Recovery Issues at Start-Up

Our company has ~40 machines and we mostly mold PBTs and Nylons. On some of the PBTs and Nylons it takes up to a half hour to get the material to feed into the screw and purge properly. Once the material starts purging well we have no other process issues once in cycle. We’ve tried raising the rear zone temperatures, hotter feed throat, high/low manual BP, high/low RPMs, pulling the screw back then trying to extrude, starve feeding material, and forcing the material down into the feed section using a piece of wood (seems to work the best but we cannot do this on modern machines as we cannot get to the feed throat due to the large purge shields on the machine).

Some info on our machines:
Barrel sizing: 22mm to 40mm
All electric Roboshots
GP screws

This does not happen for every Nylon or PBT, it seems to be just a few grades. It’s not a terrible problem, just annoying and you end up wasting a good bit of time and material trying to get the machine to purge. When we’re having this issue and pull all the material out to look at the screw it is never wrapped and in good condition. Any ideas?

Can you tell whether the material is getting to the screw or hanging up in the throat? If the latter it could be coming out of the drier too hot and soft and bridging. You could try drying at a lower temperature for longer. A technique that is supposed to help with screw feed is to have grooves cut in the back end of the barrel (essentially right under the feed throat), but I don’t have personal experience with it.

Interesting - it's fine once it's running though. So what would cause dosage issues when screw is empty vs screw is full?
The material enters the screw same way in either condition. So that sort of eliminates any potential issues above the screw, right?
You mentioned most solutions that came to mind.
I might try a colder (like cold) rear barrel temp and profile up to a hot temp to the nozzle
if that doesn't work try the other way around

Tried with high and low back pressure? or try to simulate what the screw would do in fully auto?
block the nozzle (with cold slug in sprue maybe?), use back pressure extrude the screw. I might try pulling screw back and trying to extrude in different stages as well; feed zone, mixing zone.

let us know

Have you confirmed all is correct inside injection unit? Maybe wear or pellets stuck to the screw after shutting down. How do you shut these machines down and for how long? Run them completely empty one shutdown and see what you get.
I’ve seen this with incorrect barrel temperatures (t/c wrong/damaged) and when the cold screw prevention times were too low. Is there a torque setting on charging drive? Maybe running high speed versus high torque.
Could there be a setting in machines that causes low torque and screw RPM on restarts or a set idle timer? I’ve seen that on several new machines.
Rick

I had this same issue at a company I worked for a while back. We were running 30% GF Nylon and two things were happening over time on the old Van Dorn beast.

1) We were using an older worn out GP screw for 30 GF Nylon so it wouldn't build a shot. It would just allow the material to flow out through the tip.

2) The back pressure solenoid was malfunctioning. So there was continuous back pressure, even when disengaged at the switch.

The company was cheap and not ready to replace the screw. The solenoid they said they would find some ebay or somewhere. Temporary fix was having a thick steel plate block the tip while making a shot. Or close the mold and manually make the first shot into the mold, knowing it would be short and run fine after.

Since the problem is only at startup, maybe the barrel heats are not "soaked"
Instead of trying to purge for that 30 minutes, try just waiting.

I also have experience this . Most times it is pellets stuck to the screw . instead of force feeding try to clip some runners about the size of one flight of the screw and feed the down the screw first then followed up by the resin .

I would assume that the rear heaters are all functioning properly along with the feed throat water . If you have the covers off the barrel you could try to measure on the rear zone what the temp spread is this would mean measure on feed throat side of the heater band and on the barrel side of the heater closes to zone 2 usually the rear zone has at least 30 degrees differential within the one zone and is usually worse as things are heating up as heat is being pulled away by feed throat while heater band is duty which also over heats the region between zone 2 and 3 . As we start to cycle the heat source is more from friction and the heating rate is decrease which helps to balance this rear zone better . Most newer presses will try to compensate by having a larger wattage heater next to the feed throat depending on the size of the press .