Tài liệu Mold Selection P3 docx

137  Because of the better cooling, the, mold will cycle faster for higher productivity  Interchangeability of components is easier to achieve and repairs are less costly.  Heat expansion is not a problem; by having the cores mounted to float a limited amount to self-align with the cavities, regardless of any tempera- ture difference between the plates, the size of the mold (number of cavities) is not limited.  If this design is used for molding products that can only be ejected with air, it becomes a really simple mold, with very few mold shoe components. This method is of special advantage in stack molds, which are then much simpler. Disadvantages of the modular mold design are:  The cavity spacing is larger than a comparable retainer plate design, because the wall thickness of the cavity must be strong enough to withstand the injection pressures. Therefore, the mold will be larger, for the same number of cavities and more expensive.  Because the mold will be larger than a comparable mold with retainer plate design, the cost of the mold will be somewhat higher, but this additional cost can be easily recovered by the increased productivity and lower maintenance cost.  Modular molds cannot be used for 2-plate molds, because the cold runners would be interrupted between the modules. However, they can be used for 3-plate molds. 4.1 Selection of an Appropriate Mold Today, most molds for small containers (dairy, etc.) are of modular construction 1281han04.pmd 28.11.2005, 11:14137 Previous Page 138 4 Mold Selection A A B C E F G H I K L M N O O G U P P V R L T S Figure 4.49 Modular 6-cavity mold (Courtesy: Husky) Figure 4.50 Underside of core backing plate (Courtesy: Husky) Figure 4.49 shows a modular 6-cavity mold for a container with core lock design. The cavities (A) are set into the cavity retainer plate (B), the hot runner system is in the plate (C) behind the cavities. In front is a cavity (A) with cavity bottom (E). The core assemblies (F) are mounted with float, on top of the core backing plate (G) that is also the frame for the ejector mechanism shown in Fig. 4.35. The core assembly (F) consists of the core (H), a BeCu core tip (I), the core retainer block (K), and the stripper ring (L). All water supply to the cores and the cavities is cross-drilled in the cavity retainer plate and the core backing plate. Some of the water connections can be seen at the bottom (M). The three leader pins (N) are just for mold handling and to protect the cores from damage; the final alignment is by the individual tapers (O) between cavi- ties and cores. Note the numerous venting groves and channels, cavities, and stripper rings. Figure 4.50 shows the underside of core backing plate (G) in Fig. 4.34. Note the substantial supports (P) under the cores and the center of the mold. Note also the three guide pins (R) and bushings (S) for supporting the stripper ejector plate. The stripper rings (L) are driven by four pins (T) each, which are guided in bushings (U) in the plate (G). There is no need for a mold mounting plate. The mold can be mounted directly by screws into the tapped holes of the machine platen or by clamps, entering the recesses (V). 1281han04.pmd 28.11.2005, 11:14138 139 4.1.7.2 Cavity Spacing Note that the cavity spacing in 2-plate molds for most products is usually greater than in either a 3-plate or a hot runner mold because of the space required for the runners between the cavities. This means that for the same number of cavities, a somewhat larger mold shoe will be required; in addition, more clamping force will be needed because of the additional projected area of the runner system. Very small cavities, flat or with little depth, can often be located very close together and can take less space than hot runner molds, even when adding the space required for the runners. But hot runner technology has con- siderably advanced by providing drops to feed into more than one gate, therefore the products can be spaced much closer together. 4.1.7.3 Hot Runner Edge Gates There are two other gate configurations not mentioned earlier in the section on gates, because they are used only in hot runner systems with two or more cavities. The hot runner edge gate (HREG) is an open hot runner gate into the sidewall of the product. The principle (and construction) of the HREG is simple and explained in detail in [5]. Gates from one drop from the manifold can feed into just one cavity, two cavities (most frequently used), three, or even four small cavities. There are standard HREG nozzles commercially available. It is very important that the design suggestions by the manufacturers are closely followed. The system is very reliable and trouble-free, but must be used with absolutely clean, preferably virgin, plastic material to avoid plugging the small gates by dirt. Molds with HREG are frequently used in cases where gating into the top surface of the product is not acceptable, because the end use of the product or appearance reasons prohibit it. Typical examples are earlier molds for Petri dish bases and covers that need top surfaces with optical clarity, but also for clear small boxes for packaging delicate items, such as jewelry, cosmetics, and so forth. As stated above, drops can feed more than one cavity. For example, for an 8-cavity mold, rather than using a standard hot runner system with eight drops (one per cavity), a HREG mold can be selected with four drops only, each feeding two cavities. This could represent a considerable saving in mold and hardware cost. Note: HREG molds can be more expensive to design, build, and maintain. Today, Petri dishes are often molded using valve gates, placed near the edge of the top surface. The problem of hot runner gating small products, or where two products are placed closely side by side, can also be solved by using hot runner drops and nozzles as shown in Fig. 4.53. This method is less expensive than the use of hot runner edge gates, but will leave a gate vestige on top of the product, near its edge. There are also similar nozzles with three or even four gates. Figure 4.51 Schematic of hot runner edge gate Figure 4.52 Typical hot runner edge gate vestige (A) on the side, near the bottom of the product Figure 4.53 One “drop” feeding more than one gate. This picture shows two gates, but there are also drops for 3 or 4 gates 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14139 140 4 Mold Selection 4.1.7.4 Insulated Runner Molds Insulated runner molds are a further development of the through shooting principle. It is based on the insulating properties of plastics, as we have already seen in the through shooting nozzle (Section 4.1.6.2). Using large diameter flow channels, in the order of 16–19 