Why Should the Injection Mold Be Equipped with An Exhaust System ?

The exhaust of injection mold is an important issue in mold design. Especially in rapid injection molding, the requirements for the exhaust of injection mold are more stringent.

The necessity of Exhaust System

1. The source of gas in the injection mold.

1) The air in the pouring system and mold cavity.

2) Some raw materials contain a small amount of moisture, which vaporizes into water vapor at high temperatures.

3) Due to the high temperature during injection molding, some uniform plastics decompose and produce gas.

4) Gas produced by volatilization or chemical reaction of certain additives in plastic raw materials.

2. Harm of poor exhaust

Poor venting of the injection mold will bring a series of harms to the quality of plastic parts and many other aspects. The main hazards are as follows:

1) During the injection molding process, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will be difficult to fill the melt. Then, it results in insufficient injection volume and the cavity can’t be filled.

2) The gas that is not smoothly removed will form high pressure in the cavity and penetrate into the plastic under a certain degree of compression. Finally, it causes the quality defects like pores, voids, loose tissue, and silver streaks.

3) The gas is highly compressed and the temperature in the cavity rises sharply. Then, the surrounding melt decomposes and burns, causing local carbonization and scorching of the plastic parts. It mainly appears at the confluence of the two melts, the corners and the cracks in the gate.

4) The gas is not removed smoothly and the speed of the melt entering each cavity is different. Therefore, it is easy to form flow marks and fusion marks, and the mechanical properties of the replacement plastic parts are reduced.

5) Due to the restriction of the gas in the cavity, the filling speed will be reduced. Besides, the molding cycle will be affected, and the production efficiency will be reduced.

3. Distribution of bubbles in plastic parts

The sources of gas in the cavity are mainly divided into three categories:

  • The air accumulated in the cavity
  • The gas produced by decomposition in the raw material
  • The vapor of the residual water in the raw material

The location of the bubbles generated due to different sources.

1) The air bubbles generated by the accumulation of air in the cavity are often distributed on the part opposite to the gate.

2) The bubbles generated by the decomposition or chemical reaction in the plastic raw materials are distributed along the thickness of the plastic part.


3) The air bubbles generated by water vaporization remaining in the plastic raw materials are distributed irregularly on the entire plastic part

Judging from the distribution of air bubbles in the plastic parts, it is possible to estimate the nature of the air bubbles and whether the exhaust position of the mold is correct and reliable.

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