How to Design a Mold Venting System to Improve the Surface Quality of Injection-Molded Parts?

How to Design a Mold Venting System to Improve the Surface Quality of Injection-Molded Parts?

Why is venting necessary? During the plastic filling process, the original air in the mold cavity, along with gases produced from the decomposition of plastic under high temperature, must be fully expelled. If these gases cannot be released quickly enough, they can affect product formation and ejection, leading to various defects such as trapped air. In severe cases, the gas may ignite under high temperature and pressure, causing localized carbonization and burn marks on the part.

1. Venting of the Mold Base

For example, in a three-plate mold, a nylon gas plug is used, and an air vent hole is opened on the A plate to ensure smooth mold closing. At the same time, a 6×0.5 mm venting area is often provided on the guide sleeve to facilitate proper mold alignment and closing.

2. Venting at the End of the Runner

3. Venting on the Parting Line

For ease of machining and cleaning, venting is generally placed on the cavity side (front mold insert) in most cases.

4. Venting Using Inserts

At locations where filling is difficult or at the final filling areas, venting should be provided on the insert. The venting must be directed to the mold base. The dimensions are as shown below:

5. Venting for Round Inserts

The venting for round inserts should be designed with the dimensions as shown in the figure below:

6. Venting Using Ejector Pins or Core Pins

In practice, this type of venting is often achieved by intentionally leaving a slightly looser fit during pin assembly, rather than deliberately grinding a step into the pin. This is because the outer diameter of the pin beyond its effective length is already clearance-sized. It's common to add a pin at the bottom of deep solid posts or ribs, serving dual purposes of venting and assisting in EDM flushing.

7. Venting for Large Shut-off Areas

In addition to the 8-12mm sealing area at the shut-off, the rest of the surrounding area can be milled down by 0.2mm. This not only facilitates mold fitting but also improves venting. In addition, a separate vent hole should be added to direct the gas out of the mold.

8. Porous Steel (Venting Steel)

Its venting effectiveness is inversely proportional to its thickness, so it is typically designed with a thickness of 30–50mm. During final machining, it cannot be processed using conventional machining methods and must be finished by EDM (Electrical Discharge Machining). It's important to note that due to cost considerations, porous steel is rarely used in practice.