Tài liệu Mold Selection P4 docx

142 4 Mold Selection gate must be opened to access the mold for servicing. The rear gate is usually either screwed locked, or does not have safety features such as drop bars. Opening the rear gate often stops the operation of the machine completely, even shutting down the hydraulic pumps. Over the years, with these safety features in molding operations, the number of personal injury-accidents has greatly decreased. Referring back the Fig. 4.54, plates (a) and (c) must be locked together solidly even though they are clamped together during injection by the clamping force of the machine. If plastic is injected (inadvertently or deliberately) while the clamp is open, these plates could see a large separating force; the plastic could escape through a gap between these two plates and spray into the open, causing injury to bystanders. With any such molds, to ensure that these plates are held together securely, pairs of large, solid screws or clamps are provided on the operator’s side of the mold and at the rear of the mold. To pull the plates (a) and (c) apart is relatively easy; after unlocking them, cavity plate (a) can be latched to the core plate (b); the clamp is then opened so that plate (a) moves with the core plate (b). As seen above, for startup of an insulated runner mold it is necessary to open the mold between plates (a) and (c), remove the runner, re-close and lock the plates together, and restart after closing the safety gates. This presents a serious problem. Since the screws or clamps are on both sides of the mold and this whole “operation” of unlocking, opening, cleaning, and re-locking must be accomplished within about 15 seconds; therefore, operators are required on each side of the machine. This can be very unsafe, unless other safety measures for the rear gate than the ones mentioned above are provided. There is always a risk of having more than one operator starting up any machine, but here, they are also being rushed, while only one of the operators is at the controls of the machine. The challenge is to find a method to allow locking the two plates (a) and (c) together from the operator’s side only, with only one operator used for startup. Once a method of safe operating conditions is created, this very economical system can be used for many applications. 4.1.8 Single- or Multi-Level Molds? “Stack” (multi-level) molds have a long history. Back in the 1950s, a stack mold was used for making matched color toilet seats and covers. This was an ideal application: both pieces are very heavy-walled and require similar, very long cooling times. In addition, the projected area of each piece was about the same and the color match is (inherently) perfect. Later, stack molds were used for smaller products, using elaborate systems of hot nozzles and cold runners; today, mostly thin-walled products are made on stack molds and practically every stack mold uses a hot runner system. The cycle times are often in the order of 5–8 s and the output of these molds 1281han04.pmd 28.11.2005, 11:14142 Previous Page 143 For the same product, and the same number of cavities per level, a stack mold essentially doubles the output of single level mold. The stack mold requires only one rather than two molding machines, provided the shot- and plasticizing capacity of the machine can provide for the increased number of products made on the stack mold A Figure 4.57 A 2 × 1 cavity stack mold for a large container can be enormous. The theory and design features of stack molds are explained in much detail in [5]. Here, we are more concerned with why and when to use them. Understanding the following will allow the decision maker to make the right choice between single- and two-, or more-level molds. 4.1.8.1 Two-Level Stack Molds for High Production A 2-level stack mold consists of two essentially identical, conventional molds, placed back to back, but with one, common hot runner system feeding the two cavity plates. The two core sections are mounted on the stationary and moving platen, respectively. The center section of the mold, also called the “floating” mold section, (two cavity plates and the hot runner assembly in between them) is supported (directly or indirectly) on the machine base or on the tie bars. It is moved, usually half the length of the machine stroke, by levers, gears, or other mechanical methods in synchronization with the clamp motion. Alternatively, hydraulic cylinders could move the center section, independently of the clamp motion. The core half on the moving platen can be exactly like it would for a single-level mold and can use the existing machine ejection mechanism. The core half mounted on the stationary platen presents two problems: (1) In most stack molds, the sprue supplying the center hot runner system passes through the center of the (stationary) core half, so that there must be a passage large enough for a long, heated sprue bushing to move through so that the cavity stacks closest to the center may have to be spaced farther apart. With the above mentioned toilet seat and cover mold, this problem was resolved by placing the open gap of the horseshoe-shaped seat on the side nearer the injection side of the machine, so that the sprue bar could pass through it and the seat could fall freely without hanging up on the sprue bar Figure 4.57 shows a 2 × 1 cavity stack mold for a large container. This mold is built for a machine equipped with the actuating mechanism and with special supports for the floating center section. Note the sprue bar (A) of the hot runner system (only partly shown) is located at the side of the cavity to engage in a special, offset hot runner manifold behind the core plate near the stationary platen (not visible in this photo). 