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Injection Molding Defects (Jetting (Jetting occurs at high fill rate into…
Injection Molding Defects
Short Shot
Unfilled mold
Ejector Pin Marks
Marks are often left on the part in the area where the ejector pins pushed
Pin below flush
Clearance around pin
Pin above flush
Eject the part before it solidifies and pins push through the
part
Place the ejector pins on hidden areas of the part
Warpage/Residual Stresses
Variation in thickness differing shrink rates due to melt orientation
Uneven cooling Differences in the mold cavity pressure
Weld Lines
Weld lines are created when two flow fronts come together in the mold
Weld lines decrease the strength of the part
Jetting
Jetting occurs at high fill rate into large open cavity between the gate
and the opposite wall
To reduce the chance of jetting,
gates should be located so that
entering material flows into wall,
not into open section
Material stream shoots to the opposite wall and freezes. Fresh
material folds over cooled flow lines, traps air and creates weak joints.
Flashing
Over-filling of the mold forces mold open
flash around the edges of the part at
the parting line
Sink Marks
occur at excessively thick wall section
where there are abrupt changes in thickness- thick sections solidify too late and shrink away from the wall
Proper design reduces/eliminates sink marks (ribs, core out sections)