When selecting gate location (regardless of whether the tool features a cold or hot runner system), balanced and even filling of the molded part is the top priority. Cold runner gating provides the moldmaker with simplified integration in terms of selecting the optimal gate location,

as the runner feeding the cavity can be more easily routed to avoid interference with mold features. Conversely, when selecting gate location with a hot runner system, the designer must consider maintaining a steel safe condition between the nozzle bore/gate detail and mold features. In certain situations, this may require additional work to modify the mold layout to satisfy the steel safe condition and maintain balanced/even filling. Advances in hot runner gating technologies and increasingly smaller hot runner nozzle diameters have expanded the list of gating options available to the designer, increasing the flexibility of gating options.

  Although cold runners provide the designer with simpler integration upfront, the trade off is the improved gate quality that the hot runner offers. As the part is ejected in a cold runner mold, the runner must be trimmed from part. This can occur either manually or automatically via in-mold de-gating apparatus or via a three-plate mold. The resulting vestige is evident as a small bump (imperfection) on the molded part.

 Hot runner systems provide molders with a variety of gating options that offer improved part quality relative to the cold runner alternative. Hot runner gating options fall into two main categories: thermal gating and valve gating.

 Thermal Gating
This approach relies on the thermal properties of the tip and gate area to cause the resin to freeze (solidify) at the gate location. When the melt within the cavity has cooled sufficiently, the part is ejected and the resin at the gate location is sheared—leaving a very small gate vestige on the molded part. As well, reduced injection pressures relative to a cold runner produces a more robust part since there is less risk of molded-in stress.

 Valve Gating
A valve stem mechanically shuts off the melt flow by making direct contact with the gate steel . The molded part separates from the gate without breaking or shearing the plastic; thus, discoloration or deformation due to gate break is unlikely. Physical properties of the molded part are improved with the use of a valve gate. Molded-in stress is a result of molecular orientation, which is caused by shear. Valve gates, due to the larger gate diameters, generate less shear compared to thermal gates. The result is a part with less warpage and better physical properties.                

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