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.
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