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

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Figure 4.13: Insert height allowance

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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

4.2 Cavity and Core Insert Creation

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.

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Figure 4.15: Core and cavity inserts for cup

Figure 4.16: Core and cavity inserts for bezel

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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.