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It is not an uncommon issue. An injection molder notices strings of hardened melt hanging from his molding machine's nozzle when the gate pulls away from it. How to beat this problem? Ask your peers.That's what one molder did in our online forum, and we decided to bring the helpful feedback he received here to the front page to help other processors facing the same issue. The comments have been mildly edited for clarity; you can skip to the original string here.Starting the string of comments was this problem:

PlasticsToday Staff

April 8, 2011

3 Min Read
Plastics processors' peer-to-peer forum: How do I avoid PC/ABS strings?

"Ok, help me out here as I've got some stringing issues. When the gate pulls from the nozzle, there is some plastic string hanging there; this naturally is not optimal.

The material is ABS and PC and PC/ABS for now. What I tried so far is to lower the nozzle temperature from 10 deg. to 50 deg. (Fahrenheit) and also raised them as well so they'd suck back less, then more. I switched from a full taper nozzle tip to a GP tip, and lowered the mold temperature. We also tried adjusting the back pressure-reducing it as well as increasing it, and done the same with screw rpm speeds (faster and slower).

My head has a dent in it from banging it on the mold; that did not help. The most problematic material is an ABS, gray in color. Thanks for any help."

The first response from a fellow molder came in quickly:

"Here are the majority of reasons for stringing, listed in no particular order.

  1. Wrong type nozzle tip for resin.

  2. Wrong orifice size for mold sprue bushing.

  3. Wrong nozzle tip radius.

  4. Charge time of screw is complete more than 5 seconds before cooling time is finished.

  5. PID of nozzle heat control is wrong.

  6. Too much or too low watt density on nozzle body.

  7. Poor or incorrect placement of nozzle thermocouple.

  8. Barrel/shear heat excessive causing resin to drool.

  9. Decompression speed too high or low for mold/resin/machine combo.

  10. Wet resin.

I hope these help."

Injection molding consultant Bill Tobin added his thoughts too:

"First things to try.

  1. Use Eliminator nozzle tip.

  2. Increase your decompression.

  3. Go though that shopping list above as it pretty much covers all the bases.

If that fails, you have a machine problem."

The original poster replied with the type of machine he uses, and a question to one of the 10 tips above:

"The press is a Battenfeld. What is the PID of the nozzle heat? The T/C cannot be moved as it's already maxed out.

So you're saying that the watts for the nozzle is tot much? The dryer is on top of the press and it is big, drying 6 hrs worth or more of material. The mold opens right after recovery, maybe 3-sec or a bit more."

[Ed. Note: PID refers to a hot runner temperature control.]

Another reader chimed in, but be forewarned as maybe he works for the company he's pushing:

"Bill (Tobin) meant to say try an 'E' Tip from MoldersChoice instead of the Eliminator, since the Eliminator is a lesser value for the money. (Ed. Note: The Forum includes quite a few comments about Eliminator nozzles, and this one I found particularly interesting.)

The 'E' tip was designed to deal with the problem you're having right now without sacrificing your molding window. You still must do the right things as far as nozzle body temperature control, but this tip really gives you a shot at repeatable melt delivery. Visit the website then read the patent info to learn why it works. John Bozzelli just had an article in Plastics Technology about nozzle temperature control and how much it cost molders. This tip takes the bite out of that loss."

What the heck, here's the link to that competing publication's article.

Here's hoping those strings are gone. When you need answers to your plastics processing issues, give the Forum a try. Your competitors are also your peers.

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