:  The gate can be much larger than the openings possible with pinpoint or circular open gates. Gate diameters of 4 mm (5/32 in.) or even larger are quite common.  The vestige of the gate is a circular mark, similar to that of an ejector pin mark. Because of the large opening, dirt is much less of a problem. Most dirt will pass through the gate when open and end up being encapsulated into the product, which may – or may not – be accept- able.  The cycle time is shorter than with a comparable open gate diameter because of the longer time required to freeze a larger open gate There is an exception to this: with very thin-walled products, the gate area with a valve gate would be hotter than with an open gate and could slow down the otherwise faster molding cycle possible, because the thin walls cool faster. The main disadvantages of this system are  Larger costs compared to open hot runner gates (approx. 40%)  Possibly added space requirement for the valve actuating mechanisms. This can affect the spacing between the stacks, especially if the products are small. 4.1.5.4 Combination of Hot and Cold Runners In some molds, often for smaller products and with a large number of cavities, but also with larger ones, as the example shown in Section 4.1.3.2, a combination of cold and hot runner systems can be of great advantage:  It can eliminate a large portion of the cold runner and thereby significantly reduce the mass of plastic to be reground or lost. Figure 4.34 Typical valve gate (Courtesy: Husky) A B 1281han04.pmd 28.11.2005, 11:14128 129  There is much less pressure drop between the machine nozzle and the gates, because the pressure drops in the hot runner manifold is smaller than in a (long) cold runner.  It can be used when very small cavities cannot be located very close to each other, at a “pitch” (distance) for which there are no standard-spaced hot runner nozzles available, or where it is not possible or practical to use hot runner gates. Typically, a cluster of 2–6 (or even more) very small products can be gated from a small runner or a disk, which is fed from a hot runner drop.  It will shorten the cycle time. Large distributing (cold) runners take much longer to cool than the final runners feeding the cavities. Especially if the products cool rapidly, such heavy runners significantly slow down the molding cycle. The cold runner portion in such cases can be treated as any cold runner mold; it could be a 3-plate arrangement (rarely used) or a 2-plate system with edge or tunnel gates into the product. 4.1.6 Single Cavity Molds Many products, and in particular large products, are molded in single cavity molds. This allows the use of the simplest mold construction, with simple injection methods as well as simple methods of ejection. In addition, large products are often not required in very large quantities, and if they are, it is usually more economical to use two or more machines, each with single cavity molds. Such machines can later be used for other molds and give the plant more flexibility. Very large machines (15,000 kN or 1,500 tons and over) are usually dedicated for molds for specific, very large products, which cannot be fitted into a smaller machine because of the physical size of the mold, the clamping force required, and the required large shot capacity and plasticizing capacity of the injection system. 4.1.6.1 Single Cavity Cold Runner Molds Single cavity molds have been used since the beginning of the injection- molding era, for any product size from small containers to large pails. As explained in Section 4.1.3.2, edge gating a single cavity is often not practical or even possible; therefore, we will consider only gating into the outside on top of the flat or deep product. In a typical mold, the plastic enters the cavity space from a sprue inside a sprue bushing. The machine nozzle presses against the sprue-bushing seat while injecting; the sprue bushing must be well cooled to ensure that the plastic within is stiff enough for ejection before the mold opens up. Unfortunately, this cooling time for the sprue is often longer than the cooling time required for the product and will unnecessarily increase the cycle time. The mass of the tapered sprue increases as the length of the sprue increases (Fig. 4.36). This can be easily improved by using a shortened sprue bushing, Figure 4.35 Typical 16-cavity mold; top: cold runner layout; bottom: hot runner supplying 4 drops to small cold runner, each supplying 4 cavities Figure 4.36 Cold sprues (top) standard length, (bottom) shortened, into top of product or into runner system 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14129 