Tài liệu Mold Selection P6 ppt

153 When considering the use of stack molds, especially for fast cycling products, special measures must be planned to remove the huge amounts of products. Depending on the shape of the product, these measures could be any of a number of controlled ejection methods, conveyors, stackers, packing ma- chines etc. Figure 4.70 shows a typical cup stacker. The stacker is positioned at the rear of the molding machine. It consists of an unscrambling vibratory conveyor (A) that receives the randomly ejected, scrambled containers from under the mold. The cups are then elevated to the orienting section (B) and moved to the stacking section (C), where they are counted to the required stack length. The stack is moved to the staging table (D). From there the stacks are removed by hand and packed into boxes. Production, depending on product size, ranges from 2,400 to 12,000 pieces/hour. Figure 4.71 shows a typical lid stacker for round lids, in a range from 3 to 6.5 in. diameter. The randomly ejected lids are moved by in-press conveyors onto a twin-disc unscrambler (A). The lids are then single-filed from each disc and oriented ready for stacking in two lanes up the elevating conveyor (B), to the stacking system “waterfall” (C). Next, the flow is split again and four lanes (D) enter the spin bar stacking section (E). Stacks are cut to length ready for transfer to the tray loading section (F). The automatic tray feeder (G) moves the cardboard trays under the stacked lids, four stacks at a time, and moves the loaded boxes to the unloading station (H) from where they are hand packed into cartons. Figure 4.72 shows a (CHSV) retrieval, transfer, and stacking system, seen from the side, where the molding machine would be located. Arms enter between the open 2 × 8 container stack mold. Eight suction cups face the cores in each level. The ejected containers are held in these cups, which then retract out of the molding area. Another array of suction cups in the Figure 4.70 Typical cup stacker (Courtesy: CBW) A B C D E F G H Figure 4.71 Typical lid stacker for round lids (Courtesy: CBW) 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14153 Previous Page 154 4 Mold Selection “out-side” position picks up the cups from the transfer arm. After two shots, the receiving station rotates and stacks the cups to a predetermined number. From there, the 16 stacks per level move on a conveyor to the packing station. 4.1.9 Semi or Fully Automatic Operation? With extremely rare exceptions, every mold can run automatically. “Auto- matic” in this context means that the mold cycles without an operator’s intervention, such as opening the safety gate to remove the products, or lubricating (spraying) the molding surfaces, and so forth. The following reasons make it desirable for the mold to run automatically:  Safety considerations. Every time an operator needs to open the gate and reach into the molding area, there is a safety hazard, even with all the safety features on today’s machines. Safety features have been known to fail and, worse, they have been deliberately tampered with or voided to save costs to repair a problem. The best and safest policy is to have no operator near the operating parts of the machine. Note that in general, mold operators employed to “run” a machine are often not educated in machine operations and are therefore more difficult to train in safe procedures around a machine. Usually, well-trained setup personnel are employed to install the molds and to get the machine up to production. They are better educated and well trained on the job and are less likely to run the machine in an unsafe condition or to condone unsafe practices. Figure 4.72 CHSV retrieval, transfer, and stacking system (Photo courtesy: CBW, USA) 1281han04.pmd 28.11.2005, 11:14154 155  Quality of product. Uniformity of the quality of product depends much on the uniformity of the cycle time. Some operators are more skilled than others, but even with the same operator, the time to open the safety gate, to remove the product, and to close the gate before starting the next cycle can vary greatly, depending on the time of the day or night, the length of time working on the machine, and is affected by fatigue and boredom. If the product tends to hang up in the mold, or worse, if a portion of the piece sticks in the core or the cavity and must be removed with a tool (e.g,. with a pointed piece of brass wire), it may take a while before the mold can be restarted. These undesired stops and variations in the molding cycle affect the melt quality and the cooling temperatures in the mold and can result in unequal quality of the product. Also, if such stoppages are too long, the extruder and any plastic in the mold may have to be purged, before the mold can be restarted.  Absenteeism. This is often a serious problem, especially with the available labor force required to operate the molds. To be operated most eco- nomically, molding machines should be run uninterrupted, 24 hours a day, for at least 5 days a week. Many plants operate 7 days a week and never stop a machine, except for mold changes and scheduled mold and machine maintenance. The problem is that absenteeism can create labor shortages, which are difficult to control and affect the continuous operation of a machine  Labor cost. The elimination of machine operators is often quoted as the reason for running fully automatic, but in fact this reason is much less significant than the others given above. With very few exceptions, every mold can be designed so that it will run fully automatically. The problem is sometimes that the mold has been designed correctly to run auto- matically, but was installed and set up poorly so that it will not eject properly, as intended. 