(Updated April 2019)

Cold runner systems are used for plastic injection molding processes. This system cools the material in the channel between the nozzle and the cavity (the runner) and is then ejected along with the part. These systems offer a simpler and less-expensive alternative to hot runner systems. Cold runner systems offer advantages such as reduced maintenance requirements and rapid color changes.

Cold runner systems are used on either two or three plate molds.  Two-plate molds have one parting line along which the mold is split into two halves.  Three-plate molds have two parting lines.  When a part is ejected, the mold splits into three sections. 

Three-plate cold runner plastic injection molding provides for greater design flexibility and allows gates to be installed according to application requirements. Cold runner systems also differ in terms of product specifications and features. 

Regardless of whether the alignment uses two or four pins, or whether the assembly screw orientation for cold runner systems is from the parting line - or from the back side, the obvious disadvantage of this system is the wasted plastic that gets generated. 

The runners are either disposed of - or they are reground and reprocessed with the original material.  This adds steps in the manufacturing process. 


Considerations for Designing Cold Runner Molds

It is important that the cold runner dimension is thicker than the component.  This ensures that molten material can be packed into the component as it cools without any restriction. 

The most efficient and easy to machine cold runner design is the round runner, with each half of the mold containing one half of the runner diameter.  Another runner design is a round trapezoid runner milled either in the core-side or the cavity-side of the tool. 

Gating is also important. Tunnel-gates are frequently used in combination with cold runner systems because trimming the gate from the part happens automatically.  Typically, the action of either the mold opening or part ejecting will remove the gate from the part.  If the gate is removed using an ejector pin, the distance between the gate and the ejector pin needs to be maintained carefully so the gate breaks properly. 


The Advantages of Hot Runner Plastic Injection Molding Systems

With hot runner plastic injection molding systems, your runner is internal in the mold, and is kept at a temperature above the melting point of the plastic.  This means the runner-scrap is reduced or eliminated. 

A hot runner system might be much more expensive than a cold runner system, yet they reduce cycle times. With the elimination of the runner, your result is a reduction in the cooling portion of the cycle, thus reducing the overall cycle time.  

Also - the reduction of the overall shot-weight means that injection time is reduced and the resin’s flow path is much shorter.  Elimination of the runner-plate movement reduces the clamp motion allowing for full clamp speed to be incorporated, which reduces cycle times.  Mold open-dwelling time is reduced since the system doesn't have to eject the runner, further reducing cycle times.

Material and color changes are more difficult in hot runner systems because it is virtually impossible to remove all the plastic from an internal runner system.  Yet hot runner plastic injection molding systems are popular for high-production runs. Also, hot runner systems are used when the injection mold has a lot of cavities, when expensive plastics are being used and in high-quality molding situations where gate vestiges should be minimal.


Understanding Hot Runner System for Plastic Injection Molding

Hot runner molds are two-plate molds.  Hot runner systems are divided into two parts: the manifold and the drops.  The manifold has channels that carry the plastic on a single plane, parallel to the parting line - to a point above the cavity.  The drops are placed perpendicular to the manifold - and carry the plastic from the manifold to the part.

Hot runner systems are designated by how the plastic is heated. Either internally or externally heated drops and manifolds are used. Externally heated hot runner channels have the lowest pressure-drop of any runner systems - they are better for color changes, as none of the plastic in the runner system freezes.  Since there are no places for the material to hang up and degrade, externally-heated systems are good for thermally-sensitive materials.

Internally-heated runner systems require higher molding pressures - making color changes very difficult. Internally heated drops offer better gate-tip control and also better separate runner heat from the mold - because an insulating frozen layer is formed against the steel wall on the inside of the flow channels. 


Crescent Industries’ Cold Runner and Hot Runner Molding Capabilities

Crescent Industries builds and utilizes both cold runner and hot runner injection molds to produce quality plastic components.  Our engineers will guide you through the process to ensure your project is designed for manufacturability and all of your product specifications are met. 


For additional information, please fill out the form below to get our white paper “10 Factors that Impact Tooling Costs”. 

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Sources for article content:

  • IHS GlobalSpec Cold Runner Systems Information
  • Kenplas.com/mold/injectionmold/moldtype.aspx
  • Tips for perfect Cold-runners Injection World; published June 2013.


Topics: injection mold building