Impacts of Poor Mold Venting During Injection Molding

Factors Leading to Gas Formation in Injection Molds

Gas formation in injection molds is often related to the following factors:

  • Air exists in the injection system and mold cavity.
  • Some materials may contain moisture that has not been properly dried and can vaporize into steam at high temperatures.
  • Certain plastics with unstable properties can undergo decomposition and produce gas when injection temperatures are too high.
  • Gases generated by the evaporation of certain additives or chemical reactions in the plastic material.
Impacts of Poor Mold Venting-1

Impacts of Poor Mold Venting

Simultaneously, the reasons for inadequate venting need to be identified promptly. Poor venting in injection molds can cause various issues in plastic products, including:

(1) During the injection process, molten plastic replaces the air in the mold cavity. If the air is not removed promptly, it can lead to difficulties in molten plastic filling, resulting in inadequate injection volume and incomplete cavity filling.

(2) Inadequate air evacuation can create high pressure within the mold cavity. Besides, under certain compression conditions, gases may penetrate into the interior of the plastic, causing defects such as voids, pores, loose structures, and silver streaks.

(3) The highly compressed gases in the cavity cause a sharp increase in temperature, leading to the decomposition and burning of the surrounding molten plastic.

This results in localized carbonization and charring of the plastic, mainly occurring at the confluence point of two molten streams and at the gate flange.

(4) Inadequate gas venting leads to variations in the speed at which molten plastic enters different cavities, making it prone to flow marks and weld lines, reducing the mechanical properties of the plastic parts.

(5) Gas obstruction in the mold cavity slows down the filling speed, affecting the molding cycle and overall efficiency.

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Distribution of Bubbles in Plastic Parts:

  • Bubbles formed by trapped air in the mold cavity are often distributed on the side opposite to the gate.
  • Bubbles generated by decomposition or chemical reactions in the plastic material are distributed along the thickness of the plastic part.
  • Bubbles resulting from residual moisture in the plastic material are irregularly distributed throughout the entire plastic part.

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