Hot runner manifold channel designis essential for the operation of an injection mold, impacting change-out time, part quality and even the number of possible drop locations in the mold. The key to advancing manifold design is having the ability to manufacture and implement curved channels in blocks of steel.
Cross-drilling and plugging a block of steel produces the simplest manifolds. Advanced manifolds may use brazed plug inserts to round internal corners or two-piece brazed manifolds to achieve curved internal channels without plugs. However, plugs are a complicated solution that can still leave drag spots. Brazed plates can be successful, but they have limited size and potential quality issues because of the weakness of braze joints (which are basically a metal “glue”). Manifolds are typically subject to the hottest temperatures and the highest pressure of either the mold or the hot half, making steel strength critical for maintaining the mechanical integrity of the hot half.
Curved manifold channel designproduced by metal additive manufacturing (AM) is a new method capable of achieving a better balance of fill across drop locations (or equalized residence time of the polymer), according to a recent research project by Oak Ridge National Laboratory (ORNL) and HTS International Corp. with the simulation effort headed by Dr. Prashant K. Jain and Dr. John A. Turner of ORNL. The findings in the research mean faster color changes, improved hot runner system stability and reduced pressure requirements. Hybrid metal additive manufacturing equipment has made it possible to produce smooth internal surface finish for H11, H13 and 420 SS variants of components. Hybrid AM involves a machining step during layer buildup, which offers superior surface finish to the standard printed finish from power-bed processes.
To date, metal additive manufacturing has produced advanced, curved manifolds H11 (1.2343) at a footprint of 600 millimeters by 400 millimeters. This is three to five times larger than the size of previous generations of curved manifolds produced using brazing technologies.
Curved manifold channel design produced by metal additive manufacturing is a new method capable of achieving a better balance of fill across drop locations.
The R&D began in 2017 in an effort to understand and publish guidelines for proper hot runner manifold channel design. These guidelines are relevant for hot runner manifold construction, independent of the method of manufacturing. Some of the basic findings are summarized here to showcase the considerable difference between a drilled, straight manifold design and a curved channel design.
Assumptions. This research was performed under the assumptions that polypropylene melts at uniform temperature, which is equal to 250°C (plus or minus 30°C) and that the polypropylene had a Newtonian, laminar flow. The pressure boundary was at an inlet of 10 kilopounds per square inch, and the research was performed using an isothermal steel manifold.
The curved manifold was designed for equal flow channel length. Other parameters could be optimized, including pressure drop and residence time. The results of the simulations were significant: residence time (or the time that the polymer spends in the manifold channels) was nearly equalized along the different flow paths. The residence time was cut from 400 milliseconds to 200 milliseconds in the outer channels , while the residence time was increased from 100 milliseconds to 200 milliseconds in the interior channels .
A look at the normalized, surface-concentration profile further validates the equal filling profiles in the curved manifold compared with the conventional manifold. While the cases shown represent two extreme ends of the manifold design spectrum, the benefits of removing constraints on the shape of the manifold channels are clear. For multi-cavity molders, new production capabilities with metal AM can enable the design of higher-density injection gates and can simplify the manifold stack. For single-cavity molders (as in many automotive molds), these new capabilities enable the strategic placement of drops in complex arrangements.
These benefits occur because patterns are developed without regard for intersecting cross-drill patterns, and curved channels can add path length into a reduced footprint for drops closer to the manifold inlet. Curved channels also reduce the path length for drops located further from the manifold inlet. Lastly, curving channels in a single manifold body reduce stack height instead of having to stack symmetrically drilled manifolds to achieve uniform channel length in high-volume tools.
With today’s rapid growth in production, high demands and short lead time can cause additional stress in both the design and launch process. Hot runners are such a common ingredient of injection molding that design of tooling is dependent on the feed system being provided.
The curved manifold was designed for equal flow channel length. Other parameters could be optimized, including pressure drop and residence time. The results of the simulations were significant: residence time (or the time that the polymer spends in the manifold channels) was nearly equalized along the different flow paths. The residence time was cut from 400 milliseconds to 200 milliseconds in the outer channels, while the residence time was increased from 100 milliseconds to 200 milliseconds in the interior channels .
this type of runner requires extremely thick runner channels to stay molten during continuous cycling. These molds have extra-large passages formed in the mold plate. During the fabrication process, the size of the passages in conjunction with the heat applied with each shot results in an open molten flow path. This inexpensive system eliminates the added cost of the manifold and drops, but provides flexible gates of a heated hot runner system. It allows for easy color changes.
