Method of Providing Air Vents

Air is present in the space in the cavity of a mold. In injection molding, molten plastic is poured inside the cavity, that is, it is also possible to say that the process is that of replacing air with molten plastic.

Therefore, if it is possible to discharge the air from inside the cavity to the outside efficiently, then it is possible to make small the resistance at the time of filling; therefore it is possible to suppress the filling pressure to a low value, and as a result, it is possible to make small the residual stress in the molded product. In addition, it is also possible to prevent the breakage of core pins, etc., due to the filling pressure.

On the other hand, if it is possible to discharge from inside the cavity to the outside the volatile constituents or gas generated from molten plastic, it is possible to improve the quality of the molded product.
"Air vent" is one that carries out these functions. An air vent can be provided on the parting surface as shown in the example in Fig. 1, or can be provided on the core pin side surface as shown in the example in Fig. 2.

One end part of the air vent should be in contact with the cavity. The depth of the air vent at this contacting part is generally about 0.002 to 0.02 mm. Although the efficiency of discharging air or gas becomes better when this is deep, care should be taken because it is possible that burrs are generated in the molded product. Select an appropriate value depending on the plastic material and the cavity surface temperature, etc.
It is sufficient if the length of the air vent at this part is about 2 to 5 mm.

The important factor is that the other end part of the air vent is connected to a much deeper air vent. The depth of this deeper air vent should be about 0.2 to 0.5 mm and becomes a path for the air or gas to be exhausted to atmosphere from the side surface of the mold plate or of the packing plate.

Since slag or tar components generated from the plastic during injection molding is deposited in the air vent, it is necessary to clean the air vents periodically, such as about once in a fortnight to once in a month.
The position of providing the air vent can be near a part where a weld is generated, near the final last filling part, or in the middle of the runner. It is also a practical approach to make one trial and then decide the detailed location of providing the air vents.