Injection Mold Sprue: What You Need to Know

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This blog post will explain what an injection mold sprue is, why it is important, and how to design it properly. You will learn about the function, design, and location of the sprue, as well as the different types of sprues and their advantages and disadvantages.

What is an injection mold sprue?
What is the function of the sprue?
How to design an injection mold sprue?
What are the different types of sprues?
What are the best practices for sprue location?
Conclusion

What is an injection mold sprue?

An injection mold sprue is a passage that allows molten plastic to enter the mold cavity from the injection machine nozzle. It is usually a cylindrical channel that runs through the entire thickness of the mold (the A plate). The mouth of the sprue is sealed tightly to the nozzle with a sprue bushing.

The sprue is one of the components of the injection mold gating system, which also includes runners and gates. The gating system controls the flow and distribution of molten plastic into the mold cavity, and affects the quality and performance of the molded parts.

What is the function of the sprue?

The sprue plays a crucial role in the injection molding process as it serves as the primary pathway for delivering the molten plastic material from the injection molding machine’s nozzle into the mold cavity. When the molding process begins, the molten plastic is injected into the mold through the sprue, which acts as the entry point for the plastic material to flow and fill the mold cavities. The size and design of the sprue are carefully considered during the mold design phase to ensure proper flow of the plastic and efficient filling of the mold.

One of the main functions of the sprue is to control the flow rate and direction of the molten plastic material. The sprue is typically designed to have a larger cross-sectional area than the runner and mold cavities, which helps in regulating the flow rate of the plastic material as it enters the mold. This controlled flow ensures that the molten plastic fills the mold cavities evenly and uniformly, reducing the risk of defects such as flow marks, voids, or inconsistent part dimensions.

Additionally, the sprue acts as a reservoir for the molten plastic material during the injection process. When the injection molding machine’s screw pushes the molten plastic into the mold, any excess material that is not immediately required to fill the mold cavities accumulates in the sprue. This accumulation of excess material helps in maintaining a steady and continuous flow of plastic into the mold, preventing interruptions or inconsistencies in the molding process.

Furthermore, the sprue aids in the purging process during color or material changes. When switching to a different color or material, it is necessary to clear any residual plastic from the machine and mold. By purging the machine through the sprue, any remaining material is pushed out, allowing for a clean transition to the new color or material without contamination.

After the molding process is complete, the sprue is removed from the final molded part along with the runners as waste material. This is a standard practice to separate the finished parts from the leftover material, which is typically recycled or reused in the manufacturing process to reduce waste and material costs.

In summary, the sprue in injection molding plays a vital role in the efficient and controlled delivery of the molten plastic material into the mold cavity. It regulates the flow rate and direction of the plastic, ensures even and uniform filling of the mold cavities, and acts as a reservoir for excess material during the injection process. The sprue is an integral part of the injection mold design and helps in achieving high-quality and consistent molded parts, making it a fundamental component of the injection molding process.

How to design an injection mold sprue?

The design of the injection mold sprue affects the quality and efficiency of injection molding. Some of the factors to consider when designing a sprue are:

Diameter: The diameter of the sprue should be large enough to allow sufficient material flow into the mold cavity, but not too large to cause excessive pressure drop or material waste. A general rule of thumb is to make the diameter of the sprue 1.5 to 2 times larger than that of the runner.
Length: The length of the sprue should be as short as possible to minimize heat loss and material waste. However, it should not be too short to cause excessive shear stress or friction on the molten plastic. A general rule of thumb is to make the length of the sprue equal to or slightly less than its diameter.
Taper: The sprue should be tapered in the direction of the nozzle end, meaning that it should have a smaller diameter at the nozzle end than at the runner end. This helps to control material flow and prevent air bubbles. The taper angle should be between 3° and 5°.
Radius: The sprue should have a smooth radius at both ends, especially at the nozzle end. This helps to reduce stress concentration and prevent cracking or breaking of the sprue during ejection. The radius should be between 0.5 and 1 mm.

What are the different types of sprues?

There are two main types of sprues in injection molding: cold sprues and hot sprues.

Cold Sprues

Cold sprues are made of steel or other metals that have a lower thermal conductivity than plastic. They are attached to a cold runner system that also cools down and solidifies during injection molding. Cold sprues are cheaper and simpler than hot sprues, but they have some disadvantages:

They require more material and energy consumption, since they need to be filled with molten plastic every cycle.
They generate more scrap material, since they need to be trimmed off from the molded parts after ejection.
They can cause defects such as jetting, weld lines, or cold slugs due to inconsistent temperature or flow rate.

Hot Sprues

Hot sprues are made of steel or other metals that have a higher thermal conductivity than plastic. They are attached to a hot runner system that maintains a constant temperature and flow rate during injection molding. Hot sprues are more expensive and complex than cold sprues, but they have some advantages:

They reduce material and energy consumption, since they do not need to be filled with molten plastic every cycle.
They eliminate scrap material, since they do not need to be trimmed off from the molded parts after ejection.
They improve quality and performance of the molded parts, since they provide a uniform temperature and flow rate.

What are the best practices for sprue location?

The location of the sprue affects the filling and cooling of the mold cavity, and therefore the quality and performance of the molded parts. Some of the best practices for sprue location are:

Locate the sprue 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 warpage.
Locate the sprue at the center of the part, where it can provide a balanced and symmetrical filling and cooling pattern.
Locate the sprue away from any critical features or dimensions of the part, where it can avoid causing stress concentration or distortion.
Locate the sprue on a flat or slightly curved surface of the part, where it can easily be trimmed off or hidden.

Conclusion

An injection mold sprue is a passage that allows molten plastic to enter the mold cavity from the injection machine nozzle. It is an important component of the injection mold gating system, which controls the flow and distribution of molten plastic into the mold cavity. The design and location of the sprue affect the quality and efficiency of injection molding. There are two main types of sprues: cold sprues and hot sprues, each with its own advantages and disadvantages. By following some best practices for sprue design and location, you can optimize your injection molding process and produce high-quality parts.