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David O. Kazmer Injection Mold Design Engineering ISBN-10: 3-446-41266-2 ISBN-13: 978-3-446-41266-8 Leseprobe Weitere Informationen oder Bestellungen unter http://www.hanser.de/978-3-446-41266-8 sowie im Buchhandel 4 Mold Layout Design During the mold layout stage, the mold designer commits to the type of mold and selects the dimensions and materials for the cavity inserts, core inserts, and mold base. Mold bases are only available in discrete sizes, so iteration between the inserts’ sizing and mold base selection is normal. The goal of the mold layout design stage is to develop the physical dimensions of the inserts and mold so as to enable procurement of these materials. Mold material selection is also an important decision, since the material properties largely determine the mold making time and cost as well as the mold’s structural and thermal performance. The mold layout design assumes that the number of mold cavities and type of mold has been determined. To develop the mold layout, the mold opening direction and the location of the parting plane are first determined. Then, the length, width, and height of the core and cavity inserts are chosen. Afterwards, a mold base is selected and the inserts are placed in as simple and compact a layout as possible. It is important to develop a good mold layout design since later analysis assumes this layout design and these dimensions are quite expensive to change once the mold making process has begun. 4.1 Parting Plane Design The parting plane is the contact surface between the stationary and moving sides of the mold. The primary purpose of the parting plane is to tightly seal the cavity of the mold and prevent melt leakage. This seal is maintained through the application of literally tons of force (hence the term “clamp tonnage”) that are applied normal to the parting plane. While the term “parting plane” implies a flat or planar surface, the parting plane may contain out-of-plane features. The mold designer must first determine the mold opening direction to design the parting plane. 4.1.1 Determine Mold Opening Direction Examination of any of the previous mold designs (e.g., Figure 1.4 to Figure 1.8) indicates that the mold opening direction is normal to the parting plane. In fact, the mold usually opens in a direction normal to the parting plane since the moving platen of the molding machine is guided by tie bars or rails to open in a direction normal to the platen. Accordingly, guide bushings and/or mold interlocks are almost always located on the parting plane to guide the mold opening in a direction normal to the parting plane. It may appear that there is nothing about the mold opening direction to determine since the mold opens normal to the parting plane. However, it is necessary to determine the mold 68 4 Mold Layout Design opening direction relative to the mold cavity. There are two factors that govern the mold opening direction: 1. First, the mold cavity should be positioned such that it does not exert undue stress on the injection mold. The mold cavity is typically placed with its largest area parallel to the parting plane. This arrangement allows the mold plates, already being held in compression under the clamp tonnage, to resist the force exerted by the plastic on the surfaces of the mold cavity. 2. Second, the mold cavity should be positioned such that the molded part can be ejected from the mold. A typical molded part is shaped like a five-sided open box with the side walls, ribs, bosses, and other features normal to its largest area. If so, then the part ejection requirement again supports the mold opening direction to be normal to the part’s largest projected area. Consider the cup and lid shown in Figure 4.1. A section of the core and cavity inserts used to mold these parts was previously shown in Figure 1.6. There are only two potential mold opening directions relative to the part. One mold opening direction is in the axial direction of the cup, while the second direction is in the radial direction of the cup. Figure 4.1: Sectioned isometric view of cup assembly 4.1 Parting Plane Design Figure 4.2: Axial mold opening direction for cup 69 Figure 4.3: Radial mold opening direction for cup A section of a cavity block with an axial mold opening direction is shown in Figure 4.2. The two bold horizontal lines indicates the location of the parting plane where the two halves of the insert are split to form the cavity insert (top) and the core insert (bottom). Consider next the same cavity block but with a radial mold opening direction for a portion of the cavity insert as shown in Figure 4.3. For this design, four bold lines separate the sides from the top and bottom. Since the metal core is located inside the molded part, there is no way to remove the core other than in the part’s axial direction. The cavity insert, however, can be separated into three pieces that move along two different axes in order to remove the molded part. Of these two designs, the axial mold opening direction shown in Figure 4.2 is the simplest design and is usually preferred. However, the second design is sometimes used in practice since it allows for a more complex part design as well as more options in locating the parting line. For instance, the second design might be required if a handle were added to the cup, or if it was necessary to move the parting line to a location away from the top lip. This second design is known as a “split cavity mold” and is discussed in more detail in Section 13.9.1. As another example, consider the laptop bezel shown in Figure 3.5. There are again two potential mold opening directions. The first opening direction is in the screen’s viewing direction, as indicated by the section view shown in Figure 4.4. In this case, the mold section is split by two horizontal lines into a cavity insert forming the outside surface of the bezel and a core insert that forms the inner surface and ribs of the bezel. When the core and cavity inserts are separated as indicated by the arrows, the molded bezel can be readily removed. Figure 4.4: Normal mold opening direction for bezel 70 4 Mold Layout Design Figure 4.5: Complex mold opening directions for bezel Alternatively, the cavity block for the PC bezel can be split as indicated with the three vertical lines shown in Figure 4.5. In this case, the former cavity insert is split into two pieces, resulting again in a split cavity mold design. The two halves of the former cavity insert must now be removed in oblique directions in order to remove the molded part; the mold opening direction is inclined in order to allow the mold surfaces to separate from the molded part without excessive surface friction or shearing of features on the molded part. This movement requires several additional mold components to control the moving cavity inserts, which add significantly to the cost of mold design, manufacture, and operation. 