Plastic injection molding is a great choice for manufacturing parts. However, contains some concepts and mechanics that prospective users should understand when it comes to the designing and building of molds. Molds have many parts and options for parts, and it’s important to understand how the various options contribute to the performance, cost, and complexity of a mold. In this article we look at the runner systems found in injection tools and how selecting hot or cold runners will impact the final product.
What are Runner Systems in Injection Molding?
To understand the differences between hot and cold runners, it’s first important to understand how runners factor into a mold. Molten plastic is injected into the mold via the sprue, where the nozzle meets the mold. The plastic then travels through passageways called runners to reach the actual cavity or cavities of the mold via the gateways. Most molds have at least two or more gateways and runners that lead to them, but some molds, especially those that cause many parts, have much more.
Hot Runner Molds: Speed at a Price
Hot runners exist for one reason: once the plastic has filled the mold cavity and cooled, it has also filled the runners. If left alone, it will cool as well and be ejected along with the part (depending on the mold type, they either remain attached to the part or are separated before ejection). Heated runners make use of a heated manifold that keeps the plastic in the runners molten, which has some significant advantages and disadvantages.
Hot Runner Advantages
- Eliminates waste plastic because the runner plastic doesn’t need to be ejected with the part.
- Have plastic ready to go in the runners leads to faster cycle times and higher part volume.
- Without the resistance of the cooling plastic in the runners, less pressure is needed.
- Without having to worry about plastic cooling in the runners, molds can make larger parts.
Hot Runner Disadvantages
- The mold has higher upfront costs because of the more complex design.
- Some plastics that are thermally sensitive (thermally conductive) will not work in hot runners.
- The mold must be powered for the heating system.
- The added complexity of the mold means higher inspection and maintenance needs.
Cold Runner Molds: Jack of All Trades
In a cold runner system, the runners operate at the same temperature as the rest of the mold without additional heating systems. The sprue, runner, and gate are all cooled along with the mold and the additional plastic left attached to the part or ejected separately (depending on the complexity of the mold). The plastic can be recovered and recycled if it’s made out of a thermoplastic resin.
Cold Runner Advantages
- Due to the lack of a complex heating system, cold runner molds cost less to create.
- Can accept a wider range of polymers as it doesn’t have to worry about heat sensitivity.
- Lower maintenance and inspection needs due to a more simple design.
- Easier design changes (movement of gate and runners) than hot runner systems.
Cold Runner Disadvantages
- Depending on the mold, it will need manual or automated removal of the extra plastic.
- Longer cycle times due to the need to eject the sprues and runners plastic.
- More waste creation from extra ejected plastic (if not recycled).
- Limited in part design and size due to the limitations of cooling plastic in the runners.
So, which type of runner is the right choice for your mold? Get a quote and consultation from us here at Midstate Mold. We can go over the various options for molds and plastics to build you the perfect mold for your needs. With more than 100+ years of injection mold design, tooling, and part production experience, we can help you with every part of the process. Contact us today to get started.