US 4,908,169 AGranted

Method for plasticating using reciprocating-screw having a melt channel and solids channels

OwnerGeorge J. Galic, Steven M. Mausinventors

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Abstract

Method for improved melt quality of thermoplastics processed in reciprocating-screw molding, especially optical injection molding processes. In a reciprocating-screw injection-drive unit (4) is a barrel (53) and screw (15). Screw (15) is double flighted throughout its melting or transition zone (2), to form melt channel (48) and solids channel (50) separated by melt-filtering flight element (47), and kept in fluid communication by melt-transfer apertures (47 or 55 or 56) or substantially nonvertical ramped barrier flight geometry (not shown). The largest energy input for melting is directly provided by thermal conductivity, via electrical resistance heating elements. The primary source of such heat is by such elements (35) mounted internal to screw (15), and operating in an open loop control with respect to melt temperature sensor (22); secondary heat sources are elements (12, 13, 14, 29) mounted external to barrel (53) and nozzle (33) and operating in a closed loop manner with respect to barrel temperature sensing device (17, 19, 21, 30) and/or melt temperature sensor (22). By screw geometry designed to keep the solids bed (24) continuously under compression and by suitable numbers and sizes of apertures (47 or 55 or 56), melt films are continuously transferred away from the screw's and barrel's heated surfaces as fast as they are formed, thereby minimizing residence times at peak temperature and maintaining maximal heat transfer rates.

A method for plasticizing thermoplastic polymer using a reciprocating-screw injection molding machine having a barrel with a screw operatively mounted therein, the screw having a root, shank and tip and forming a solids channel and a melt channel, the method comprising:

(a) feeding the polymer in its solid form into the barrel proximate the screw shank thereby establishing a solids bed;
(b) rotating the screw within the barrel thereby causing a melt barrier flight to engage the solids bed and effecting relative motion between the barrel and the solids bed;
(c) externally heating the barrel thereby producing a barrel wall melt film on an outer surface of the solids bed proximate the barrel wall;
(d) internally heating the screw to at least the melting temperature of said thermoplastic polymer thereby producing a screw surface melt film on an inner surface of the solids bed proximate the root of the screw;
(e) transferring the barrel wall and screw surface melt films from the solids channel to the melt channel, by providing means of maintaining fluid communication therebetween, thereby substantially eliminating melt in the solids channel and accumulating melt mostly in the melt channel;
(f) maintaining the solids bed under substantially continuous compression forces between the barrel wall and the solids channel's screw surfaces, thereby maximizing the relative motion of a steadily diminishing solids bed to the barrel wall and minimizing the relative motion of said solids bed and said screw surface of the solids channel;
(g) reducing continuously the solids bed channel depth at a rate equal to the rate at which the solids bed's occupied volume is reduced by melt formation and transfer, to produce said substantially continuous compressive forces;
and
(h) maintaining said continuously reduced solids bed to be in close thermal contact with the heated barrel and the heated screw surfaces throughout the plasticizing process, with said plastication melting process thus minimizing breakup of the solids bed before melting is substantially complete, whereby the polymer proximate the screw tip is substantially devoid of objectionable solids.

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Abstract

Claims20