mm diameter, the plastic layers closer to the walls will freeze, but the center of about 5–10 mm diameter remains molten to allow an effective flow of hot plastic toward the gates. While the principle of operation is simple (Fig. 4.54), the start-up of an insulated runner mold needs a certain amount of skill. At the first shot into the empty, clean mold, the sprue (d) and the rather large runner (e) are filled, together with some or all of the cavity space. If the first shot is insufficient to fill all cavities, after quickly removing the incomplete shot, a second shot immediately following will be usually sufficient to have the whole system filled. After removing this second, by now probably complete shot, the mold is ready for production. If not, a third shot may be required The main problem with insulated runner systems is the startup of the mold: after the first injection that usually does not fill the runner and all the cavities immediate action is of the essence. The incomplete first shot must be removed if it has not properly ejected, and the mold must be started again quickly so that the plastic in the sprue and runner does not have enough time to freeze. If the plastic does freeze, the mold must be opened between the plates (a) and (c) to remove the plastic in the runner; after closing and locking these plates together, the startup will be repeated. By then, the plates have warmed up a bit and will make the following startup shot(s) easier. Preferably, the cooling water is turned off during startup. Once the mold can run on cycle automatically, the molding conditions (pressures, temperatures, and times) can be adjusted for best productivity. With PS, the time frame available is approx. 15 s, for PP and PE it is approx. 30 s, before the runner will freeze. The main requirement for successfully running these molds is that the ejection method must be absolutely reliable to avoid any delays due to failure to eject. Once the mold runs on cycle, it could run without stopping until the production run is completed. Molds for 2, 3, 4, 6, 8, and even 12, or 16 cavities can be quite easily made and operated. A typical example is a 16-cavity mold for PE chair leg protectors that runs on a 25 s cycle; once started, the molder did not stop it at all for two consecutive years. This is not necessarily practical with other molds that need interruptions (time for maintenance), but it shows how well these molds can run. The advantages of this system are  Very simple and inexpensive construction  There is no need for any heaters in the system, although there are variants to the “true” insulated runner mold where pointed nozzle heaters inside the drop keep the gates from freezing. This is used with plastics such as PS that tend to freeze easier because of their better heat conductivity. Both Fig. 4.55 and 4.56 show the use of such heater probes with insulated runners. Figure 4.54 Schematic of cross section through a two-cavity insulated runner mold; (a) cavity plate; (b) core plate; (c) runner plate; (d) sprue; (e) insulated runner; (f) gate A B C D Figure 4.55 Section through a 2-cavity insulated runner “slug” D E ABC Figure 4.56 Partial section through an 8-cavity insulated runner “slug” 1281han04.pmd 28.11.2005, 11:14140 141 Figure 4.55 shows a section through insulated runner “slug” of a 2-cavity mold for nylon gears (A). Note the flow of the last injection (blue, B) inside the white, older plastic (C). Note also the cylindrical hollows (D) where electric heated “torpedoes” keep the melt hot in each drop, especially with its pointed torpedo tip extending right into the gate. Such heated torpedoes are suggested for all molds requiring longer cycles and with plastics other than PP or PE. Figure 4.56 shows the removed insulated runner of an 8-cavity mold for PVC products (A). The cut-open section of one runner branch clearly depicts the flow of the dark plastic (B) within the surrounding, colder plastic (C). For illustration purposes, the color of the plastic was changed from white to dark. Note the pockets (D), where the heated torpedoes are located (see also note with Fig. 4.55). Color changes are easy; there are two methods: 1. Stop the mold, remove the runner, close the now clean mold, and restart with fresh new color after the extruder has been purged. This takes some time and effort. 2. An even easier method is to remove the old color plastic from the machine hopper but to continue to run the extruder while the machine is pro- ducing pieces of the old color. When the new color is fed into the hopper; it will gradually replace the old color in the extruder and after 15–20 shots products with the clean new color should be ready. The production of the shots during the changeover will have a color mix and will have to be scrapped, unless the plastic can be reground and used where the color mix does not matter. The runners inside the mold will change their hot inside core (where the plastic flows) to the new color, while the frozen plastic near the outside of the runners is still of the old color. This can be clearly seen in the two photos above. Note that with conventional hot runners, color changes are always done by gradually purging through the manifold while the mold is producing. A good hot runner design will permit changing from a lighter to a darker color in about 50 shots, and longer when changing from a darker to a lighter color. Safety Considerations with Insulated Runner Molds With all these substantial advantages of insulated runner molds, why are they not used more frequently? As these molds are built today, they are inherently unsafe as will be explained below, and with the rapid development of reliable hot runner systems and the associated standard hardware, the insulated hot runners have been put on the sidelines and have been either completely forgotten or are being avoided. Any “regular” mold can be operated from the operator’s side of the machines, without the need to open a safety gate, except during startup when the front 4.1 Selection of an Appropriate Mold Insulated runner molds are ideal for molds requiring multiple color changes a day Insulated runner molds seem to be ideal – so why are they not used more frequently? The reason is that these molds need special skills for start-up and are inherently unsafe, as explained in the text 1281han04.pmd 28.11.2005, 11:14141 Next Page . gates 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14139 140 4 Mold Selection 4.1.7.4 Insulated Runner Molds Insulated runner molds are. Appropriate Mold Today, most molds for small containers (dairy, etc.) are of modular construction 1281han04.pmd 28.11.2005, 11:14137 Previous Page 138 4 Mold Selection A A B C E F G H I K L M N O O G U P P V R L T S Figure