System and apparatus for injection molding articles of amorphous polyethylene terephthalate and similar materials

A system for the runnerless injection molding of articles of substantially amorphous, crystallizable polymeric materials, said system comprising:

• a source of molten, crystallizable polymeric material under intermittent positive pressure, said material having a characteristic minimum glass transition temperature, a characteristic maximum crystal melting temperature and a characteristic crystal formation temperature range;
• a mold comprising (a) first temperature control means for maintaining said mold at a first preselected temperature below said characteristic minimum glass transition temperature of said polymeric material;
• (b) an outer surface;
• (c) a longitudinal axis perpendicular to said outer surface;
• (d) a first cavity centered on said axis, said first cavity extending into said outer surface of said mold to an inner end, and defining an outer portion between said outer surface and an intermediate end and an inner portion between said intermediate end and said inner end;
• (e) an article formation cavity;
• and (f) a gate having an inlet orifice associated with said inner end of said first cavity and an outlet orifice associated with said article formation cavity such that said gate connects said inner portion of said first cavity to said article formation cavity;
• and
• transport means adapted to be substantially completely filled with said molten, polymeric material for intermittently receiving preselected quanities of said molten, polymeric material from said source under said positive pressure at a location remote from said inlet orifice of said gate, and simultaneously ejecting quantities of said molten, polymeric material substantially equal to said preselected quantities of said molten, polymeric material from a location proximate to said inlet orifice of said gate;
• said transport means comprising second temperature control means for maintaining said transport means at a second preselected temperature above said characteristic maximum crystal melting temperature of said polymeric material and an elongate bushing;
• said elongate bushing having a proximal end defining a material receiving orifice, a distal end defining a material discharge orifice and an unobstructed longitudinal lumen therethrough connecting said material receiving orifice to said material discharge orifice;
• said elongate bushing being at least partially telescopically and axially inserted into said first cavity of said mold such that said discharge orifice is located within said inner portion of said first cavity in close proximity to said inlet orifice of said gate;
• wherein (i) said inner end of said first cavity has a periphery, (ii) said inner portion of said first cavity is defined by a side wall which curves inwardly from said intermediate end to said periphery of said inner end such that said inner end and said side wall together define a portion of a truncated, substantially semi-spherical volume, and (iii) said gate defines a substantially cylindrical portion extending from said inlet orifice to a frusto-conical portion flaring outwardly from said cylindrical portion to said outlet orifice;
• whereby during the intervals between said intermittent ejection of said quantities of said molten, polymeric material through said discharge orifice of said elongate bushing, crystallization of the next preceding one of said quantities of said molten, polymeric material occurs substantially only in a small volume located between said discharge orifice of said bushing and said outlet orifice of said gate;
• and
• whereby, upon the removal of a completed article from said article formation cavity of said mold, said polymeric material neither strings be between said outlet orifice of said gate and a vestige protruding from said article, nor drools from said outlet orifice of said gate.

Apparatus for the runnerless injection molding of articles of substantially amorphous, crystallizable polymeric materials having a characteristic minimum glass transition temperature, a characteristic maximum crystal melting temperature and a characteristic crystal formation temperature range, said apparatus comprising;

• a mold comprising (a) first temperature control means for maintaining said mold at a first preselected temperature below said characteristic minimum glass transition temperature of said polymeric material;
• (b) an outer surface;
• (c) a longitudinal axis perpendicular to said outer surface;
• (d) a first cavity centered on said axis, said first cavity extending into said outer surface of said mold to an inner end, and defining an outer portion between said outer surface and an intermediate end and an inner portion between said intermediate end and said inner end;
• (e) an article formation cavity;
• and (f) a gate having an inlet orifice associated with said inner end of said first cavity and an outlet orifice associated with said article formation cavity such that said gate connects said inner portion of said first cavity to said article formation cavity;
• and
• transport means adapted to be substantially completely filled with said molten, polymeric material for intermittently receiving preselected quanities of said molten, polymeric material under said positive pressure at a location remote from said inlet orifice of said gate, and simultaneously ejecting quantities of said molten, polymeric material substantially equal to said preselected quantities of said molten, polymeric material from a location proximate to said inlet orifice of said gate;
• said transport means comprising second temperature control means for maintaining said transport means at a second preselected temperature above said characteristic maximum crystal melting temperature of said polymeric material and an elongate bushing;
• said elongate bushing having a proximal end defining a material receiving orifice, a distal end defining a material discharge orifice and an unobstructed longitudinal lumen therethrough connecting said material receiving orifice to said material discharge orifice;
• said elongate bushing being at least partially telescopically and axially inserted into said first cavity of said mold such that said discharge orifice is located within said inner portion of said first cavity in close proximity to said inlet orifice of said gate;
• wherein (i) said inner end of said first cavity has a periphery, (ii) said inner portion of said first cavity is defined by a side wall which curves inwardly from said intermediate end to said periphery of said inner end such that said inner end and said side wall together define a portion of a truncated, substantially semi-spherical volume, and (iii) said gate defines a substantially cylindrical portion extending from said inlet orifice to a frusto-conical portion flaring outwardly from said cylindrical portion to said outlet orifice;
• whereby during the intervals between said intermittent ejection of said quantities of said molten, polymeric material through said discharge orifice of said elongate bushing, crystallization of the next preceding one of said quantities of said molten, polymeric material occurs substantially only in a small volume located between said discharge orifice of said bushing and said outlet orifice of said gate;
• and
• whereby, upon the removal of a completed article from said article formation cavity of said mold, said polymeric material neither strings between said outlet orifice of said gate and a vestige protruding from said article, nor drools from said outlet orifice of said gate.