Plastics Technology

DEC 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Let's start by defining what an RTO and an RSO is. An RTO is a Runner Turn-Off. It is a cylindrical mold component that can be rotated to prevent the flow of molten material from continuing down its path, or divert it to a different path, or both. RTOs are often located opposite a sprue bushing, but they are also com- monly located at the intersection of a runner branch. An RTO is a specific type of the more generic Runner Shut-Off, or RSO. An RSO can be round, square, rectangular, or any shape imaginable. If it blocks or diverts the flow of material, it's an RSO (see Fig. 1). It can be a small insert with a portion of a runner cut into it. Flip it over and it blocks the flow of material. Or it can be extremely large. Say, for example, you had two MUD inserts in an "H"-Frame and one of the inserts had a problem, or you just wanted to mold one of the inserts. You could rotate the unwanted insert 180°, which effectively makes that entire cavity and core set an RSO— preventing the flow of material from entering it. RSOs are usually used in family molds, because it is common not to need all the different parts during every production run. Molding unneeded parts, and then throwing them out or grinding them up is anything but efficient. RSOs are typically used in two- plate molds, but function equally well in three-plate molds. Tricks of the Trade on RTOs, RSOs One advantage to RSOs is that they eliminate the need to use unhealthy methods of blocking off a gate. How many of you have ever pounded a piece of brass, copper, aluminum or electrical solder into a sub-gate hole? (See Fig. 2.) Maybe you prefer to apply cyanoacrylate (aka Krazy-Glue) onto a small portion of a molded part and strategically place it on the core near the gate. Or maybe you go as far as leaving an unwanted part in the cavity and either removing the ejector pins to that part, or drilling holes in the part where the ejector pins are located. These methods are not foolproof; they take time to install or perform and can cause damage to the mold—particularly during removal. Before I discuss other types of RSO designs, let's briefly review the different types of cold-runner shapes. There are only three types that should even be considered for injection molds. In order of priority, they are: full round, parabolic and trapezoidal. Full-round runners are the most efficient. They have the lowest ratio of cross- sectional perimeter to cross-sectional area. If you are running only virgin material, full-round runners are the way to go (Fig. 3). However, there are advantages and disadvantages to parabolic and trapezoidal-shaped runners. These runner shapes need only The more you know about these runner diverters and shutoffs, the more you'll use them. Here's how to put them to work in your mold. Get more insights on tooling from our expert authors: Learn more at KNOW HOW TOOLING By Jim Fattori PART 1 OF 2 Shown here are subgates blocked with copper, which unfortunately is a common technique. A basic RSO blocks or diverts the flow of material. FIG 2 FIG 1 32 DECEMBER 2018 Plastics Technology K now How TOOLING

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