Cold slugs

Section provides opposite sprue opening of the injection mold, used for trapping cold slug. 

Cold slug First material to enter an injection mold. 

A problem had arisen with molded parts from a medical products company. Some injection-molded sheets were exhibiting small translucent areas (gels) within the opaque polymeric material, which had been designated as non-melt areas. One theory for the source of the non-melt was that a contaminant was present, while another postulate was that these areas represented a cold slug coming from the sprue. One sheet was chosen for analysis since the area of non-melt within it was largest. Both the non-melt area as well as a "normal" area were tested. 

The Auto-Shut Valve requires more soak time than conventional non-retum valves. This valve has a spring loaded poppet that rides in the body of the valve. There is a pool of plastic contained inside the valve by the poppet. The pool of plastic in the center of the valve, and the poppet shaft, are the last front end components to reach operating temperature. The plastic around the screw will reach operating temperature before the internals of the Auto-Shut Valve. If screw rotation occurs too soon, the plastic from the screw will ftow into the valve, and either the poppet will resist opening, or the cold slug of plastic in the valve will block the flow. If the screw continues to rotale, the Auto-Shut valve may become damaged. 

A conventional nozzle (Fig. 6.31) with a reverse taper can be used with FEP, PFA, and ETFE. The size of the bore should be as large as possible and it should be tapered in order to prevent stagnation and abrupt changes in the melt velocity. The sprue should extend into the nozzle a sufficient length (13-25 mm) to minimize the likelihood of cold slug formation in... 

Hot or cold runner systems can be used. For cold runners, the sprue should contain a cold-slug well to receive the cold melt emerging first from the nozzle of the extruder. The cold well is typically the largest diameter of the sprue with a depth of 1.5 times this diameter. Runner size 6.3 to 9.5 mm diameter. ... 

Mdt fracture at the gatefr [melt too cold] /temperature sensor error/injection rate too fast/gate too small/sharp edge in gate area/cold slug well in the runner too small. 

Space provided directly opposite the sprue opening in an injection mold to trap the cold slug. 

In injection molding, a portion of the plastic composition held in the cold slug well by an undercut, used to puU the spme out of the bushing as the mold is opened. 

In order for the sprue assembly to be ejected cleanly from the mold use a 3 to 6 degree (inclusive) taper on the sprue and incorporate a T type, sprue puller pin in the cold slug well ensure that the pin is secured or keyed in the correct position so as to enable the feed system to fall away, under its own weight, from the undercut on the pin. 

Full round runners are preferred where typical diameters of 6 to 8mm (0.236 to 0.315in) (for component wall thicknesses of up to 4mm (0.157in)) and 8 to 12mm (0.315 to 0.472in) (for component wall thicknesses of greater than 4mm (0.157in)) are recommended. Z type, cold slug well pullers are preferred for cold runner molds. 

Cold slug n. The first material to enter an injection mold, so called because in passing through the sprue orifice it is cooled below the effective molding temperature. 

In some molds, a small well in the mold opposite the sprue catches the cold slug and thereby prevents it from entering the ruimer system. 

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