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suitable taper with an ideal orifice size, based on the parts and material to be molded. As it stands now, there's little choice on the internal angle and only four choices on the orifice size (table at right). Standardization of mold compo- nents has greatly helped our industry reduce costs and lead times. But in the case of items such as sprue bushings, I believe it has in part hampered the mold designer's ability to design a better mold. Material manufacturers recom- mend using large tapers in sprue bushings. That makes sense because they're in the business of selling material. While it's true that large sprue tapers are less likely to cause a sprue to stick inside a bushing, if you incorporate some key features in the design and construction stages, large tapers are not always necessary. Two parts having a conical fit with a taper of 7° per side or less are considered to be a "self-locking" or "self-holding" taper. That's the angle where the coefficient of friction between the two surfaces is greater than the retraction force required to separate them. The smaller the angle, the more the force. Obviously, a sprue bushing with its small internal taper is theoretically self- locking, but we rely on the plastic material to shrink away from the bore of the bushing as it cools—negating this taper lock. As I noted in my February column, material suppliers specify the shrinkage factor of their materials based on ASTM D955, which uses a 1/8-in.-thick test bar. What many suppliers neglect to mention is that if you are molding something thinner than 1/8 in. it will shrink less than the speci- fied shrinkage factor. Conversely, if you are molding something thicker than 1/8 in., it will shrink more. It doesn't matter if it's a part or a sprue—plastic is plastic, and Orifice Size, in. Sprue Bushings Nozzle Tips 1/16 — Yes 3/32 — Yes 1/8 — Yes 5/32 Yes Yes 3/16 — Yes 7/32 Yes Yes 1/4 — Yes 9/32 Yes Yes 5/16 — Yes 11/32 Yes Yes 3/8 — Yes 1/2 — Yes American standard nozzle tips come in 12 different orifice sizes, but sprue bushings are only available in four sizes. It's not too expensive to ream and polish a sprue- bushing orifice larger, but you can't make it smaller. Wouldn't it be nice if someone sold a sprue bushing with a 3/32-in. orifice? Standard Sprue Bushing and Nozzle Tip Orifice Sizes n s i g n E Q U I P M E N T ENSIGNEQ.COM | 616.738.9000 | SALES@ENSIGNEQ.COM • SYSTEMS INTEGRATION • CONTAINER FILLING/UNLOADING • MATERIAL CONVEYING • ADVANCED DUMPING TECHNOLOGY • WEIGH SYSTEMS • MIXING AND BLENDING Smart Solutions To Keep You Moving! BULK MATERIAL H A N D L I N G S Y S T E M S E N S I G N D E S I G N S & M A N U F A C T U R E S @plastechmag 29 Plastics Technology T O O L I N G