As a traditional melt delivery system, cold runners still have a place in injection molding, but when demand for plastic parts and required output rises, transition to hot runners can bring countless benefits to injection molding plants.
In injection molding process, one of the most important functions of mold is to transport melt from barrel to cavity of mold. Melt delivery management has an important influence on final quality of parts and final success of production line. Precise control of resin flow, pressure and temperature is crucial in this process. Therefore, a more in-depth comparison of hot runner and cold runner solutions is very important for initial investment of project and economics of subsequent production.
Cold runner is a melt plastic flow channel processed on parting surface of mold. Main purpose is to provide a channel for molten resin to flow through gate and enter cavity. After entering cavity, molten resin cools and hardens, is ejected together with part, next injection cycle starts again. Cold runner system usually has a small impact on initial mold cost or initial capital expenditure, that is, initial investment cost of project will be small. They are usually used in short-term or infrequent production runs and low-cavity molds.

 

With increasing demand for injection molding output, application of high-cavity molds has become more and more widespread. When this happens, cold runner will have many adverse effects on processing and cost, such as waste of runner material and so on. Complex cold runner system means more resin is injected into mold, cooled and processed. Layout of cold runners may be more complicated. In order to accommodate complex runners, spacing between mold cavities must be larger.
In addition, cold runner will also bring longer cycle time, because melt will still be filled into cavity during feeding stage. Thickness of cold runner is usually greater than thickness of part, so it takes longer to cool runner to ensure safe operation. In addition, gate of cold runner may require a larger mold opening stroke to be ejected smoothly, which will also extend cycle time. When mold is opened, it takes time to remove runner from mold by gravity or a robot. All of these will extend cycle time of injection molding.

Hot runner is heated to ensure that plastics in runner and gate remain molten. Since there are heating rods and heating rings near or in the center of runner, entire runner from nozzle exit of injection molding machine to gate is in a high temperature state, so that plastic in runner is kept molten, and there is generally no need to open runner to take out condensate after shutdown. When starting machine, you only need to heat runner to required temperature. Therefore, hot runner process is sometimes referred to as a thermal header system, or as runnerless molding. Its function is similar to cold runner, because it provides a path for molten resin through a high-tech and well-designed system without allowing molten resin to solidify in runner. Melt flow channel is enclosed in a steel manifold and will not be exposed to air. Manifold distributes molten resin to nozzles to enter gate and cavity. Temperature of manifold and nozzle is adjusted by thermocouples and heaters to keep temperature of molten plastic consistent. Relatively speaking, cold runner can be compared to an open-air downtown street, while hot runner is more like a subway tunnel.

 

Following figure shows a typical injection molding cycle and breakdown of its steps, comparing cycle time required for cold runner molds and hot runner molds.

 

It can be clearly seen from above figure that under same conditions, cycle time of hot runner mold will be shortened by about 20% or more than that of cold runner mold.
Hot runners are generally considered to be one of the most complex systems in injection molding process. Various materials and their properties have an impact on overall engineering and final process performance. There are many considerations for gate design and temperature control, depending on plastic and application. Needle valve can be thermally controlled by a hot nozzle or mechanically controlled by a valve nozzle. Temperature controller can be used to monitor, control and fine-tune process temperature.

 

In contrast, hot runners do not need to be processed during mold processing. There is no need for time, personnel or robots to remove solidified runner waste or dispose of recycled materials during injection molding production process. All of this means reducing costs, reducing equipment maintenance, making better use of floor space, and saving valuable time .
Shortened cycle brought by hot runner can increase production. In some injection molding plants, you can still see some promotional posters that read "Changing cycle time from 10 seconds to 9 seconds can increase output by 11%." To estimate impact of cycle time changes on production, divide current time by required time (in same unit). Following table illustrates changes in cycle time and resulting energy changes.

 

For example, if cycle time is reduced from 15 seconds to 13 seconds, capacity will increase by 15%, and so on.

We all know that hot runner is more economical for long-term production, but first investment is more expensive. So how to decide more scientifically whether a project should use hot runner or cold runner? Suppose there is a 64-cavity cold runner mold with a cycle time of 14 seconds. It is planned to produce 7 days a week and run 3 shifts a day. With 90% uptime, it will produce about 2 million cycles and produce about 129.3 million parts per year. Weight of a single part is 2 grams, total weight of 64-cavity product is 128 grams, and weight of cold runner is 192 grams, which is equivalent to total injection weight of 320 grams.
This project requires 656,900 kilograms of materials each year, but only 259,000 kilograms of materials are useful products. Remaining 60% must be reground, processed, counted and transported before it can be recycled. If this project directly uses hot runner gates, there are much less materials that can be purchased and cycle time can be shortened. Assuming that cycle can be shortened to 10 seconds, 2.8 million cycles can be produced every year, and 181.1 million products can be produced every year. There is no doubt that Hot Runner is the best choice for this project.

In addition to reducing waste and increasing output, hot runners can also better control part quality, dimensional accuracy and surface finish. Gate position is no longer subject to many restrictions, gate position can be moved to guide melt flow, thereby better affecting critical dimensions or other characteristics affected by filling characteristics. Hot runner will also bring a shorter melt flow length, which can reduce filling pressure requirement of cavity and other process factors that affect warpage of part. Clamping force requirements may also be reduced.

Gate location of hot runner can make parts more functional and have a better surface finish.
Since hot runner gate allows repositioning to any position on part, it can eliminate runner trimming, secondary operations and their inconsistencies. This will bring greater design freedom for products in many precision industries.
With advancement of hot runner technology, we continue to see a strong momentum of transition from cold runner to hot runner. With continuous advancement of design of plastic parts, reduction of resin consumption is still primary consideration for all molders. We can foresee that through advanced hot runner technology, melt transportation management will play an increasingly important role in injection molding process.