130 4 Mold Selection and providing the molding machine with a longer nozzle. Shortening the nozzle length by half will reduce the mass by almost 75% of the mass of the longer one, resulting in shorter cycles. In addition, the gate vestige, after the sprue is trimmed from the product, is much smaller. This type of gate is used frequently for large parts, as long as they can be filled from only one gate. 4.1.6.2 “Through Shooting” System Through shooting is a better method than using a sprue; however, this method is only applicable, if (a) the cycle time is short and (b) if the L/t ratio is such that one nozzle alone will be sufficient to fill the cavity space (this method is really a hot runner, in its simplest form). It can also be called “single cavity insulated runner method” (see Fig. 4.37). The principle of this sprue is simple and the method can be used for most types of plastic. It is particularly suitable for plastics, such as PE and PP, but also for PS and other plastics if the cycle is short enough. The machine nozzle seats on a sprue bushing, with a large (approx. 15 to 18 mm diameter) “well”, deep enough to reach the short, open, circular, tapered gate, which leads to the cavity or to a runner system. The well can be as deep as 75 mm (3 in.), and even longer wells have been used successfully, but it is recommended to keep it shorter (approx. 25–30 mm). As the plastic is injected the first time, the well is filled with a plastic “slug”. The outer surface of the slug, in contact with the cooled cavity steel, will freeze, but because of the good heat insulating properties of most plastics, the melt around the axis of the slug stays hot long enough that even though the gate will freeze, the plastic injected during the next shot will easily traverse the still hot center of the slug. The heat of the incoming plastic will then melt the frozen gate and the cavity will be filled for the next shot. This system of runner and gate works very well for most plastics if the cycle time is 15 seconds or less; with PE and PP, molds with cycles as long as 30 s are running successfully. If a stoppage is long enough to completely freeze the plastic slug, it can be easily removed by pulling back the injection unit about 15–20 cm (6–8 in.) and then pulling the slug out with a heated, hooked wire or some special tool made for the purpose. It is suggested to provide the walls of the well with a draft of at least 3° per side and good polish so that the slug slides out easily. As soon as the slug is removed, the injection unit can move forward into the molding position, and production can resume. This through shooting system is very inexpensive to make and very reliable. Color changes are easy: As soon as one color is finished, the slug can be removed and the new color finds a clean mold. The gate vestige is small; it looks similar to the vestige of a 3-plate gate or an open hot runner gate. Other benefits of this method are: (a) there is no sprue to regrind or to scrap and (b) even though any dirt in the plastic larger than the diameter of the gate will plug the gate, this dirt is easily removed together with the slug and the interruption in production is just the short time required to remove the slug with the molded-in dirt and to restart. Figure 4.37 Two examples of the through shooting system; long slug (top) and short slug (bottom) 1281han04.pmd 28.11.2005, 11:14130 131 The size of the gate is determined like any other gate, from a small pin point gate up to a gate of about 6 mm diameter. Larger gates are possible but they might not freeze and the gate could drool while the mold is open for ejection of the product. If a larger gate is required because of the large amount of plastic that must enter the cavity space, either a cold sprue as described in Section 4.1.6.1 or a single cavity, hot runner system (“hot sprue”) will be required, as described in the following. 4.1.6.3 Single Cavity Molds for Large Products There are several options for single gating a large product, apart from the old fashioned standard cold sprue (for multiple gates into one product see Section “Single cavity mold, multiple gated” see p. 134). (1) Use a short, cold sprue, as described in Section 4.1.6.1 Since the product is large, the cycle will probably be long (25 s or more). The sprue can be cut after molding, e.g., during stacking, assembling or packing, without adding labor cost. If the gate is large and needs to have a good appearance, it may have to be milled in a fixture. Many molders and designers overlook this simple, and inexpensive solution when making such large products and select more expensive methods. (2) Use the through shooting method, as described in Section 4.1.6.2 This method will work well, as long as – The cycle time fits in the time frame described in Section 4.1.6.2, – The gate size is smaller than approx. 