October 8, 2021

Hot Runner Fundamentals

How melt delivery technology helps minimize waste and increase production efficiencies


What is a hot runner system? This is probably the first question for those who are not fully familiar with the injection molding process. Hot runner systems are a critical component of the injection molding process. Since their introduction to the industry, hot runners have continued to play a key role. While hot runners increase the initial cost of a mold, their many production enhancements make them the more economical choice over the long term. 

What is a hot runner?

Hot runners allow for increased productivity and system performance, producing better part aesthetics, greater flexibility and improved process monitoring. In addition, they improve energy efficiency and eliminate scrap plastic, resulting in faster cycle times and reduced part cost. They also deliver melt to the mold with high gate quality, excellent cavity-to-cavity balance, fast color changes, total design freedom and greater customizability. Hot runner solutions help processors achieve the highest levels of quality and part volume at the lowest part cost. And they are available in both hot tip and valve gate configurations, optimized for a processor’s specific application.

Applications with high output and long product life have the most to gain by using hot runner technology, but so do lower cavitation molds where speed and consistency are important to continuous improvement. This article provides an overview of how hot runner melt delivery systems work and how they can improve the injection molding process.

Cold Runner Limitations

During the injection molding process, one of the most important requirements of the mold is to deliver melt from the barrel to the cavity. Melt delivery management has a major impact on final part quality and the ultimate success of the production line. Precise control of resin flow, pressure and temperature is critical. The success of the molding process is directly related to the design of the melt delivery system, so a close examination of a hot runner vs. cold runner solution is helpful.

Cold runners are solidified plastic flow channels found inside molds, and they must be removed from the mold and plastic part after the molding process is completed. Cold runners are the result of channels machined in the mold’s parting surface that provide access for molten resin to flow through the gate and into the cavity. The molten resin cools and hardens and is ejected with the part, and the cycle begins again. Cold runner systems typically have little impact on starting mold cost or initial capital expenditure. They are often used in molds designed with short or infrequent production runs and low cavitation.

However, some molded parts generate increased demand and eventually move to higher-cavitation molds. When this happens, a cold runner can have increased impact on processing and its related costs. A larger runner system equates to more resin to inject, cool and handle. Cold runner layouts can be complicated, and cavities must be spaced further apart to accommodate the runner and venting.

Cold runners can be a detriment when it comes to cycle time, since melt is still supplied to the cavity during the pack phase. Cold runners also typically have greater thickness than the part and therefore take the longest time to cool. Clearly, one of the most important considerations is that cold runners are a costly waste of resin. Processors are forced to throw away the runner, recycle it through regrind for use in other processes or sell it as bulk scrap for pennies per pound.

The advantages of a two-plate cold runner mold include compactness, low capital cost and low residence time. It is also simple to maintain, offers rapid color change and has easily machined gates. Among disadvantages are the production of scrap plastic, costly secondary operations for de-gating, extended cycle time, and quality issues including poor surface finish and dimensional stability.

Scrap from a three-plate cold runner system can be reground, but the process creates other issues. Processors are faced with many questions surrounding future use of cold runner regrind in new applications. Does the part require virgin material? Is only a small percentage of regrind allowable? Is there unwanted material in the scrap? Can foreign debris plug gates? Meanwhile, there are numerous de-gating issues such as whether the runners will break off or must be cut off. There is the possible need for manual labor and related capital expenditures for automated de-gating machinery. This investment can be expensive with extended return-on-investment (ROI) ramifications.

Hot Runners Bring Optimization

The alternative melt delivery system is a hot runner. Its function is similar to that of the cold runner, as it provides a path for molten resin to flow through a highly technical and carefully engineered system. The most important difference is that a hot runner never allows molten resin to solidify. The distribution network is completely encased in a steel manifold and never exposed to air.

The manifold distributes molten resin to nozzles (nozzles are often called “drops”) that provide access to the gate and cavity. The manifold and nozzles are regulated with thermocouples and heaters to maintain the temperature of the molten plastic. Where a cold runner can be likened to an open-air downtown street, the hot runner is more like a subway tunnel. While a hot runner increases mold price, it should be considered an investment because of its ability to eliminate waste, reduce cycle time and decrease part variation.

