Warpage is always a great deal during product development. In different papers I have checked, packing pressure determines hightly warpage when it's not influenced by cooling. If increasing pressure increases as well shear stress and temperature of the part, which parameter will give the same result? Found that decreasing flow rate, but this affects importantly cycle time and thus cost.
In the case cooling is an important factor, is better to have a faster cooling or a slower cooling? How far from mold temperature should go lines?

It really depends on the part design and the material used.

There are three different causes for warpage:
1.) Compressive Stresses
2.) Orientational Stress, or Cooling stresses (same thing)
3.) Tensile Stress

COMPRESSIVE STRESSES:
Since plastic is compressive we lose pressure going through the part geometry. When this happens there can be more pressure (more polymers) on one end of the part and less on the other. Therefore you can have a differential in shrinkage. This can cause warp.

COOLING STRESSES:
If you are using a semi-crystalline material then cooling rate and time will be a huge factor that can influence the warp on your parts. When we flow the resign we orientate the polymers and in the semi-crystalline resign those polymers want to organize themselves and fold this is called crystallization. This stress can cause warp inside or outside the tool if the polymers are frozen in the orientation state. Sometimes in the field these parts if heated past the deflection temperature can cause the polymers to move and that can cause warp as well.When we talk about cooling rate that can be both melt temperature and coolant temperature that can influence the rate. However the location of the cooling lines on the mold can influence the steel temperature that the plastic is in contact with. So we should really be looking at the temperature on the steel not so much the coolant.

TENSILE STRESS
This is typically when you have sharp corners on your part design. When the plastic flows around the corner it forms a lot of shear stresses. This stress can cause warp as well.

So therefore it would be hard to determine a one size fits all strategy for fixing warp. 1.) What material are you using
2.) Do you have thick walls or thin?
3.) Can you see the in cavity pressure to determine the pressure loss or have you done a pressure loss study?
4.) Do you have a consistent steel temperature across the molded surface taken in between shots?
5.) Does the part have an excessive flow length?

These are just a few questions I would ask before I can suggest anything.

Dan is making good points to warpage. Most of time, adjusting molding parameters will not erase warpage significantly, changing part design or changing mold steel is more realistic and practical. And make some simulation eg moldflow before mold fabrication, can help to see potential warpage, then change part design or set some safe steel for later adjusting, this is one of our strategies.

Initially we need to understand that all molded parts have differential shrinkage factor in flow direction and traverse to flow this can cause warpage. If we increase packing pressure and time we minimize the shrinkage effect having lower warpage but if is too much can cause overpacking and warpage when part is ejected. The cooling system should be capable to have sufficient pressure and water volume in g.p.m to ensure turbulent flow, adding baffles can help. If cooling lines are too close to cavities we have better cooling but can cause leaking problems in short time due weak steel condition if is too far away the cooling will be not sufficient. A good approach can be to design cooling lines 1.5 times cooling line diameter from center to cooling line to the cavities. Other option is to add a nucleating agent or a processing aid.

Hope this info help you
F.Gallegos
Carling Technologies

Thanks guys, I'm mold flow analyst, but when the design engineers don't want to change anything cause they already have the part released, it's quite difficult to work with those parts and I just have options for parameters.
Responding the questions:
1. I'm using natural PP UC9X005C-Natural B96704
2. Thin walls
3. I made a Moldflow simulation, so I have access to pressure drop at different points
4. No, there are differences in temperature up to 40 celcius
5. In fact, making the revision, flow lenght is pretty high

You have a few things working against you.

MATERIAL
1.) There is an old joke about PP (Polypropylene) that we use in the field at times. I call polypropylene - polywarpolyene. This is because it is a semi-crystalline material so it will be very sensitive to temperatures differences and cooling times.

THIN WALLS
2.) Since you have thin walls I am sure the walls are starting to set up while you are filling the parts so fill speed will definitely be a contributor to your warp as well.

PRESSURE DROPS
3.) If you have a large pressure drop through the part you will find that the pressure can be a huge influence on the warp as well. Try not using gate seal to allow some of the pressure to leak out from through the gate to try and equalize the pressure from the gate side to the end of fill. That sometimes works.

STEEL TEMPERATURES
4.) I think F. Gallegos said it best where you really need to have good control over the temperature differences throughout the molding surface. Especially since you are running Polywarpolyene. :)

Happy processing....

As you are using Moldflow, try different gate positions and different packing profiles in order to have a uniform volumetric shrinkage through the part.
Unfortunately, if you have very thin walls or ribs, they will freeze very fast and the packing changes will have no effect on them.
Hope this helps.
Cheers