Imbalance during pack

[Valentin Leung]

Hello everyone,
I have a new 16 cavities hot runner valve gated tool in a 4x4 configuration and I had a hard time working with it. I manage to established balance in the Fill phase by profiling the hot runner temperatures to reduce weight difference throughout the cavities (weights are within 2% of the average weight across all cavities). But once I pack the parts to get rid of the sinks, it all goes sideways, cavities in the middle two rows get ahead and all the plastic goes into these 8, top and bottom rows receive less and I have up to +9%/-8% difference with the average weight.
Ultimately, I find a solution by using a timer on the valve gates but I would like to inquire about possible reasons behind that. What dictates the motion of the plastic during the pack phase? I did try to profile the gates temperature to take in consideration the pack phase but still, even with shorts, once I hit the pack, the two middle rows will get ahead.
It's processing general purpose PP and temperatures are within processing range (200-235 with 250 on one odd cavity)
Thanks.

[brentb]

Question:
In fill phase, what % of final part weight is achieved ?

KOM

brent

[Valentin Leung]

89%, all geometry has received plastic at that point. After that, I am trying to get rid of sinks by adding the material during pack.
Val

[JayDub]

The problem you are seeing seems to be a difference in the relative pressure losses in the flow path form nozzle to cavity between the high-shear portion of the cycle (Fill) and the low-shear portion of the cycle (Pack). Brent, as usual, is on the trail of a solution. Typically you want to have the fill-only parts at 95 to 98% of final weight. If you can get more material into your cavities during Fill, then the pressure differentials during Pack should have much less effect.