Injection Mold Venting System

What is an Injection Mold Venting System and Its Design Principle?

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The injection mold venting system is a very important part of the injection molding process. Although the design of the venting system is not complicated, it plays a very important role in the normal injection molding of the mold. They ensure the smooth flow of molten material into the mold and the expulsion of air or gases. Understanding their significance and proper design principles is crucial for manufacturing high-quality molded parts.

Why is an Injection Mold Venting System Important?

When injection molding starts, the melted plastic flows into the mold cavity at the gate, and the air in the cavity can no longer be sent out, turning into “trapped air”. When the melted plastic continues to flow in, the air is compressed under pressure. When the air is continuously compressed, the air temperature and back pressure will continue to increase. The air temperature rises high enough to scorch the plastic. Furthermore, no matter how much pressure is applied, the trapped air cannot be compressed to zero. In other words, no matter how much injection pressure the injection molding machine increases, it will never be able to fill the entire cavity, making the size of the product inaccurate. This is not allowed for products with high precision requirements. And not only the air in the cavity but also the water vapor coming out of the plastic material, the volatile gases decomposed in the melted plastic, etc., will all have an impact on the product. So all designs should try to eliminate it and solve it.

What are the Common Issues Without a Proper Venting System?

Improper venting system will lead to various issues in injection molding, affecting both the quality and integrity of the molded parts.

Here are some of the typical problems with inadequate venting:

Air Traps:

Trapped air within the mold cavity. That will cause the formation of air pockets in the molded part, leading to surface defects and compromised structural integrity.

Burn Marks:

Incomplete venting causing overheating. That will cause visible burn marks on the molded part, affecting its appearance and potentially weakening the material.

Visible Weld Lines:

Insufficient venting around obstacles in the mold. That will cause the formation of visible lines on the molded part where molten material rejoins, affecting the part’s aesthetics and strength.

Insufficient Filling:

Air entrapment prevents the complete filling of the mold cavity. That will cause uneven distribution of material, leading to incomplete parts, voids, or variations in thickness.

Types of Injection Mold Venting Systems

Open Venting Grooves on Parting Surfaces:

Opening venting grooves on the parting surfaces of the front and rear molds is the most commonly used method for exhausting air.

Almost every set of molds must use this venting method because this venting method has the largest venting volume. It is also the easiest and most effective.

Each set of molds has a parting surface, and this exhaust method is accomplished by slotting between the front and rear panel inserts.

Venting Through Ejector Pins:

Ejector pins, which are used to eject the molded part from the mold, can also serve as channels for venting. They create openings through which trapped air can escape.

It is a conventional exhaust design. Utilizes existing components (ejector pins) for venting, reducing the need for additional features in the mold design.

Venting Through Insert Mating Surface:

In this approach, venting channels are integrated into the mating surfaces of mold inserts. These channels allow air to be expelled as the mold closes and molten material fills the cavity. Effective venting without compromising the external appearance of the molded part.

Venting with Craft Inserts:

Craft inserts, which are removable components in the mold, can be designed with built-in venting features. These craft inserts are strategically placed to enhance venting in specific areas of the mold. The design of process inserts is a venting design method usually used when the product structure is special, the exhaust requirements are high during injection molding, and the product size requirements are high.

The Venting System Design Principles

The Venting Groove Can Only Allow Air Out and Keep Melt Flow In:

Venting grooves should be designed to facilitate the release of air or gases from the mold cavity during the injection process. Simultaneously, they must prevent the escape of molten material to ensure proper filling and formation of the molded part.

Design Different Depth Venting Grooves for Different Plastics:

Different types of plastics have varied properties, including melt viscosity and flow characteristics. Tailoring venting grooves to the specific properties of the plastic being used ensures optimal venting and helps prevent issues such as incomplete filling or surface defects.

Plastic Material

Vent Depth Range (in mm)









PET, PBT, Polyesters








High Impact Polystyrene(HIPS)






T/P Elastomer


Design Venting Grooves for Mold Cavity, Runner System, and Cold Slug Well:

Not only the cavity design with venting grooves but also the runner system and cold slug well should be designed with the venting grooves to promote uniform air evacuation throughout the molding process.

The Venting Grooves Should Extend to the Mold Base:

To enhance the overall efficiency of the venting system, extend venting grooves to the mold base, especially when venting through an insert, venting through an insert venting pin, or venting insert.

Try to Use a Milling Machine to Process the Venting Grooves:

Utilizing a milling machine for processing venting grooves. After processing, polish it with 320 sandpaper to remove the knife marks. tye to avoid using a grinder to process the venting groove. The too-smooth surface leads to a poor venting effect.

Key Considerations of Injection Molding Venting System Design Guide

Number of Vents:

Consideration: Determine the appropriate number of vents based on the size and complexity of the mold, as well as the volume of the molded part.

Importance: Having an adequate number of vents helps ensure efficient air evacuation, reducing the risk of air traps and other related defects.

Positioning of Vents:

Consideration: Carefully position vents in areas where air is likely to be trapped during the injection process. Consider the geometry of the part and the mold, identifying locations prone to air entrapment.

Importance: Properly positioned vents allow for effective air escape, minimizing the likelihood of defects such as burn marks and incomplete filling.

Melt Viscosity:

Consideration: Consider the melt viscosity of the plastic being used in the injection molding process. Different plastics have varying flow properties, and the venting system design should accommodate these differences.

Importance: Tailoring the venting system to the specific melt viscosity helps ensure consistent filling, preventing issues such as uneven distribution and incomplete parts.


In conclusion, an injection mold venting system is a critical part of the injection molding process. Addressing common issues through proper venting system design principles and considering key factors in the design guide contributes to the production of high-quality molded parts. Manufacturers should venting system optimization for improved overall efficiency and part quality.

Zhongde is an experienced on-demand manufacturer. We offer custom injection molding services, compression molding services, CNC machining, mold and tooling production, etc. If you are looking for a reliable partner for your project, welcome to upload your file and make your design come true.

Except for the injection mold venting system, there are other important parts about injection mold in the injection molding process. Welcome to our previous blogs about injection mold design.

Hot Runner System vs Cold Runner System

Injection Molding Slider Design

Injection Molding Lifter Design

Injection Molding Gate Design

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