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.

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Placing all the cavities along a line, as shown in Figure 4.17, is a simple but poor design. Unless the insert geometries are long and narrow, the resulting mold layout produces a mold that has a high aspect ratio. In general, the width to length ratio of the bounding envelope around all cavities should be kept less than 2 : 1. Higher aspect ratios will require the use of large molds that are significantly under utilized while at the same time producing structural loadings across the mold for which molding machine platens may not be designed. Furthermore, the use of such a line layout requires an unbalanced feed system which can result in poor melt control as discussed in Chapter 6.

As an alternative to a linear layout of all cavities, it is common to place cavities in a grid as shown in Figure 4.18. This design is most common for applications requiring high production volumes when the number of cavities is a multiple of 2, i.e., 4, 8, 16, 32, etc. There are two primary benefits to a grid layout. First, the grid layout will result in a compact mold with an acceptable aspect ratio. Second, the grid layout lends itself well to naturally balanced feed system layouts as discussed in Chapter 6.

While the grid layout is compact and very common, it can result in a feed system design with multiple branches. To reduce the feed system complexity and ensure more balanced melt filling, a circular layout is sometimes used when the molded parts are relatively small or when the number of mold cavities is relatively low, for example 8 or less. Figure 4.19 shows one such layout in which all the cavities are provided at an equal distance from the center of the mold. The primary disadvantage is that such a circular layout requires a larger mold surface area than the previously discussed grid layout.

While the previously discussed layouts are the most common, there is nothing to prevent a mold designer from utilizing other layout designs. Some applications may best utilize a combination of the above layouts. For example, Figure 4.20 shows a combination of a line layout plus a circular layout. The resulting layout is a very compact design for six cavities.

Again, the designer should develop the layout that is best for the application’s geometry and requirements.

Figure 4.17: Series layout of cavities

Figure 4.18: Grid layout of cavities

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4.3.2 Mold Base Sizing

The size of the mold base is determined primarily by the area required to accommodate the designed cavity layout. A primary issue, however, is the potential for conflict between the placements of the cavities and other mold components (such as leader pins, guide bushings, and others). Furthermore, there is the potential for conflict between cavity support systems (such as cooling lines, ejector pins, support pillars, etc.) and other mold components (such as leader pins, guide bushings, and others.) Due to these conflicts, mold bases are often sized larger than what would first be considered.

The shaded area in Figure 4.21 represents the usable area of the parting plane into which the core and cavity inserts can be placed. Ejector return pins are located to the left and right of this area, while guide pins and socket head cap screws are located above this area. A dimensional allowance equal to at least one-half of each component’s diameters is provided between the mold cavity and the surrounding components to avoid excessive stress during the mold’s operation.

Figure 4.19: Circular layout of cavities

Figure 4.20: Hybrid layout of cavities

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Given the cavity layout and its geometric envelope, a mold base with a feasible length and width is selected. Standard mold bases are widely available from 200 mm up to 1000 mm on a side. When specifying a mold base, it is also necessary to specify the height of the “A” plate, the height of the “B” plate, the height of the support plate “S”, and the distance of the ejector travel, “E”, as shown in Figure 4.22. The total stack height is defined as the distance from the bottom of the rear clamp plate to the top of the top clamp plate.

Figure 4.21: Usable parting plane area

Figure 4.22: Height dimensions to specify

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With respect to mold base selection, the height of the “A” and “B” plates are respectively matched to the height of the cavity and core inserts as previously discussed. The height of the support plate, “S”, is normally determined from the mold base supplier based on the height of the “A” and “B” plates, though the height of the support plate can be special ordered to varying dimensions. The travel of the ejector plate should be selected to eject the part from the mold. Often, the ejector travel is set to be equal to the depth of the molded part. From the ejector travel, the height of the ejector housing, dimension “C”, is assigned by the mold base supplier.

When selecting a mold base, it is also necessary to specify an orifice diameter for the sprue, which is not shown in Figure 4.22. This dimension is of lesser importance since the sprue bushing may be replaced or machined, or the molding machine nozzle changed, to match the sprue to the nozzle as later discussed in Chapter 6.

4.3.3 Molding Machine Compatibility

When selecting a mold base, the mold designer should verify that the mold will fit in the available molding machine(s). There are many requirements that should be considered when matching a mold to a molding machine. First, the mold must physically fit in the machine.