2 mm diameter. This method could yield an even lower product cost because (a) there is no gate cutting required, and (b) the cycle time can be less than with a cold sprue, because the sprue may take longer to freeze than the product thus controlling the cycle time. (3) Use a hot sprue, as described in Section 4.1.6.4 below A “hot sprue” is essentially a heated cold runner sprue. The melt within the sprue is kept hot with electric heaters. The gate could be an open gate as shown in Figs. 4.33 and 4.34 or a valve gate as shown in Figs. 4.41 and 4.42. Hot Sprue, Center Gating The hot runner suppliers sell hot sprues of various designs as standard hardware, ready for incorporating into the mold. It is important to follow exactly the interface dimensions and tolerances specified by the manufac- turers, as well as the operating instructions to ensure trouble-free operation. Hot sprues with an open gate (annular or circular) or with a valve gate can be purchased in a variety of lengths. They are most suitable when it is necessary to gate inside a deep product, too deep to reach for even a very long extended machine nozzle. 4.1 Selection of an Appropriate Mold Figure 4.38 Single-cavity pail mold with a hot sprue (Courtesy: Topgrade molds) 1281han04.pmd 28.11.2005, 11:14131 132 4 Mold Selection Figure 4.39 shows a hot sprue with heater bands for the top (A) and for the nozzle extension (B). The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume. Figure 4.40 depicts the schematic of the open hot sprue. There are heater bands for the top (A) and for the nozzle extension (B). The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume. A great advantage of a valve gated hot sprue is that the gate can be of any reasonable size, 4 mm diameter and even larger; the gate vestige is circular. Hot sprues are quite expensive (in the order of $3,000.00) and need heat controls, wiring, and air pressure lines and controls for the actuating mechanism. However, in some molds, these expenses are warranted. Hot Runner Manifold with Offset Gate(s) In some single cavity molds it may not be acceptable, or even possible, to use a center gate into the product; often, for appearance reasons. Two typical examples:  The cavity must be edge-gated, as shown in Section 4.1.3.2, where several drops from a hot runner manifold are located to feed one or more cold runner systems outside the circumference of the product.  The gate could be positioned away from the center, but still within the outline of the parting line (or the circumference) of the product (see examples in Fig. 4.45). Figure 4.39 Hot sprue with heater bands A B C Figure 4.40 Schematic of the open hot sprue Figure 4.41 Valve gated hot sprue Figure 4.42 Schematic of the valve gate hot sprue (all: Courtesy: Husky) 1281han04.pmd 28.11.2005, 11:14132 133 Molds for Very Large Products and Limited Production Single-cavity molds, such as for large automotive products, can weigh many tons, and because the products change almost yearly with the model changes, it is important that all possible shortcuts be taken to keep the mold cost low, while still guaranteeing the best quality of the product. Because of the large mass of plastic entering the mold, today, all such molds use several valve gates, located usually where suggested by flow analysis. This may appear to be expensive but it is necessary to ensure the quality of the product and for achieving a reasonable molding cycle. On the other hand, great savings can be achieved by proper selection of the mold steels used. The cavities and cores are often cut right out of steel blocks, each of which could weigh several tons. “Conventional” mold plates are rarely used. There is no need for expensive mold steels or pre-hardened steels, especially if the specifications of the product do not require high gloss finish. Mild steels are often acceptable, but there could be inserts required for places where wear is expected. However, expensive beryllium-copper inserts are used frequently in locations where it is important to provide better cooling to reduce cycle time. Other good mold making practices, such as cross drilling for cooling channels, are replaced by the use of flexible hose connections from channel to channel. Simple horn pins, wedges, or hydraulic actuators can move side cores. Alignment is provided with leader pins and bushings. Even the parting line match of the usually complicated shape of the products need not be perfect. Excessive but reasonable gaps can provide good venting for the large flow of plastic entering the mold, and ensure proper filling. Any unwanted flash occurring could be scraped off by hand. The large, sometimes unwieldy products are often removed from the molding area by robots, but additional handling is usually done by hand and, if necessary, any excessive or unsightly flash can be removed at that time. Figure 4.44 shows a mold for a rear door of a car. The mold weighs more than 17,000 kg and runs on a 2,000 ton machine. The wall thickness of the panel (in TPO) is approx. 