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Cold runner injection mould’s gate types
Bernát Apatóczki
Introduction
The function of the gate is to establish a connection between the runner and the mould cavity. During the injection phase, the material in its thermoplastic (molten) state flows through the gate into the cavity, filling it. After the cavity is filled, during the packing (holding pressure) phase, additional material is fed through the gate to compensate for volumetric shrinkage until the melt “freezes” (solidifies) at the gate. Beyond this point, the processes occurring inside the cavity can no longer be influenced.
When moulding large parts, multiple gate locations are often applied on the same component; in many cases, even different gate types are used within a single part. In the following sections, we present the most commonly used types of cold runner gates.
Direct Sprue
The first and simplest gating method is direct sprue gating with a cold runner. In this case, it can hardly be considered a gate in the strict sense, as the runner (sprue) is directly connected to the part. It is typically used in single-cavity moulds for large parts with relatively low quality requirements. This type of gating is a preferred solution for prototype tooling due to its low cost and ease of implementation. The runner is removed in secondary (post-processing) operation.
Standard Gate
The next gate type is the standard (edge) gate. It is located at the edge of the part and is typically used in two-plate moulds. Its design can vary in several ways, and it is simple and cost-effective to manufacture. It is generally applied for moulding thicker-walled parts with relatively low surface or aesthetic requirements. The gate removal requires a secondary (post-processing) operation.
Fan Gate
The third type discussed is the fan gate. It differs from the standard (edge) gate in that it introduces the material into the mould cavity over a wider area. It is typically used for moulding two-dimensional, thin-walled parts. In the case of fiber-reinforced materials, it improves fiber orientation. Its machining is simple and cost-effective. Similar to the standard gate, it is used in two-plate mould designs, and its removal is performed as a secondary operation. A subtype is the film gate, where the gate thickness is only a few tenths of a millimeter. The use of film gates is not recommended for fiber-reinforced materials, as the fibers may break due to high shear, or may clog the gate.
Overlap Gate
The next gate type is the overlap (tab) gate. In this case, a simple auxiliary feature (tab) is essentially added to the part, and the gate is placed on this feature (typically a standard gate, but it can also be a submarine gate or banana gate - see later). The main advantage of this solution is that <strongresidual stresses generated around the gate are not transferred into the final part, as they remain in the added tab feature.</strong This type is <strongcommonly used for optical components</strong, where residual stresses near the gate could cause light refraction. It is also applied in parts subjected to high mechanical loads, where residual stress in the final product is not acceptable.
From a manufacturing standpoint, this gate type is easy to machine; however, the removal of the added tab can be difficult and complex. It is advisable to position the tab in a concealed area of the part after assembly, so that only the runner needs to be removed. It is typically used in two-plate mould designs.
Umbrella / Spoke Gate
For gating rotationally symmetric parts or features, umbrella gates or spoke (radial) gates can be applied. Their main advantage is that they preserve the circularity of the product. Due to the uniform filling, parts exhibit little to no ovalization. In the case of an umbrella gate, the cavity is filled uniformly over the entire circumference. In contrast, with a spoke (radial) gate, the melt enters the cavity at multiple points along the circumference. The greater the number of gates used, the lower the risk of an oval-shaped final part.
The umbrella gate has a simple design; however, gate removal can be difficult and may require special tooling. The spoke gate can also be simple in design - for example, when it is not self-degating and uses standard gates to connect to the surface - but submarine gates may also be applied (see later). In such cases, the mould may need to open in multiple directions (multi-parting or multi-action tooling).
Submarine Gate
In two-plate moulds, the first type of gate that does not require post-processing removal from the part - that is, a self-degating gate - is called a submarine gate. There are several design variations of this gate, but in all cases, the gate separates from the part either during mould opening or ejection. Its design is more complex than that of the previously discussed gate types, which also makes it more difficult to repair. It is recommended to place the gate in a separate mould insert.
Submarine gates are generally used in two-plate mould constructions, but they can also be implemented in sliders.
Banana Gate
The next type of self-degating gate, which typically appears in two-plate mould constructions, is the banana gate. In Anglo-Saxon literature, it is sometimes referred to as a cashew gate or a whip gate. It is generally positioned at the bottom of the part, in an area that is not visible. Its design is complex, usually implemented as a unit composed of two insert halves, although some standard manufacturers offer it as a monolithic block. It is not suitable for materials that are too brittle or too elastic due to performance limitations.
Pin Gate
The last gate type discussed in our article is the pin gate. The gate is located on the top surface of the part and separates during mould opening, making it another type of self-degating gate. It is used in three-plate moulds, typically (but not necessarily) in multi-cavity tooling. Its design is simpler than that of tunnel or banana gates, but repair can be difficult, so it is recommended to place it in a separate mould insert.
Summary
Based on the above, it can be concluded that selecting the appropriate gate when designing an injection mould is by no means a simple task, as numerous factors must be considered. The challenge is further increased because it is not only necessary to choose the right gate type but also to determine the correct position, geometry, and dimensions. To simplify this process, most standard component manufacturers now offer ready-made solutions. Self-degating gate types are typically available in pre-inserted form. Using these can speed up design time, and their easy replaceability allows for straightforward handling of any necessary modifications.
In addition to experience and literature, most designers today have access to simulation software, which aids in making informed decisions. The use of such tools has become almost indispensable, especially for complex part geometries, as a few iterations can lead to an acceptable solution while eliminating options that are clearly unsuitable.
This table summarizes the key characteristics of the gate types listed above.
Gate Type | Location | Design Complexity | Removal Method |
Direct Sprue | Top Surface | Simple | Post-processing |
Standard Gate | Side | Simple | Post-processing |
Fan Gate | Side | Simple | Post-processing |
Overlap Gate | Side | Simple | Post-processing |
Umbrella / Spoke Gate | Top Surface | Simple | Post-processing / Mould Opening / Ejection |
Submarine Gate | Side | Complex | Mould Opening / Ejection |
Banana Gate | Bottom | Most Complex | Ejection |
Pin Gate | Top Surface | Complex | Mould Opening |