Injection Molding PressureInjection pressure. The plastic inside of the hot runner stays hot. This means the hot runner injection pressure drop is significantly smaller than for cold runner systems. You should perform mold flow simulation to get the data necessary to design the proper injection pressure, especially for materials that have poor melt-flow performance and large parts, which require long melt flow and a complete fill.
Injection Molding HeatingHeating. The primary difference between the various types of hot runner systems has to do with the method used to heat the melt. External heating systems keep the material. Externally, the raw material flows through the runner without any carriers. This method provides more reasonable melt shear force curves. Internally heated systems mount right on the melt channel and heat the material inside.
Injection Molding Gate TypeGate type. Hot runner systems have a wide variety of gate types. Aspects you should consider include the gate marks, gate location, and injection of material types. Non-crystalline and crystalline thermoplastic elastomers require different types of gates.
Standard or Custom Injection MoldingStandard or custom-made system. You need to assess whether to go with a standard hot runner system or a custom-made solution. In most cases, the standard solution makes the best choice. The standard length, nozzles, runner boards, gate inserts, and other components means the parts are available. It also reduces delivery time, costs less, and is easier to maintain.
Injection Molding Plastic TypeTypes of plastics processing. When choosing hot runner systems for injection molds, the user must consider the plastic resin. For example, glass-reinforced plastic requires the gate inserts to provide reasonable wear resistance. If the plastic resins easily discompose, you should employ an external heat system runner to avoid dead corners.
Size of Hot or Cold RunnerRunner size. The runner size has a significant effect on the overall performance of the hot runner. Get this wrong and it can cause degradation of plastic injection molded components or even an incomplete filling. To determine the optimal size of the runner, you need to consider the pressure drop, residence time, temperature, shear rate and frequency, as well as other factors.
Injection Molding TemperatureMulti-zone temperature control. When it comes to large, complex hot runner systems and temperature-sensitive plastic resin with tight processing parameters, you should employ a multi-zone temperature controller system. The system can account for heater quality and heat loss.
In recent years, much attention has been paid to the effects that shear has on the melt as it flows through a cold runner system. Of specific interest is how shear heated melt is distributed by the cold runner geometry. Research in this field has led to a greater understanding of shear-induced variances as they apply to cold runner systems and led to the introduction of technologies that are aimed to help address molding issues which have irritated molders for years.
Hot runner system Uniform balance is always desirable. If a mold is significantly imbalanced, it will be difficult to start up and may have a narrow process window. The balance that can be achieved between cavities on a multi-cavity mold will have a bearing on the part to part consistency. That being said, there are some applications that will require a higher degree of balance than others. These will include parts that have a demanding dimensional requirement or parts that will be difficult to eject if they are over packed. Here uniform balance is important to ensure that all cavities are uniformly filled. It is important to recognize applications where balance will be critical.
However, it’s important to recognize that there are some fundamental differences between hot runner system and cold runner system designs. Cold runner systems are more prone to the effects of shear due to their inherent design.
Hot-runner manifolds that contain one or more types of non-melt channels in addition to melt channels, and injection-molding systems containing such hot-runner manifolds. The differing types of non-melt channels include: coolant channels for carrying a coolant for cooling the tips of hot-tip nozzles, for example, during hot latching operations; heating-fluid channels for carrying a heating fluid for heating melt within melt channels within the hot-runner manifolds; and actuation-fluid channels for carrying actuation fluid to valves of valve-actuated nozzles. In each case, nozzles can be formed unitarily monolithically with the hot-runner manifolds and one or more of the various types of non-melt channels can be continuously routed within such unitary monolithic nozzles. Freeform fabrication processes can be used to form hot-runner manifolds of the present disclosure, which often contain complex/intricate internal passageways that form the various types of melt and non-melt channels.
When using the plastic injection molding process, the fabrication of any part or product starts with the mold. Molding systems are divided into two categories: hot runner molds and cold runner molds. Hot runners use a screw nozzle that is fed by a barrel using a pump, while cold runners use a closed, thermoset mold. The primary task of any injection runner system is to direct the material flow from the sprue to the mold cavities. The system requires additional pressure to push the material through the runner. Frictional heat, generated within the runner mold by the material, flows through the runner and raises the temperature, which facilitates the flow. One of the best pay-offs for any project relates to the proper sizing of the runner for a given component and mold design.
