Understanding Gate Design in Injection Molding

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Injection molding  constitutes the obtaining of mold products by injecting molded materials  molten by heat into a mold, cooling and then solidifying it. During this process the molten plastic goes through several channels called sprues  and runners until it reaches a gate, an opening serving as the entry point of molten material into the mold cavity.

There are different types of gates in the injection molding process mold  depending on the part located and the materials being used. Gates can take different forms, some are wide, narrow, tapered and others maintain a defined diameter.

In this article, we will explore  the various gate designs, roles and types of gates in injection molding and considerations to note when choosing a gate design.

Gate Design 2

Anatomy of a Mold Design (Image source: Pinterest)

The Role of Gates in Injection Molding

In understanding the different modes of gate design we will first have to understand the roles the gates play in injection molding. Gates are located between the runner system and mold cavity. The runner system delivers molten plastic from the injection unit to the mold cavity. 

Injection molding gates are important as they allow the molten plastic to flow into the cavity. These gates control pressure, flow and distribution of materials used to ensure uniformity and the end products meet design measurements and precision. Proper gate design and location placement is crucial in achieving high quality products and optimal molding results.

Gates play a significant role in part packing, cosmetic appearance and structural integrity. Let’s dive in and see how gates have an impact on them.

1. Part Packing

The gates regulate the flow of molten material into the mold cavity affecting how the materials fill and pack in the cavity. Proper packaging ensures that the cavity is filled with materials appropriately avoiding inaccuracies, sink marks and distortion in the final product.

2. Cosmetic Appearance 

During injection molding the gate leaves a vestige on the surface of the molded part. The appearance of this gate mark is determined by the shape, size, and location of the gate. The desired gate design should be one that reduces the visible appearance of these gate vestige ensuring a smooth surface of the end product.

3. Structural integrity

Proper gate design results in uniform material distribution and packaging in the cavity leading to the end product having consistent density and strength. This ensures the final product is of high integrity and mechanical properties.

Types of Gates in Injection Molding

In injection molding there are various types of gates that are used in different scenarios to achieve the design requirement of the mold. Below we are going to look into the manually trimmed gates and automatically trimmed gates.

Manually Trimmed Gates

Gates that are manually trimmed are where employees spit the molded parts after every production cycle from the runners. These gates bring in controlled production and flexibility.They also bring in additional labor costs and increased production downtime.

Manually trimmed gates include:

  • Tab gates
  • Fan gates 
  • Direct gates
  • Disc gates
  • Ring  gates
  • Spoke gates
  • Edge gates 

Automatically Trimmed Gates

These automatically trimmed gates are designed to separate from the molded part without need for manual trimming. With automation of gate trimming there is reduced labor cost, uniformity and precision is maintained on the parts and overall desired  quality of the product is achieved.

Thee automatically trimmed gates include

  • Hot tip gates
  • Sub gates
  • Pin gates
  • Tunnel 

Manually Trimmed Gates

Here we are  going into a detailed description of different types of  manually trimmed gates and their operations and their advantages.

We have seen that manually trimmed gates are where the molded part is separated manually by workers. Below are the different types of manually trimmed gates and where they are suited for. 

Direct Gates

This type of gate is suited for single cavity molds and non aesthetic parts.They are designed to allow efficient and direct flow of molten material into the cavity.

Disc Gates

These gates are circular in shape, they resemble small disks.They are suitable for parts that require cosmetic appearance precision since they leave minimal markson the surface. The advantage of using disc gates is that they ensure uniform material distribution enhancing quality and part appearance.

Fan Gates

These gates expand outward in the form of a fan through the direction of the mold cavity maintaining consistent width. They help establish stable flow into large parts and are suitable for parts with complex geometry.

Gate Design 3

Fan Gate Design (Image source: Pinterest)

Edge Gates

They are preferred for their simple and effective use in part molding in the injection molding process.They are easily flexible and customizable according to the design requirement. They are filled from the side and suited for production of  flat parts.

Advantages of Manually Trimmed Gates 

  • Flexibility in gate design: Manually trimmed gates offer flexibility in that they can easily be customized according to the parts design requirement.
  • Enhanced quality control: Through use of manually trimmed gates quality of the mold parts can be enhanced. During the trimming process operators can identify any defect and address it.
  • Suitable for low cost production: In a scenario where the parts to be produced are in low volume manually trimmed gates are suited ensuring cost effectiveness.

Gate Removal and Potential Impact on Part Quality

Gate removal is a crucial step in injection molding and can impact part quality. You should consider several factors before degating. These factors include gate location, design and trimming technique. 

