Mold Parting Line and Gate in Plastic Injection Mold

In Injection Molding (or almost in any type of molding), one of the most basic concepts is Parting line. A Mould is divided into two parts (more for complex designs) so that it comes apart in order to remove the product and reuse it again and again. These two parts are, namely, Core and Cavity. While doing injection molding the cavity needs to be placed inside the core before pouring the molten plastic in. The line where these two parts meet is called the mold parting line. The component produced thus has a single continuous line around its perimeter. This line is usually visible naked eye as a slight discontinuity on the surface of the element after it is removed from the mold.

In Injection molding, one of the other most important factors, while manufacturing a component is the mold gate. It is the part through which molten plastic flows to the final piece. After removing the mold, the gates are usually trimmed which leaves a mark on the surface of the manufactured component called the gate mark. Different types of gates are used to make the part desirably. For example- Point gate, Banana Gate, Submarine gate, Tab gate, Fan gate, Side gate, and Direct (Sprue) gate. All types have different pros and cons that affect the manufacturing process as well as costs significantly.

These are the two types of irregularities that appear on the surface of a component manufactured by the injection molding process. In order to separate the element from the mold easily, any part of the element must not have any indentation or protrusion that prohibits part ejection from an image. This type of intrusion is called an undercut feature. Any overhanging features must also be avoided during the process although these intrusions are used in processes like overmolding or double shot injection molding. In order to prevent these intrusions and to remove the part easily from the mold, a draft angle is needed. The draft angle is defined relative to the parting line. If these measures are not taken while the mold manufacturing process, more complex types of moulds can be made, but this adds significantly to the cost of manufacturing.

In all, Parting lines are one of the main aesthetic problems that can happen with Injection molded plastic parts. In order to have an aesthetically pleasing design which is robust at the same time, one needs to take care of the parting line as it affects how fast the molten plastic cools and solidifies as well as how many layers it forms which affects the friction coefficient of the part.
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