Cold Runner vs Hot Runner Molds

Plastic injection molding uses two different mold types: cold runner and hot runner. Each type has its benefits and drawbacks.

Cold Runner Molds

The mold base of a cold runner mold typically has two or three plates held within it. Plastic is injected into the mold using a sprue. This plastic fills up the runners and travels into the mold cavity.

• In two-plate cold runner molds, the runner system is attached to the mold, and an ejection system is in place to separate them.
• In three-plate cold runner molds, the runner is separated on its own plate, so only the finished part will be ejected.

In both two- and three-plate cold runner systems, these runners can be reground and recycled. This process increases overall cycle times but also reduces plastic waste.

Advantages of cold runner molds include:

• Cost-effective
• Low maintenance costs
• Can use many different polymer resin types
• Engineered or commodity
• Facilitates fast color changes
• Improved cycle times when robotic assist is available to remove runners

Disadvantages of cold runner molds include:

• Slower cycle times compared to hot runner systems
• Increased waste if plastic runners aren’t recycled

Hot Runner Molds

Hot runner molds are built from two plates heated by a manifold system. The manifold heats the plastic, and then the mold cavities are filled with molten plastic via nozzles.

While a variety of hot runner systems exist, they can largely be split into two categories: internally heated and externally heated.

• Internally heated hot runner systems offer better control of plastic flow.
• Externally heated hot runner systems are best for polymers that are sensitive to changes in temperature.

Hot runner systems eliminate runners, so there is no need to recycle or regrind runner plastics.

Insulated runners are variations on the hot runner system which are used for injecting semi-crystalline polymers with low levels of thermal conductivity. Instead of using heat to keep the plastic in its melted state, the insulation on the runners keeps it from cooling.

Advantages of hot runner systems include:

• Less waste
• Potential for faster cycle times
• No need for automation to remove runners
• Can handle large parts

Disadvantages of hot runner systems include:

• Molds are more expensive to create
• Difficult to change colors
• High maintenance costs
• Higher potential downtime

• Often not suitable for thermally sensitive resins