Injection molding polymer through small gates creates complex three-dimensional fluid flow behaviors. Jetting occurs in injection molding when polymer material is injected through a nozzle, runner, or gate at high velocity, into open, thicker areas, without forming contact with the mold wall.
The jetting effect is like water flowing through a hose. If you place your thumb on the opening of the hose the water will push out in different directions. The amount of water that sprays out depending on how tightly you close the hose opening. This is the same as a gate during molding. A small gate can create high polymer injection velocity, forcing the material to push forcibly into the cavity.
Jetting occurs when molten polymer enters a cavity and does not establish a circular flow pattern. A strand of polymer squirts into the cavity is unable to adhere to the mold wall. The material cools quickly and the material that follows is unable to fuse with the cooled strand, preventing a homogenous melt structure.
The jetting molding defect can appear as a snake or worm track inside a mold cavity and show up as a gloss variation, splay, blush or dull spot near the gate of a molded part. Because the internal molecular structure is not homogenous, part strength and mechanical challenges may also be apparent. Finally, uncontrolled and turbulent material flow cause cavity air entrapments.
How can jetting be prevented? There are several considerations such positioning the gate so that material entering the cavity flows against a wall or object in the mold, using the appropriate gate type and size, and ensuring the injection flow velocity can develop a circular fountain flow pattern. But remember, reducing injection velocity can create other molding challenges so should be considered carefully.
It is important to evaluate molding defects before part and mold designs are finalized.