BASIC TYPES OF INJECTION MOULDS

There are two main types of injection molds:

1. Cold runner.
2. Hot runner.
A runner is the channel in the mold that conveys the plastic from the barrel of the injection molding machine to the part.

1. Cold runner

In a cold runner mold, the runner is cooled and ejected with the part. Every cycle, a part and a runner are produced. The obvious disadvantage of this system is the waste plastic generated. The runners are either disposed of, or reground and reprocessed with the original material. This adds a step in the manufacturing process. Also, regrind will increase variation in the injection molding process, and could decrease the plastic's mechanical properties.
Despite these disadvantages, there are many significant advantages to using a cold runner mold. The mold design is very simple, and much cheaper than a hot runner system. The mold requires less maintenance and less skill to set up and operate. Color changes are also very easy, since all of the plastic in the mold is ejected with each cycle.

2. Hot runner

In a hot runner mold , the runner is situated internally in the mold and kept a temperature above the melting point of the plastic. Runner scrap is reduced or eliminated. The major disadvantages of a hot runner are that it is much more expensive than a cold runner, it requires costly maintenance, and requires more skill to operate. Color changes with hot runner molds can be difficult, since it is virtually impossible to remove all of the plastic from an internal runner system.
Hot runners have many advantages. They can completely eliminate runner scrap, so there are no runners to sort from the parts, and no runners to throw away or regrind and remix into the original material. Hot runners are popular in high production parts, especially with a lot of cavities.