injection molding jetting

Definition
Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. The buckled, snake-like jetting stream causes contact points to form between the folds of melt in the jet, creating small-scale " welds".
Jetting leads to part weakness, surface blemishes, and a multiplicity of internal defects.

Causes
Excessive ram speed
Poor gate position
Lack of melt contact with the mold allows jetting to occur.
Inadequate hot runner system design

Remedies
Optimize gate design and position
Direct the melt against a metal surface by repositioning the gate or use an overlap or a submarine gate.
Use a tab or fan gate to slow down the melt with a gradually divergent flow area. This reduces the melt shear stress and shear rate. molding jetting
where:
a = overlapping gate
Optimize the ram speed profile
Use an optimized ram-speed profile so that melt-front velocity is initially slow when the melt passes through the gate, then increases once a dispersed flow is achieved.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

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