Parting surface is surface from which product is removed from mold. Choosing parting surface is the first step in mold design, influenced by many factors such as product's shape, appearance, wall thickness, dimensional accuracy, and number of cavities. For most products, determining large parting surface is generally straightforward. However, for products with side core pulling, or those involving inserts, bumpers, or through-holes, choice becomes more complex. Therefore, let's discuss how to select parting surface.

 

1) General Principles for Parting Surface Selection
Selection of parting surface not only affects molding, demolding of plastic part but also involves mold structure and manufacturing costs. Therefore, it is crucial to pay attention to selection of parting surface. Generally, there are three main principles for selecting parting surface:
1) Ensure quality of plastic part. This is the most basic principle; quality of plastic part must meet predetermined requirements.
2) Facilitates demolding of plastic parts. Easy demolding increases productivity, prevents deformation of plastic parts, and improves yield rate.
3) Simplifies mold structure. Same plastic part can have vastly different levels of structural complexity due to different choices of parting surfaces. A reasonable selection can simplify mold structure.

 

(2) How to Choose a Parting Surface
Relationship between cavity and mold can be basically divided into three categories: cavity is entirely within moving mold; cavity is entirely within fixed mold; cavity is located in both moving and fixed molds. Since plastic parts come in many shapes, choice of parting surface varies greatly. To provide a basic understanding of parting surface selection, some typical parting surface choices are introduced below.
1) Parting of long components, as shown in Figure 1. If plastic part is long, such as a tubular, cylindrical, or rod-shaped part, placing cavity in a single mold plate will result in an excessively large draft angle (Figure a). Arranging cavities separately in moving and fixed molds (Figure b) can reduce draft angle, prevent excessive dimensional differences between two ends of plastic part.

 

Figure 1 - Parting line for long components
a) Parting line with excessively large draft angle b) Parting line with reduced draft angle
2) Parting line with plastic part on moving mold side, as shown in Figure 2. Keeping plastic part on moving mold side makes it easier to set up, manufacture a simpler demolding mechanism. Therefore, plastic part should be kept on moving mold side as much as possible. For cap-shaped plastic parts, parting line selection shown in Figure a is more reasonable; for plastic parts with inserts, since inserts will not shrink and will tightly enclose core, parting line selection can be shown in Figure b; if core of plastic part is symmetrically distributed, it should be as shown in Figure 1. Parting lines force plastic part to remain on moving mold. For plastic parts with side holes, parting should be done according to Figure d to avoid core pulling from fixed mold.

 

Figure 2 - Parting line that leaves plastic part on moving mold side
a) Cap-shaped plastic part b) Plastic part with inserts c) Plastic part with symmetrical core d) Plastic part with side holes
3) Parting lines that ensure appearance quality of plastic part, as shown in Figure 3. Appearance quality of plastic part needs to be guaranteed. Careful consideration should be given when selecting parting surface. For example, parting surfaces should be avoided as much as possible on smooth surfaces or curved surfaces. Figure a shows a parting line that maintains a smooth curved surface, Figure b shows a parting line that reduces flash, and Figure c shows a parting line that reduces overflow.
4) Parting lines that facilitate venting, as shown in Figure 4. In injection molds, parting surface is often used as a venting channel. To ensure smooth venting, parting surface should be located at the end of melt flow, and care should be taken that there should be no obstruction at its end. Structures in Figures b and d are more reasonable than those in Figures a and c.

 

Figure 4 - Parting lines that facilitate venting
5) Parting lines that ensure coaxiality, as shown in Figure 5. Many plastic parts have coaxiality requirements. This requirement should be ensured in mold design. Generally, parts of plastic part with coaxiality requirements should be designed within same moving mold plate to meet accuracy requirements. Figure a meets coaxiality requirement, while Figure b has inaccurate mold closing and cannot meet coaxiality requirement.

 

Figure 5 - Parting lines ensuring coaxiality
a) Parting lines ensuring coaxiality b) Parting lines where coaxiality is difficult to ensure
1 - Moving mold 2 - Fixed mold
6) Parting lines with side holes, as shown in Figure 6. When selecting parting surface, side core pulling or parting lines should be avoided as much as possible. If necessary, following principles can be referred to: Figure a places side core on moving mold, which is convenient for core pulling, while placing it on fixed mold makes core pulling more difficult; Figure c places core pulling distance with longer distance in mold opening direction and core pulling distance with shorter distance in side direction, which is more reasonable. However, if parting line is as shown in Figure d, it will make demolding difficult.

 

Figure 6 - Parting lines with side holes
1 - Moving mold 2 - Fixed mold
7) Parting lines ensuring accuracy, as shown in Figure 7. Some plastic parts require high surface precision, such as threaded surfaces and mating surfaces. In these cases, parting surface cannot pass through these surfaces, otherwise it will affect precision of plastic part and may even make it unusable. As shown in Figure 3, parting surface ensures thread precision; parting surface in Figure b passes through thread axis, thus failing to guarantee thread precision. Parting surface in Figure c ensures integrity and smoothness of circular surface, while parting surface d will have a noticeable seam line around its perimeter, affecting appearance and usability.

 

Figure 7 - Parting Surface Guaranteeing Precision
a) Parting surface guaranteeing thread precision b) Parting surface not guaranteeing thread precision c) Parting surface guaranteeing a smooth appearance d) Parting surface not guaranteeing a smooth appearance
8) Parting surface for thin-walled plastic parts requiring uniform wall thickness, as shown in Figure 8. To achieve uniform wall thickness, a conical stepped parting surface, as shown in Figure a, is not used as usual flat surface, as in Figure b.

 

Figure 8 - Parting line for thin-walled plastic parts requiring uniform wall thickness
a) Conical stepped parting line b) Planar parting line
Selection of parting line is quite complex and involves many factors. Above introductions can serve as important references when selecting parting line. In actual operation, selection should often be made flexibly according to specific situation.