By definition, an injection molding gate is an orifice through which the molten plastic is injected into the mold. The type and size of the gate plays a very significant role in the process of injection molding.

Gate Types

There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates. Manually trimmed gates require an operator to separate the parts from the runners manually after each cycle. Manually trimmed gates are chosen for several reasons:

•  The gate is too bulky to be automatically sheared by the machine

•  Shear-sensitive materials such as PVC cannot be exposed to high shear rates

•  Flow distribution for certain designs that require simultaneous flow distribution across a wide front   Automatically trimmed gates incorporate features in the tool to break or shear the gates when the tool opens to eject the part. Automatically trimmed gates are used for several reasons:

•  Avoiding gate removal as a secondary operation, reducing cost

•  Maintaining consistent cycle times for all parts

•  Minimizing gate scars on parts

Common Gate Designs

The largest factor to consider when choosing the proper gate type for your application is the gate design. There are many different gate designs available based on the size and shape of your part.

Below are three of the most popular gate designs used by Quickparts’ customers:

The Edge Gate is the most common gate design. As the name indicates, this gate is located on the edge of the part and is best suited for flat parts. Edge gates are ideal for medium and thick sections and can be used on multicavity two plate tools. This gate will leave a scar at the parting line.

The Sub Gate is the only automatically trimmed gate on the list. Ejector pins will be necessary for automatic trimming of this gate. Sub gates are quite common and have several variations, such as a banana gate or tunnel gate. The sub gate allows you to gate away from the parting line, giving you more flexibility to place the gate at an optimum location on the part. This gate leaves a pin sized scar on the part.

The Hot Tip Gate is the most common of all hot runner gates. Hot tip gates are typically located at the top of the part rather than on the parting line and are ideal for round or conical shapes where uniform flow is necessary. This gate leaves a small raised nub on the surface of the part. Hot tip gates are only used with hot runner molding systems. This means that, unlike cold runner systems, the plastic is injected into the mold through a heated nozzle and then cooled to the proper thickness and shape in the mold.

The Direct or Sprue Gate is a manually trimmed gate that is used for single cavity molds of large cylindrical parts that require symmetrical filling. Direct gates are the easiest to design and have low cost and maintenance requirements. Direct gated parts are typically lower stressed and provide high strength. This gate leaves a large scar on the part at the point of contact.