June 2006 Design Tip

Parting without Sorrow

Here’s something you probably don’t think about much: parting lines. And why should you? After all, it’s your job to design the part and ours at Protomold to design a mould that will successfully make the part you designed. Right?

Well, sort of. In fact, understanding the mould design and manufacturing process can help you design more effective parts. Take for example the part shown below. There are two ways in which it can be designed which will result in two very different mould designs because of where the parting surfaces of the mould need to be located. (Note the different locations of the parting lines shown in red in the two examples.)

Figure 1

In the example on the left (Fig. 1), the walls of the part are formed entirely in the bottom half of the mould (the walls are “ribs”). To allow ejection, they have to be drafted, slanting the inner wall outward and the outer wall in. This results in a thicker wall at the base of the part. In addition, when cutting tools have to reach deep into a narrow cut, the cut may have to be widened to allow the tool to operate, further thickening the part wall.

In the example on the right, the walls are formed between the core in the top half of the mould and the cavity in the bottom half of the mould (the part has “core/cavity” construction). The benefits are that the walls can be drafted but be constant thickness, and the mould is easier to make because larger cutters can be used.

As a rule, if any area is to be thickened to provide draft, the thickest section will be at the parting line. In the case above, moving the parting line completely eliminated the need to thicken walls. The following example demonstrates an additional option.

Figure 2

Figure 3

Figure 4

Now, instead of designing a part with one closed end, we are designing a cylinder with two
open ends.

The first example (Fig. 2) is like the “rib” designed part with surfaces drafted toward one another. The parting line is at the bottom, the wall at one end of the cylinder has to be significantly thicker than the wall at the other end, and the mould requires potentially problematic deep narrow cuts.

The second example (Fig. 3) is very similar to the “core/cavity” part. The main parting line is at the bottom of the diagram, the inner parting line is at the top. The flare would provide draft for the mould core coming out of the cylinder and for the cylinder coming out of the upper mould cavity.

The final example (Fig. 4) is not like either of the parts. Because it is open at both ends, cores can extend from both mould-halves, placing the parting lines halfway up the inside and outside of the cylinder. The depth of the cylinder’s sidewall is split between the two mould halves, so each of the narrow cuts is only half as deep, making them easier to mill. And while the draft angle remains the same, the reduced depth of the cut into each mould half reduces the amount of wall thickening needed to produce the required draft. Note again that the thickest part of the wall coincides with the location of the parting lines.

The core-cavity version has distinct advantages both in mould-making and wall thickness, but there may be times that your design requirements make one of the other choices preferable. If you need rib construction, additional wall thickness may be unavoidable. But knowing the impact of parting line placement gives you more options in designing your part and may allow you to get what you need with less need to compromise.