A technician encountered this interesting problem yesterday…
We have been working to achieve 95-97% fill during first stage fill with zero hold time and pressure. As soon as I apply hold pressure and time, the screw continues at speed after transfer. My machine has a hold speed minimum setting, but no option to turn this setting off. I am actually observing a pressure spike from 15 bar to 25 bar at transfer. To get an acceptable part, I need to profile a drop in injection speed before transfer as well as increase the transfer position in 5mm increments until the pressure spike at transfer was eliminated.
Can you offer any thoughts as to why this is happening? Is there something I missed?
Many machines have a bad habit of allowing the screw to overshoot when 2nd stage pressure is turned on, but stop immediately when 2nd stage is turned off. On most of the newer electric and counter-pressure molding machines you can actually watch the screw stop instantly when the second stage is turned off. Unfortunately, a few of these fancy machines are factory set to overshoot the moment the second stage is turned on… this means that the same screw which stopped on a dime will now overshoot and flash just by turning on the second stage time (even when the pressure setting is at zero).
Basically, this is a silly attempt to mimic older hydraulic machine behavior. In such a case, you should turn on the second stage time to a normal value, then set the lowest value possible for pressure and minimum speed. You should now see the screw overshoot on your particular machine. This will allow you to establish a more representative 1st stage short shot.
On any machine, you must establish a 1st stage short shot which represents the machine’s behavior during production… this will allow you to fill and pack out the part more consistently.
note: Whenever possible, you should turn off the ability for these newer machines to overshoot. The more accurately the machine performs the desired actions, the more consistent the process.
Although each machine is different, many newer machines have the option to turn on and off the injection overshoot such as a choice between sharp or gradual transfer. Other machines may have this option buried in the maintenance portion of the controls. You may have to contact the manufacturer to find out where this option is located.
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