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Sprue bushings are available in more than a dozen different types and styles. Most mold designers select the "B" series bushings because the 7/8-in. head thickness is often the same thickness as the injection clamp plate and it works nicely with standard locating rings. Then they select the nozzle seat radius, one of the four available ori- fice sizes, and the overall length. That's often the extent of the consideration given to the sprue bushing during the mold-design phase. The most massive area in the melt- delivery system of a two-plate mold is almost always at the parting line where the sprue, runner, and cold well intersect. This mass frequently—and often unnecessarily—controls the cycle time. The contribution of the sprue bushing to this massive area is directly related to its internal taper, length, and orifice size. Length: With the industry-standard taper, if you can reduce the length of the sprue bushing by just one inch, you reduce the wall thickness of the sprue at the parting line by 0.042 in. That may not sound like a lot, but thermoplastics are good insulators and they Reducing the Mass In Sprue Bushings have non-linear ratios of thickness to cooling time. For example, a 0.050-in.-thick part may solidify in 10 sec, but a 0.100-in.-thick part may take 25 sec—not 20. It may sound strange, but it is more important to reduce the length of a sprue bushing having a small orifice then it is with one having a large orifice. For example, reducing the length of a sprue bushing from 6 in. to 5 in. reduces the area at the parting line by 24% for a small 5/32-in. orifice, but only 16% for a larger 11/32-in. orifice. Most people assume the opposite and only focus their attention on large sprues. There are a few other mold-design considerations to think about when trying to reduce the length of a sprue bushing, such as: a) The locating ring may need to be an "Extension Type" (see my March column); b) You can pick up ¼ in. just by using an "A" series sprue bushing with its 5/8-in.-thick head instead of a "B" series with its 7/8-in.-thick head; c) Never let the head of the bushing seat directly against the back of a cavity block, runner bar, or any other mold insert, unless the insert can be sufficiently retained with heels and /or multiple large screws. d) The head of the bushing should be supported by at least 3/8 in. of mold-base steel for small molds and 5/8 in. of mold-base steel for large molds. Internal Taper: Back in the 1800s, the Brown and Sharpe Co. invented a taper with an included angle of ½ in./ft for end mills, collets, and machine-tool spindles. More than 150 years later, this is still the adopted industry-standard taper for sprue bushings in injec- tion molds. This taper equates to an angle of 2.39° included, or 1.19° per side. Personally, I would prefer that sprue bushings were sold with a 1/8-in. through hole and allow the tool maker to machine a Important and frequently overlooked details of sprue-bushing design can improve the process and profits for an injection mold. Get more insights on tooling from our expert authors: short.ptonline.com/toolingKH Learn more at PTonline.com KNOW HOW TOOLING By Jim Fattori The most massive area of a 2-plate mold is typically at the intersection of the sprue, runner, and cold well. Shallow tapered sprue bushings can reduce cycle times. 28 JULY 2017 Plastics Technology PTonline.com K now How TOOLING