Common Gate Designs in Injection Molding

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The gate design, number, and location impacts everything from cycle time, to aesthetics of the product, and the products’ structural integrity. This makes gates very important components in injection molding projects. 

That said, there are various types of gates that you can use in injection molding based on the process. These types include direct or Sprue Gates, edge gates, submarine gates, cashew gates, diaphragm gates, valve gates, thermal gates and fan gates. 

Careful selection of the design of your gates can improve resin flow, which in turn prevents weld lines and flow marks. Read on to learn more about the different gate designs and their distinct roles in injection molding

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Injection molding gate design (image source: Rapiddirect)

Edge Gates

Edge gate is the most famous gate design in injection molding because of its simplicity and effectiveness in the injection molding process. The gate is located at the edge of the part as the name indicates and is best suited for molding flat parts. This gate is ideal for medium and thick sections and can be used on multicavity two plated tools. 

An edge gate is easier to produce when building a mold. It can also be easily modified or customized if need be. The gate is used to fill larger parts or parts where thicker wall sections cannot be avoided. Unlike other gates, edge gates leave smaller marks on the part thus you can utilize them for thin-walled parts or parts with delicate features. 

There are two other variations of edge gates named fan gates and tab gates. Fan gates are located along the sides of a mold and are usually wider than the runner leading up to them. In contrast, tab gates have a consistent wall thickness with the runner.

Fan gates allow more molten plastic into the mold. This makes them suitable for molding large parts. For tab gates, their design makes it possible to absorb the shear stress in the gate as opposed to it being transferred to the part during molding. 

With edge gates, you can have larger cross-sectional areas compared to other gates. This design comes in handy to facilitate more plastic to flow into the mold.  The gates also have longer hold times because you can freeze the gate for long.

For edge gates, the circular shape of the runner tapers into the rectangular shape of the gate. This shape of the gate helps with plastic flow, dimensional stability, and may avoid some cosmetic issues.  

Edge gates are usually designed to minimize the material flow distance, resulting in faster cycle times and reduced waste. However, they can create knit lines or weld lines in the part where the flow fronts meet.

Sub Gates 

Sub gates or rather submarine gates, are the only automatically trimmed injection molding gates. These gates are usually machined below the parting line to ensure that the gate is trimmed automatically as the part is ejected. Normally, sub gates allow the molten polymer to flow into the cavity from the bottom, thus helping in reducing visible gate marks on the product. 

Sub gates are very common for injection molding where high cavities are used. They are also used in projects that require the gate to be trimmed automatically for high level aesthetic finish. These gates are also used for parts with thick walls and small parts like electronic housings. 

Sub gates are very common and are available in several variations such as tunnel gate, banana gate, and smiley gate. Let’s have a deeper look into the three variants!

1. Tunnel Gate

Tunnel gate is one of the most common variants of submarine gates. This gate is usually placed at the side of the part or on the movable parts of the injection mold. 

However, the gate can also be placed underneath the part, which is better than on the side because it doesn’t leave any marks on the surface. Tunnel gate design is used when there is a need to automatically cut the part as the mold opens. 

2. Banana gate

A banana gate is an injection molding gate that is shaped like a banana. This gate is a newer type of a sub gate that doesn’t leave any marks on the surface of the part. 

While this gate functions more like a tunnel gate, it is shaped in a manner that allows it to reach below the part and allow the plastic to be injected into hidden areas of the part. 

3. Smiley Gate 

A smiley gate is an injection mold gate that resembles a smile, hence the name. This sub gate variant is difficult to machine and is used right at the bottom of a plastic part. 

The gate is truncated or cut off at the bottom to allow it to be right at the bottom of the part. Smiley gates are used in high production applications where the part must break free from the gate for purposes of automation.

Basically, sub gates allow you to mold away from the parting line, giving you more flexibility to place the gate at an optimum location on the part. This allows for optimal product quality especially the ones that need to be aesthetically pleasing. 

Hot Tip Gates

Hot tip gates are small circular holes on the “A” side of the injection mold which allow the molten polymer to flow into the cavity. These gates derive their name from the thermal heater that is placed at the back of the mold in order to heat the polymer to temperatures that ease flow through the small gates. 

Hot tip gates can be thought of as direct extensions of the molding press barrel and screw. Because the resin is hotter at the point of injection, the opening can be smaller. Additionally, no runner is required thus the part can use virtually all of the allowable mold X-Y space. Hotter resin also means material may be pushed further into a thin cavity.

These gates are typically located at the top center of a part (as opposed to on the parting line) and are ideal for round and conical shapes where uniform flow can improve concentricity. However, the hot tip gate leaves a small raised nub on the surface of the part. 

