From the perspective of the mold, when the molten plastic reaches the gate through a larger-sized runner, the flow resistance of the melt is greatly increased because the gate size is usually very small. Study shows that the root cause of jetting is related to the mold design on the one hand and the viscoelasticity of the material on the other. Since the melt is immediately cooled the moment it makes contact with the cavity wall (mold surface) which has a relatively low temperature, making the temperature of the jetted material lower than that of the fountain stream that comes later, thus causing poor welding and obvious jetting on the surface of the product. The melt that enters the cavity later fills the remaining space in the mold cavity with a normal fountain flow and is welded to the jetted streams.

However, when the molten plastic flows at a high speed through narrow areas, such as the nozzle, the runner and the gate, and then suddenly enters an open and relatively wider area, the plastic melt will be injected from one end to the other of the cavity in the form of jet streams, creating folded strips. Under normal circumstances, the molten plastic fills the mold cavity in a “fountain flow” manner, as shown in Figure 2, which depicts the “fountain flow” in detail. Jetting, also known as jet, worm track, or snake-like pattern, refers to the snake-like curves on a plastic injection molded part along the flow direction from the gate, as shown in Figure 1.

Plastic injection molding