Gravity effect in injection molding?
In injection molding bigger TV cabinet, there is less shrinkage in lower wall, in spite of thicker wall, Some say higher packing in thicker part results in small shrinkage. And the other say gravity effect makes small shrinkage in lower wall. CAE result shows bigger shrinkage in lower (thicker) part, for that higher temperature at ejection makes bigger shrinkage. Any comment on the matter will be appreciated.
The difference in shrinkage is due to the amount of packing pressure throughout the part and how the plastic cools. Shrinkage is stress relief. Uneven stress will give you uneven shrinkage. You can actually map regions of the part as a function of the distance from the gate as to the amount of shrinkage.
Remember, your mold is the world's most expensive shrink fixture. Keep the part in the mold for 20 minutes you'll get a large part. Make the same TV cabinet in 30 seconds and the shrinkage will show itself as warpage.
WHEN TO EJECT: forget the CAE models. Look up the heat distortion temperature from the material manufacturer. Get a fast response pyrometer. The fastest cycle you can have with minimal warpage (with the mold at the proper temp and your material's injection temp being at the midpoint of the melt point range) will be a surface temperature taken immediately upon opening that is 20% lower than the Heat Distortion Temperature.
Multicavity molds are usually unbalanced. While I'm not a believer in the bold balancing software with a little 'tuning' they can be balanced.
Hope this helps.
In many cases, we solved in the past, making CAE analysis, problem in parts where simulations were previously done.
Everyone is on the side "no gravity effect", and I would join the party. I would look for the reason outside. I would look for what makes the error in CAE analysis.
Now I am making CAE analysis reference for the part to predict warpage result for the future models, matching CAE results with real product, so I cannot ignore CAE models (^^).