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What are Injection Mold Sprues: Types, Design, and Process Optimization

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Injection molding is a highly efficient method for manufacturing complex plastic parts. It involves injecting molten plastic into a mold, which then cools and solidifies to form the desired shape. The injection mold sprues act as the channel for plastic to enter the mold cavities. However, the placement and design of the sprue can greatly affect the finished part’s quality and the molding process efficiency. Let’s delve into sprues in injection molding and important considerations for their design and use.

What are Injection Mold Sprues?

The injection mold sprue is the first place where the plastic melt enters the injection mold cavity. Specifically, the sprue is the entry point of the molten plastic, the vertical passage which plastic then flows through into runners and gates. It introduces the molten plastic that is injected from the injection molding machine nozzle into the runner or the injection mold cavity. Its shape is conical, which facilitates the smooth flow of the molten plastic to the injection mold cavity. forward flow. The size of the injection mold sprue directly affects the flow rate of the molten plastic and the mold filling time.


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Is Injection Mold Sprue Necessary?

Yes, injection mold sprues are necessary. Next, we will explain why it is necessary.

Sprues and Their Functions:

Guide Molten Plastic Distribution: They act as the pathways that guide the molten plastic material from the injection unit of the machine into the mold cavities. This ensures that the plastic fills every detail of the mold.


Help to Balanced Filling: Sprues contribute to even distribution, preventing issues like short shots (incomplete filling) and minimizing variations in part quality.


Maintain Consistent Pressure Control: They help maintain consistent pressure during the injection phase, which is vital for ensuring a defect-free final product.


Waste Reduction: Efficient sprue design minimizes material waste, as they are often the only parts of the plastic components that are discarded.

Impact on Quality:

Without sprues, mold cavities might not be filled uniformly. That will lead to defects, weak spots, or incomplete components.

Types of Injection Mold Sprues

There are two primary types of injection mold sprues: Cold Sprue and Hot Sprue in Injection Molding.

Cold Sprue in Injection Molding:

A cold sprue is a type of injection mold sprue where the sprue and runner system remain at or near room temperature during the injection process. The molten plastic is injected into the mold cavity through a channel that is not preheated. The plastic cools rapidly as it enters the mold cavity.


Advantages of Cold Sprue:

Simplicity: Cold sprue systems are straightforward to design and implement.

Lower material cost: Since there is no need for a heated sprue, it can lead to cost savings.


Disadvantages of Cold Sprue:

Cooling effect: Rapid cooling can sometimes lead to cosmetic defects in the molded parts.

Limited to certain materials: Not suitable for all types of plastics, particularly those that require precise temperature control.

Hot Sprue in Injection Molding:

A hot sprue involves a heated sprue and runner system, where the sprue remains at a higher temperature during the injection process. That temperature makes the plastic material remain molten as it travels through the sprue, ensuring a more controlled and uniform flow into the mold cavity.


Advantages of Hot Sprue:

The elevated temperature in the sprue can help prevent issues like flow lines and cosmetic defects. That hot sprue is suitable for a wider range of materials and complex molding processes.


Disadvantages of Hot Sprue:

Increased complexity: Designing and using hot sprue systems can be more complex and costly. Increased Energy consumption: Requires additional energy to maintain the elevated sprue temperature.

Choose the Right Injection Mold Sprue for Your Project

The choice between cold and hot sprue systems depends on the specific requirements of the molding process, the type of material used, and the desired part quality.


Cold sprues are simpler and cost-effective but may be less suitable for materials that require precise temperature control.


Hot sprues provide more control over the flow of molten plastic and are versatile but come with added complexity and energy consumption. Selecting the appropriate sprue type is a critical decision in optimizing the injection molding process.

Injection Mold Sprue Design Standards

Injection mold sprue design standards are essential to ensure that the sprue functions optimally and that the overall injection molding process is efficient and produces high-quality parts. Here are some key design standards and best practices for injection mold sprues:

Tapered Design:

The injection mold sprue is designed to be tapered or conical. This shape helps to reduce shear stress and pressure drop while allowing for smoother material flow.

Shorter Length of Injection Mold Sprue

Under the condition of ensuring that the plastic parts are well formed, the length of the injection mold sprue should be as short as possible. Otherwise, the condensate in the sprue will increase, and the pressure loss will increase, which will cause the molten plastic to cool down fast and affect the injection molding.

Proper Injection Mold Sprue Radius

The big end of the injection mold sprue is rounded, and its radius is r=1~3mm to reduce the resistance of the flow velocity turning transition.

Closely Connected with the Nozzle

In order for the molten plastic to completely enter the injection mold sprue without overflowing, the sprue should be closely connected to the nozzle of the injection machine, and the joint of the sprue should be designed into a hemispherical pit.

Material Compatibility

Ensure that the sprue material is compatible with the molding material to prevent contamination or undesirable interactions.

Suitable Sprue Location

Typically, the sprue should be set at the thickest section of the part, where it can effectively fill the cavity and minimize the risk of defects such as voids, sink marks, and warpages.


In conclusion, injection mold sprues have a direct impact on the quality, efficiency, and success of manufacturing complex plastic parts. Understanding the importance of sprues, their functions, and their impact on the final product’s quality is vital for manufacturers and engineers. By choosing the right sprue type, and setting the proper location, size, length, and shape, the manufacturer can optimize the mold design and deliver high-quality plastic parts. We have introduced the other components of the injection mold design and provided valuable insight into their function, design principles, and optimization techniques. Stay tuned and learn more about how these components work together to achieve a successful injection molding process.


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FAQ about Injection Mold Design

What are the Differences Between Sprues, Runners, and Gates?


The differences between sprues, runners, and gates lie in their roles and locations within the injection molding process. Sprues are the initial channels connecting the machine to the mold, runners distribute material within the mold, and gates control the entry points into individual cavities.