3 mm and the molding cycle is approx. 40 s for a productivity of 700,000 pieces per year. Injection is with two Synventive sequential valve gates. The parting line matches perfectly, therefore no scraping is required. Finish is SPI #4 on the inside, and SPI #2 on the visible outside. Ejection is by ejector pins, with hydraulic actuators inside the mold. The panels are removed by robot, directly to a conveyor. In these cases, either a standard size (listed in a catalogue) hot runner manifold can be selected or a specially designed manifold will be required. The drops (nozzles) to the product or the cold runners could be open or valve gates. Note that today, many hot runner manufacturers offer standard sizes of many manifolds and all other hot runner hardware, from stock, at lower cost and faster deliveries, than special sizes. Figure 4.44 Mold for a rear door of a car (Courtesy: Accurate Molds) 4.1 Selection of an Appropriate Mold Figure 4.43 Large automotive mold for bumper fascia (Courtesy: Accurate Molds) 1281han04.pmd 28.11.2005, 11:14133 134 4 Mold Selection There is an often overlooked and much less expensive solution for products such as the one shown in Fig. 4.45 (top). This solution is depicted schematically in Fig 4.46: Instead of using an expensive offset hot runner, this large product with an opening in the center can be easily filled from a cold sprue as shown in Fig. 4.36 (or a through- shooting sprue as in Fig. 4.37) that both feed into a cross-shaped or multi- spoke runner or even into a shallow disk in order to edge-gate or to feed a continuous gate all around the inside of the opening. Now we will turn our attention to the (small) cold runner and the problems possibly associated with it: (a) To make sure that the runner stays with the product from where it will be broken off or cut by an operator, (b) We will have some scrap, which may represent a very small percentage of the product weight, depending on the size of the opening. Economically, both the cost of the runner and the labor of removing it, and regrinding the scrap, may still be less than the cost of the otherwise necessary hot runner system. This can be easily calculated. If the total number of products from the mold is small, the cold runner system is preferable. If the number is very large, the added cost of a hot runner can be easily justified Single Cavity Mold, Multiple Gated (Hot Runner or 3-Plate Mold) The following applies mainly to hot runner molds, but also to 3-plate molds. In a large single-cavity mold, several gates are often used to provide better plastic flow into the cavity. This approach must be selected if the L/t ratio for a single center gate would be too high or if one gate would not allow enough plastic to flow into the cavity. By choosing suitable locations for two or more gates, far enough spaced from each other, the L/t ratio per gate can be much reduced. A product that otherwise could not be filled at all, or only with very low viscosity (very hot) plastic and with very high pressures, can be filled much easier from several gates. Special care must be taken to ensure that the cavity space is well vented where the streams from the various gates are expected to meet, e.g., by placing ejector pins or vent pins there. One problem with multiple gating with open hot runner gates is that if one of them freezes more solidly than the other(s), the incoming stream at the next cycle will not be able to dislodge the frozen slug in that gate. The plastic will then not use all gates as intended; this can result in unfilled products. The most effective solution in such cases is to use valve gates. The advantages of valve gating have been described earlier. The disadvantage is the added cost of the valve gate systems and more controls in the machine. When using multiple gating for 3-plate molds, typically when very small (pin point gates) are desirable for appearance, it is important to use very clean plastic; if one gate gets plugged, the other gate(s) will not be large enough to fill the cavity space. There is also a method of sequentially programming Figure 4.45 Schematic illustrations of two large, single-cavity molds with offset hot runner gates within the outline of the products Figure 4.46 Large product with center opening, with cold runner gating 1281han04.pmd 28.11.2005, 11:14134 [...]