Screw pulling air through nozzle

[LSasso]

I recently realized that when the screw retracts it is pulling in air after a few seconds. What seems to happen is when there is melt in the nozzle it seals it off and the screw retracts normally filling with resin, but after a few seconds the melt in the nozzle drops away and with the nozzle open the cylinder pulls air through it. This explains why I've always needed very fast screw speeds. If the screw speed is not very fast the cylinder fills with more air each time the screw retracts and eventually the shot will be almost nothing. Is there some mechanism to stop air from flowing in that might have failed? Or am I missing something about how this process is supposed to work?

[brentb]

Well if the resin goes out the nozzle, air would go in.. Do you mean barrel when you say cylinder? If all the resin leaks out, where is it drooling to? What resin are you running? Have been molding long?

KOM
brent

[Maheshkumar Hiremath]

Hi,
Is this problem are you facing with and without nozzle retract ? Please check this effect without nozzle retract if you are using the same one.

[LSasso]

Brent: Yes I mean barrel (I'm used to working on engines). The resin is Zeonor COP. I've been molding for about a yeah in my garage, entirely self-taught from print materials and trial-and-error, so some details have been missed.

MaheshKumar: I was experimenting with the nozzle fully retracted when I discovered this problem, but the behavior is the same during a normal cycle where the nozzle is not set to retract. There's no way for me to see what's happening when the nozzle is not retracted.

Is there no system in a nozzle to block air from being pulled in, it just relies on the previous sprue to act as a valve? In that case maybe I need to adjust the process in some way to ensure that the nozzle is blocked off when the screw retracts? I guess I'm also wondering if this would be normal behavior for the machine, let's say with the nozzle retracted?

Thanks for your help!

[kelley]

With enough back pressure the melt should fill the front of the barrel completely, preventing any air from entering. decompression should also be kept to a minimum.

If your drooling during sprue break, try lowering your nozzle heat a bit, perhaps even trying a slight reverse profile for your barrel heat.

What type of machine are you using?

kelley

[brentb]

There are shut-off nozzles, but be careful
KOM
brent

[rickbatey]

There is some air pulled into the nozzle, that is typical. That is why you use as short a stroke and slow a speed that makes good parts.
The typical process has the screw recharging and finishing decompression 1-5 s before the cooling timer runs out. Also monitor the nozzle temp to minimize drooling.
Rick.

[Maheshkumar Hiremath]

One of the interestiing topic everyone contributing their experience / thoughts.....

LSasso,

Please watch carefully whether the screw ( NOT NOZZLE ) is retracting before refilling starts, in this case there is chance that air can enter into the barrel.I have seen Some machines have this option of screw retract before refill.