Our six steps for a perfect simulation

---

Plastic components are in use by every industry and manufacturing these components through injection molding has come a long way. A wide range of equipment options exist depending on your application and capabilities. Generally speaking you have a choice between traditional cold runners or the more advanced hot runners. Each option comes with its own unique sets of pros and cons and so understanding the differences and how they relate to your application could have a big impact on your productivity and overall profitability.

Cold Runners

In a cold runner mold, the molten thermoplastic is injected into the mold which fills the runners that distribute the molten plastic to the individual mold cavities. The cold runner mold then cools the sprue, runner, and gate along with the molded part.

Cold runner molds are certainly more economical to manufacture and can be easier to maintain, however they have several major limitations compared to molds with hot runner systems:

Longer cycle time

Creates waste (sub-runners)

Require additional auxiliary processing equipment (robotics, re-grinding machines/employee labor to remove runners, etc.)

Secondary operations (degating, removal of cold runners, re-grinding etc.)

Why Choose Hot Runners

While hot runners often come with a higher upfront cost and require some additional maintenance, their more efficient design can often easily provide a valuable return on this investment. Hot runners significantly overcome the inefficiencies of its cold runner counterpart.

Hot runner systems produce less wasted plastic, have shorter cycle times, use less energy, improve gate quality, use fewer auxiliaries and require less manual labor for runner handling, trimming and regrinding.

Wasted Plastic & Energy

Depending on the part design, the cold runner can equal 50% to 250% of the mold part weight with regrind typically limited to 15% at most, so the remaining 85% is waste or has minimal salvage value. Re-grinding also adds a step in the manufacturing process and could decrease the plastic’s mechanical properties

For some markets, this waste could be much higher. The medical market requires 100% virgin resin, so all of the runner would be scrap. The energy consumption of a cold runner can double due to extra heat, cool and regrind wasted.

For many applications, the wasted runner can double the part cost.

Cycle Time

Cycle time is typically dominated by part cooling, with cooling time being dictated by part wall thickness or cold runner thickness. Even optimized cold runners cause typically 50% to 100% longer cycle times than hot runners.

Hot runners offer higher productivity yields due to significantly reduced process cycle times.

Capital Equipment

Cold runners mold with 3 plate design, trimming equipment, regrinding equipment, added chilling/cooling capacity and metering blender. Hot runners only require a manifold, nozzles and plates as well as a temperature controller, which is reusable.

Managing additional overhead and operational factors such as added chilling capacity and the noise and dust related to grinding scrap runners.

Labor Costs

Cold runner costs include runner handling, trimming, re-blending and scrap. They are prone to occasional stick in molds interrupting overall operation. Maintenance is also required on numerous auxiliaries.

Hot runners are highly automated and are ideally suited to scheduled preventative maintenance.  Interruptions are possible with failed heaters or thermocouples but depending on the hot runner manufacturer, these interruptions can be minimal.

Eliminating the cold runner saves the added labor from runner handling, gate trimming and regrinding.

For years, natural balance has been the cornerstone of a successful hot runner balance. This means that the melt experiences the same flow length and the same diameter melt channels from when it leaves the machine nozzle until it enters each cavity within the mold.  This approach has served the industry well.

In recent years, much attention has been paid to the effects that shear has on the melt as it flows through a cold runner system. Of specific interest is how shear heated melt is distributed by the cold runner geometry. Research in this field has led to a greater understanding of shear-induced variances as they apply to cold runner systems and led to the introduction of technologies that are aimed to help address molding issues which have irritated molders for years.

It has also led to a greater awareness of the topic by the general injection molding community. While the majority of published research on this phenomenon has been based on cold runner systems, it has naturally called into question the validity of natural balance principles as applied to hot runner systems.

Balance Sensitive Applications

Uniform balance is always desirable. If a mold is significantly imbalanced, it will be difficult to start up and may have a narrow process window. The balance that can be achieved between cavities on a multi-cavity mold will have a bearing on the part to part consistency. That being said, there are some applications that will require a higher degree of balance than others. These will include parts that have a demanding dimensional requirement or parts that will be difficult to eject if they are over packed. Here uniform balance is important to ensure that all cavities are uniformly filled. It is important to recognize applications where balance will be critical.

However, it’s important to recognize that there are some fundamental differences between hot runner system and cold runner system designs. Cold runner systems are more prone to the effects of shear due to their inherent design.