Plastics processors' peer-to-peer forum: How do I avoid PC/ABS strings?
By PlasticsToday Staff
Published: April 8th, 2011
Published: April 8th, 2011
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:
"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.
- Wrong type nozzle tip for resin.
- Wrong orifice size for mold sprue bushing.
- Wrong nozzle tip radius.
- Charge time of screw is complete more than 5 seconds before cooling time is finished.
- PID of nozzle heat control is wrong.
- Too much or too low watt density on nozzle body.
- Poor or incorrect placement of nozzle thermocouple.
- Barrel/shear heat excessive causing resin to drool.
- Decompression speed too high or low for mold/resin/machine combo.
- Wet resin.
I hope these help."
Injection molding consultant Bill Tobin added his thoughts too:
"First things to try.
- Use Eliminator nozzle tip.
- Increase your decompression.
- 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.
Matt, You're welcome Matt,
Matt,
You're welcome Matt, but the link is still incorrect (Pay Attention to Your Nozzle Body & Tip by John Bozzelli, March 2011, Plastics Technology) I would post the link but the system won't allow me.
And to the gentleman who is lucky with the GP tip, more power to ya and good luck.
If you medical molders have ever had a rejected shipment due to molded in strings, then you know you can't afford even one string to lay across the mold face. If you watch the process - you'll see that the sprue is NOT the same each shot. One shot it will snap clean like a fracture - leaving a cold slug in the tip, the next shot may pull out a huge chuck of semi-frozen resin. This is what I mean when I say " repeatable melt delivery ".
If you try an 'E' Tip, you soon realize that each shot the frozen slug is removed with the runner. If you read this Forum, you will notice that some members have gone across the board and put this type tip (old eliminators) on all their machines. The 'old eliminators are the same as the 'E' Tip but there will be no more 'old eliminators' - just 'E' Tips from Molders Choice. And they're not just for strings!
'E' Tips solve a whole bunch of other problems - one day they will be considered the 'Universal' nozzle tip much like a GP and molders will only buy GP's for hot runner molds.
To avoid confusion,
Inventor of the Eliminator and the 'E' Tip,
Samuel Kern
Many thanks to all of you for
Many thanks to all of you for weighing in and adding your insight on ways to overcome this problem.
My apologies for the bad link to John Bozzeli's article (although, agreed, the one I did link to also is pertinent). Here is the correct link:
http://www.ptonline.com/articles/balanced-filling-is-critical-for-holdin...
Matt
Stringing ABS, generally I
Stringing ABS, generally I use a GP nozzle w/ zero decompression and proper nozzle tip temp. Start lowering it until you get a nice "rounded" break off of the sprue from the nozzle.
I know, I know, your suppose to use a R.T. but G.P. is what works for me.
Stringing ABS, generally I
Stringing ABS, generally I use a GP nozzle w/ zero decompression and proper nozzle tip temp. Start lowering it until you get a nice "rounded" break off of the sprue from the nozzle.
I know, I know, your suppose to use a R.T. but G.P. is what works for me.
Not trying to hide anything
Not trying to hide anything so I will first state that I work for Milacron. To the stringing problem, the Roboshot S2000iB series has a new feature called "Precision Metering" where at the end of recovery the screw can be used to decompresses the resin by "counter-rotating" the screw rather than pulling the screw backwards. By this action, the material experiences a positive pumping action to decompress the pressure in front of the tip. This feature was not developed to solve the stringing problem but found to be one of the positive benefits.
The link is incorrect though
The link is incorrect though that article is pertinent. Bozzelli's article regarding nozzle temp control is in the March 2011 edition of Plastics Technology, titled: Pay Attention to Your Nozzle Body & Tip
The reason one reader (me) chimed in regarding the 'E' Tip and it being a better value is because he designed both. The Eliminator used to be exactly like the 'E' Tip but no longer is because Spirex lost their license to manufacture the patented tip. They have a Trademark on the name - but that's it. Their new design, in my opinion is not as robust, leaves 'dead zones' for contamination and degradation and the tip is not heat treated for endurance. The 'E' Tip is exactly like the inventor (Samuel Kern) designed and will remain that solid value.
You decide, pay for the original heat treated patented design that works great ('E' Tip) or buy a name that is a knockoff of what it used to be.
For string issue also
For string issue also controll your injection unit.
Somethimes after the suck back pressure are reamin and this cause the problems.
most of the time you could not see on your controll unit only if you put manometer directly on the right place.