Injection Molding Gate Types and Design

What is an Injection Molding Gate?

The injection molding gate is an opening designed to control molten plastic flow into the injection mold cavity under high pressure. The gate design purpose is let the molten plastic flow smooth entering mold cavity and ensure complete filling, and form the final shape of the plastic parts.

There are various types of injection molding gates, choosing the right gate is a vital factor in the success of plastic parts injection molding.

After the mold cavity is fully filled, the gate will solidify first as a restriction point and form a small mark on the part surface, which is called the gate mark. The gate mark needs post-processing or removal in some cases.

The injection molding gate is the boundary between the finished part and waste material. The gate location, size, and shape will have an obvious influence of finished part’s structural integrity, molding quality, and appearance.

The gate is usually designed smaller than the cross-section, it can enable controlled filling and prevent flash or overflow of excess material.

Types of Injection Molding Gate

Direct Gate

Direct gates have the advantages of short flow paths, rapid material feeding, and good molding quality. It is often applied in single cavity molds, suitable for low aesthetic demands and large products with deep cavities. Because the elevated stress is high near the gate, for shallow and flat products, they are more prone to warping and deformation due to shrinkage and residual stresses.

Edge Gate

Edge gate is the most basic type of injection molding gate, it is suitable for flat or thin-walled parts. It has a simple structure and a stable molding effect. The edge gate not only has the advantage of low manufacturing cost but also is convenient during mold modifications. Moreover, it is suitable for large parts or parts that can’t avoid a thick wall design. Due to the long gate freeze, it allows more plastic flow into the mold cavity and achieves longer holding pressure.

Submarine Gate / Tunnel Gate

Submarine gates (also known as Tunnel gates) are a common gate type, it usually need to be used with an ejector pin mechanism. When the part is ejected, there is an automatic trimming at the gate. Submarine gates or tunnel gates are typically located under the parting line. It is usually conical in shape, with the narrowest end connected to the part.

If the submarine gate size is too large, during the automatic shearing, there could be cracking or cosmetic defects. If the size is too small, it can lead to excessive shear heating and increase filling difficulty. To ensure the gate can separate without breaking the parts or leaving too large a gate mark, the gate size must be within the proper range.

Submarine gates provide good aesthetics, minimising visible gate marks, and are suitable for parts with complex geometries. But the successful application requires careful structural design and molding process analysis. The manufacturing cost and subsequent maintenance expense are higher.

Cashew Gate

The cashew gate is a variant of the submarine gate, it can achieve gating in the area of a standard submarine gate can’t reach. It is often located at the back of the product surface. As for the size restrictions, cashew gate has similar limitations to submarine gate.

Tab Gate

Tab gate can be used to reduce jetting or other gate-related issues by reducing melt velocity when it exits the gate. The melt will be separated between the main gate and the overflow area. This will reduce the folw front velocity as it enters the mold cavity from the gate, but there could be hesitation at the gate. Therefore, whether the gate is suitable or not, it needs to be determined by trial molding and practical experience.

Fan Gate

Fan gate is an injection molding gate with a triangle or fan opening. It can let molten plastic be distributed evenly and increase product molding quality and aesthetics. The disadvantages is it could leave an relatively obvious gate mark on part surface. Fan gate is usually applied when balanced filling and high demand for appearance.

Diaphragm Gate

The diaphragm gate is usually used for parts with a large opening, especially suitable for cylindrical parts that are open at both ends. When concentricity is a main concern, the diaphragm is usually applied. Even plastic flow is helpful for part shrinkage as uniformly as possible during the cooling. The advantages of the diaphragm gate include the elimination of weld lines, decreasing the risk of core deflection, and enabling an ideal pattern to minimize warpage by achieving predictable shrinkage.

Injection Molding Gate Design Considerations

Gate Location

The best injection molding gate should be located at the area with the maximum area, which is helpful for smooth melt flow and ensure fully filling of the mold cavity. Injection from the area with maximum wall thickness can have the best filling performance and holding pressure performance. When the holding pressure is not enough, thinner areas solidify more rapidly than thicker walls. Therefore, we should avoid setting the gate location in areas where the wall thickness rapidly changes to avoid molding defects such as flow stagnation or short shots.

When it is possible, it is recommended use a central gate injection. Placing the gate at the part center can form equal flow lengths, the flow length will affec required injection pressure directly. A central gate makes holding pressure more even in all directions and avoids uneven volumetric shrinkage. When locating the gate, we should reduce gate marks and weld lines on the surface of the finished parts. If it is not avoidable, we should optimize gate location to reduce the influence on part aesthetic and performance.

As for the gate direction, best practice is to locate the gate in front of a standing core on the mold, which can prevent gate defects and jetting, as it will let the molten plastic flow evenly.

Part Quality and Appearance

Gate location and design will affect the finished parts’ aesthetic and overall quality directly. When it is required to reduce or hide the gate marks, edge gates or submarine gates are typically preferred. In the case when gate visibility is not strictly demanded, a sprue gate or a fan gate could be more suitable.

Gate Size

The gate size must enable proper shearing during the injection molding process. Also, it should ensure cavity can be filled fully and correctly. For the basic function, the gate cross-section should be as small as possible and the length should be as short as possible. Only like this, the requirements such as melt flow velocity increase, rapid cooling and sealing, smooth part separation, and minimizing gate marks can be achieved. A small gate will form higher shear heating, but if the gate size is too small or too large, the flow pressure can be increased accidentally. Therefore, we should choose proper gate sizes to get the best injection molding performance.

Material Selection

Different plastic materials have their own unique flow characteristics and viscosity requirements. Some materials need a large or small gate size to achieve good flow. When choosing gate types and sizes, we should consider the material’s melt temperature, viscosity, and relevant processing characteristics.

Crystalline polymers (such as PP, PE, etc.): it is better to use a relatively large size gate, and relatively extend the holding pressure time.

High-viscosity materials (such as PC, PPS, etc.): need to enlarge the gate cross-sectional area to reduce the flow resistance.

Amorphous polymers (such as PS, PC, etc.): it is better not to use too large a gate size.

Shear-sensitive materials (such as PVC, POM, etc.): need to control the gate size strictly and avoid excessive shear.

Production Volume and Cycle Time

Gate design will affect production throughout and the molding cycle time. For massive production, it is better to use gate types that can provide rapid cavity filling and fast cooling. Besides, the gate should be easy to remove or require as little post-processing as possible, which is helpful to simplify the molding process and increase efficiency. 

We can help you with choosing the best gate types. By understanding the advantages and disadvantages of different injection molding gates, you can make sure the success of the plastic project, whether it is prototyping, low-volume production, or high-volume manufacturing.

Project Evidence: Gate Location Is an Approval Item

The de-identified DFM image identifies a proposed point-gate location and size. The decision connects the visible gate mark with filling, packing and downstream trimming requirements.

A gate selected for one part should not be copied to another without flow, resin, appearance and ejection review.