Selection of parting surface is not only related to molding and demolding of plastic parts, but also involves mold structure and manufacturing cost. Therefore, it is necessary to pay attention to selection of parting surface. Generally speaking, there are three general principles for parting surface selection:
1) Ensure quality of plastic parts. This is the most basic one. Quality of plastic parts must meet predetermined requirements.
2) Facilitate demolding of plastic parts. Easy demolding can increase productivity, prevent plastic parts from deformation, and improve quality rate.
3) Simplify mold structure. Same plastic part can have a very different degree of structural complexity due to different parting surface selections. Reasonable selection can simplify mold structure.

Relationship between cavity and mold can basically be divided into three categories: cavity is completely in movable mold; cavity is completely in fixed mold; cavity is in both movable and fixed molds. Due to wide variety of plastic part shapes, choice of parting surface varies greatly. To provide a basic understanding of parting surface selection, following describes some typical parting surface options.

1). Parting for Long Components, as shown in Figure 1. For long parts, such as tubes, columns, and rods, placing mold cavity in a single mold plate will result in an excessively large draft angle (Figure a). Separating mold cavity in movable and fixed molds (Figure b) can reduce draft angle and minimize dimensional difference between the two ends of part.

 

Figure 1 - Parting for Long Components
a) Parting with Excessive Draft Angle b) Parting with Reduced Draft Angle
2) Parting with Part Retained on Moving Mold Side, as shown in Figure 2. Leaving part on movable mold side makes it easier to set up and manufacture a simple demolding mechanism. Therefore, it's best to keep part on movable mold side whenever possible. For lid-shaped parts, parting surface shown in Figure a is more appropriate. For parts with inserts, since inserts will not shrink and wrap around core, parting surface shown in Figure b can be selected. If core of part is symmetrically distributed, parting should be performed according to Figure d, forcing part to remain on movable mold. For parts with side holes, parting should be performed according to Figure d to avoid core pulling from fixed mold.

 

Figure 2 - Parting that retains part on movable mold side
a) Lid-shaped part b) Part with insert c) Part with symmetrical core d) Part with side holes
3) Parting that ensures appearance quality of part, as shown in Figure 3. Appearance quality of part must be maintained, so careful consideration should be given when selecting parting surface. For example, on smooth or curved surfaces, a parting surface should be avoided as much as possible. Figure a shows a parting that maintains a smooth surface, Figure b shows a parting that reduces flash, and Figure c shows a parting that reduces flash.

 

Figure 3 - Partings that Ensure Part Surface Quality
a) Partings that Smooth Surfaces b) Partings that Reduce Flash c) Partings that Reduce Flash
4) Partings that Facilitate Venting, as shown in Figure 4. In injection molds, parting surface often serves as a venting channel. To ensure smooth venting, parting surface should be located at the end of melt flow, ensuring that there is no obstruction at the end. Structures in Figures b and d are more reasonable than those in Figures a and c.

 

Figure 4 - Partings that Facilitate Venting
5) Partings that Ensure Coaxiality, as shown in Figure 5. Many plastic parts require coaxiality, and this requirement should be maintained during mold design. Generally, parts of part requiring coaxiality should be designed within same moving platen to meet precision requirements. Figure a meets coaxiality requirement, while Figure b exhibits inaccurate mold closing, making it difficult to achieve coaxiality.

 

Figure 5 - Parting to Ensure Coaxiality
a) Parting to Ensure Coaxiality  b) Parting Where Coaxiality is Difficult to Ensure
1 - Moving Mold 2 - Fixed Mold
6) Parting with Side Holes, as shown in Figure 6. When selecting parting surface, avoid using side core pulling or parting. If necessary, refer to following principles. Figure a places side core on moving mold, making it easier to pull, while placing it on fixed mold makes it more difficult. Figure c places core with the longest pull distance in mold opening direction and core with shortest pull distance laterally, which is more reasonable. Parting according to Figure d will make demolding difficult.

 

Figure 6 - Parting with Side Holes
1 - Moving Mold 2 - Fixed Mold
7) Parting to Ensure Precision, as shown in Figure 7. Some surfaces on plastic part require high precision, such as threaded surfaces and mating surfaces. In this case, parting surface cannot pass through these surfaces, otherwise it will affect precision of part and even make it difficult to use. Parting pattern in Figure 3 ensures thread accuracy, while parting pattern in Figure b passes through thread axis, which cannot guarantee thread accuracy. Parting pattern in Figure c ensures a smooth and complete circular surface, while parting pattern in Figure d creates a noticeable seam around perimeter, affecting appearance and usability.

 

Figure 7 - Parting Patterns that Ensure Accuracy
a) Parting Patterns that Ensure Thread Accuracy; b) Parting Patterns that Do Not Ensure Thread Accuracy; c) Parting Patterns that Ensure Smooth Part Appearance; d) Parting Patterns that Do Not Ensure Smooth Part Appearance
8) Parting Patterns for Thin-Walled Parts Requiring Uniform Wall Thickness, as shown in Figure 8. To achieve uniform wall thickness, conventional flat parting surface, as shown in Figure b, is replaced by a tapered step parting surface, as shown in Figure a.

 

Figure 8 - Parting Patterns for Thin-Walled Parts Requiring Uniform Wall Thickness
a) Tapered Step Parting Surface b) Flat Parting Surface
Choosing a parting surface is complex and involves many factors. Above descriptions serve as important references when selecting a parting pattern. In practice, flexibility is often required based on specific situation.