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14143 144 4 Mold Selection If the sprue bar cannot be in the center of the mold, as, e.g., in Fig. 4.57, the solution is to relocate the sprue bar outside the molding area. This requires an additional hot runner system in the mold half, located on the stationary platen. The machine nozzle enters at the usual center of the mold into the (additional) hot runner manifold leading to outside of the molding area where the long sprue can supply the melt to the center section from outside the molding area. In this arrangement, the main hot runner system is supplied near its end instead of the usual point in the center of the manifold. This method will allow molding large products even on that side of the mold where the sprue bar is located. This method is also useful for example for a 2 × 2-cavity mold or a 2 × 6-cavity mold, with the center cavities arranged one above the other, for free-fall ejection, so that the upper products will not hit the hot sprue bar when falling down. This off-set sprue bar arrangement is also useful when extracting the molded products with robots, should more clearance for the path of the robot arm be required. Injection-molding machines, as a rule, don’t have ejection systems on the in- jection side; actuators must be added either to the core section located on the stationary platen, or to the stationary machine platen. The ejection system can also be mechanically linked to the moving platens or to the operating mecha- nism moving the floating section. This is sometimes done (very crudely) by connecting the ejector mechanism (stripper plate or ejector plate) with chains or (somewhat better) with lost-motion links to the center section of the mold. A better solution is to connect the ejector mechanism with links to the “pro- peller” that actuates the center section. One disadvantage of these methods is that the machine stroke must be closely controlled to avoid damage to the links. In addition, the ejection takes place only close to the end of the opening stroke, and MO time is necessary to allow the products to clear the molding area. A newer design avoids these problems. Figure 4.59 shows a stack mold for lids. The profile of the rocker arm (A) is designed so that, as soon as the roller (B) on the cavity-side is engaged, the shorter arm of the rocker pushes on roller (C) to move the stripper plate (D) forward to eject. This can take place early during the opening stroke so that the products have enough time to clear the molding area before the mold closes again. Usually, no MO time is required and the mold will cycle faster, for higher production. Note that springs (not visible) inside the core plate return the stripper plate as soon as the rocker (A) leaves the roller (B). This method also makes the mold independent of an accurate opening stroke of the machine, because there are no fixed links between the mold sections. The rocker arm system is also less expensive (by about $10,000) than an independent ejection system on the mold half mounted on the injection side of the machine. The ideal solution (where possible) is to use only air ejection from the cores, requiring no mechanical ejection system at all; but this only is possible with certain product shapes and plastics, see [5]. Figure 4.60 shows an early design (year 1975) of a self contained stack mold with center section supported and aligned on the upper tie bar (A), actuated by levers (B) and (C). Figure 4.58 Typical air eject stack mold (2 × 4) for a round container (Courtesy: Topgrade Molds) 1281han04.pmd 28.11.2005, 11:14144 145 CDA E B Figure 4.59 Self-contained 2 × 32-cavity stack mold for threaded lids. The lids are stripped off the cores. The mold is equipped with rocker arms (A) in each corner of core plate and the matching rollers (B) and (C) for the actuation of the arms and motion of the stripper plate. Note the BeCu core caps (E); random ejection; product mass: 14 g; cycle time: 11.0 s; productivity: 21,000 pieces per hour A B CC Figure 4.60 Self contained stack mold (Courtesy: Husky) 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14145 146 4 Mold Selection Figure 4.61 shows a recent self-contained 2 × 4 stack mold for dairy containers (modular construction). It features special supports for the center section (A) and rack and pinion actuation (B) for its motion. The mold is equipped with servomotors (C) and suction cups (D) to remove the air-ejected containers. The mold runs without MO time, at a 2.75 s cycle, for a pro- ductivity of 10,473 pieces/h. Figure 4.62 shows a recent self-contained 2 × 8 stack mold (modular con- struction) for fairly heavy (55 g) stadium cups. The mold features rack and pinion actuation (A), air ejection, random free-fall. Note the center sprue (B) and how the machine tie bars (C) are supported (D) on the base to take the mass of the heavy center section (E). The mold has a cycle time of 10 s, for a productivity of 5,760 cups/h. A B C D E Figure 4.61 Self-contained 2 × 4 stack mold for dairy containers A B C D E Figure 4.62 Self-contained 2 × 8 stack mold (modular construction) (Courtesy: Husky) 1281han04.pmd 28.11.2005, 11:14146 Next Page . photo). 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14143 144 4 Mold Selection If the sprue bar cannot be in the center of the mold, as,. 4.60 Self contained stack mold (Courtesy: Husky) 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14145 146 4 Mold Selection Figure 4.61 shows