What is a Venting System?

Injection molds are categorized as cavity molds, where a significant amount of gas is present within the gating system and the mold cavity. These gases include air, moisture from the plastic that turns into gas due to high temperatures, and gases resulting from the decomposition of the plastic and its additives under high temperatures. The venting system is a structural feature designed to promptly expel gases from the cavity and gating system during the injection molding process and to introduce gases during mold opening and part ejection, preventing a vacuum from forming between the molded part and the cavity walls.

Specifically, the venting system in an injection mold includes the following two aspects:

During the injection molding process, the venting system expels air from the mold cavity and gating system, as well as low-molecular-weight volatile gases produced by the plastic due to heating or solidification.
For large products, the venting system is designed to prevent the product from sticking to the fixed mold or deforming during ejection.

Areas in the Mold Prone to Gas Trapping

Thin-walled cavity structures, at the end of melt flow.
Thick-walled cavity structures where air is easily entrapped, forming bubbles, posing challenges for venting design.
Junctions where two or more melt flows meet, often leading to weld lines or short shots due to poor venting.
Ends of melt flow in the cavity.
Bottom of blind holes in the cavity, often at the ends of solid pillars in the product.
Bases of reinforcing ribs and screw pillars in the molded product.
Parting lines of the mold.

Consequences of Inadequate Venting

During the design phase of an injection mold, the design of the venting system should be given sufficient attention. If gases in the mold cavity are not promptly expelled, it can severely affect the quality of the molded parts and the injection molding cycle.

Surface Defects: Flow marks, gas streaks, and seams can form on the product surface, making the surface
Filling Difficulties or Local Flashing: If gases cannot be expelled in time, the injection pressure must be increased, leading to the cavity being forced open and causing flashing.
Local Carbonization and Burning: Compressed gases during melt filling can generate high temperatures, causing local carbonization and burning at trapped

Any gas follows the principle: Pressure × Volume = Constant.

If gases in the cavity have no escape route, as the volume is compressed, the pressure and resistance to melt flow increase. Compressed air concentrates its enthalpy in a small volume, causing a significant temperature rise, sometimes reaching several hundred degrees Celsius, which can burn the leading edge of the melt.

4. Gas Entrapment Forming Bubbles: Especially in thick sections, leading to a porous structure and reduced strength. If air in the gating system and cavity is not expelled in time, bubbles often form in the runners or thick sections. Bubbles from decomposed gases usually distribute along the wall thickness, while those from moisture are randomly distributed on the product.

5. High Residual Stress and Poor Welding: Inadequate venting leads to decreased injection speed, rapid cooling of the melt, increased injection pressure, higher residual stress, and increased warpage risk. Raising the material temperature to reduce injection pressure requires very high temperatures, which can cause plastic degradation.

6. Reduced Filling Speed and Extended Cycle Time: Severe cases can result in short shots. Proper venting allows higher injection speeds, better filling and packing, without excessively increasing barrel and nozzle temperatures, significantly improving product quality.

If the above issues cannot be resolved by adjusting injection parameters, the venting system design is likely inadequate. Properly designed venting slots can eliminate defects, reduce injection pressure, injection time, packing time, and clamping force, making molding easier, improving production efficiency, reducing costs, and lowering energy consumption.

Venting slots are crucial for thin-walled parts and areas far from the gate. They are also important for small or parts, as they prevent surface burns, short shots, and other defects, reducing mold contamination.

For molds with minor venting issues, that can be taken after trial molding, such as adding venting slots in areas with filling issues or burns. However, for severe venting problems, even adding venting inserts may not help. For large products, if the venting system is overlooked during design, will be difficult after trial molding.

In fast and precision injection molding, the design of the venting system plays a pivotal role.