Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1994-11-02
2001-07-17
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S045500, C264S250000, C264S259000, C264S328100, C264S328700, C264S328180
Reexamination Certificate
active
06261488
ABSTRACT:
SUMMARY OF THE INVENTION
The present invention relates generally to plastic injection molding techniques and more specifically to such techniques which reduce or eliminate weld lines and their weakness problems.
Weld lines are the “lines” where flow fronts of injected heat softened plastic materials meet in the filling of pre-closed molds. These fronts are cooled by flowing over cool mold surfaces and therefore bond less well and give a weakened inter-flow-front surface bonding.
Since there is no mixing, interlocking, interleaving or knitting of the flow front materials, this weakening effect is particularly apparent when fiber reinforced thermoplastic materials are molded. The reinforcing fibers enhance the part's strength everywhere except at the weld lines where the fibers fail to intermesh.
In U.K. Patent 2,170,142 attempts to avoid weld lines or seams by providing pistons positioned about a die cavity which operate in synchronism to pump the plastic back and forth generally perpendicular to the direction of extrusion so that as a melt passes through the die cavity the pistons operate causing the extrudate to move circumferentially or transversely about the die cavity thereby eliminating seams, voids and other defects. Special heating of the runners would be required for some part configurations. The piston configuration needs to be tailored to a particular part configuration and large gates along with extensive trimming of scrap would be frequently required. For many part configurations, the process appears to be relatively slow.
The present invention may advantageously utilize molding materials such as disclosed in my copending application Ser. No. 08/333,504, entitled FOLDED FIBER FILLED MOLDING MATERIAL, filed on even date herewith, the entire disclosure of which is specifically incorporated herein by reference; however, other materials may be utilized in the practice of the present invention.
Among the several objects of the present invention may be noted the reduction or elimination of weak regions in fiber reinforced injection molded plastic parts; the avoidance of the above noted prior art drawbacks; the provision of a technique for forming foamed plastic parts having selected low density regions; and the provision of a process of operating an injection molding machine to manufacture, in a mold cavity, a substantially weld line free plastic part. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter.
In general, after injection molding of a prior part, a compression type mold is partly pre-closed sufficiently to form an enclosing compartment. The enclosing compartment is then explosively filled with a molding material followed by rapid compaction of the material within the enclosing compartment. By “explosively filling” is meant a highly turbulent flow through open gates and runners (if used) under high enough injection pressures to turbulently fill the mold very quickly, e.g., in from a fraction of a second to under five seconds and providing fill and compaction time short enough to permit satisfactory part finish before cooling causes blemishes. Preferably, the compaction immediately follows mold fill.
Also in general, a new molding method for virtually eliminating weld lines and flow orientation in molded plastic parts utilizes an almost explosively rapid turbulent injection fill of a partly closed compression-type mold with a suitable amount of foamed plastic followed by compression molding of the foamed material to the desired density.
Still further in general and in one form of the invention, a foamed plastic part having selected low density regions is made by injection molding the part with a uniform density and subsequently reducing the density in the selected regions. The subsequent reduction may be effected by selectively heating the selected regions or by selectively reducing the pressure to which the part is subjected during, or immediately after, a compression molding step.
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patent: 6030573 (2000-02-01), Matsumoto et al.
Kuhns Allan R.
Materials Research Innovations Corporation
Taylor & Aust P.C.
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