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High Cavity Valve Gating
An increasing number of injection molders demand annual production rates of several hundred million parts from a single, high-cavity mold. Small plastic parts produced at such high volumes are commonly found in medical devices, packaging solutions or precision electronics and generally involve short cycle times well under 10 seconds and extremely small shot weights of under 1 gram. Demanding high cavity applications in a 24/7 automated injection molding environment require an extremely reliable valve gate hot runner system.
 
Faster Cycle Times
For example, the annual production of 400 million disposable syringe plungers each with a shot weight of 0.7 grams at a 90% output rate, requires 128 cavities and a cycle time of 8 seconds. This can be achieved using a valve gated system that can deliver the consistently crisp, clean gates that medical parts demand. The added benefit of valve gated systems in high cavity settings is that they help reduce cycle times dramatically without wasting valuable material by producing cold runners.
Precision Control
The main advantage of hot runner valve gating is its consistency from cavity to cavity, and from shot to shot. Valve gates offer more processing control than traditional thermal gates using hot tip nozzles because small changes in the processing parameters and fluctuations of the melt viscosity do not affect their performance. A larger processing window is of extreme importance in an automated 128-cavity mold running small parts. When molding micro parts (weighing less than 1 gram), a specific challenge is to maintain uniform melt distribution from the machine barrel through the melt channels (manifold) and the hot runner gates. In the example of the syringe plunger, a total of only 90 grams of material is distributed through the melt channel with each cycle to the 128 individual mold cavities. To make this possible, the valve pins have to open and close at exactly the right time to control the flow. In order to produce absolutely consistent parts, the packing pressure and shear rate have to be balanced. The melt also has to maintain a uniform temperature for each cavity. Accurate control of these parameters and the reduction of cycle times are the key functions of the hot runner.
Better Gate Quality
Using a cylindrical valve shut off instead of a conical valve pin tip eliminates the mechanical stress around the gate opening, especially over millions of cycles. The extreme heat expansion and force placed on a traditional conical valve pin can easily lead to damage of the steel around the gate opening. A cylindrical shut off, on the other hand, pre-guides the valve pin towards the center of the gate, engaging it at a precise tolerance in the micro meter range. Not only does this relieve the stress in the area around the gate opening, it enables the effective heat transfer from the gate cooling to the pin shut-off surface. In order to ensure a crisp, clean gate on the part, the surface temperature of the front end of the valve pin cannot be higher than the melting temperature of the polymer at the time of part ejection. This is true even for the smallest of valve pins, measuring only 0.5 mm in diameter.
 Rheo-Pro® N04 valve gate hot runner nozzles
Tight Pitch Gates and Cavity Spacing
Micro parts require special, compact nozzles, since space inside a high-cavity mold is extremely limited. The 128 tightly spaced hot runner nozzles (18 mm tip-to-tip distance) can not be allowed to affect the strength or the cooling of the mold plates through which they direct the flow of plastic. This becomes especially challenging if the application requires extremely long and slender nozzles. In the case of the syringe plunger project, the nozzles have to accommodate a 178 mm ejector plate. The cavities must have ejector pins on both sides of the mold in order to securely eject all 128 parts during each cycle. Therefore, the nozzles must span the relatively long space of the ejector plates, each constructed with 128 cut-outs. Rheo-Pro® N04 nozzles with an 18 mm pitch make this possible. The compact construction of the nozzle body is subject to extreme internal melt pressure over millions of molding cycles. Special material selection and surface treatment enhance the robustness and longevity of these small valve gate nozzles.
Consistent Melt Viscosity
Long, compact nozzles also present a special challenge for the nozzle heater which must maintain a precise and even temperature distribution throughout the length of the nozzle body. When molding small parts, it is not uncommon for the melt residing inside the nozzle channel to equal several times the part volume. The nozzle heaters have to be carefully controlled to keep the melt temperature balanced so that all 128 cavities fill exactly the same way. The 230VAC power-profiled heater on a Rheo-Pro® series nozzle is encased in a thermally conductive sleeve assembly that creates a highly responsive heater circuit. Uniform temperature is conducted to the melt channel and nozzle tip by the snug fit of the heater around the nozzle body. Along with the heater tube, durable 1mm Fe/CuNi thermocouples are fully front-replaceable for ease of maintenance.
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Faster cycle times, increased productivity |
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Better part quality |
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More process control |
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Ideal for automated, high volume production |
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Increased performance and reliability |
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It is important to avoid nozzle heaters that are permanently integrated into the nozzle body and/or depend on inaccessible wiring. Serviceability must be a critical factor when considering a hot runner design, especially in small, high-cavity systems which are more prone to particle contamination in the melt that can lead to blocked gates. The front-wired and front-removable heaters make it possible to clean or replace nozzle elements without removing the entire mold from the machine. The cavity plate can easily and quickly be detached and shifted to the core side on the plate guide pins. Anyone working with the mold on the shop floor will understand how important this feature is.
Rheo-Pro® VA Cylinders
An important advantage of a Rheo-Pro® valve gate system is the reliability and the serviceability of its pneumatic valve gate cylinders. By simply removing the top clamp plate, all cylinders are immediately accessible. The 128 cavity syringe plunger system, discussed earlier, uses 32 manifold-mounted cylinders, each responsible for actuating a set of 4 valve pins. This grouping is made possible by the tight gate-to-gate distance of 18 mm. The air supply to actuate the cylinders is run through channels in the top clamp plate. The extreme accuracy of these pneumatic cylinders and the valve gates they control, is in the millisecond range. This ensures an absolutely consistent injection of melt, even when dealing with very small parts.
The Importance of Flow Analysis
Since most polymers can only spend a limited amount of time exposed to high processing temperatures, it is crucial that a detailed flow analysis is conducted before machining any melt channels. Especially in high cavity molds for small parts, where these channels branch off multiple times, an accurate rheological calculation is necessary to ensure an ideal melt residence time. In order to maintain their properties and integrity, most materials should not spend more than a few minutes traveling the length of the melt path. Flow simulation software is used to model a balanced system that optimizes melt residence time, pressure drop and shear rates inside the manifold and nozzle channels. The runner geometry, combined with the model of the mold cavities creates accurate results that simulate realistic processing conditions. The final design is derived and validated by this analysis. The importance of this step cannot be overstated since it eliminates any risk or uncertainty in the early stages of a project.
In the example of the syringe plunger, the total shot weight equals 89.6 grams (128 x 0.7g) and the runner channels contain 800g of melt. At a cycle time of 8 seconds, it takes approximately 9 shots to clear the entire melt path, resulting in a melt residence time of only 72 seconds. This means that this runner layout will accept even very temperature sensitive polymers and additives without risk of degradation.
When molding small and micro parts, only minute quantities of material are displaced with each shot cycle, generating very little shear heat. Since this does not have a noticeable impact on the melt viscosity, the channels can be kept small to facilitate a shorter melt residence time. A balanced melt flow is maintained by reducing the diameter of the channel each time it branches off inside the manifold. In special cases, the walls of the channel are treated to reduce the adhesion of polymer.
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iVG™ Stack Molds
The Rheo-Pro® iVG™ is an internally air actuated valve gate system with no external cyclinder, no seals and no lubricant. It represents the latest and most innovative valve gate technology in the industry and is ideal for ultra compact stack mold solutions.
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