C-4-Runner System

Runner is a system (path) of mold for plastic material flow travel from outside of mold into cavity area, see concept in pic.-illustration of mold.

Illustration of mold
  • In which
    Red: Runner system, Orange: location ring (a part of mold to connect to injection unit), Joint of red line and part: Gate (entrance of cavity)
  • During production, plastic inside mold could be divided into 2 parts: runner and part.

Here discuss 3 runner systems

  1. Cold runner
    a. Introduction to Cold runner
    b. Mold type for different Cold runner
  2. Hot runner system
    a. Introduction to Hot runner system
    b. Valve gate type Hot runner system
  3. Semi Hot runner system

  1. Cold runner
    The injection molding process with Cold runner design mold produces parts and Cold runner. The runner part plastic mostly become waste, or lower grade raw material after recycling.
Part with Type-1 cold runner
Part without cold runner
Part with Type-2 cold runner
Part without cold runner

Generally, Cold runner could be divided into areas below, see detail in pic.-illustration of Cold runner type-1

  • Injection entrance
    Where material enters mold. (close to location ring)
  • Sprue
    Let material flow travel to same level of Main runner.
  • Cold slug well
    Let lower temperature material flow stay instead of into cavity.
  • Main runner
    Let material flow travel to Branch runner.
  • Branch runner
    Let material flow travel to Gate.
  • Gate
    Let material flow travel into cavity.
Illustration of Cold runner type-1

Some parts need many even distributed Gates to fulfill injection molding process requirement. The Gates are at different level with Main runner, material flow needs to travel through second Sprue then reach Gate, see concept in pic.-illustration of Cold runner type 2.

Illustration of Cold runner type 2

  1. Hot runner system: The function of Hot runner system (also known as HRS) is to keep plastic in this system stay melted by providing heat. The material flow travels inside HRS from Injection entrance to end of Hot tip, see concept in pic.-illustration of Hot runner system inside mold. It brings following effects to injection molding process:
  • Reduce waste of raw material: Plastic material flow in HRS stays melted and keep joining next injection molding cycle. Each shot weight of mold with HRS will be less comparing to mold with Cold runner.
  • Reduce machine load: Less shot weight stands for less plastic shall be injected by inject molding machine during production, this can reduce injection pressure requirement.
Illustration of Hot runner system inside mold

Note: Red: a. Plastic material flow, b. This is a concept for basic 2 Gates/Cavities design.

HRS is an “Additional” system to be installed inside mold (mostly at fixed side of mold), generally it consists of following items:

Manifold: keep plastic material inside melted.
Hot tip: keep plastic material inside melted.
Temperature controller: control temperature inside HRS.
Connector: connect HRS to Temperature controller and other equipment.

Valve gate type Hot runner system: The major purpose of Valve gate type Hot runner system (also known as HRS) is to give user ability to adjust the timing of when material flow leaves each Hot tip. Weld line is the joint of material flow come from different directions; it could lead to part strength / cosmetic issue. By controlling the timing of material flow enter into cavity area, it is possible to move Weld line to an area on part where is acceptable to have defects.

Without valve gate type hot runner system
With valve gate type hot runner system

Valve gate is an “Additional” system to be installed into HRS, generally it consists of following items (See concept in table-4)

  1. Valve pin: control material flow.
  2. Cylinder: control the movement of Valve pin.
Valve gate status: Close
Material flow can NOT leave Hot tip & enter cavity area.
Valve gate status: Open
Material flow CAN leave Hot tip & enter cavity area.

  1. Semi Hot runner system: This is a combination of Cold runner and Hot runner system. When material flow leaves Hot tip, it enters a small piece of Cold runner, then into cavity. Semi Hot runner system allows most flexibility of gate location and less waste of raw material. Some parts design has higher cosmetic requirements, this system is an option as well, see concept in Pic.-Semi Hot runner system concept.

Mold with Semi Hot runner system able to move / hide Gate and fulfill part design requirement.

Semi Hot runner system concept

Note: Blue: part, Orange: area accept no defects (Gate, Eject mark…etc.), Green: Cold runner


In the end, we compare 3 runner type and list items shall be considered during mold design stage.

Table Comparison of 3 runner system

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