Injection molds sprue

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

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. 

Tube flow is encountered in several polymer processes, such in extrusion dies and sprue and runner systems inside injection molds. When deriving the equations for pressure driven flow in tubes, also known as Hagen-Poiseuille flow, we assume that the flow is steady, fully developed, with no entrance effects and axis-symmetric (see Fig.5.13). 

Inevitably, in multi-cavity injection molds, some of the mold cavity gates are located further than others from the sprue that delivers the melt from the plasticating unit. If the runner system that distributes the melt from the sprue to the individual cavities has a constant... 

The part taken from the mold is, in most cases, a finished product ready to be packed and shipped or ready to be used as a part of an assembled unit. In contrast to metal forming, there is very little if any wasted material in injection molding. For cold runner TP systems most runners and sprues are reground and reused. By using hot runner molds, the sprue and runner systems remain in a melted state in the mold and become part of the next finished part (Chapter 17). The hot runners can be thought of as an extension of the plasticizing chamber. 

Little material is wasted since there are usually no sprues or runners [when not compared to runnerless injection molding (Chapter 4)]. 

The SCORIM unit is placed between the nozzle of the injection molding machine and the mold (Figs. 14.11-14.13).The gates (i.e., the sprues and runners) are located so that all potential weld lines are placed between them from the flow point of view. 

Sprue. (1) The main feed channel that runs from the outer face of an injection mold to the gate in a single-cavity mold or to the runners of a multiple-cavity mold. The hquid polymer is forced through this orifice from a nozzle till the mold is filled to capacity. Some polymer remains in the sprue after the mold is closed, leaving a projecting piece that must be removed after the product is ejected. The viscosity of the polymer must be low enough to permit it to pass through the sprue readily. 

Fig. 5 (A) Schematic of screw extrusion of a ceramic shape. The ceramic feed material dispersed in a binder flows from the hopper into the barrel where the rotating screw transports the material through the die opening. (B) Schematic of injection molding of a ceramic shape. The ceramic feed material dispersed in a binder flows from the hopper into the barrel where the rotating screw transports the material through the sprue into a closed mold. After the mold is opened, the part is ejected.

The sprue is the channel from which the resin will flow through the gate and into the mold. Runners are perpendicular to the direction of draw, and the sprue is parallel to the direction of draw. This portion of the system is often recycled because typical OEMs allow up to 15% of regrind material (the use of polymer salvaged from the runner or sprue and not used in the original part). When injection molding takes place, automotive thermoplastics must be cooled under the Tm or T before ejection. 

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