The injection molding industry generally perceives companies that use hot runner systems as more advanced than companies that use cold runner systems. Often, shops make the decision to use a hot runner without considering a cold runner’s possible cost and quality benefits, or management dictates the decision as part of a cost-savings initiative. 

On the surface, using a hot runner seems perfectly valid. Hot runners require less material, which decreases part cost. However, hot runners can actually create more scrap because of downtime, leaks or other issues. The savings on paper do not always add up to real money in the bank.

Many companies that only run hot runners. Others run a mix of hot and cold runners in their facilities, and they have one thing in common: they all complain about their hot runner systems. The complaints are often about temperature uniformity, quality, support, downtime, maintenance and the narrow process window they often run into with these systems.

Digging into the design of a hot runner system reveals all the places from which potential issues could stem. Understanding the source of these hot runner issues and their impact on a shop’s bottom line can help a shop determine if a cold runner is the better option for some of its applications.

Evaluating Sources of Variation

Here is a checklist of several common hot runner challenges and sources of variation to consider in deciding between a hot runner or cold runner solution:

1) High pressures at assembly points

2) Required co-existence with a cold mold and platens

3) Gating issues (for example, cooling, or molten material on one side and frozen material on the other, clogging, drooling, stringing and control systems)

4) Thermal expansion and alignment of drilled channels

5) Stack-up tolerances

6) Hang-up/dead flow in channel geometry

7) Higher skill-level requirement

8) Smaller shot size yielding less velocity control during injection

9) Cycle interruptions, which potentially create more issues

10) Leaking issues

11) Controller performance

12) Heater resistance

13) Thermocouple placement and contact

14) Dimensional variations of the gate tip

15) Nozzle bore details

16) Cross drill alignment

17) Valve pin seating

18) Flow imbalances

  1. Power consumption

Evaluating Costs

A hot runner is an investment, and it should be viewed accordingly. If the investment does not meet the criteria for payback over a desired return on investment timeframe, then a cold runner may be a better option. When evaluating the cost of a hot runner system, consider both the direct and  indirect costs.

Direct costs include the hot runner system, spare parts (tips, heaters, thermocouples and nozzles) and controller components. The indirect costs include maintenance issues, residence time, color changes (time, material, purging compound), potential variation in temperature throughout production, effect on part quality and process window and training requirements