The hot runner is often considered one of the most sophisticated systems in the injection molding process. Various materials and their functional capabilities contribute to the overall engineering and final process performance. Several options are available for gate design and temperature control, depending on resin and application. Gates can be controlled thermally with hot-tip nozzles or mechanically with valve-gate nozzles. These have their own design requirements for how they interact with the mold. Temperature controllers can be used for process monitoring, control and fine adjustment. Husky’s own Ultra® hot runners feature Ultra HelixTM valve gates, a technology that delivers a gate vestige so clean, it is virtually unmeasurable.

Hot runners eliminate the need to handle the cold runner before, during and after processing. Manual labor or robotics are not needed to remove solidified runners or handle regrind, resulting in reduced cost, less equipment to maintain, better use of precious floor space and valuable time savings. With no runners to manage, this eliminates de-gating or facilitating sorting. For processors, these cycle time advantages can translate into significantly increased output.

Clear difference in the cycle time

Here’s another hot runner vs cold runner consideration: While both cold and hot runners go through the same stages of the injection molding process, they influence cycle time in very distinct ways. Cold runners can add cycle time, since melt is still supplied to the cavity during the pack phase. Cold runners typically have a greater thickness than the part and take the longest time to cool for safe handling. In addition, the cold runner’s sprue may require greater open stroke to remove, again adding time to each cycle. While the mold is open, time is needed to remove and clear the runner from the mold, either by gravity or a picker. While the next cycle starts, the cold runner remains.

The clamp opening and closing stroke is reduced with a hot runner system. The part cools quickly and is ready for ejection, versus the cold runner, which must cool before ejection. The hot runner system also has a reduced screw recovery time. The cold runner requires a larger shot size compared to the hot runner, which can use a smaller barrel requiring less energy.

To justify the cost of a hot runner, the overall savings must outweigh the hot runner cost. Various factors must be considered in the cost savings calculation, including part quantities, part weight vs. runner weight, resin cost, machine hourly cost, number of cavities, labor rates and regrind acceptance. Again, Husky Ultra® hot runners feature Ultra HelixTM valve gates, which last for millions of cycles—the best longevity in the industry, significantly reducing machine hourly costs.

There is always a tipping point when processors carefully consider a potential conversion to a hot runner system. For example, consider a 64-cavity cold runner mold with a 14-sec cycle time scheduled to run three shifts a day, seven days a week. With 90% uptime, it will run about 2 million cycles and make about 129.3 million parts annually. A part weight of 2 grams and cold-runner weight of 192 grams equates to a total shot weight of 320 grams.

Hot Runners Enhance Quality

In addition to reducing waste and increasing production, hot runners allow greater control over part quality, dimensional accuracy and surface finish. The gate location is no longer limited to the perimeter and can be moved to direct melt flow in a way that better influences critical dimensions or other features affected by filling characteristics. A shorter total flow length may be possible, which can reduce cavity fill pressure requirements and other process elements that impact part warpage. The projected total area and clamp tonnage requirement may also be reduced.

Of all the components and equipment in the injection molding process, the hot runner provides the greatest flexibility in making adjustments that influence individual cavities in a mold. While a cold runner also can be tuned, the investigation, adjustment and validation can have a long lead time unless the mold is designed to permit quick removal or replacement of cold runner inserts. Changes are often made by hand, making them iterative and difficult to replace. Reverting to the original state can be time-consuming. Steel can be added by welding but requires refinishing, and material properties may not be as resistant to fatigue.

Injection barrel and machine clamp influences are process-encompassing. Swapping mold cavities requires coolant evacuation and can be mechanically intensive, especially on high-cavitation molds. A hot runner’s nozzle tip can be quickly swapped out, heater position changed or zone temperature adjusted; the effect on overall quality is almost immediately observed.

Enhanced Design and Aesthetics

Surface quality is another important factor in hot runner vs cold runner. Well-designed hot runner gate placement can result in a more functional part with better surface quality. Since the gate can be relocated to anywhere on the part, runner trimming, secondary operations and their inconsistency can be eliminated.

Enhanced surface quality can be especially important for applications such as irrigation, where the flow of individual drops of water are controlled, and in medical, where the risk of tearing a surgeon’s glove on a gate/trim vestige must be minimized. Similar performance is required in healthcare, where a catheter must be as unobtrusive as possible. Some iconic packaging and consumer electronic parts known and recognized today would have never been possible without optimal gate design and placement.

