Limiting the Injection Speed on PBT?

[zshupe]

Hey all,

I am training to be a process engineer, and today I was shooting a new tool that uses Toray 1401X PBT. One of our sales guys stops by and freaks out at how fast I was injecting it. He told me, per our customer, that PBT should not be shot faster than 60mm per second (seriously? go sell some parts and stay out of processing ).

Now I haven't been processing for more than a week, but how can you limit the LINEAR VELOCITY instead of the VOLUMETRIC FLOW RATE? Am I right in saying that unless your nozzle tip and barrel area are the same, the linear velocity tells you very little about the effects of shear. A 1" barrel is going to shoot a lot less material than a 10" barrel at the same linear velocity.

Does anybody know of a shear limit on PBT? And wouldn't they give me that figure in mm^3/s (or maybe s^-1)? Tell me if I'm wrong, but since the viscosity is reduced the faster you inject, wouldn't the shear rate be lower at faster speeds as well? When I shoot it slower, it puts the process very high on the Rheology curve. My understanding is you want the process to lie towards the low end (the more linear section) to make a stable process. At the slower speed, my injection time goes from .75 seconds (determined by scientific molding) to almost 2 seconds!

Feel free to lecture me on any of these topics! I got put on the processing fast track when our last engineer quit, so I'm trying to soak up as much as I can!

Thanks,
-Zach

[Suhas]

Hi Zach,
Yes, you are correct. One needs to know Volumetric flow rate. May be he asking you to limit the velocity since it could be glass filled. Of course there is a practical limit on how fast you can inject but under the regular dia screws between 25 and 45 mm, 60 mm/sec is not much at all That is about 2.4 in/sec. I have used higher speeds more that a few times.
Wish you all the best in your career. Keep Posting!
Regards,
Suhas

[rickbatey]

My two plugged nickels: I'm going to fill every mold I can at the fastest injection rate I can; few exceptions! Time is money and slow fill rates allow inconsistent melt paths inside the tool. Then you end up chasing issues every time you shoot that mold.
Rick.

[rickbatey]

Unless you see glass fiber breakage, gas traps or poor surface finish. Otherwise I'm putting the pedal down as far as I can.
Rick

[zshupe]

Thanks for the answers.... exactly what I was hoping to hear! The resin isn't glass filled and my injection speed ended up being around 2.5 inches per second, we'd be better off setting the mold on its side and filling the mold using gravity.

[Suhas]

2.5 inches/sec is no issues at all. (But always think flowrate).
2.5 in/sec on a 50 mm screw is way different than on a 25 mm screw.
Suhas