A well-designed runner system delivers a number of benefits, such as:
Achieving the optimal number of cavities
Delivering melt to the cavities
Balancing filling of multiple cavities
Balancing filling of multi-gate cavities
Minimizing waste
Allowing for easy injection
Providing efficient energy consumption
Controlling the filling/packing/cycle time
Selecting a particular type of hot runner system is influenced by the product design and production requirements. There are many hot runner component and tool manufacturers available. If possible, utilize a system or component supplier with experience in styrenic TPEs.
Manifold Design
Externally heated systems are best.
Internally heated manifolds are not suitable for TPEs. These systems typically have hot spots and stagnation zones that cause partially solidified material to cling to the cooler manifold walls.
For maximum flexibility, the design should be naturally or geometrically balanced. Rheological balancing is possible, but only for a specific grade or rheometric curve.
All passages should be highly polished circular cross sections with gentle bends to minimize the possibility of stagnation zones.
In order to maintain high shear, minimize residence times and promote flow, the passages should have a diameter of 0.250″ to 0.375″.
Individualized zone controls for the hot runners are recommended to allow the operator to adjust the balance slightly to make the parts more uniform.
For example, A hot runner X style manifold,For the injection molding system,manifold is the center part. When you design the manifold,you make mold-flow analysis to determine the runner size to make sure the best effctive.Manifold heat fast and well control the temperature. A manifold design are based on nature balance and rheology balance theory.Which can make sure the mold melt with reasonable shearing speed and allowable pressure lost.A designed and produced manifold,which are praised by changing color fast,easy heating,precision temperature control,runner banalance and other advantages.
Hot-runner manifolds that contain one or more types of non-melt channels in addition to melt channels, and injection-molding systems containing such hot-runner manifolds. The differing types of non-melt channels include: coolant channels for carrying a coolant for cooling the tips of hot-tip nozzles, for example, during hot latching operations; heating-fluid channels for carrying a heating fluid for heating melt within melt channels within the hot-runner manifolds; and actuation-fluid channels for carrying actuation fluid to valves of valve-actuated nozzles. In each case, nozzles can be formed unitarily monolithically with the hot-runner manifolds and one or more of the various types of non-melt channels can be continuously routed within such unitary monolithic nozzles. Freeform fabrication processes can be used to form hot-runner manifolds of the present disclosure, which often contain complex/intricate internal passageways that form the various types of melt and non-melt channels. …
An injection-molding manifold distributes one or more molten materials, or one or more “melts,” such as one or more plastics, from an injection-molding machine to injection-molding nozzles via a network of melt-channels within the manifold. Each melt is intermittently delivered to one or more mold cavities via the injection-molding nozzles during molding operations. The melt in each melt-channel is typically heated using electrical heaters located on the exterior of the manifold. If the nozzles are of a valve-gated type, actuators that reside on the side of the manifold opposite the nozzles are typically used. Sometimes equipment operators disengage and reengage the nozzles with a mold plate/gate inserts while the nozzles are still hot. This is known as “hot latching” and can lead to excessive wear and damage to the nozzles and/or mold plate/gate inserts where the components engage one another.
Hot Runner System Advantages: 1) Easier fully automatic operation. 2) No loss of melt. 3)Pressure losses minimized. 4) Increase of mechanical efficiency. 5) Improvement of product quality. 6) Extension of mold life.
7) Shorter cycles.
Hot runner systems are feed systems for injection molds which convey molten plastic from the machine nozzle into the cavity.
Hot runner systems are composed of different parts and mechanisms:
Nozzles
Nozzles are the melt delivery system. Nozzles are designed to inject and distribute molten polymer to a number of cavities.
Manifolds
It is a heated melt-distribution system with channels. This structure can be in different shapes as I, H, X, Y or any demanded shape.
Flow control
Valve technology to control the melt flow.
Temperature controller
Connections/Parts
All relevant connections as electrical, resistors, thermocouple…
Hot runner manifold is a part that must individualized design in hot runner system. A good hot runner system depend on hot runner manifold structure design. Hot runner manifold design must consider the following aspects:
port hole size and flow path;
Hot runner thermal expansion
Multi-cavities manifold rheology balance theory
Manifold steel material choosing
Hot runner manifold adopt of small diameter runner,this design is reduce heat-sensitive plastic melt residence time in the runner and guarantee filling quality. Hot runner manifold use big diameter runner can transfer the pressure and it’s benefit for big parts products injecting and molding. Big diameter runner is suit for high viscosity plastic. Hot runner manifold structure design avoid arising death end in conner of runner and nozzle.
Runner diameter and length design should consider the following factors:
allowing pressure less than 35MPa.
