October 2010 Design Tip



Keeping Your Part in Line

When you are developing a 3D CAD model to be injection moulded, you may not spend much time thinking about where its parting line will be, but it’s worth keeping in mind as the location can affect your part in several ways. On some parts the location for the parting line is obviously right down the middle, while for more complex parts it may not be so obvious. Take, for example, a simple cup. The outer face is formed by one mould half (A-side), while the cup’s inner surface and brim will be formed by the other mould half (B-side). The parting line occurs along the outside edge of the brim of the cup (see Figure 1).

Figure 1

Figure 1


For other designs, the parting line location is not so obvious. These tend to be “free-form” shapes with soft edges. An example would be the familiar green moulded-plastic toy soldiers. Most are designed to be injection moulded in two-part, straight pull moulds, and if you look carefully, you can see the seam around each figure where the two halves of the mould meet (see Figure 2).

Figure 2: The highlighted “seam” is a result of the parting line (the location where the two halves of the mould meet). A slight mismatch in the mould will leave a raised seam (flash).

Figure 2: The highlighted “seam” is a result of the parting line (the location where the two halves of the mould meet). A slight mismatch in the mould will leave a raised seam (flash).


When a design is a complex shape, determining where the parting line is can be a lot more complicated than for the cup mentioned earlier. You can assume that if you send an undrafted 3D CAD model of such a part, Protomold will determine where the parting line should be. However, in designing such a part, you might want to think about the parting line anyway for one simple reason: designers and moulders look at parts differently. Moulders share your interest in producing the best possible part, but the focus is on moulding it correctly, whereas designers focus on how it will function after it comes out of the mould. The location of the parting line can affect both.


First of all, the location of the parting line determines the direction of mould opening and, consequently, the direction in which features must be drafted for easy ejection. Second, it affects where any vestiges left by the mating surfaces of the mould halves will be, and potentially, how those vestiges will look. Third, parting line location can also impact the cost of mould making and the type and cost of secondary operations needed to finish the part.


The plastic toy soldier is an obvious example of a design with a challenging parting line, but the issue can come up on simpler parts as well. For example, a straightforward geometric design with radiused or rounded edges can also be problematic. The parting line has to trace the path along which a tangent to the surface is parallel to the direction of mould opening. In Figure 3, the parting line has been placed across an otherwise smooth surface. Any mismatch in mould edges will create a fairly obvious seam, so a parting line in this location would create a need for tighter tolerances and potentially increase milling costs. The increased likelihood of flash could also impact both the cosmetics and functionality of the part, potentially making assembly of finished parts more difficult. If we instead place the parting line along a sharp edge, any potential seam would be camouflaged, and we would avoid the undesired manufacturing, functionality, and cosmetics issues outlined above (see Figure 4).

Figure 3: Both mould halves form the parting line, so any mismatch in the mould will leave a seam, changing the shape of the part at the parting line. We recommend placing the parting line on a sharp edge.

Figure 3: Both mould halves form the parting line, so any mismatch in the mould will leave a seam, changing the shape of the part at the parting line. We recommend placing the parting line on a sharp edge.

Figure 4: Both mould halves meet to form the sharp edge of the part.

Figure 4: Both mould halves meet to form the sharp edge of the part.


There are several ways to address parting line challenges. Simple awareness of the significance of the parting line is a good start. As stated earlier, on many parts, that location is obvious and not a problem. For more complex parts, you may be able to use tools within some CAD packages to locate and evaluate split lines. Or, upload an undrafted 3D CAD model and Protomold will propose a parting line and suggest appropriate draft based on that orientation. (Note: our software may also point out that the part cannot be made in a straight pull mould and that design changes, side actions and/or pickouts may be required.)


Whether you get your information from a CAD package or from Protomold, keep in mind that the suggested parting line may not be your only option. Neither your CAD program nor Protomold’s design analysis software knows how you intend to use the part. Look carefully at the suggested parting line and consider whether its location will work both cosmetically and functionally. If not, there may be other options for your existing design, or you may want to change the design to allow a more suitable parting line for your application. If you need assistance, you’ll find it at +44 (0) 1952 607447, where Proto Labs Customer Service Engineers are standing by to help.