Where the gate is placed on the molded part affects how it is removed and how it affects the part quality. Gates that are situated in arrears where they have minimal gate marks are preferred. In terms of design, the design affects how it is removed and the simplicity of the removal process and how this will impact the part quality.

The trimming technique also matters a lot. Automated trimming offers consistency in operations and accuracy in achieving part requirements. On the other hand, manual trimming offers controlled gate removal but can lead to variations in gate marks.

Automatically Trimmed Gates 

We have seen that automatically trimmed gates have automated features that are added so that mold parts are separated from the mold without the need for manual input. Some of the automatically trimming gates include:

  • Sub gates: This molding gate is located beneath the parts surface so as to reduce gate marks on the surface of the parts.
  • Pin gates: This is a type of injection molding gate designed to use a small diameter pin that breaks off cleanly as the part separates from the mold. This helps reduce material waste and efficiency in production.

These gates offer various benefits including reduced labor costs, increased production efficiency, increased quality and consistency, reduced production downtime and cost effectiveness.

Automatically trimmed gates minimize cosmetic defects. These gates do so by promoting uniform dimension and precision. This ensures that the measurement and accuracy of the mold parts are maintained throughout the injection molding process. 

They also reduce gate vestige whereby marks left behind after injection molding are minimized through the use of automatic trimming for smooth part surfaces. 

There is controlled gate design and placement which is aimed at ensuring a clean separation from part. Quality is maintained when using automatically trimming gates leading to no production of substandard parts 

Gate Design Considerations 

The injection mode gate design is very important in making sure part quality and productivity is achieved. The gate design can be the determinant of producing great molds or defected molds. 

By putting these considerations into gate design optimal production can be achieved. The following are the several consideration for gate design:

Gate Size 

The size measurements of the gate has a great influence on material flow, pressure distribution and gate removal. Gates should have a standard size that ensures effective flow of the materials into the cavity. The sizes of gates vary according to the design specification of the parts. 

Gates that are extremely large increase the chance of dimension variation and inaccuracy. Proper size of the gate ensures uniform filling and packing of  the mold cavity.

Gate Location

The location of the gate in injection molding is important in making sure proper filling and packing of the mold cavity. The gate should be situated at the widest and uniform areas of the part to ensure efficient material flow. Improper gate position can result in uneven material flow which can lead to production of  distorted or defected parts.

Material Used

Properties of materials that are used should be put into consideration. These include the viscosity rate. Gates should be designed to meet the flow characteristics of the molten materials. Some materials are more prone to dimensional changes under high stress resulting  in inaccurate parts measurements.

Gate Design 1

Injection molding material (Image source: Pinterest)

Gate Type 

When designing a gate there needs to be a clear vision of what type of gate is needed and the application the gate will play in injection molding. The gate type depends on factors such as measurements and parts consideration.

Choosing the Right Gate Type

When selecting the right gate type it’s important to note the materials property, part design and production requirement. You can choose the right gate type in your injection molding process based on these factors.

Part Design 

Part measurements and dimensions determine  the type of gate choice you will use.parts that require complex measurements  require a gate type that meet these needs in terms of material flow to reduce defects on parts. Pin gates can be used to achieve these properties.

Cosmetic Properties

Parts that require smooth exterior surfaces on the final product require a gate type that can minimize visibility of these gate vestiges. Gates that can be used in this instance are edge gates, tunnel sub gates .

Production Requirements

For large volume production, it is important to consider the gate type that offers minimal material waste and effective material flow to optimize production. When selecting a gate type production costs such as labor and material used should be put into consideration. 

To increase production output the gate type that facilitates a fast run time in filling and packing of mold cavity should be considered to reduce cycle time. An example of such a gate type is edge gates.

Materials Properties 

For materials that have high rates of contractions, gate types that allow controlled flow and packing can be used to avoid part distortion. Materials that are highly sensitive to heat need a gate type that reduces heat exposure to reduce distortion. Materials with a high density need a specific type of gate that provides enough pressure to feel the mold cavity uniformly. 


Gate design plays a significant role in injection molding. Proper gate design impacts on uniformity in filling and packaging of the mold cavity which leads to production of high quality products with minimal defects. Each of the different types of gates, whether manually trimming or automatically trimming, is best suited for a particular use case. 

You need to put various factors such as material, product design, product size and aesthetical requirements when making a decision for your injection molding gate. Overall  gate design greatly influences part quality, efficiency and production cost. 


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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