Lower Cycle time, Increase Output
The cooling time influences greatly in any mold cycle time-how fast the resin can be sufficiently cooled so that the part can be ejected without permanent deformation. Generally, those areas take longest time to cool are those with thickest wall section.
Parts moulded with hot runners better lend themselves to automated part removal. With no runner to interfere with part removal, secondary mold processing times involving manual labour, including part/runner separation, part trimming and packaging, are reduced or eliminated entirely.
Collectively, these individual time reductions reduce the overall moulding cycle, contributing to increased output per unit of time. Depending upon the production quotas, you may be able to free up the injection moulding machine for another run often days, weeks, or even months earlier than anticipated. Figure 1 shows the components of a typical moulding sequence and where time savings when using a hot runner tool can be expected.

Reduce Production Costs Significantly
Although a more expensive capital investment upfront, a hot runner system is a significantly more cost-efficient. It keeps production costs to a minimum over the long run as following:
* Resin Savings
Since there is no cold runner to discard or recycle, resin consumption is reduced. Resin savings can be significantly shown for moulded parts like medical components or parts requiring FDA approval, which the product may require 100 percent virgin material.
* Energy Savings
Energy is wasted plasticising, cooling and regrinding each cold runner that is produced. Increased energy consumption also is a direct result of extended cycle times.
* Labour Savings
Secondary operations―such as manual part de-gating and trimming―are eliminated entirely with a hot runner system.
* Mold Cost Savings
A smaller cavitation hot runner equipped mold may be able to satisfy production quotas using a smaller number of cavities since it runs at a faster cycle. The smaller mold frame size may enable installation into a smaller press.
* Injection Press Costs
Hot runners allow reduced injection pressures during packing, as the system does not have to deal with injecting resin through a cooled runner. Melt in the cold runner may lose heat en route to the gate, possibly requiring higher heats and/or pressures from the injection moulding machine. By reducing the injection pressure and clamp tonnage required, it is often possible to run the same part in a smaller tonnage machine as the clamp tonnage required is not as great.

Improved Part Quality
Although cold runners provide the designer with simpler integration selecting the optimal gate location 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, residue gate material) on the moulded part.

Hot runner systems provide mold makers 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 moulded part. As well, reduced injection pressures relative to a cold runner produces a more robust part since there is less risk of moulded-in stress.

Valve Gating
A valve stem mechanically shuts off the melt flow by making direct contact with the gate steel. The moulded 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 moulded part are improved with the use of a valve gate. Moulded-in stress is a result of molecular orientation, which is caused by shear. Valve gates, due to the larger gate diameters, generate fewer shears compared to thermal gates. The result is a part with less warpage and better physical properties.

Long-Term Benefits
Despite the higher initial cost, the long-term benefits of equipping a mold with a hot runner system can be easily justified. A hot runner-equipped mold can effectively reduce moulding costs without significantly increasing the complexity of the mold design. Generally, mold build lead times are not impacted as the hot runner is designed and manufactured in parallel with the mold.

References: http://www.moldmakingtechnology.com/articles