Every time after finish shooting,clearly clean the melt plastic in the runner.
Melt plastic flow time from injection machine screw to mould cavity is equal to 10%~20% plastic decomposing time.
In hot runner manifold structure design,hot runner system normally adopt of circular flow channel section because manifold is processed by drilling holes.Hot runner manifold processing need handle care,even a scrap iron left in the runner,it will influence the whole hot runner system operating.
A hot-runner system is provided with a manifold body including a manifold melt channel, and a melt-flow control structure communicating with the manifold melt channel. The melt-flow control structure is integrally formed with the manifold body.
For years, natural balance has been the cornerstone of a successful hot runner balance. This means that the melt experiences the same flow length and the same diameter melt channels from when it leaves the machine nozzle until it enters each cavity within the mold. This approach has served the industry well.
In recent years, much attention has been paid to the effects that shear has on the melt as it flows through a cold runner system. Of specific interest is how shear heated melt is distributed by the cold runner geometry. Research in this field has led to a greater understanding of shear-induced variances as they apply to cold runner systems and led to the introduction of technologies that are aimed to help address molding issues which have irritated molders for years.
Advantages of hot runner Mold
Runner is not attached with molded part. Therefore material cost is reduced.
Reduced Cycle time.
More control over injection process. Therefore part quality improves .
Low pressure is required to push molten plastic.
Recommended for larger parts
Disadvantages of hot runner molds
Higher mold cost.
Higher maintenance cost.
Complete mold cleaning is required to change material.
Not recommended for thermal sensitive materials.
When using the plastic injection molding process, the fabrication of any part or product starts with the mold. Molding systems are divided into two categories: hot runner molds and cold runner molds. Hot runners use a screw nozzle that is fed by a barrel using a pump, while cold runners use a closed, thermoset mold. The primary task of any injection runner system is to direct the material flow from the sprue to the mold cavities. The system requires additional pressure to push the material through the runner. Frictional heat, generated within the runner mold by the material, flows through the runner and raises the temperature, which facilitates the flow. One of the best pay-offs for any project relates to the proper sizing of the runner for a given component and mold design.
large runners facilitate the flow of material at relatively low pressure requirements. However, they require a longer cooling time, consume more material and scrap, and need additional clamping force. On the other hand, using the smallest runner system as required for the project will maximize efficient use of raw material as well as energy consumption during the injection molding process. Ultimately, reducing the runner size depends on the molding machine’s injection pressure capability.
Hot runner molding systems consist of two plates heated by a manifold system inside one-half of the mold that sends the melted material to nozzles, which feed the part cavities. The system consists of two parts: the hot manifold and the drops. The manifold moves the rubber on a single plane and parallel to the parting line to a location above the cavity. Positioned perpendicular to the manifold, the drops move the rubber from the manifold to the component.
hot runners are not required for injection molding processes, they can be useful to ensure a higher quality part. They are particularly beneficial with challenging part geometries that require lower margin of error in the flow properties of the molten plastic (i.e. where inopportune cooling or temperature deltas might result in uneven flow). Further, hot runners can be beneficial in reducing wasted plastic during high volume shoots. Because cold runners are unheated, the channel needs to be larger and thus more plastic needs to be shot during each cycle. If you are shooting a large number of parts while iterating to get the design correct you could easily run up the cost of plastic above the cost of a hot runner assembly.
Hot runners are designed to maximize manufacturing productivity by reducing cycle time. One of the reasons they didn’t take hold when they first came out is that they needed to maintain the molten plastic at a uniform temperature while the injection mold cavity is simultaneously being cooled. This requires a fair level of complexity. The initial (now obsolete) designs implemented internal heating with isolated heaters inside channel cavities. Internally heated hot runner designs resulted in solidified plastic on the internal boundaries of the channel with molten plastic much more localized to the specific heater location. By contrast, externally heated runners utilize heated nozzles and a heated manifold and based on the high thermal conductivity of metal they are able to maintain much more even flow properties for the internal plastic.
Like every entity in the world, hot runners also have some disadvantages as well.
It is comparatively difficult to change the colors in hot runner system.
Hot runner system is more complex because the melt is to required to keep at certain temperature while the injection mold cavity is requires cooling at the same time. Being complex in turn means higher maintenance cost.
Hot runner system may not be suitable for the materials which are sensitive to heat. Moreover thermal expansion of various mold components should be considered.
Of course for best result in quality production, the professionals have to analyze the type of runner for best injection molding system.
