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April 1, 2002

6 Min Read
Coping with sprues: Trivial or critical?

Editor's note: Consultant Bill Tobin of WJT Assoc. is a regular contributor to IMM and here offers his insights on the importance of sprue details and how to get the best results with the right sprue.


It is always the "Gotchas" that getcha. Something as simple as a stuck sprue is a Gotcha. It causes scrap, slows down cycles, and erodes profit. If you get on a good search engine the Internet can provide 50-plus sources of sprue bushings. This should be a catalog item as simple as buying peanuts, right? Wrong. Most people think the sprue bushing mates with the machine and marries with the runner system. True. But there is a lot more to it, and with a few tricks, you can substantially improve your profits. Here are a few:

  • Ball radius. Wouldn't it be nice if the radius of the bushing exactly matched the machine nozzle? You'd be amazed at how many people jam sheet lead, copper, or even cardboard between the nozzle and the bushing because they can't buy a bushing or a nozzle that match. A mismatch causes pressure loss and unwanted material shear during injection.

  • Nozzle orifice/sprue diameter. There's an interesting rule of thumb having to do with sticking a sprue in a mold. The exit orifice in the machine nozzle should be 20 percent less than the diameter of the sprue. Here are a few reasons: First, if the machine's injection unit isn't directly in line with the sprue bushing, this provides a little forgiveness. Second, if the sprue's orifice is smaller than the nozzle, the material in the sprue will harden (which it is supposed to do) to the cold slug in the nozzle. Since it can't pull anything out of the nozzle it either breaks free of the machine nozzle or breaks from the runner and stays in the mold. (Trivia question: Ever seen somebody drill out a machine nozzle with the hopes of improving injection flow? Ever wonder why so many sprues stick after this piece of genius engineering? Answer: Reread this paragraph, then go whack the guy who did it with a rolled-up newspaper.)

  • Sprue taper. We generously taper a sprue because we want it to shrink and come out easily when the mold opens. However, most mold designers look for the easiest route. They pull a standard sprue design out of a catalog and specify it. Why should they care? They only build molds. It's your problem to make parts. But if you consult with your material supplier you will probably find that a much different taper is ideal for your material. While this may mean you need a new machine nozzle, it's cheap compared to the scrap generated by shear and stuck parts.

  • Sprue length. It's simple geometry: Anything with positive draft gets bigger the farther away you go. While most people will give you rules of thumb or FEA results about cycle times and part cooling characteristics, you can't open the mold until the thickest part is cool enough to eject. This is usually the sprue and runner system. A simple problem has a simple answer: Shorten the length of the sprue.

It's not a sin to purchase an extended nozzle for the machine and use a short sprue. Nor is it written that you can't eliminate the sprue bushing altogether and use a heated sprue. Better yet, buy a machine nozzle that has a sprue tip configuration and eliminate the expense of a hot sprue bushing. The shorter the sprue length, the smaller the cross section of the sprue and the less time it will take to solidify.

Also, don't forget that metals expand when heated. Ever notice that when a mold runs for any length of time there is flash around the end of the sprue bushing and the runner? Most people think flash appears there because this spot marks the highest pressure point of the plastic.

What really happens is that because the sprue bushing is rarely cooled, it gets very warm during production. As it heats, it gets longer. With the bushing's end sticking out of the parting line, the mold closes and pushes it and the injection unit back. After a while, the mold base loses this pushing match and begins to "hob" or dent from the sprue bushing. When the mold is started next time, the bushing is cool (not expanded) and there exists a small gap between the edge of the sprue bushing and the mold base. Because most molders overpack during startup, flash develops. And so it goes.

So, how do we fix this problem? Simple: Build the bushing .020 to .030 inch short in the first place. You'll see a circle of molded plastic in each shot but the tool will not flash. (If you're doubtful of my opinions regarding expansion, run your thumbnail over the sprue bushing of a new mold. You'll feel it is flush. Start it up and after a few hours run your thumbnail over it again. You can feel it is actually higher than the parting line.)

  • Sprue cooling. Our brothers in the thermoset business have an interesting problem: Thermosets use heat to initiate a chemical reaction to make the part crosslink. Yet, they definitely don't want the sprue to crosslink. These processors have developed a simple solution: They use sprue bushings with cooling circuits in them. They buy these from the same suppliers from which you buy noncooled bushings. For the additional cost of this bushing—and the minor silliness of hooking up waterlines for the sprue—you'll see cycle times decrease substantially because the sprue is solid in a shorter period of time. Now you are molding parts, not sprues.

  • Sprue life. Everyone should believe in preventive maintenance. If you don't use preventive maintenance, then it becomes periodic maintenance. If you don't follow periodic maintenance, then you pull the mold to fix it when it's broken. Ever wonder what happens to steel when it is flexed under pressure each time a shot is made? It hardens and becomes brittle. Ever watch your setup tech go after a stuck sprue with a heated screw, heated paper clip, heated screwdriver, or a pair of needle nose pliers? Have you ever seen them whacking away with a rod (hopefully soft brass) trying to push it out? If you say no, you've been on too many coffee breaks.

Now take a look at the molded sprue: Is it smooth as glass or do you see little dings in it? The point here is that sprue bushings are relatively cheap. If you stick a sprue a couple of times per run, buying and installing a new bushing will pay for itself with the savings from not having to remove the first sprue that didn't stick. Only the molders who are obsessed with maximizing profits change sprue bushings periodically.

Why pay all this attention to a mold component that, in the end, costs so little? Simple: You buy the tool once and do maintenance periodically. However, you pay for bad mold design in lost profits every time the tool opens and closes.

Contact information
WJT Assoc.
Louisville, CO
(303) 604-9592
Fax: (303) 604-0319
[email protected]

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