Adding a hot tip dimple to the design may help shift the nub below the surface of the part, which might allow something like a decal to be applied over it with little or no need for trimming beforehand. 

Using a hot tip gate promises you top quality parts. This is because this type of gate minimizes defects like cold slugs, weld lines and short shots. The fact that the polymer is not remelted in every cycle also reduces wastage on material. This method can be used if you want to have efficient production with reduced cycle times. 

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Hot tips gate (image source: Sage Journals) 

There are several factors to consider when designing hot tip gates. These factors include type of material, part size and shape and flow characteristics of the polymer. Controlling the process properly by ensuring optimum pressure and temperatures  is vital for optimal performance hot tip gates. 

Direct or Sprue Gates

Direct gates or rather sprue gates are among the most common gates for injection molding projects. These types of gates are usually easy to design and ensure high tensile stress. 

Sprue gates are normally placed on the mold’s parting line, especially at the thickest area of the plastic. These gates are designed this way to allow the polymer to flow into the mold seamlessly and efficiently. This allows uniform filling of the cavity which further translates to high quality products.

Normally, sprues allow the polymer to move and melt directly into the cavity thus promoting quick injection of the polymer in huge volumes. These gates normally need short filling times and minimal injection pressures.

The fact that these gates are easy to design makes them suitable for economic injection molding. While they offer various benefits, sprue gates have a drawback because they leave marks on the end product. 

Additionally, direct gates must be removed manually by operators which further slows down the injection molding process. 

Direct gates are usually suited for box or shell molds which have a single deep cavity and are used to manufacture products that do not have to be aesthetically pleasing. These gates are used in injection molding projects for making items like bins, washing machine parts and TV parts. 

One of the main benefits of direct gates is that they allow production of parts with little or no gate lines. This is because lines on items can be easily minimized or concealed when performing post processing. Nonetheless, this might lead to aesthetic defects on the parts. This disadvantage is what makes it well suited for non aesthetic parts.

Sprue gates have a basic and conventional design that entails circular cross sections and a small taper. This makes the connection with the injection molded part have a large cross section. 

Choosing the Right Gate Design 

As an injection molding manufacturer, you know that gate design is a vital component in the injection molding project as gate designs determine factors such as costs, aesthetics and cycle times. 

With these holes, you can have them either narrow or wide to regulate flow of the molten polymer. Additionally, you can place them throughout the cavity or along the mold separation lines. 

Choosing the right gate design can be very daunting due to the wide selection of gates available to use in injection molds. However, a decision should be guided by various aspects such as material, part shape and size.

For optimal gate design, the size of the gate needs to be wide enough to enhance proper shearing when the polymer goes through the mold. The gate dimensions need to be optimal to allow correct filling of the cavity with the polymer. 

Additionally, small cross-sectional gates promote higher shearing rates. If the gates are too small or too large, they can increase pressure to enhance flow of the polymer. For optimal results on parts, you need to go for gates that are of the right size and shape. 

You need to choose a specific gate design based on the shape and the final output desired on the part. For instance cashew gates are perfect for molding small parts that require fine and smooth surface finishing. 

When dealing with complex injection molding projects, you need to take into consideration factors such as undercuts which can hinder free flow of gate design. This factor necessitates you to go for a gate shape that eases flow and minimizes part defects. 

For optimal product quality in terms of aesthetics, the gate should always be thinner towards the orifice. This helps in preventing hindering polymer flow which if it occurs can lead to distorted products due to flow and fill pressure imbalance. 

Gate Design and Mold Maintenance 

In injection molding, the quality of the final product is determined by various factors such as material and gate design. However, other factors such as mold maintenance also play a crucial role in ensuring that the mold is functioning properly. 

That said, different gates have different use cases. Simply, every gate design is used in injection molding of certain products based on requirements. For instance, edge gates do not require materials with specific shrinkage thus are a good choice when you plan to simplify your injection molding design. 

Injection is a very efficient manufacturing method. However, this method can be faulty if the mold is not maintained. Some of the defects that might arise due to poor maintenance include weld lines, sink marks, discoloration, and blisters. 

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Injection molding maintenance (image source: Pinterest) 

Normally, these defects can be prevented by having a preventive maintenance routine. The routine should involve cleaning, dusting off, greasing and lubricating. Preventive maintenance also makes the mold last longer, improves efficiency and reduces costs. 


Paying close attention to injection molding gate design can transform your injection molding project into tangible reality with perfect form, structure, and essence of your molded products. A thorough understanding of the product requirements and material properties will help determine the optimal gate type for specific products. 

This will further result in high-quality and consistent products that meet or exceed customer expectations. Overall, choosing the right gate design is essential for ensuring a successful injection molding process. 


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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