Wearing out of the barrel

[Jendalf]

Dear colleagues,

I have question:
Do You have experience with wearing out of the barrel? Or have You ever been forced to solve problems at parts by changing of the barrel?

I have large product from PP long glass fiber (mixing 60%LGF 1,5mm pellets with Nonfilled granulated PP at gravimeter to concentration 30% in (melt) part).
The glass content in the final product is ok but problem is in its distribution where clusters of non-dispergated glass fibers appear on the surface and comes thru the whole thickness of the part. Although the part is non visible, the nondistributed glass content in some area causes that the part is scrap and can´t be used in next process.

We´ve tested a lot. Even with the changing of the screw by new special one for glass content = no change. Than put back the old refurbished = no change. Closing ring is ok, no wearing.
The last thing that had never been changed is barrel itself.

But I dont want to misslead You in decission by this, just asking about experience and possible cause of poor distribution of glass content in the part


I´v contacted supplier as well as the other IM producers - yet without clear answer.

Thanks for reading/comments in advance.

J.

[Jendalf]

Photo attached. non-disperged LGF filament on the surface of the part

[joeprocess]

We run a long fiber Stainless Steel filled PC/ABS and sometimes we experience the same thing. The root cause to our issue is hang ups of the glass in and around the check ring and tip. Are you using a free floating check ring? How aggressive is the screw tip? Does it have areas that could collect tramp glass? With the long glass fiber material I would also look at increasing the gap between the check ring and the retaining ring to allow the material to flow through easier.

[Airwinghma]

Greetings;

You need to determine if you barrel is bad/worn by measuring the zones and match to the manf specs, lots of $ need to have data. Have you tried a mixing head or section of your screw?

Scott

[Suhas]

Hi J,
It looks like the plastic is have not melted sufficiently and so the glass and the plastic are staying together. When you inject it, the long glass fiber pellet/strand is getting smeared like you put butter on bread. Have you tried increasing the melt temps and mold temps?
Suhas

[rickbatey]

I'm with Suhas. Your not hot enough to disperse the glass fibers in the melt. You should use moderate back psi and a medium screw rpm. What's the shot volume compared to press and residence time? That might be another contributor to your problem. Have you measured the actual melt temperature?
Rick.

[Jendalf]

Hi all,

screw tip is standard tip without any mixing elements. also head of the screw isn´t special and it has only several cross sections around the diameter.

The temperature set is enough for PP LGF (generally I use 270 - 260°C) profile from begining to tip.

There is one thing about the melting of the LGF:
Generally the most heat to plastic goes from friction not from the heaters.

Problem here is that the friction can´t be applied in full influence bc of higher gap between screw and barrel. This comes from experience of screw/barrel producer.
Another point that we discussed togather is that glass content makes the layer on the surface of the barrel (this layer is almost glass only!) and appears on the metering section of the barrel. Also from this area can all the problems come.

Barrel/screw producer confirmed all of these things with their own experience.

Now I´ll come with this to management - but still will present it like a probability of solving problems.

[rickbatey]

What's the residence time? If it's very short your standard barrel heats that are 'ok' really aren't good enough. Too much back psi or screw rpm will just break the glass fibers and they can rise to the surface. Also what's the mold water temperatures? That can help hide the glass fibers. Also I prefer full internal taper nozzle bodies and tips to eliminate the possibility of cold slugs.
Rick.

[JayDub]

Here’s a random thought – do you see glass-rich areas in the sprue and runner, or just in the part itself? If the problem only shows up in the part, then maybe the segregation is happening at the gate, not in the barrel.

[Jendalf]

@Rick - residence time is +- 6 shots it means 7 minutes. I´v tested back preassure several times - the highes what machine can give is around 9-10 bar in real. Thats strange even if I set the 20bar. This wasn´t discussed yet with anybody. Good point.
Mold has 30°C both sides. Problem is that I don´t want to hide it I need not to be there bc next proces is supposed to perform the wall half by laser.

@Jay - Another good point. Recently I´v made the test where I slowly "injected" the whole batch out under the injection unit to see what comes out during the entire injection. In the strap of material there wasn´t observed any conglomerations of glass!
Mistake: the injection didn´t follow exact injection conditions as it had to be slowed to be able catch the batch into strap. So i dont know what exactly I´v got. and how to evaluate this.

The tool is hot runner (12 nozzles) with 10 nozzles exatly to the part and 2 only has small sprues (measured glass content is queal to the percentage of glass set on gravicolor). The segregation at the gate is possible, but from the Picture we could see that the conglomerates are exact length of PP/glass pellets.

J.

[rickbatey]

Jendalf, I think your still not hot enough. Do you know actual melt temp? What's compression ration and L/D ratio of screw? The glass showing could be from melt fronts. You mention nozzles but not valve gates so I figure all the gates are filling at the same time. Again could be melt fronts. BTW-the glass on surface shouldn't matter to laser score since it must cut it while on surface or buried in substrate.
Rick.

[Josue]

Im with Suhas comments also,, looks like you don't have good melting,, may be the back press could help? , how about dust on your recycle?
we also run same kind of material and GF, recycle, but the only reason that we change the barrel, its due that the barrel its already worn some areas causing mainly short shots,, not by this fiver issue,

Best regards