"Hot runner" is a term used in injection molding that refers to the system of parts that are physically heated such that they can be more effectively used to transfer molten plastic from a machine's nozzle into the various mold tool cavities that combine to form the shell of your part. Sometimes they are called "hot sprues." You can contrast the term "hot runner" with its opposite, and the historically more common "cold runner." Cold runners are simply an unheated, physical channel that is used to direct molten plastic into a mold tool cavity after it leaves the nozzle. The primary difference is that hot runners are heated while cold runners are not.
Hot runners use a screw nozzle that is fed by a barrel using a pump, while cold runners use a closed, thermoset mold. The primary task of any injection runner system is to direct the material flow from the sprue to the mold cavities. The system requires additional pressure to push the material through the runner. Frictional heat, generated within the runner mold by the material, flows through the runner and raises the temperature, which facilitates the flow. One of the best pay-offs for any project relates to the proper sizing of the runner for a given component and mold design.
Cold Runner Molds
Advantages
● Inexpensive and easy to maintain
● Suitable for more types of polymers, since you don't have to worry about heat sensitivity
● Color changes can be made quickly
Disadvantages
● Longer process than hot runner system
● Need for either robotics or a person to remove runners
● Creates waste if the runners cannot be melted down and recycled
Hot Runner Molds
Advantages
● Faster cycle time
● Eliminates waste because there are no runners
● Can accommodate larger parts and a higher volume of production
● Better consistency and quality of the parts
● Less pressure is needed to push the molten mixture into the mold cavity
Disadvantages
● Higher cost to produce molds and maintain equipment
● Color changes cannot easily be made
● May not be suitable for some thermally sensitive material
When considering which injection molding system is better-hot or cold runners-there are many things to consider. A hot runner system eliminates much of the material waste created during cold runner injection molding but is a more expensive system. However, the fact that hot runners create less waste could off-set the cost of maintenance. The cold runner molds, on the other hand, can create waste if the runners are not recycled, but this system is less expensive and easier to maintain.
