Nylon 6-6 resin (glass reinforced) 1

[henks580]

the machine used for processing is a reciprocating screw type of machine. Lately, I have been experiencing "melt cushion" problems. Apparently, from what I understand, this means that I am losing some of my plastic sowhere...my shot seems to contain less plastic? we use shut off nozzles to prevent drooling and hence reduce the possibility of losing plastic through the tip of the injector. Could it be possible, assuming all my parameters have remained constant for the past few weeks, that my barrel may be leaking internally? Or that my screw in wearing out?

temperatures and injection/holding pressures have been constant all throughout.

thanks,

 

[patprimmer]

Your problem will be the check valve on the front of the screw, and possibly also wear in the screw and barrel.

Wear in the screw tends to show up as the screw not moving back on recharge.

Loss of cushion with no material appearing outside the nozzle is lack of seal at the check valve. This can be due to wear of the valve or wear of the barrel or both. It is nearly always wear in the valve.



Regards

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[injectionmolder]

Patprimmer is exactly right, sure sound like your problem is the check valve. What about your molded parts? If you are running a natural resin do, you see any burning at all? That will be an indication of the mentioned problem. Also is your part weight going up and down? Before you get to pull the screw to check overall dimensions, try this: get a shot into the cavity tool and interrupt the cycle while on holding, wait for sprue, and gate(s) to freeze off then manually, inject with nozzle making mold contact keep the nozzle shut off on closed position too. After all this if you see the screw position moving forward and going to zero then you’ll know is time to replace your check valve and perhaps even the barrel. Now if the screw holds inertly about on the same position, then we are dealing with a different issue.
If you are not losing plastic upon injection, perhaps you are not “gaining” the same amount of plastic upon recovery, were you running virgin and now is re grind. If that is the case, you might have to increase your back pressure.
Check also nozzle shut off heater bands and that shut off system is opening 100% and there is no obstruction on your nozzle tip, also check for contaminated polymer with heavier molecular weight plastic obstructing the tip and gates, keeping transfer to holding on pressure or time might work against you for this issue, is always best to keep it on position.
Have your maintenance person to also check the drain valve for the back pressure system making sure is damping the same amount of oil back into the tank as the screw is recovering.
Note: if you in fact have your check valve is defective, what some companies do to kind off cover the problem is an increase on melt viscosity by decreasing melt temperature, it works up to a point however not recommended.


Best regards