Perhaps the most common limitation is that the mold will not fit between the tie bars. The tie bar spacing is easily measurable on an available molding machine, or can be checked in a machine drawing for a potential molding machine. For instance, Figure 4.23 shows the tie bar spacing and bolt pattern for a Battenfeld HM320 molding machine. It can be viewed that the horizontal tie bar spacing is 800 mm, and that the vertical tie bar spacing is 630 mm. This

Figure 4.23: Typical tie bar and bolt pattern (dimensions in mm)

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means that the maximum mold width, including cooling plugs, hot runner connectors, etc., is 800 mm (less some relatively small clearance between the mold and the tie bars to provide for mold insertion).

A cross-section view of the same machine platens is shown in Figure 4.24 with the same orientation as shown in Figure 1.1. In this view, the nozzle of the molding machine enters the stationary platen on the right side of the drawing. The machine’s moving platen and the ejector unit are on the left side of the drawing. For the mold to be operable in the machine, the mold height must be greater than the indicated A dimension and smaller than the indicated B dimension, or between 350 and 800 mm for this machine. If the mold is smaller than 350 mm, then the molding machine platen can not fully close the mold and build clamp tonnage. If the mold is larger than 800 mm, then the mold will not fit between the two platens when the moving mold platen is fully open.

Even if the mold fits in the molding machine, the molding machine may still not be operable with the mold. For instance, the injection unit of the molding machine must have sufficient shot volume and provide enough melt pressure to fill the mold cavity with the polymer melt.

On the other hand, if the injection unit has too large a shot size, then the melt may degrade in the barrel of the molding machine. For the Battenfeld HM320, the maximum shot volume is 490 cc. To provide melt homogeneity without degradation, this machine is ideally suited for molds requiring a shot volume between 120 cc and 250 cc.

The molding machine must also provide sufficient clamp tonnage to hold the two halves of the mold together when pressurizing the polymer melt. For this machine, the clamp tonnage is 3200 kN which is equal to 326 metric tons, 360 English tons, or 720,000 pounds. If the molding machine does not provide sufficient clamp tonnage, then the mold will open during operation and the melt will flow across the parting plane and shut-offs. If the molding machine provides too much tonnage to a very undersized mold, however, the mold may be damaged by the imposed compressive stresses.

Figure 4.24: Minimum and maximum daylight (dimensions in mm)

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4.3.4 Mold Base Suppliers

The development of standardized mold base designs is considered a significant advance in the history of the plastics molding industry. A majority of mold makers in the U.S. use standard mold bases to reduce the time and expense of creating molds. Furthermore, mold maintenance is simplified through the availability of standard mold components that are replaceable at the molder’s facility.

It is noted that many mold makers do not use mold bases for various reasons. Mold bases for very large parts, such as automobile body panels, may not be available as a standard product and so may require custom design and manufacture. Some mold makers believe that standard mold bases are inferior in quality, and strive to provide a better mold with higher quality or lower lifetime cost through the development of custom designs with proprietary components.

At the other extreme, some mold makers can produce a simple but fully functional mold for less cost than just the standard mold base could be purchased in the United States.

There are numerous mold base suppliers from which mold bases and mold components can be purchased. When selecting a mold base, the mold designer should consider:

The range of mold base sizes and materials that are available. Not only should mold bases of varying plate lengths, widths, and heights be available, but these mold bases should also be available in different types of materials.

The portfolio of mold components that can also be purchased. The mold base supplier should be able to provide insert materials, ejector pins, cooling accessories, etc.

The native system of units that in which the mold base was designed and the quality of the associated drawings. If the mold designer prefers U.S. customary or metric units, the mold base drawings should reflect a compatible same system of units through the use of round numbers, fractions, etc. Drawings should fully detail the design of the various mold components and, when appropriate, document their customization and operation.

The inventory availability and delivery terms. Standard “quick ship” mold components should be in the supplier’s inventory. Customized mold bases with varying plate dimen-sions and material specifications should be custom manufactured and shipped within one week. Orders that are placed before noon should be shipped the same day and no later than the next day.

The quality of the supplied mold bases. All mold plates should be supplied finish ground, heat treated, and ready for machining at the mold maker. Guide pins, ejector pins, and other mold components should be finished, hardened, and/or coated as appropriate to ensure low wear.

The previous experience with the mold base supplier. If a company or mold designer has past favorable experiences with a supplier, then there may be risk or a significant learning curve associated with switching suppliers.

The pricing should be competitive with commodity material prices. Clearly the mold base supplier adds value to the raw materials included in the mold base, and is entitled to recover their costs and reasonable profit. Still, the mold designer must compare the

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strengths and weaknesses of various mold base suppliers to determine whether to sole source from one mold base supplier or choose from a few qualified suppliers.