Depending on the application, there are many factors that impact the suitable design of the hot runner system. These include gate type (wear, cooling, tolerance and dimensional design), manifold requirements (flow balance, thermal balance, melt channel sizing, mold size and number of cavities) and processing variables (pressure, injection speeds, mold and processing temperatures, resin type, and system timing).

Gating Options Deliver Functionality and Quality

The gate is a tiny restricting feature where plastic enters the cavity. Today, thermal (hot tip) gating and valve gating are the two hot runner options for injection molders. A thermal gate is a simple gating method with no moving parts. The probe is made of copper, a highly conductive material that transfers heat. Static plastic freezes at the gate during hold, creating a membrane/skin that creates a tiny frozen slug to block the gate. The frozen slug is blown out at the start of the injection process. Temperature and position of the hot tip probe are important considerations, and precise machining is necessary.

There can be gate quality issues when thermal gating is employed. Specifically, variation in tip position relative to the gate can impact the gate vestige, sometimes resulting in stringing (angel hair), drooling and even a non-fill part. A clean gate vestige is a competitive edge that Husky designs into its Ultra® hot runners.

The other gating option, valve gating, is a more controlled method that offers higher gate quality. A valve stem actuates to mechanically open or close the gate. Plastic is injected with the valve stem in the “back” position. After hold, the valve stem is actuated forward, mechanically closing the gate. Valve gating offers a wider operating window and greater shot-to-shot consistency compared to thermal gating. The gate diameter is larger, and there is less gate shear and reduced stress in the part. Valve gating also results in no vestige issues.

Valve gating can use two different shutoff methods—conical (taper type) and cylindrical (plunger type). A conical shutoff is used in non-precision applications and requires more strength in the gate area to avoid premature cracking, while the cylindrical shutoff delivers higher gate quality for precision parts by incorporating a tight tolerance slip-fit pin that requires no preload. 

Depending on application requirements, there’s a variety of actuation options for valve-gated hot runner systems. These include hydraulic, pneumatic and electric (servo motor). Valve stems can be moved individually or as a synchronized group with plate actuation. The pneumatic option is the most popular because it is low-cost, cleanroom friendly, and easiest to manage and maintain. 

Manifold requirements also play an important role in initial system design implementation. Hot runner manifolds are available in an unlimited number of custom sizes with flexible drop locations. Manifold shapes and sizes are dependent on the number of mold cavities and spacing. Channel layouts must be balanced to achieve optimal flow conditions. Melt channels have customized sizes depending on the application. Heater profile designs are optimized and validated for every shape and for every resin property.

While most traditional hot runner systems operate at straight angles, there are complex systems that handle more intricate nozzle configurations. Among these options, bolted manifold systems offer varied angled melt delivery for contoured surfaces like headlight bezels. 

Process Controllers Provide Accuracy and Consistency  

Every hot runner system requires a temperature controller to manage temperature and mechanical settings. Husky Altanium® mold controllers provide highly accurate temperature and servo control and are the best fault recovery solutions in the industry. They feature easy-to-use navigation on large full-color touch monitors and are available in a variety of configurations that can be implemented in any injection molding environment.

All Altanium® controllers use Active Reasoning Technology (ART), providing optimized control for greater shot-to-shot and cavity-to-cavity consistency and repeatability. ART delivers tighter control and minimized variability through best-in-class power output delivery, fully isolated thermocouple inputs and industry-leading thermocouple sample rates that ensure the integrity of temperature readings. 

Conclusion: The Importance of Hot Runners 

Hot runner technology has been an invaluable tool in the production of injection-molded parts. It has optimized the molding process with superior melt control and processing performance, yielding higher-quality parts and greater output capability.

Hot runner systems offer significant performance advantages over cold runner systems. Major savings can be achieved through reduced resin usage, cycle time improvements and post-processing elimination. Hot runners can be customized with a variety of engineered solutions configured for a particular application.

The injection molding industry continues to see strong movement in conversion from cold runners to hot runners, along with advances in hot runner technology. Hot runner solutions are positioned to meet new industry challenges as plastic part designs continue to push boundaries, and reduced resin use continues to be a focus for all molders. The expectation is melt-delivery management through sophisticated hot runner technology will play an increasingly critical role in the injection molding process.