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 plastic's viscosity is so high and the injection speed is so fast there is no practical evidence to support gravity effect. I've only seen it in slow fill molding of liquid silicone.

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.

Everytime I hear about 'gravity effect' there's a simple test: Turn the mold 90 or 180 degrees and mold parts. DON'T CHANGE the waterline hookups. If the argument is correct, it will prove itself.

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.

Normally the effect of gravity should be negligible, according the Bill's indications. From the other side, I think that the complexity of the problem (unexpected difference of shrinkage) and of the system (mould + part) can not be described in few words. I think that rheological analysis could be very helpful but be careful: simulation could describe the situation only if they model in a correct way the system.
In many cases, we solved in the past, making CAE analysis, problem in parts where simulations were previously done.

Forget the "gravity effect" this has been debated for years. There is no such thing, nor data to prove it exists. The inject portion of the average molding process takes place so fast that "gravity" has no practical impact on the process.

Thank you all!
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.

Thank you Bill. I agree on your comment. Calculated pressure difference just tens of kPa order, very small. But the molders who treated similar cabinet urges gravity effect with their experiences which include multi-cavity molding results, lower cavity showed larger dimension(small shrinkage).

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 (^^).

Thanks again.

Won-gil, there are many points that could drive to difference between the CAE results and the real situation: starting from the experience and skills of the people, arriving to the material's data and the process conditions... Most of peoples think that is the CAE system that finds the problems and fixes the issues but in my 20 years CAE experience is telling that it is not in this way. As I wrote to you before, your issue seems to me deeper and it should be analyzed in a deep and detailed way.