4.1.10 Insert Molding There are many applications where “inserts” must be used in a mold. Here, we will highlight a few examples (see Fig. 4.73). The real challenge is to design these molds to run automatically. Inserted labels at the bottom of products. There are different methods of adding labels to the mold before it closes. Label insertion is always done automatically, often with multi-cavity molds. They use automatic label dispensers reaching into the molding area and placing the label into the cavity or on the core while the mold is open for ejection Inserted labels at the sidewalls of products. More difficult is the application of labels on the sidewalls of containers, which are usually tapered, with little or large draft angles. Figures 4.74 and 4.75 show a typical system for applying labels on the side of containers. Figure 4.74 shows a view (from the rear of the machine) of an automatic inserting mechanism, attached to a standard Figure 4.73 Variety of plastic lids with labels applied automatically in the molds 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14155 156 4 Mold Selection A B C D E Figure 4.74 View (from the rear of the machine) of an automatic inserting mechanism (Courtesy: Hekuma) Operator side Fixed plate Inserting labels Taking out cups 2 cavities 2 cavities 4 cavities 8 cavities Movable plate Takeout-head of EDAT for cups Inserting-head of EDAT for labels Cavities arrays: cups 663890 3087 3750 Cup stacking Pivoted chute for manual packaging Horizontal stackingstation Taking over labels High speed side entry robot HELI 1-1700 560 1420 6388 Electric cabinet Overhead cable trunking Label separation Label magazine refilling position Label preforming Push and wind device (Patent depending) Non-operator side Label magazine refilling position 1000 Ferromatic K 155 D Figure 4.75 Schematic of the system shown in Fig. 4.74 (Courtesy: Hekuma) 1281han04.pmd 28.11.2005, 11:14156 157 ABCD Figure 4.76 Molded box molding machine. The mold (A) can be seen behind the tie bars. In the foreground, left, is the dispensing mechanism (B) for the pre-formed cup- shaped label blanks (C). Vacuum in the transfer arm (D) picks up the inserts, ready for the next cycle. After the mold arrives in the open position, the arm moves the inserts into a position opposite the cavities (left) and the vacuum changes to air pressure to push the inserts into the cavities. At the same time, the opposite face of the transfer arm receives the finished products as they are ejected from the cores and holds them with vacuum. When the transfer arm arrives in the OUT position, the finished products are then air ejected and stacked to the right (E). Figure 4.75 schematically shows the same system as in Fig. 4.74, including other features, which are not visible in the photo. It shows the label magazines, the label separating, and the label pre-forming devices, and the stacking of the finished products. The top right corner gives layouts of molds suitable for this operation with 2, 4, 6, or 8 cavities. Printed strips. Some molds feed printed strips through the cavity side of the mold and cut the label as the mold closes. This too is performed in a fully automatic mold. Paper and plastic containers. Some products (e.g., low-cost boxes) consist of a printed cardboard blank in a shape that, when folded, forms a box. As the mold closes, the core folds the cardboard while pushing it into the cavity. Plastic is then injected so that the cardboard along the open joints and corners is sealed by narrow, thin, molded plastic. These molds run also fully auto- matically, with a dispenser supplying the blanks to the open mold. Figure 4.76 shows a molded box (A), consisting of printed cardboard bottom and sides, but with a molded plastic rim (B) and plastic edges (C). Also shown is the flat cardboard blank (D), which is fed automatically into the mold. Metal inserts. The inserts can be loaded with independent robots or with dispensing and inserting attachments to the mold. Unfortunately, there are still molds for which the inserting is done by hand or with suitable loading boards, requiring an operator at the machine. Wire inserts. Molding plastic over electrical connectors and electrical wires is a special industry. This is done manually by using shuttle molds with at least two cavities (or cores), depending on the design of the product and the mold. While one mold half with the inserts is in the molding position, an identical mold half is outside the molding area, where an operator places the inserts into their respective locations. This is done mostly in vertical clamp machines, so that the shuttle can slide horizontally in and out of the molding area. This is relatively safe, because the (sitting) operator never needs to reach into the molding area. The molding cycle is usually fairly long and the operator has ample time to place the inserts before the next shot is ready; the finished products are removed when the other shuttle reaches the OUT position, before reloading. 4.1 Selection of an Appropriate Mold Figure 4.77 Metal insert over-molded parts 1281han04.pmd 28.11.2005, 11:14157 158 4 Mold Selection 4.2 Summary Up to this point, we have learned about the things to be considered by the decision maker in order to make the proper selection of a mold, i.e., how to look critically at the product, the importance of the expected production and productivity, and how to select the type of mold (and molding machine) most suitable for the job. All this applies to the technology of injection molding as it stands today, but is also valid for any future developments in this field. The most important principles are always common sense, simplicity and safety, and the target to produce the best product, at the best quality and the lowest cost. 1281han04.pmd 28.11.2005, 11:14158 . with labels applied automatically in the molds 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14155 156 4 Mold Selection A B C D E Figure 4.74. of the product and the mold. While one mold half with the inserts is in the molding position, an identical mold half is outside the molding area, where an