Selection of parting surface is not only related to molding and demoulding of plastic part, 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 main general principles for selection of parting surfaces:
1) Guarantee quality of plastic parts. This is the most basic one, and quality of plastic parts must meet predetermined requirements.
2) Facilitate demoulding of plastic parts. It is easy to demould, can improve productivity, plastic parts are not easy to be deformed, and improve rate of genuine products.
3) Simplify mold structure. For same plastic part, because of different parting surface selection, complexity of structure is very different. A reasonable choice 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 movable and fixed molds respectively. Due to various shapes of plastic parts, there are many changes in selection of parting surfaces. In order to give everyone a basic understanding of selection of parting surfaces, following introduces some typical parting surface options.

1) Classification of long members, as shown in Figure 1. If length of plastic parts is long, such as tube-shaped, cylindrical, rod-shaped plastic parts, if cavity is placed in a mold plate, demolding angle will be too large (Figure a), if cavity is arranged separately In movable and fixed molds (Figure b), demolding angle can be reduced, so that size difference between two ends of plastic part will not be too large.

 

Figure 1-Classification of long members
a) Parting with too large draft angle b) Parting with lower draft angle
2) Parting that leaves plastic part on movable mold side, as shown in Figure 2. By leaving plastic part on the side of movable mold, it is easy to set up and manufacture a simple demoulding mechanism. Therefore, try to keep plastic parts on the side of movable mold. For plastic parts with lids, it is more reasonable to select parting surface according to Figure a; for plastic parts with inserts, because insert will not shrink and pack core tightly, parting surface can be selected according to Figure b; if cores are symmetrically distributed, should be divided according to figure c to force plastic part to stay on movable mould; if it is a plastic part with side holes, it should be divided according to figure d to avoid core pulling of fixed mold.

 

Figure 2-Parting with plastic parts left on the side of movable mold
a) Lid plastic parts b) Plastic parts with inserts c) Symmetric core plastic parts d) Plastic parts with side holes
3) Classification to ensure appearance quality of plastic parts, as shown in Figure 3. Appearance quality of plastic parts needs to be guaranteed. Consider carefully when choosing parting surfaces. For example, you should try not to set parting surfaces on a smooth surface or an arc surface. Figure a is a parting to keep surface smooth, Figure b is a part that can reduce flash, and Figure c is a part that can reduce flash.

 

Figure 3- Classification to ensure appearance quality of plastic parts
a) Parting to make surface smooth b) Parting to reduce flash c) Parting to reduce flash
4) Conducive to classification of exhaust, as shown in Figure 4. In injection molds, parting surface is often used as an exhaust channel. In order to smooth exhaust, parting surface should be set at the end of melt stream, and it should not be blocked at the end. Structure of Figure b and d is more reasonable than Figure a and c.

 

Figure 4- Classification that is beneficial to exhaust

5) Parting to ensure coaxiality, as shown in Figure 5. Many plastic parts have coaxiality requirements. This requirement should be guaranteed in mold design. Generally, parts with coaxiality requirements in plastic parts should be designed in same movable template to meet accuracy requirements. Figure a can meet coaxiality requirements, while mold clamping in Figure b is not accurate, and it is difficult to meet coaxiality requirements.

 

Figure 5-Parting to ensure coaxiality
a) Parting that guarantees coaxiality b) Parting that is difficult to guarantee coaxiality
1 a moving mold 2-a fixed mold

6) Classification when there is a side hole, as shown in Figure 6. When selecting parting surface, try to avoid using side core pulling or parting. If necessary, refer to following principles. Figure a sets side core on movable mold to facilitate core pulling, and if it is set on the fixed mold, it is more difficult to pull core; Figure c is to place long core-pulling distance in mold opening direction, and place core-pulling distance small in lateral direction, which is more reasonable. If it is typed according to figure d, it will bring difficulties to demolding.

 

Figure 6-Parting with side holes
1—moving mold 2—fixed mold

7) Parting with guaranteed accuracy, as shown in Figure 7. There are some surface precision requirements on plastic parts, such as threaded surfaces, mating surfaces, etc. At this time, parting surface cannot pass through these surfaces, otherwise it will affect precision of plastic parts and even be difficult to use. Parting as shown in Figure 3 guarantees accuracy of thread. Parting in Figure b passes through thread axis and cannot guarantee thread accuracy; parting in Figure c ensures integrity and smoothness of circular surface, while d has an obvious seam line around its periphery, which affects appearance and use.

 

Figure 7-Classification to ensure accuracy
a) Parting to ensure thread accuracy b) Parting to ensure thread accuracy
c) Parting to ensure smooth appearance of plastic parts d) Parting to ensure smooth appearance of plastic parts
8) Parting of thin-walled plastic parts with uniform wall thickness, as shown in Figure 8. To make wall thickness uniform, do not use usual plane as parting surface, as shown in Figure b but take tapered stepped parting surface shown in Figure a.

 

Figure 8-Parting of thin-walled plastic parts requiring uniform wall thickness
a) Conical step parting surface b) Plane parting surface
Selection of parting surface is more complicated and involves many factors. Above various introductions can be used as an important reference for parting selection. In actual operation, it is often necessary to choose flexibly according to specific conditions.