... press fit The mold is usually less expensive, because cavities, cores, and plates are smaller than corresponding size modular molds Figure 4.47 Three typical cross sections of conventional multi-cavity molds (A) 2-plate mold; (B) 3-plate mold; (C) hot runner mold Next Page 136 4 Mold Selection There are several disadvantages with retainer plate design, even when precision (“jig-”) boring the locations Accuracy... expansion differentials (see Section 2.5) Molds with retainer plates are usually only recommended if they are small and the productivity is less important than the mold cost All cold runner 2-plate molds, many 3-plate molds, and some hot runner molds have been, and are being, built with the retainer plate design and are still very common Modular Molds Figure 4.48 Modular mold construction for a thin-walled... the mold and the cavity on the in moving half of the mold (see Section 4.1.3.5) The hot runner mold (C) is also shown in its most common arrangement, but it could also have inside center gating at a higher cost and with much less productivity The example shows a simple open gate The layout for a valvegated system would be similar 4.1.7.1 Modular Molds or Retaining Plates? From the earliest days of molding... trade shows, I have seen such molds with more than 100 hoses for the IN and OUT of coolant and still more hoses for air supply Such a mold may be less costly to build, but is expensive for set-up and service There is also the problem of unequal heat expansion of the backing plates, resulting in misalignment From these rather crude molds, the present system of modular molds was developed, where the... molds are: Cooling water for all cavity modules is supplied through drilled channels in the cavity retainer plate and the cooling (and any air channels for ejection) for the cores are provided through drilled channels in the core backing plate This makes for a simple, well-cooled mold with a minimum of hoses encumbering the space around the mold However, it requires more thought in designing such mold. .. thin-walled container Note that the cavities are pocketed into a plate while the cores “float” (Courtesy: Dollins) A more recent method of designing multi-cavity molds is to consider molds that are assemblies of a number (2, 3, 4, or more) of single-cavity molds (modules), mounted on common backing plates Each such module has its own cooling circuits, but they share a common injection (hot runner, or, rarely,... Figure 4.47 shows typical cross sections for (A) 2-plate, (B) 3-plate, and (C) hot runner molds with two or more cavities The 2-plate mold (A) is shown with a simple edge gate, but the layout would be similar with any other cold runner gate into the side (fan, tunnel) or underside (submarine) of the product The 3-plate mold is shown in its most frequently used arrangement, i.e., outside center-gated The...135 4.1 Selection of an Appropriate Mold the time when each of two or more valve gates open (by Synventive Corp.) This can be of advantage for properly filling cavities of complicated shape and to prevent the formation of weld lines where they are visible or where they could weaken the product’s strength 4.1.7 Two and More Cavities, Cold or Hot Runner Molds Figure 4.47 shows typical... cavities without leaking into the open The mold can be shallower, which is of advantage with machines with little shut height Stacks can often be placed closer together, because the cavity walls can be thinner and still resist against the forces created by the injection pressure, but only if the cavities are forced into the plates with properly calculated press fit The mold is usually less expensive, because... productivity The example shows a simple open gate The layout for a valvegated system would be similar 4.1.7.1 Modular Molds or Retaining Plates? From the earliest days of molding technology, in multi-cavity molds (unless they could be cut right into the plates) both cavities and cores were inserted into pockets or bores of matching plates, the so-called “cavity-and coreretainer plates” Retainer Plate Design . multi-cavity molds (A) 2-plate mold; (B) 3-plate mold; (C) hot runner mold 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14135 136 4 Mold Selection There. sizes. Figure 4.44 Mold for a rear door of a car (Courtesy: Accurate Molds) 4.1 Selection of an Appropriate Mold Figure 4.43 Large automotive mold for bumper