The product being formed is affected by the two main technologies of hot runners. Temperature and flow of the hot runner system are closely related. Any change in temperature varies the flow of the melt and flow change can resulting temperature disturbance hence making the molding process difficult.
The control of temperature
The temperature controller not only heats the hot runner system and the plastic in the barrel but also maintains the temperature of the components to the desired value. Temperature has substantial effect on flow than pressure because the increased temperature can force the gate tip further to the gate orifice caused by the thermal expansion. Hence the flow is disturbed because of the change in effective radius of the gate.
The control of plastic flow
Flow channels are closely related to the pressure of the molten plastic flow. Variations in pressure through the regions of a manifold cause dead spots in the product. These spots do not wash out well when purged and result in color change problems.
Hot runner system saves injection cost in many ways. As the material is always in molten form, the pressure reduces in the flow. This allows simplicity in vast production because realization of the low pressure is easy for injection in multi gate and multi cavity molds.
In the hot runner system, as the gate is hot, it results in good pressure transmission which keeps the product away from imperfect falling, cavity shrinking and product distortion due to lack of feeding.
As there is no recycling in hot runner systems, it reduces the cost of selecting and picking up, wrecking and dyeing. The quality of the product is also improved.
Injection cost is reduced also because of the faster cycle time of the hot runners.
When the product size gets bigger and needs various injections, hot runner molds are not appropriate as much as necessary.
Three plate mold serves the purpose here. It has more central points to hit upon the gate. The gate is located at the base rather than the side of the part allowing more smooth supply of the melt in the part.
As the three plate mold system has three plates with two notches. It separates runner form the part and the melt drops into the molds giving automatic working.
But alongwith the advantages of three plate systems, there are some shortcomings of this system. The complex apparatus of three plate systems makes it less stable. It has higher scrap rate, longer run distance and delayed cycle time than hot runner systems.
Melt injected by the plasticizing unit enters the manifold through a single large channel. If the manifold is designed for a multicavity mold, the main channel in the manifold feeds a multilevel or tiered hot-runner system. The tiered configuration allows for more cavities while keeping channel lengths uniform. However, the melt has to travel, either from one level within the mold to another, or when it branches from a channel to an individual nozzle that injects the polymer into a mold cavity. Gun-drilled channels have sharp corners wherever the melt is directed to another distribution level or is split into two smaller, uniform flow paths into the mold.
Hot runner manifold is a part that must individualized design in hot runner system. A good hot runner system depend on hot runner manifold structure design. Hot runner manifold design must consider the following aspects:
port hole size and flow path;
Hot runner thermal expansion
Multi-cavities manifold rheology balance theory
Manifold steel material choosing
Hot runner manifold adopt of small diameter runner,this design is reduce heat-sensitive plastic melt residence time in the runner and guarantee filling quality. Hot runner manifold use big diameter runner can transfer the pressure and it’s benefit for big parts products injecting and molding. Big diameter runner is suit for high viscosity plastic. Hot runner manifold structure design avoid arising death end in conner of runner and nozzle.
Runner diameter and length design should consider the following factors:
allowing pressure less than 35MPa.
Every time after finish shooting,clearly clean the melt plastic in the runner.
Melt plastic flow time from injection machine screw to mould cavity is equal to 10%~20% plastic decomposing time.
In hot runner manifold structure design,hot runner system normally adopt of circular flow channel section because manifold is processed by drilling holes.Hot runner manifold processing need handle care,even a scrap iron left in the runner,it will influence the whole hot runner system operating.
Hot runner manifold adopt of small diameter runner,this design is reduce heat-sensitive plastic melt residence time in the runner and guarantee filling quality. Hot runner manifold use big diameter runner can transfer the pressure and it’s benefit for big parts products injecting and molding. Big diameter runner is suit for high viscosity plastic. Hot runner manifold structure design avoid arising death end in conner of runner and nozzle.
Runner diameter and length design should consider the following factors:
allowing pressure less than 35MPa.
Every time after finish shooting,clearly clean the melt plastic in the runner.
Melt plastic flow time from injection machine screw to mould cavity is equal to 10%~20% plastic decomposing time.
In hot runner manifold structure design,hot runner system normally adopt of circular flow channel section because manifold is processed by drilling holes.Hot runner manifold processing need handle care,even a scrap iron left in the runner,it will influence the whole hot runner system operating.
thanks for sharing your experience, i am a hot runner system engineer, i learned something new from this article
thanks for sharing, as a hot runner system engineer, I really learned a lot of professional knowledge from your blog