Tunnel gates inject material below the parting line. The runner system snaps off as the part is ejected from the mold, eliminating the need for manual removal. This makes this type of gate ideal for very large volumes.

Post gates inject the material from the backside of the cavity, hiding the small imperfection left from breaking the other gate types. These gates are used for parts that require excellent visual appearance.

Hot tips are directly connected to the spur and inject plastic from the top side of the part. No material is wasted this way on the runner system making them ideal for large scale production, but a dimple will be visible at the injection point.

The vestige

At the point where the runner system connected with the part, a small imperfection is usually visible, called the vestige.

If the presence of the vestige is not desirable for aesthetic purposes, then in can also be "hidden" in the functional B-side of the part.

The clamping and ejection system

On the far side of an injection molding machine is the clamping system. The clamping system has a dual purpose: it keeps the 2 parts of the mold tightly shut during injection and it pushes the part out of the mold after it opens.

After the part is ejected, it falls onto a conveyor belt or a bucket for storage and the cycle starts over again.

Alignment of the different moving parts of the mold is never perfect though. This causes the creation of 2 common imperfections that are visible on almost every injection molded part:

Parting lines which are visible on the side of a part where the 2 halves of the mold meet. They are caused by tiny misalignments and the slightly rounded edges of the mold.

Ejector (or witness) marks

which are visible on the hidden B-side of the part. They are created because the ejector pins are slightly protruding above or indented below the surface of the mold.

The image below shows the mold used to manufacture both sides of the casing for a remote controller. Quick quiz: try to locate the core (A-side), the cavity (B-side), the runner system, the ejector pins, the side-action core and the air vents on this mold.

Benefits and limitations of injection molding

Injection molding is an established manufacturing technology with a long history, but it's constantly being refined and improved with new technological advancements.

Below is a quick rundown of the key advantages and disadvantages of injection molding to help you understand whether it's the right solution for your application.