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Video Series: Introduction to Hot and Cold Runners for Injection Molded Parts

The feed system is the section of the mold that allows molten plastic to flow from the nozzle of the injection molding machine to the mold’s cavity. Runners can either be cooled and ejected with the part or kept molten and retained in the mold between cycles. These options are referred to as cold runners and hot runners, respectively. This video explores the pros and cons of each option, so you will know which runner type is most likely to be used for the parts you design, as well as how the runner type will affect the quality of your parts.

Cold runners are a great option for low-volume parts because they are easy to machine, so they’re the most economical choice up front. They allow for gating either on the edge or on top of the part. Cold runners are typically seen as scrap, since they are ejected from the mold with the part, though that can be mitigated by granulating them and mixing them with virgin resin.

To mitigate the cost of cold runner scrap, molders and toolmakers will turn to hot runner feed systems. They are complex pieces of equipment that require heating elements, insulated flow channels, and their own controllers. The main benefit of hot runner systems is that they eliminate scrap from runners and sprues, which saves on material cost for the part. This makes them a perfect fit for high volume parts, and high-cost resins.

The choice of runner type will depend on the material, the number of parts that will be made, and the allowable budget for the mold. Understanding these differences will allow you to plan your gating strategy to optimize the quality of your parts.

Feed system designs can be compared with mold-filling simulation software, such as Moldflow. Click here to learn about Moldflow analysis and manufacturing support at The Madison Group.

Want to learn more check out How Do Gates Affect Part Quality and How Does Gate Size Affect Part Dimensions and Performance.

Amanda Nicholson

Amanda Nicholson received her B.S. in Chemical Engineering from the University of Akron.  She worked at The Madison Group as part of TMG’s Manufacturing and Product Development Group. If any of her articles interest you, or if you have any questions regarding the topics covered, please click on the contact tab and send us a short message requesting more information.