In many cases, the cold runner system is a more viable option. In some applications, it may provide a mold and a process that is capable of producing higher-quality parts. Although this is counter intuitive to what many think, the fact remains that with a cold runner, a mold designer controls the design, and a mold builder controls the manufacturing of the system. This means they can more easily see and measure the channels in a cold runner system over the channels in a hot runner system, which gives them the ability to inspect the cold runner system for variations. If they find a problem or need to change runner sizes or gate location (or locations), they can do this much more easily in a cold runner than they can in a hot runner system. This is because the hot runner gate drop locations are fixed for a given manifold.

The advantages of cold runners include:

1) Lower initial investment (no system or electronic controls to purchase).

2) Flexible gate locations and design options.

3) Easily changeable gate locations and designs.

4) Better shot control because of larger screw strokes.

5) Easier steel measurement.

6) Simpler variation diagnosis because of less complex melt delivery systems.

7) Fewer thermal variations in the process, because thermocouples, heaters and controllers are not required.

8) Easy rheological control implementation.

9) Freedom from color changes in the cold runner section, since they are ejected every cycle.

The challenges of cold runners include:

1) A higher material cost than that of a hot runner.

2) Runner and regrind handling.

3) Limited cycle time if the runner is sized improperly.

4) Reduction in the shot size capacity that is available for parts, which the runner volume causes.

5) Increased clamp tonnage considerations for the projected area.

Runner Conversions

Many companies that have participated in our education courses have used both hot and cold runner systems. Some companies converted from cold runners to hot runners. Others made the same switch only to return to cold runners, because hot runners presented quality and cost issues for the parts of those particular companies.

We need a cost analysis for a product that a manufacturer was going to produce in an eight-cavity mold for which the purchaser desired a two-year return on investment. This example strictly examines the material savings from the use of a hot runner in comparison to the use of a cold runner. Some would argue that hot runners provide additional savings, such as cycle time, energy, clamp tonnage and so on, and they could be right. However, those benefits are not a guarantee in every case, so we excluded them from this cost analysis. We encourage students to develop a spreadsheet and alter the numbers to determine what they must achieve to meet their return on investment requirements. They should then make a judgement on whether those improvements are actually viable.