What is the parting line in injection molding?

- Sep 14, 2017-

Parting line  – Tampere University of Technology A parting line is the border in which draft angles change direction (See image below). It is also the line, which splits the part into core and cavity halves. Parting line can be a starting point for the mould parting surface, but not necessarily. Image 1. A parting line, pulling direction and the part core and cavity sides. The part is generously drafted. The part designer defines the parting: the parting line, the shape of a parting surface and the corners, which can be round and which can not be round. The tool designer follows the shapes and finds the pre‐defined parting line from the model. This division of duties is important to pay attention to. If the parting is not well done, the tool designer contacts the part designer either directly or through the injection mould‐ ing company or high pressure die casting foundry. The tool designer negotiates changes together with the casting or moulding company, but it is the part designer or the part designing company, who has to make the modifications.   The images in the next pages show some examples how drafting influences the part‐ ing line and the shapes in the mould or die cavity and core sides. The shape is rather similar, but the tool costs vary significantly. The tool costs are explained next to each image. As a part designer you should learn how to use the CAD software draft analysis tools. Make the draft analysis every now and then and see if there is anything to fix. It might be very difficult and slow to start repairing the mistakes only in the final phase of the design process. The parting line and parting surface should be the starting point. Grow the part below and above the parting with drafted (not orthogonal) surfaces and try to figure the cavity shapes while designing. Change the way of thinking: You are not designing holes and cavities to the part. Instead of these you are designing cored surfaces with a draft and selecting from which side of the tool these cores are extend‐ ing.   The cavity filling has to be considered when selecting the drafting directions and the place for parting. High pressure die castings are always filled from the parting line. In injection mouldings the place for filling can vary. See the texts about cold runner systems and hot runner systems in injection moulding.