The Critical Role of Exhaust Systems in Injection Molding

  An efficient exhaust system is a vital component in injection mold design, especially for high-speed molding where trapped air cannot escape quickly enough. The gas in the mold cavity originates from several sources: (1) air present in the runners and cavity; (2) water vapor from insufficiently dried resin; (3) gas from thermal decomposition of the polymer; and (4) gases released by additives. The harm caused by poor venting is severe and multifaceted. It can lead to short shots (incomplete filling) as the trapped air prevents melt flow. It causes visible surface defects like burn marks (diesel effect), silver streaks, and internal voids or bubbles. The high pressure and temperature of compressed gas can also lead to local decomposition and charring of the plastic, particularly at weld lines and blind corners. Furthermore, trapped air can disrupt flow patterns, leading to prominent flow marks and weak weld lines, and ultimately decrease production efficiency by extending the molding cycle. Therefore, proper venting through well-designed vents or porous inserts is essential for producing high-quality, cosmetically sound, and structurally robust plastic parts.