Hot Runner Molds replace the runners in conventional or three-plate molds with a system that maintains the runners at a constant heated temperature. This innovative design eliminates the need to remove runner material after each molding cycle. The appropriate timing for using hot runners can be examined from the following four perspectives.

Structural Differences in Mold Design:

For molding exceptionally large parts, such as automotive interior panels or appliance housings, hot runners are essential to ensure balanced plastic flow. This is particularly critical when requiring simultaneous or sequential injection at multiple gates.

For side gates offset from the injection molding machine’s center, using hot runners simplifies mold construction, facilitates easier molding, increases cycle speed, reduces molding time, and eliminates sprue waste—achieving multiple benefits at once.

Disadvantages of Three-Plate Molds:

In three-plate molds, the heavy moving plate must slide along guide pillars with every shot, a motion that even a new mold may struggle to sustain, leading to reduced mold lifespan.

During each ejection cycle in a three-plate mold, the plate movement required to remove the runner is greater than what is needed to eject the part itself.

Conditions for Using Hot Runners:

Hot runners are used when gating from the ejection side or when a long sprue is necessary, avoiding issues caused by excessively long sprues. They are also suitable for large parts or products that permit off-center gating.

In three-plate molds, the moving plate must shift to remove the sprue. With hot runners, the mold opening stroke can be shortened, eliminating the need for large molding machines typically required for three-plate molds and allowing smaller machines to be used instead.

Hot runner systems address challenges in molding difficult materials, such as high-viscosity, low-stability, or high-temperature plastics. Practical examples include metal powder injection molding (MIM), ceramic injection molding (CIM), plastic magnet molding, plastic bearing molding, and thermoplastic rubber processing.

Material Savings:

Cost of Cold Runner Waste:

For instance, if cold runners account for 68% of scrap (where 1 kg of material yields only 320 g of product, with 680 g as runner waste), although runners can be recycled, factors such as labor and blending ratios mean that a stock of cold runners must be maintained for stable operation, tying up capital.

High-Speed Injection Molding:

High-speed injection is essential not only for improving efficiency but also for molding thin-walled products like cups and containers.

Stack Mold Applications:

For thin, high-volume products such as CD cases or small items, stack molds can increase output by up to 80% with only a 15% increase in clamping force and the same injection time.

Environmental and Efficiency Advantages of Hot Runners:

Hot runners generate no "waste," eliminating waste handling issues. This "waste" represents resource consumption, and hot runner systems, as low-carbon solutions, minimize environmental impact.

They save storage space for runners, avoid noise from granulation, and prevent material degradation:

With numerous plastic types and colors, storing runners requires sorting, dehumidifying, drying, and sealed storage. Granulation also creates noise, and poor working conditions can affect employee morale.

In summary, switching from three-plate molds to hot runners offers the following benefits:

Easy runner removal with reduced mold opening stroke.

More uniform plastic flow during injection, along with independent control of each gate, facilitating smoother injection.

Savings on plastic material and labor costs.