Gating and Runner System

The demands placed upon gating and runner system can be described in a single sentence  

The gating and runner system must be so designed, that homogenous melt Alls all cavities simultaneously and uniformly at equal pressure. 

The numer system is designed to suit the position of the cavities in the mold. Position and design of gates to cavities is primarily a function of the molded article s design and its speciflcations. 

When the molding is ejected, the sprue is pulled off at the (warmer) nozzle end. Therefore, the stem remains on the molding and must be removed by an additional operation. The sprue-gate is employed on high quality technical articles and on thickwaUed moldings. 

The ring gate is employed for round or sleeve-like articles, whose core - due to its length-needs supporting bilaterally. This enables even long-cored articles to be molded at an alround consistent wallthickness. 

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In contrast to multi-color and multi-component injection molding, co-injection molding uses the same gate and runner system. Here, the component that forms the outer skin of the part is injected first, followed by the core component. The core component displaces the first and a combination of the no-slip condition between polymer and mold and the freezing of the melt creates a sandwiched structure as depicted in Fig. 3.51. 

Figure 12.4 Example of a dam gate and runner system (courtesy of Bayer)...

As usual, the injection speed depends on the size and complexity of the part, as well as on the gate and runner system. Slow to moderate speeds give the best results — too fast speeds result in excessive shear heating, degradation and splay. For small parts or parts with relatively simple geometry, a constant filling speed is recommended, while for larger or more complex parts, it may be helpful to profile the speed. Mold... 

Figure 4-1 Gating and runner system for a four-impression mold...

All the different sprue versions shown thus far share the disadvantage, that material is wasted in the gate- and runner-system. Only a portion of this material can be returned to the process by internal recycling systems. For reasons of cost and environmental protection, an attempt is made to avoid such material wastes, by controlling the temperature of the gate- and runner-system in order to maintain the flowabihty of the melt in this area. 

Figure 5.87 Gating and runner systems demonstrating melt flow in RIM molds (courtesy of...

Capillary rheometer. The Monsanto capillary rheometer measures the viscosity properties of polymers, and provides a direct measure of viscosity and the change in viscosity with time and flow rate at plastica-tion temperatures. The capillary orifice simulates the gate and runner system of actual molding conditions, thus providing valuable flow information for molding compounds. 

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