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.
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 in your market
For maximum flexibility, the manifold design should be naturally or geometrically balanced. Rheological balancing is possible, but only for a specific grade or rheometric curve.
If possible, utilize a system or component supplier with experience in styrenic TPEs. Some material can crosslink (forming gels) if they are held at high temperatures for too long a period of time, therefore hot runner tools are not recommended for these materials.
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.
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.