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A molder gets a lecture on nozzle freeze-off before his runner system is fixed.

This month I received a runner system from an old friend I have worked with for many years. The note he enclosed in the box just said, “Hi, Bob—call me right away.”

I called him a couple of minutes later and went through the normal pleasant discussion that old friends have when they get together after not seeing each other for a few months. He then switched to business and told me he was tired of the cardboard. I hesitated for a moment, and then realized he must be talking about the small scrap of cardboard that many molders, including myself, put between the nozzle and the sprue bushing to keep the molding machine nozzle from freezing off between shots when running nylon.

	I could tell by the look and feel of the runner he sent me that he was running natural nylon. I asked, “Are you still using the little cardboard disk between the molding machine nozzle and the mold sprue bushing to keep the nozzle from freezing off when running nylon?”
An undersized runner system led to sinks and voids. Enlarging each level in the system generated good parts.	He was slow to answer. “No, I am not,” he replied. “I switched to asbestos disks a long time ago. I know I should get rid of them, but that’s not why I called you. I need you to review my runner and tell me why I’m having trouble running small nylon parts.”
	Before I addressed his question, I reminded him that it’s only the molders who run a lot of unfilled, not filled, nylon that have these freeze-off issues. Many molders have switched to 3-inch asbestos disks with a hole in the center, but that should only be a stopgap measure until the runner system is optimized. “Think about it,” I said. “You use a GP nozzle to run filled nylons, but you use a reverse-taper nozzle to run unfilled nylons.” I told him the reason the nylon nozzle freezes off is that the sprue bushing and, to some degree, the front half of the mold act like heat sinks that draw heat out of the nozzle faster than the nozzle heater bands can put heat into it. The result is that the outside 1?8 inch of the nozzle orifice cools down too much and plugs the nozzle with a cold slug for the next shot. This cold slug sets up so quickly that the next shot doesn’t have enough heat or pressure to blow it out the end of the molding machine nozzle.

Nylon, being highly crystalline, has to be heated to slightly hotter than the melt point before it will melt and let the nylon flow freely through the nozzle again. In my seminars, I explain that nylon is a lot like an ice cube: You can heat an ice cube and until you exceed 32°F, the cube will not melt. But heat it above 32°F and you have water. It’s the same with nylon, only at a higher temperature. Nylon 6 melts at 475°F and nylon 6/6 melts at 500°F. “Now you can see why nylon freezes off so quickly,” I told my friend.

Two types of antifreeze
The key to controlling nozzle freeze-off is to use better heater band control on the nozzle, or do as I do and increase the sprue bushing diameter so the time it takes for the nozzle to freeze off is longer than the mold open time. This allows the mold enough time to open, close, and shoot the next shot in a normal cycle.

The first method I mentioned for resolving nozzle freeze-off is with the nozzle heater band or the nozzle heater control. Unless we retrofit the control system, we often find that the nozzle control circuits are pretty much what they are, meaning it’s not worth the money, time, and effort to fix this problem with the heater controls.

That leaves the heater band. We can move it around on the nozzle to try to get more heat at the end of it. Even changing to a longer heater band so the entire length is covered might help. Try everything you can, but don’t think you can just raise the heat; that just makes the nozzle drool or string between shots.

Nylon is funny, though. By using a full heater band to put more heat into the nozzle, especially at the tip, we just might cause brown streaking in the runners and maybe even on the molded parts because of the excessive heat.

For a two-plate mold like this one, see if the sprue portion of the runner is a darker tan or brown than the rest of the runner system. If it is, you’re putting too much heat into the nozzle. The problem here is that if you turn the nozzle heats down by 2 or 3 deg F, the nozzle might freeze off again. So, nothing is usually gained by working with the nozzle heater band placement or coverage.

The second way to resolve the problem is by increasing the sprue bushing diameter, a method I generally prefer. Many molders and moldmakers will argue that a larger sprue bushing diameter leads to drooling or stringing at the nozzle between shots. I dismiss this line of reasoning because the real problem is that the backpressure is too high on the molding machine (for whatever reason) or the nylon is not being dried adequately.

The answer here is to avoid using any more backpressure than you need to achieve the required color dispersion or to add a little extra shear heat into the process conditions. Use decompression sparingly so you do not suck any air into the material you are about to shoot into the mold. Just use enough to stop stringing or drooling.

