Bridging near Feed Throat

[WNCDave]

I normally work with 350R2 blow molders but for the past 9 months I've been working with a newer Uniloy, the R2000. The problem really can be universal, however, and the problem we have been having is plastic bridging (or hardening) at the feed throat area during start up. The R2000 is typically down for approx. 50-60 hours on a weekend and then is heated up back to spec. for startup. We have setback temps that can be auto-enabled and is set to reheat the machine approx 2 hours before startup. So, typically, the machine is ready to go when I get on the floor.

Nothing has been changed to the machine as far as heat specs are concerned. The only way I can free the plastic is by pouring handle slugs on top of the screw through the magnet door under the resin hopper, thereby forcing it out. The plastic we get shipped in is also the same. I don't want to go the route of emptying the barrel and screw of plastic for shut down, for fear of having lots of burnt plastic for startup. I remember working on 350R2s with homopolymer that would give us burnt plastic for several days if this was done.

I'm actually at a loss for this one, but I do remember there being a simple modification or "cure" to get the machine to heat up more consistently. Last night was the 3rd weekend in a row of having the plastic bridge at the feed throat.

Any suggestions would be helpful.

[Louis]

When I think of bridging as it relates to the feed throat area, I think of the section of the machine directly below the hopper and above the screw that sometimes gets a clump of material bridged across the opening preventing material from getting to the screw. From what you posted, I gather that you can see the feed screw, and it is the feed screw itself that has a section of bridged material. That being the case, I should think that it is either one of two things. During shutdown, the machine has not cooled enough before the screw was stopped, allowing the still hot feed section to melt the material into a large mass, blocking feed. That, or the start-up heating process is reaching temperature far too early prior to the start of the feed screw. In either case, allowing material to melt on the feed section of the stationary screw will cause issues.

Of course there may be other factors, such material moisture and throat section temperature. As in any trouble shooting problem, you need to look for what changed.

Hope that helps!

~Lou

[Tom]

Have you looked at opportuniies for throat cooling and screw cooling? I would guess that residual heat frm the screw is working its way towards the feed throat when the process is stopped. Because you are stopping with the process filled the hot screw is covered in plastic. Heat will have trouble getting out of the screw because plastic is an insulator. The heat will naturally move towards the feed throat area because of the temperature differenace. If the screw is cored in the feed area you could set up some air or water cooling for the screw to remove the heat and prevent the problem.

[WNCDave]

Thanks for your replies. Shortly after I posted this someone from another shift had admitted to leaving the machine one with full heats for several hours before enabling setback temps for shutdowns. Needless to say I'm hoping they start doing it correctly.

[Louis]

That will do it!

It's all about time/temperature.

Hope it works out.

[WNCDave]

During shutdown, the machine has not cooled enough before the screw was stopped, allowing the still hot feed section to melt the material into a large mass, blocking feed.

I actually don't quite understand this. Once the extruder drive has been stopped then the shutdown process is to begin. From what i'm reading you're saying that the temps need to go down before the extruder is to be stopped? If you can elaborate on this i might understand what you're depicting better. Usually a machine will ring out and shut down if temperatures go below deviation while the machine is running, so it's not plausible to actually turn the temps down while the machine is running. Well, it is, but I feel as though I'm interpreting you incorrectly. If you know of a better way to prep shutdown besides just setting the setback temps on, whereby saving possible future clogs we will look into it.

edit: oh, good lord. Let's pretend i didn't type all that out. At what temp would it be wise to stop the screw from turning before the plastic loses too much malleability?

[Louis]

It is not uncommon for newer machines to be equipped with a programmed automatic shut down sequence where speeds and temperatures are adjusted. Older machines with little controls will allow you to lower the heats while the machine is running; careful – that could result in a two piece feed screw and we don’t want that to happen.

For those machines that have alarms that shut down the machine on low temperature, you can manually reset the first two zones to a lower temperature and monitor the amperage (Assuming that the machine will not shut down on high amps.) Let's face it; most running temperatures are set higher than the actual melt temperature of the resin.

I can recall lowering temperatures, turning cooling on manual and slowing the RPM's. You had to stand right there to monitor what happened, but it worked.

Again it is all about time versus temperature – you need to get the cooling to the plastic as quickly as possible during shut down. During restart, you may want to consider bring the temperatures up to something lower than your running point to settle in, then raise to running temperatures and start as soon as they reach set-point.

[WNCDave]

Thanks for your input. Yeah, the one thing I fear, which is why I will not mention it to them, is someone forgetting about the screw still turning while the temps reach a "too low" level, and then *kersnap*. It is good advice, however, and if we ever get to the point to where we might need to try this I now have the insight into another method.

[Wiseco]

If you have a good controller, you should have temp setpoint alarm somewhere. You could adjust them to avoid the extruder drive to start when temp is in the low alarm setpoint. For the shutdown procedure. Set a good low deviation alarm setpoint so at the shutdown, you could adjust the setpoints of the first extruder zone to below temp and when the low temp alarm setpoint is reach, the driveor the entire machine stop by itself.
My 2 cents.

As you said, you DON'T want someone forget something bad so if you can set a security do it.