Sink - Symptom vs Disease
Oh no!
There’s serious sink on your part.
Part geometry has some transitions and features but nothing that should create sink.
Sink is happening at unexpected locations.
Take a deep breath.
Tell me more.
What did you do to solve this issue?
We tried everything.
Extra packing
Increased melt temperatures
Changed material to a high flow polycarbonate
We even added direct venting under the sink area.
Nothing works.
Maybe there is some air trapped in the transition area. . .??
This is one of these situations where
You need take a step back.
Take a look at the full system.
Full system? Tell me more.
Check out the runners
Top and bottom sprue diameters
The machine nozzle
Engineers can get too focused on the PART when stuff’s not working.
Root cause analysis sounds like . . .
“The part, the part, the part.
Need to look at the part.
It’s the part! “
People forget to look at the feed system design.
Many molding issues originate in the feed system.
Are the runners the correct size?
Note: Polycarbonate appreciates generous runners.
Check out the sprue and the sprue-runner intersection?
How is pressure being managed at these initial transitions?
Oh . . .
And don’t forget the machine nozzle.
One of the most unloved molding components.
Machine nozzles are supplied with roughly a 2.5mm opening.
Here’s what no one tells you.
Machine nozzle inserts are supplied with the intention of being machined out to suit your molding application.
Again, no one tells you this.
Nozzles are meant to be machined.
Adjusted.
Customized.
What do you do?
Take a close look at the feed system
With experience your instinct will highlight the concern areas.
They will look . . . “just not right”
Nozzle opening, sprue diameter, sprue to runner ratio, runner primary to runner secondary relationship.
Your MoldFlow can help with this investigation
Make use of the pressure and pressure drop analysis filters.
You can also use the fill video by slowing it down to half speed.
This will help you catch any flow hesitation scenarios.