The Banana Gate: A Discreet and High-Precision Gating Solution for Injection Molds

In the world of high-precision plastic injection molding, the visibility of gate marks—the small scars left where molten plastic enters the mold cavity—can be the defining factor between a premium product and a rejected one. For consumer electronics like sleek smartphone casings or portable device housings, even the faintest blemish is unacceptable. While common gating methods like edge gates, submarine gates, or tunnel gates often leave noticeable marks on external surfaces, the Banana Gate (also known as a Horn Gate or Curved Tunnel Gate) offers an elegant, nearly invisible solution.

The Aesthetic Advantage: Hiding in Plain Sight

The core benefit of the banana gate is its ability to place the gate on the internal surface of a part. Unlike conventional methods that leave marks on visible exterior faces, the banana gate results in nothing more than a minuscule, almost imperceptible pinpoint mark on the interior. This makes it the go-to choice for manufacturing high-end, cosmetically sensitive products where flawless external appearance is paramount.

How It Works: Engineering the Invisible Entry

The magic—and complexity—of the banana gate lies in its unique two-piece insert design and a curved runner system that facilitates clean ejection.

1.The Two-Piece Insert: The system requires two precisely machined inserts, let's call them Insert I and Insert II. Each insert is carved with half of a curved, banana-shaped runner channel. When assembled, these halves form a complete, smooth, conical tunnel that guides the plastic from the main runner into the cavity at an internal angle.

2.The Ejection Mechanism: The real innovation is in the demolding process. The banana-shaped runner itself is not rigidly fixed. During ejection, as the part is pushed out, the curved runner flexes and deforms. This elastic deformation allows the entire "banana," along with the solidified sprue, to be pulled out of its curved channel without shearing or getting stuck. The gate is cleanly torn from the part at its delicate tip.

3.Critical Design Nuance – The "Free Length": A crucial element for success is providing a specific free length in the secondary runner—the distance between the root of the banana curve and the nearest ejector pin. This length is not arbitrary; it is the essential space that allows for the necessary elastic deformation of the runner during ejection. Neglecting this can lead to a stuck runner and a failed cycle.

Precision in Every Detail: Materials and Finish

Given the significant ejection forces involved in pulling the curved runner, every aspect of the banana gate must be engineered for minimal friction and maximum durability.

• Material Selection: The inserts are typically made from high-grade mold steels like 738 or 718H, heat-treated to a robust hardness of HRC 53–55 to withstand wear. Alternatively, NAK80, a premium imported pre-hardened steel with a hardness of HRC 38–42, is often used for its excellent polishability and inherent toughness.

• The Mirror Finish: The interior surface of the banana runner channel is absolutely critical. It must be polished to an exceptionally high, near-mirror finish. Any surface roughness drastically increases friction, raising the risk of the runner galling, deforming permanently, or breaking during ejection. This polishing is a specialized art, ensuring the plastic can slide out with minimal resistance.