Runner errors
I suppose we should get back to how I would fix the problem at hand. I started my review by checking the diameter of the subrunner that fed one of the 32 gates on this nicely balanced runner system. This subrunner should have been 11?2 times the thickest section of the part (for crystalline materials). Since I didn’t have the part, and my friend didn’t know the dimension, I suggested we use .060 inch for the part wall thickness. He could adjust the dimensions later when the mold went to the toolroom.

Incidentally, this runner had interchangeable inserts to produce several differently designed parts out of the same mold base. The runner stayed the same.

My friend told me these parts were used in many office desks, file cabinets, and other office pieces and could be molded out of nylon or in some cases acetal. I told him to hold up right there. Just remember for future reference that everything we do here for nylon will also work for acetal except for the size of the gate. Acetal requires a bigger gate size by 20-25%.

If the wall thickness was .060 inch, the subrunner feeding the part had to be 11?2 times that, or .090 inch. The next level in the runner system would be larger by .025 inch and so on up through the sprue bushing: .115, .140, .165, and the main runner would be .190 inch. The sprue O-diameter should be .050 inch larger than the main runner—.240 inch—and the nozzle orifice comes out at .216 inch (10% smaller than the sprue O-diameter). In the existing runner system, the dimensions were as follows: .075 (subrunner feeding the part), .075, .075, .090, .125 (main runner), .150 (sprue O-diameter), .090 inch (nozzle orifice).

This presents an additional problem: What is the internal sizing of the existing reverse taper or nylon nozzle? Nobody knows this kind of detail off the top of his or her head, but I can tell you what I’ve seen. I have run into reverse-taper nylon nozzles that have a .060-inch inside diameter and then open up to a .100-inch diameter at the end of the nozzle. Does this sound like it might restrict the flow of nylon for a 32-cavity mold like this one? I suggested that he increase the ID of the reverse-taper nylon nozzle to .216 inch as recommended by our sizing exercise and adjust the angle going to the outside of the nozzle so it’s just slightly smaller in diameter than the .240-inch sprue bushing O-diameter. Sometimes it’s better to go with a partial increase in the reverse-taper nozzle internal dimensions, and then increase the sizing if needed. Toolmakers are happier when you take it slow.

You can always speed up the cycle
Overall, I think we can safely say that this was a completely undersized runner system to distribute nylon into 32 cavities.

I told my friend to modify the runner system to the new sizes, check the internal dimensions of the reverse-taper nozzle, and try it out. I reminded him to turn down the pressure so he wouldn’t flash the mold face on startup. Then, almost as an afterthought, I suggested he check that his subgate size was 50% of the thick section of the many nylon parts.

I also suggested he check the runner and cavity venting. I reminded him that part vents are only .0005 inch deep, but runner vents can be .003 inch deep. We do not want flash at the parting line of the parts, but a little bit of that feather flash is OK on the ends of the runner system. A couple of weeks later he called back and said the parts were running just fine and thank you very much. He then asked if a 12.2-second cycle sounded about right for this mold. I thought for a minute, using my mental formula for nylon parts:

Wall thickness (was actually .050 inch) x 160 + 1 second for every 100 tons of molding machine tonnage (250 tons in this case, or 2.5 seconds) = 10.5 seconds.

He got quiet again, and then said, “How can I get this mold so it will run 10.5-second cycles?”

I knew the answer but stalled a bit to build up his interest. Then I told him that he should bring the barrel heats down first, and then start filling the short shots with injection pressure. They got in a hurry and were running too-low injection pressures and slightly higher-than-needed barrel melt temperatures.

I reminded him that his problem now was the same as with molders that run family molds. You have to set up the cycle for the thickest part in the mold: first barrel melt temperatures, then injection pressure, and then hold pressures. It’s the same with my friend; he just got in a hurry and forgot to go back to the basics.

It has been some time since I have heard from this molder. I guess everything turned out all right for him. As I have said before, if he is happy, then I am happy too.

September, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.

About Bob Hatch
Bob Hatch is one of the leading on-the-spot problem solvers in the molding industry. Mr. Hatch spent time as the technical programs manager at Channel Prime Alliance and managed a molding operation for more than 25 years. Currently, he writes articles for Injection Molding Magazine under the pseudonym The Troubleshooter.

The Plastics Troubleshooter