4.1.2 Determine Parting Line The term “parting line” refers to the location at which the cavity insert, the core insert, and the plastic molding meet. Since the core and cavity insert meet at this location, any significant deflection of the cavity insert away from the core insert will result in a gap into which the plastic will flow and form a thin film of plastic known as “flash”. Imperfections in the core and cavity inserts at this location, for instance due to wear or improper handling, will also create gaps into which the plastic will flow. Even with new and well-crafted molds, the location of the parting line usually results in a very slight “witness line” along its length. For this reason, the parting line should be located along a bottom edge of the part, or some other non-visual, non-functional edge. Consider the previous cup shown in Figure 4.1. Placing the parting line very close to the lip as indicated by the dashed line in the left drawing of Figure 4.6 would result in a witness line and possible flash that might make the molded cup unusable. Alternatively, a better location for the parting line is at the bottom of the rim as indicated in Figure 4.2, corresponding to the parting line shown in the right drawing of Figure 4.6. Figure 4.6: Two parting line locations for cup 4.1 Parting Plane Design 71 Figure 4.7: Parting line location for bezel For the laptop bezel, the parting line will be located around the bottom edge of the part as shown in Figure 4.7. It is observed that, unlike the cup, the parting line for the bezel is not in a single plane. Rather, the parting line follows the profile of the features on the side walls. This non-planar parting line is required to fit the core insert which hollows out the mold cavity to form the holes required for the various connectors. As will be seen in the next section, this complex parting line shape will cause a more complex parting plane. 4.1.3 Parting Plane Once the parting line is identified, the parting plane is projected outwards from the part, so as to separate the core insert from the cavity insert. The preferred parting plane for the cup is shown in Figure 4.8. The cavity insert will form the outer and top surfaces of the part, while the core insert will form the rim and inner surfaces. Figure 4.8: Parting plane for cup 72 4 Mold Layout Design Figure 4.9: Parting plane for bezel For the laptop bezel, the parting line in Figure 4.7 can be radiated outward to form the parting surface shown in Figure 4.9. It can be observed that all of the out of plane features along the parting line now become complex surfaces on the parting plane. These surfaces pose two significant issues during mold operation. First, any misalignment between the sharp features on core and cavity inserts will cause wear between the sliding surfaces if not an outright impact between the leading edge of the core and the mating cavity surface. Second, the clamp tonnage exerted on the core and cavity inserts can cause the surfaces to lock together with extreme force, causing excessive stress and potential mold deformation during mold operation. To avoid excessive stress, interlocking features on the parting plane should be inclined at least five degrees relative to the mold opening direction. The parting surface is now typically created via three dimensional computer aided design (“3D CAD”) using lofted surfaces. Each lofted Figure 4.10: Modified parting surface for bezel 4.1 Parting Plane Design 73 surface blends a curved feature along the parting line to a line of corresponding width on the parting plane. The result is a surface with the needed profile at the parting line and the necessary draft down to the parting plane. The lofted surfaces are then knit together with the parting plane to provide a parting surface, as shown for the bezel in Figure 4.10. 4.1.4 Shut-Offs Shut-offs are contact areas between the core insert and the cavity insert that separate portions of the cavity formed between the core and cavity inserts. A shut-off will need to be defined for each window or opening in the molded part. Conversely, if a part has no windows, like the cup, then no shut-offs are defined. Each shut-off is defined by a parting line, which should be located in a non-visual area where a witness line or slight flashing would not reduce the value of the molded part. For example, the laptop bezel has one large opening above the parting plane for the display. A shut-off is necessary across the entire area of the opening. As indicated in Figure 4.11, there are essentially two possible locations for the shut-off ’s parting line, corresponding to the top and bottom of the shelf that supports the display. Either location (or even any location in between) would likely be acceptable since the entire shelf is hidden from view. If the parting line is placed at the top of the shelf as indicated at the right of Figure 4.11, then a shut-off surface as shown in Figure 4.12 will result. Figure 4.11: Shut-off surface for bezel Figure 4.12: Shut-off surface for bezel 74 4 Mold Layout Design 4.2 Cavity and Core Insert Creation With the definition of the parting plane and all necessary shut-offs, the core insert and the cavity insert have been completely separated. To create the cavity and core inserts, the length, width, and height of the inserts must be defined. The length and width of the cavity and core inserts must be large enough to: • enclose the cavity where the part is formed, • withstand the forces resulting from the melt pressure exerted upon the area of the cavity, • contain the cooling lines for removing heat from the hot polymer melt, and • contain other components such as retaining screws, ejector pins, and others. All of these requirements suggest making the core and cavity inserts as large as possible. For smaller molded parts, increasing the sizing the core and cavity inserts may have little added cost. However, the