Almanac: Switching from a cold sprue bushing to a heated sprue bushing

Taking the right steps to select and install this component means the difference between success and failure.

Eventually we all will be asked to change a cold sprue bushing to a heated sprue bushing. Here are some common reasons for switching:

The problem that immediately surfaces is that the toolmaker is not told which heated sprue bushing to use. Which manufacturer should be used? Should it be a regular heated sprue bushing or one with a valve gate? What size should the orifice be? Should the replaceable tip, if it has one, be made out of titanium, tool steel, or something that conducts heat much better than these other choices? The answers are sometimes difficult to find without wading through many catalogs and websites.

Brand Loyalty?

Let’s go through the steps one at a time. First, what about the brand? From my experience I have found that most of the offerings are pretty much the same. Most of the flow channels are externally heated and for most materials that seems to be the better choice.

Gate Choices

Second, should we use a hot tip gate straight out of the heated sprue bushing, or incorporate a valve gate into the bushing to positively shut off the flow of material between shots? This is a personal issue and depends on how much money you want to spend and whether or not a little gate vestige is going to be an issue.

Are you going to feed directly into a part or into the middle of a runner system? When feeding directly into a part, you will likely choose a nonvalve-gated system, especially if the hot sprue orifice diameter is .090 to .125 inch. If volume warrants it (500,000 parts/year), then a valve gate is recommended.

One advantage of the valve-gated system is that you can start recovering the screw sooner and still keep the material from drooling out of the sprue because of the positive shutoff. Another advantage is that since you are losing the heat applied to the melt at the sprue bushing orifice instead of at the nozzle orifice, you should be able to drop the barrel and nozzle heats 10 or 20 deg F, which could result in a somewhat faster cycle.

I have also seen orifice or gate inserts that incorporate a corkscrew design that the material must pass through just prior to exiting the heated sprue bushing. This is an excellent feature if you need a little more mixing of the polymer just before injection or if you want to reduce the amount of vestige on the part. This spiral design also helps the cleanout process when color changing.

Orifice and Flow Tube Sizing

The mistakes that tend to ruin a heated sprue bushing debut are an improperly sized flow tube diameter, an orifice that is too small, or a molding machine nozzle that hasn’t been opened up to match the flow tube diameter exactly. This brings us to our third point. What size should the orifice be? The orifice in the heated sprue bushing should be the same size as the cold sprue’s O-diameter, assuming it was sized correctly.

An example of this is when you have a runner system with subrunners sized at .150 inch and the main runner at .200 inch; in this case, the cold sprue O-diameter should be .250 inch. This means the heated sprue bushing orifice should also be .250 inch. With an orifice of .250 inch, you need a flow tube diameter of at least .375 inch, or possibly larger if you think you might want to increase the diameter of the orifice some day (see Table 1).

This constitutes perfect sizing for PC, ABS, ASA, acrylic, and other stiffer-flow materials. PE, PP, and nylon do not require such a large-diameter flow path. It’s just a matter of dimensions. If the heated sprue orifice is only .060 inch, then a flow tube diameter of .187 inch or .250 inch is big enough.

Metal Selection

Now, how about the type of steel? Usually the material dictates the steel choice. For abrasive or corrosive materials, stainless steel typically gets the nod. Corrosive materials are usually thought of as acetals and PVCs. Abrasive materials are any filled material, such as glass- or talc-filled nylon or polypropylene. Sometimes it is just the gate inserts that need to be made out of stainless steel or some other longer-wearing metal, such as A-2 or D-2.

I find most heated sprue bushings are made out of tool steel, which is a middle-of-the-road heat conductor. If the orifice is inserted, it will be possible to use either titanium or a high-conductivity metal as an orifice or gate insert to conduct more or less heat to the orifice when the mold is open between shots. If you are having trouble with the material stringing between shots, then go to a lower-conductivity metal, such as titanium. If the orifice seems to freeze off slightly between shots, go to a high-conductivity metal, such as beryllium or tellurium copper (or similar) for the orifice insert.

Wiring Wisdom

I discussed these questions with some of my colleagues. Mike Gebel, the toolmaker who helped me with my boat lift pulley article (July 2002 IMM, pp. 50-53), says the only thing he has to do when switching from a cold to a hot bushing is to mill a wire way in for the electrical connections. Tim Bryant, my toolmaker buddy, agrees with Mike. He usually orders his heated sprue bushings with a little extra length so he can mill the runner connectors in. His main concern when picking a heated sprue bushing is where the electrical wires are located. He says it doesn’t happen very often these days, but it used to be that sometimes the plastic material would ooze backwards out of the nozzle seat and cover the sprue bushing wires. When this happens, all you can do is try to rewire the bushing or get a new one. He also said he is glad those days are mostly behind him now.

The first dozen or so cold sprue bushings that you switch to hot will cause you concern. After that you will probably just replace the cold sprues with hot ones prior to the mold ever going to the molding floor.