Direction of parting surface should ideally be perpendicular to mold opening direction of injection molding machine. Shape elements include planes, inclined planes, curved surfaces, and stepped surfaces. When selecting location of parting surface, it should not only be chosen in a position that does not affect appearance of plastic part, but also ensure easy removal or trimming of any flash or burrs generated at parting surface. Efforts should be made to avoid burrs at parting surface to facilitate demolding. Parting surface should not affect dimensional accuracy of plastic part; difference in size between large and small ends of plastic part caused by draft angle should be minimized. Parting surface should allow injection mold to be divided into easily machinable parts to reduce machining difficulty.
To remove product from mold, mold must be divided into two parts: moving mold side and fixed mold side. Contact surface between these two parts is called parting surface. It serves functions of mold separation and venting, but due to differences in mold precision and molding, burrs are easily generated at parting surface, affecting product's appearance and precision. When selecting a parting surface, pay attention to following:
Parting surface should generally not be located on decorative outer surfaces or rounded corners, and should not be in a prominent position that would affect product's appearance; During mold opening, product should remain on the side with demolding mechanism, generally on moving mold side, to facilitate demolding; Selection of parting surface should ensure good manufacturability of mold parts; For products with high concentricity requirements, design cavities on same side as much as possible, and place concentric portions with high concentricity requirements on same side of parting surface to ensure concentricity; To avoid long-distance core pulling, consider placing longer axial dimension of plastic part in moving mold opening direction. If core pulling is necessary, core pulling mechanism should be located on moving mold side as much as possible; Generally, avoid parting at rounded sections to avoid affecting product's appearance; For products with high height and small draft angles, parting can be done in the middle, with cavity divided into two sides to facilitate demolding.
When determining parting line, following factors should also be considered:
Conduct a detailed study and analysis of product's shape, dimensions, and wall thickness to first determine optimal mold opening direction; Understand properties and moldability of plastic, as well as layout of gating system; Pay attention to venting and demolding, simplify mold structure, make mold operation convenient, and facilitate part machining.
Although a flat parting line is simple, misalignment can easily occur between cavity and core. For precision molds, a precise positioning (tiger's mouth) must be designed, as shown in Figure 4-34.

 

Figure 4-34 Planar parting surface anti-misalignment
A sloped parting line cannot be simply designed as a slope. Figure 4-35(a) shows a poor design, lacking a reference plane for machining and inspection. It should be designed as shown in Figure 4-35(b).

 

Figure 4-35 Sloping parting surface
For a sloped parting line, due to larger force on one side of cavity, injection eccentricity occurs on both sides of cavity, leading to a tendency for relative slippage between fixed mold and moving mold. If eccentric force is small, it can be supported by guide pillars. However, if eccentric force is too large, it will cause excessive wear of guide pillars and guide bushings, resulting in dimensional deviations in plastic part. Following methods can be used to solve this problem:
Create a stop on one side of cavity, as shown in Figure 4-36(a). This can counteract injection eccentric force and maintain relative position between cavity and core. Arrange two cavities symmetrically, as shown in Figure 4-36(b). This balances injection forces on both sides of cavity and makes mold structure more compact.

 

Figure 4-36 Key Design Considerations for Sloping Parting Surface
When parting surface is curved, curved surface should be extended to increase contact area of curved surface during mold closing, see Figure 4-37.) Design of curved parting surfaces should facilitate fitting by mold makers. Chamfers can be designed on non-sealing areas, or moving and fixed molds can be used to create clearance in non-sealing areas. This involves chamfering on raised side of moving and fixed molds, creating an R-angle on recessed side, or creating R-angles on both sides, but with different radii. R-angle on raised side should be 1-2mm larger than that on recessed side. When designing molds, especially for 3D parting surfaces, attention must be paid to tool diameter (see Figure 4-38). Distance T should be greater than tool diameter. Larger diameter tools have larger feed rates, significantly improving machining efficiency. When curved surface is steep or too deep, a larger diameter tool is required. Therefore, R-radius at parting surface junction should not be too small. A larger R-radius facilitates tool machining and also benefits mold maker's final fitting.

 

Figure 4-37 Example of extending parting surface of a curved surface

 

Figure 4-38 Influence of tool diameter on machining
For molds with curved parting surfaces, a reference plane for parting surface must be designed. Typically, when designing parting surfaces, factors such as appearance and structure of plastic part often result in a combination of one or more parting surfaces with varying heights, such as steps or curved surfaces. In such cases, a reference plane must be established to facilitate mold fitting and data monitoring during machining.
For cylindrical plastic parts, parting surface must be located on maximum diameter of tube, inevitably resulting in sharp corners. Sharp corners cause draft angle at parting surface to be zero, making part prone to sticking to mold. Since sealing sharp corners can easily produce burrs, affecting roundness and rotational performance of plastic shaft parts, some modifications to parting surface can be made, as shown in Figure 4-39.

 

Figure 4-39 Parting surface of cylindrical structural elements
For curved parting surfaces, parting surface must be made smooth during 3D mold parting to facilitate CNC machining and also to aid in mold fitting.
Table 4-1 summarizes various common parting surface design problems. Left side shows correct design method, and right side shows incorrect design method.
Table 4-1 Parting surface design diagram