Pulsating gas-assisted injection molding apparatus

DETAILED DESCRIPTION The present invention resulted from the recognition that if the plastic region encompassing the interface and while still in a molten state could somehow be made to shift back and forth, this shifting could re-orient the plastic molecules in an entangled manner at the interface, effectively eliminating the weld line.
There is, however, no direct hydraulic plastic melt communication with the center as well as the perimeter of the weld section as with a solid plastic part.
But it was discovered that the effect thereof could be accomplished by pushing and pulling on these melt fronts at their interface with differential pressure immediately after the gas injection stage and while the gas pressure is in its holding stage as the plastic cures in the mold.
The apparatus for accomplishing this push/pull or shifting movement comprises a pair of nozzles or gas pins that project into the mold cavity on opposite sides of where the voidless plastic weld region occurs.
The nozzles are closed at their end and have adjacent miniature transverse holes that face into the hollow of the plastic melt that develops during the normal gas injection stage.
At the end of the latter stage and while the gas pressure is maintained for a brief interval, the nozzles are rapidly alternately connected to receive the same gas but at a higher pressure and to exhaust or vent the gas pressure.
This creates a rapidly reversing or pulsating pressure differential acting on the melt/solid weld section, causing the plastic melt at the interface to shift in opposite directions.
The amplitude and frequency of the plastic melt shift at the interface is controlled by the magnitude and rate of the pulsating gas pressure so as to cause considerable mechanical entanglement of the polymer molecules at the interface that effectively eliminates weld line defects.
When the injected plastic includes reinforcement fibers, this back and forth shifting action at the melt/solid interface also creates a shearing action that acts to align the fibers in parallel relationship transverse to and across the interface to further improve the strength of the fusion weld joint