Plastics Technology

DEC 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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to be machined into one side of the parting line and they have a lower pressure drop than a full-round runner. But depending on the included angle, they can use 20% to 35% more material than a full- round runner, which potentially can extend the molding cycle time— especially if the runner is removed by a robot or picker. Parabolic and trapezoidal runners can also cause aesthetic issues with certain types of gates, which is a subject for a future article. If you have a parabolic or a trapezoidal cold-runner design, you need only one RSO. If you have a full-round runner, you will need a second, mating RSO on the opposing side of the mold. But you can be a little creative and avoid having to use two RSO's by doing either one of two things: First, you can have a parabolic or trapezoidal runner cut into the RSO, which feeds a full-round runner (Fig. 4), but you need to make sure of two critical design features. The cross-sectional area of the trapezoidal section must be equal to or greater than that of the full-round runner; and the amount of overlap must be sufficient—usually 1.25 to 1.5 times the cross-sectional area. Otherwise, you will be restricting the material flow, which causes an undesirable spike in injection pressure. As long as the material isn't shear sensitive, this design can save a few dollars—and a few headaches. The other thing you can do is have an RSO with a protrusion extending into the opposing side of the mold to shut off that half of the flow channel (Fig 5). In this design, #8-32 flat-head cap screws are used to secure the RSO in a pocket in the mold base. The countersinks on both sides of the RSO enable you to flip it over and re-secure it to the mold base. The half-round cutout on the bottom of the RSO has the same radius as the runner channel, which when flipped over, allows for uninterrupted material flow. The small inside diameter between both sets of countersinks is drilled with a #19 (0.166-in.) drill and then tapped with a #10-32 thread. The #19 drill is required to clear the outside diameter of the #8 screws. The reason for this #10-32 thread is so a pair of #10-32 socket- head cap screws can be used to easily remove the RSO from its pocket. The ends of the #10 screws are turned down to 0.130 in. diam. This diameter is smaller than the root diameter of the #8-32 thread to prevent any damage to the start of the threads in the pocket. The length of the threaded portion is equal to or greater than two times the height of the RSO. This modified screw is called a jack screw. It's a good idea to mount these jack screws directly to the mold. One method is to drill and tap two holes in any available location, such as an ejector- housing rail or a clamp plate. This eliminates the need to spend an untold amount of time having to hunt for them later. Even though this RSO is for a full-round runner, it can also be configured for a para- bolic or trapezoidal runner. This type of RSO can be mounted on either side of the parting line, which can be very handy if a water line or other obstruction is in the way. Lastly, since mating mold components usually "seat" themselves, it is a good idea to orient the RSO. One method to ensure the RSO is always in the proper orientation is to machine the two corners on one end of the insert with a different radius than the opposing two corners. For example, if one end had a pair of 1/8-in. radii and the opposite end had ¼-in. radii, there is only one possible way the RSO can be installed in the pocket in the mold. If you look closely at Fig. 1, you can see that is exactly how those RSOs were machined. Flow areas of different types of cold-runner shapes. FIG 5 FIG 4 A parabolic RTO feeding a full-round runner. A one-piece RSO for a full-round runner. The jack-screw on the right is used to remove the RSO. Full Round Parabolic Trapezoid Area = 0.049 in.² Area = 0.059 in.² Area = 0.063 in.² 0.250 in. diam. 0.250 in. diam. 0.250 in. diam. 10 to 30° 10 to 30° ABOUT THE AUTHOR: Jim Fattori is a third-generation injection molder with more than 40 years of molding experience. He is the founder of Injection Mold Consulting LLC, and is also a project engineer for a large, multi-plant molder in New Jersey. Contact; FIG 3 @plastechmag 33 Plastics Technology T O O L I N G

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