cost of larger core and cavity inserts can become excessive with increases in the number of cavities or molded part size. 4.2.1 Height Dimension The height dimension is often determined by two requirements. First, the core and cavity insert should have enough height above and below the molded part to safely pass a cooling line. Cooling line diameters typically range from 4.76 mm (3/16″) for smaller molds to 15.88 mm (5/8″) for large molds. Generally, large inserts with larger cooling lines will provide faster and more uniform cooling as will be analyzed in Chapter 9. While cooling line design will be later discussed, the minimum height dimension between the molded part and the top or Figure 4.13: Insert height allowance 4.2 Cavity and Core Insert Creation 75 bottom surface of the insert is typically three times the diameter of the cooling line to avoid excessive stress as analyzed in Chapter 12. The initial height dimensions for the core and cavity inserts are shown in Figure 4.13. Second, the core and cavity insert should have a height that is matched with the height of available cavity and core insert retainer plates (the “A” and “B” plates). These plates are commonly available in ½″ increments in English units, and in 10 mm increments in metric units. As such, the insert heights should be adjusted up such that the faces of the cavity and core inserts are flush or slightly proud with respect to the “A” and “B” plates on the parting plane. It should be noted that the height of the core insert as indicated in Figure 4.13 is not its total height but rather the height dimension from the rear surface to the parting plane. For materials procurement and cost estimation, the total height of the core insert should also include the height of the core above the parting plane. 4.2.2 Length and Width Dimensions The length and width dimensions are similarly determined by two requirements. First, if a cooling line is needed around the exterior of the mold cavity, then the inserts should be sized large enough to accommodate such a cooling line. As for the height allowance, length and width allowances of three cooling line diameters per side are typical. Second, the width and length dimensions of the inserts should provide side walls, also known as “cheek”, that are thick enough to withstand the lateral loading of the melt pressure exerted on the side walls of the mold cavity. This requirement will become dominating for deep parts with large side walls. While the structural design will be discussed in detail in Section 12.2.4, a safe guideline is that the thickness of the side wall in the length and width dimension should equal the depth of the mold cavity. Figure 4.14 demonstrates an allowance that should be added to the length and width of the mold cavity to derive the length and width of the core and cavity inserts. It can be observed that for the laptop bezel, the requirement of fitting a cooling line will exceed the structural requirement. For the molded cup, however, the insert length and width dimension are driven by the structural requirement. Figure 4.14: Insert length and width allowance 76 4 Mold Layout Design 4.2.3 Adjustments The core and cavity inserts can now be created with the prescribed dimensions. However, it is sometimes desirable to adjust the cavity insert dimensions to provide a more efficient mold design. In general, the length and width dimensions of the inserts are more critical than the height dimension, since these dimensions will • • drive the size of the mold base in multi-cavity applications, and contribute more to the material and machining costs. As such, these dimensions may be decreased somewhat by effective cooling and structural designs, which will be supported by later engineering analysis. Figure 4.15: Core and cavity inserts for cup Figure 4.16: Core and cavity inserts for bezel 4.3 Mold Base Selection 77 Figure 4.15 provides the core and cavity inserts for the cup. Since the molded part is round, the design of the core and cavity insert may also be round. This shape provides a benefit with respect to ease of manufacturing, since both the core and cavity inserts can be turned on a lathe. While the allowances in the axial and radial dimensions are sufficient to fit cooling lines, the allowance in the radial dimension may not be sufficient to withstand the pressures exerted on the side wall by the melt. There is no fundamental requirement on the external shape of the core and cavity inserts. While the insert design in Figure 4.15 showed round inserts, the mold design for the cup shown previously in Figure 1.4 used square inserts. Rectangular inserts with or without filleted corners are also quite common. The design of the insert should be dictated by the shape of the molded part, the efficiency of the mold design, and the ease of manufacture. The core and cavity inserts for the laptop bezel are shown in Figure 4.16. In this case, rectangular inserts are designed. The length and width dimensions of the inserts have been designed quite aggressively. While the bezel is quite shallow and the inserts are structurally adequate, the thickness of the surrounding cheek may not allow for sufficient cooling around the periphery of the mold cavity while also providing space for other mold components. 4.3 Mold Base Selection After the core and cavity inserts have been initially sized, the mold layout can be further developed and the mold base selected. It is critical to order a mold base with appropriately sized plates and materials, since any mistakes in the mold base selection can consume significant time and expense. To determine the appropriate size, the mold designer must first arrange the mold cavities and provide allowances for the cooling and feed systems. Afterwards, the mold designer should select a standard size from available suppliers and verify suitability with the molder’s molding machine. 4.3.1 Cavity Layouts The goal of cavity layout design is to produce a mold design that is compact, easy to manufacture, and provides molding productivity. If a single cavity mold is being designed, then the cavity is typically located in the center of the mold, though gating requirements may necessitate placing the mold cavity off center. For multi-cavity molds, there are essentially three fundamental cavity layouts: • • • cavities are placed along one line cavities are placed in a grid, or cavities are placed around a circle.