There are two main types of injection molds: cold runner and 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.
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.
Types of Cold Runner Molds
There are two major types of cold runner molds: two plate and three plate.
A two plate cold runner mold is the simplest type of mold. It is called a two plate mold because there is one parting plane, and the mold splits into two halves. The runner system must be located on this parting plane; thus the part can only be gated on its perimeter.
Cold Runner Molding: Efficient and Economical
Significant advantages to using a cold runner system include:
- Simple mold design
- Considerably less expensive than hot runner system
- Less skill required to set up and operate
- Mold requires less maintenance
- Color changes are easy - all plastic in the mold is ejected with each cycle
With a cold runner mold, the runner (excess material retained in the feed channels) is cooled and ejected with the part. Every cycle, a part and a runner are produced.
Cold Runner Molding: A Few Disadvantages to Consider
While it is a less expensive process than hot runner systems, there are a few disadvantages to cold runner systems to consider, including:
- Waste plastic generated
- Runner material must be either disposed of or
reground and reprocessed - Additional steps in the manufacturing process
- Regrind will increase variations in the injection
molding process - Regrinding could decrease the plastic's strength
and mechanical properties
Cold Runner Molding: Get the Right Mold for Your Project
Cold runner molds are the least expensive to build for parts of simple design. Depending on part complexity, one of two types of cold runner molds may be used:
- A two plate cold runner mold is the easiest and least expensive type of mold to produce.
- Two plate molds have a single parting plane, and the mold splits into two halves at the plane
- Because the runner system must be in line with the parting plane, the part can only be gated on its perimeter
- A three plate cold runner mold differs from a two plate in that it has two parting planes, and the mold splits into three sections each time a part is ejected
- This feature provides greater flexibility of design, which allows gating to be placed in the most efficient locations
- Since the mold has two parting planes, the runner system can be located on one, and the part on the other, which allows for easier plate separation
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 is 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.
Primary advantages of hot runner molding include:
- Materials cost savings - no runner to regrind or reprocess
- Least expensive cost / piece
- Reduction of energy costs
- Shorter, faster cycle times - no runners to cool
- Smaller machines - reduced shot volume into runners
- Automated processing – runners do not need de-molding
- Gates at the best position for economical design
- Elimination of runners means
- No runners to remove or regrind
- Reduces the possibility of contamination
- Lower injection pressures
- Lower clamping pressure
- Consistent heat within the cavity
- Shorter cooling time
- Shot size reduced
- Cleaner molding process
- Eliminates nozzle freeze
Hot Runner Molding: Precise Computer-Controlled Accuracy and Advanced Processes
Hot runner molds are usually connected to needle valve nozzles, which are activated with precise computer-controlled timing for accurate reproduction of parts.
This allows for a number of advanced processes, including:
- Multi-Cavity Molds – cavities with different geometries and/or volumes
- Parts that belong together produced in one mold
- Injection valve opening and closing can be adjusted to the conditions of each individual cavity
- Injection pressure and holding pressure may be adjusted independently of each other
- Controlled Volume Balancing – a weld line can be shifted into a non-critical area of the molded part
- Stack Molding – multiple parting lines for additional cavities in a given area
Hot-Runner Systems: Weigh the Cons with the Pros
There are, however, a few disadvantages to hot runner systems that need to be considered:
- Hot runner molds are more complex and expensive to build than cold runner molds
- Higher initial start-up costs than cold runner systems
- Complex initial setup prior to running the mold
- Higher maintenance costs – more susceptible to:
- Breakdowns
- Leakage
- Heating element failure
- Wear caused by filled materials
- Risk of thermal damage to sensitive materials
- Elaborate temperature control required
