[Runner And Gating Design Handbook.pdf
Virginie Fayad
John Beaumont, president of Beaumont Technologies Inc., continually strives to enhance the knowledge of practical polymer characteristics, proper part and mold design, and the successful implementation of CAE mold filling simulation within the plastics industry. To help achieve his efforts, John has undertaken the task of both co-authoring and authoring several books published by Hanser Gardner Publications discussing those various topics.
For the first time, both the art and science of designing runners and gates are presented in Runner and Gating Design Handbook in a concise format that highlights all critical issues. This first-of-its-kind processing handbook is intended to provide the reader a better understanding of the rheological properties of polymer melt and melt delivery systems consisting of nozzle, sprue, runner and gate. Also explained in a clear manner are shear-induced melt variations, key differences between hot and cold runner molds, gating locations and molding problems related to gates and runners. This book provides a valuable resource for proper design and troubleshooting techniques for both hot and cold runner systems, as well as methods to successfully solve engineering and processing issues.
The book is a must for all designers, tool makers and molders. It will prove extremely valuable to anyone wishing to further enhance his or her plastics engineering knowledge. Full color 3D graphics, illustrations and photographs, as well as charts, checklists and troubleshooting guides complete this handbook.
Successful Injection Molding gives comprehensible explanations on the best ways to utilize simulation in the design of injection molded parts and the tools they are molded from. Strong emphasis is placed on the proper application of these tools and proper interpretation of their results. This book can be used by anyone involved with the development of injection-molded products, tooling or processes. It provides insight for many CAE analysts or anyone in the industry who encounters analysis information.
Injection Molding Handbook serves as an introductory textbook for students getting acquainted with injection molding. It presents a thorough, up-to-date view of injection molding processing equipment and techniques. It also includes sufficient related fundamental information on chemistry, physics, material science, and process engineering.
The handbook was written to serve engineers, professionals and others involved with this important industry sector. It not only covers the injection molding machine and process, but also includes the topics that directly affect the injection molding process, such as materials, process control, simulation, design and troubleshooting. The handbook presents a well-rounded overview of the underlying theory and physics that control the various injection molding processes without loosing the practical flavor that governs the manuscript between its covers. The carefully chosen set of authors includes experts in the field, practitioners and researchers in both industry and academia.
Figure 3 depicts an improved tapered edge-gate design. It has the same 0.030-in. land length as the previous examples, and then tapers out in three directions. The intersection of the tapered portions and the runner have generous radii. There is also a 5 taper on the sides to facilitate release from the cavity. This design provides a sufficient amount of steel between the runner and the part. There is a uniform land length on both the top and bottom, and from the center to the outer edges. The gate depth is easy to measure. The material flow from the runner to the gate is less restrictive, and there is less shear sensitivity due to the elimination of the sharp edges. This design is obviously more time-consuming to machine using an EDM electrode and then hand blending the intersections, but its benefits often outweigh the cost.
Most of us have been taught that runner branches in a cold-runner system should vary in diameter, as opposed to having a constant or uniform diameter. The runner branch feeding the gate would be the smallest. The subsequent branches would get progressively larger, leading up to the primary runner and sprue. This varying or graduated-diameter runner requires less pressure to fill a part than a constant or uniform-diameter runner does. However, a varying-diameter runner can have a longer cycle time than a constant-diameter runner because the larger primary runner branch takes longer to cool.
If the material is fairly viscous, such as rigid PVC, a varying-diameter runner is typically the best way to go, so as not to have a significant pressure loss and elevated shear stress. But if the material has a low viscosity, such as polyethylene, and the fill pressure is not an issue but the packing pressure is, a constant-diameter runner is often the better choice, because it will set up faster. While others may disagree, my personal preference is to use a varying-diameter runner regardless of the material type. I prefer to minimize the fill pressure and shear stress as much as possible. If the large primary runner extends the cycle time, I simply add some stiffening ribs, which also act as flash traps, and some gussets connecting the primary runner to both the sprue and the cold well, as depicted in Fig. 5. This intersection, regardless of the runner type, is almost always the most massive, which takes the longest to reach its ejection temperature.
Just so we are all speaking the same language, the hierarchy of runner sizes is primary, secondary, tertiary, quaternary and quinary. These are typical in naturally balanced two-, four-, eight-, 16- and 32-cavity molds. If you have more cavities than that, the next levels are senary, septenary, octonary, nonary and denary. Note: This same terminology also applies to the flow channels in a hot-runner system.
The diameter of the runner feeding the gate is extremely critical for two reasons. First, it has a significant effect on the processing window and the part quality. Second, because the size of the other runner branches in a varying-diameter system are directly related. A good starting point is to make the last runner diameter 1.5 times the wall thickness of the part where it is gated into. This may seem like an overly simplistic rule, which it actually is, but the alternative is to perform some intricate empirical calculations, or to perform a flow analysis. For the average custom molder, the time and cost factors for either of those approaches is often not warranted or achievable.
Do not make the runner diameter 1.5 times that of a thicker wall section located somewhere else on the part. Packing out that section is based on the wall thickness between the gate and that section. A larger runner diameter will not help pack out that section. As most of you know, one of the golden rules in our industry is to always try to gate into the thickest section of a part. Unfortunately, the part design, the aesthetic requirements, and the gate-stress considerations often require the gate to be located in a less-than-ideal location.
Molds with large shot weights or shot volume typically have longer fill times. During the filling phase, the exterior of the runner starts to solidify against the cold surface of the mold and the effective flow area gets smaller and smaller. The runner diameter must be adjusted accordingly. Despite my disdain for rules of thumb, the following guidelines are recommended: Increase the cross-sectional flow area of the runner, not its diameter, by 19% for each second of fill time greater than one, as shown in Table 1. If the runner diameter gets extremely large, it is an indication that a second gate and runner branch may be required. Why? As discussed last month, if you double the number of gates, the flow rate (in.3/sec) and the flow speed (mph) gets cut in half. Now you can double your injection velocity, which cuts the fill time in half, and maintains the same melt viscosity entering the cavity.
where Dfeed is the diameter of the runner feeding the branch, Dbranch is the diameter of the runner branches, and N is the number of runner branches (typically two and occasionally four for geometrically balanced runner designs.) There is no need to adjust these diameters for fill time or material viscosity. You already accounted for them when determining the runner diameter feeding the gate. However, common sense will dictate whether an adjustment should be made to account for any long runner lengths.
About the Author: Jim Fattori is a third-generation molder with more than 40 years of experience in engineering and project management for custom and captive molders. He is the founder of Injection Mold Consulting LLC in Pennsylvania. Contact: j...@injectionmoldconsulting.com;
injectionmoldconsulting.com