The Importance of Gate Design and Gate Size in Injection Molding

While injection molds tools are hard to design and manufacture, their core concepts are easy to understand. The mold itself consists of two faces, between which is the cavity and core set into which the plastic is injected. The plastic – which, like its namesake implies, is injected into the mold through a sprue that feeds the runner plastic enters the mold’s cavity through the gate. The gate is the literal gateway of plastic into a cavity/core and therefore can make or break a mold. Too small and the mold cannot fill completely; too large and the plastic is too hard to shear.

Gate Size, Location, and Number

There are a few factors to consider with gates: their locations, wall thickness, number, size, and design. While all will affect the product and process, the three most critical are the size, locations, and number.

Mold Gate Size

As mentioned above, the gate size is a measuring act between filling the mold exactly and keeping the smallest runner possible to improve shearing when the mold cycles in the machine. Time also plays a factor: molten resin must be able to reach all points of the mold before it starts to harden. While small gates may provide a nicer appearance to the part, they can take more time or pressure to fill, which can lead to molded in stress.

Mold Gate Location

The location of a gate can help with the above problem. Ideally, gates should be placed in the deepest cross-section to provide the best flow and minimize voids and sinking. The gate should also be on a piece of the mold where the stress and distortion of the runners and gate will not affect function or user aesthetics. Think about employing gates to reduce game sizes or dealing with thin places in the cavity that are acting as bottlenecks for resin.

Number of Mold Gates

Employing multiple gates can help with the above issues, especially for larger parts where one gate would not be enough. Think about employing multiple gates to reduce gate sizes or dealing with thin places in the cavity that are acting as bottlenecks for resin.

Three Common Gate Designs

Below are three of the most common gate designs. Besides differences in location and finished product, they also differ in how the final piece is trimmed (i.e., how the parts are separated from the runners).

• Edge Gate: A very common gate design, the edge gate is on the edge of the mold. Best used for flat parts, one or multiple gates are ideal for medium or thick parts that are along a single axis, leaving a scar along the parting line.
• Hot Tip Gate: For use with molds with hot runners (channels that keep the plastic heating within runner), a hot tip gate tends to be located at the top of the part. As such, they work ideally with radial parts such as round or conical shapes of uniform thickness.
• Sub Gate: Sometimes known as the submarine gate or one of the many variants (banana, tunnel, or smiley gate, to name a few), these gate types use pins to shear the runners off, simplifying gate removal and reducing scars, which can lead to overall lower costs.

Want to learn more about mold basics? Make sure to read our blog, 15 Plastic Injection Molding Top Key Terms, for more information on the terms here. If you’re ready to talk about tool design, part production, or mold troubleshooting, it’s time to contact Midstate Mold. With more than 100+ years of injection mold design, tooling, and part production experience, we can help you with every part of the process.