Sorting out this type of issue can be quite complicated and although the advices given above are all very helpful and sound, one would require a full picture of the issue (part, machine, material, processing conditions etc.) to resolve it. My first suggestion would be to hire someone who is experienced in resolving this type of issues. Here are four tips:
  1. Change in pellet shape (was stranded, now underwater) implies a different compounding line, so maybe mixing/uniformity is less, degradation more, etc.
  2. Grind both the parts made from old (good) compound and the new (suspect) compound, and test them the same way (can you tell the bad ones from some quick test like tensile or impact)? Record screw amps or torque to compare viscosities.
  3. Infrared on such a mix may be too complicated to read clearly. DSC is better. Test solid density, melt index (dry samples), other rheometry.
  4. Once you prove that the materials are substantially different, it's the material supplier's problem to figure out why. Look for another supplier, at least until the supplier pays for more testing, replaces the bad lot, or any other solution that you agree to. If it's a "bargain" supplier who is charging less than the average elsewhere, you are getting what you have paid for, and still should look elsewhere.
Before making an estimate of the required flow rate of the installation, you have to decide the method of cooling the water and thus the operating temperature. Water chillers can provide 10 - 13 oC input, while cooling towers in a hot summer not less than 30 oC. The coldest the water the less you need. Gather information concerning the quantity of water needed by each injection molding machine for the oil coolers and feed throats. This information is provided by the manufacturer of the injection molding machine. A good option is to bypass water cooled oil coolers with air cooled. In this way you provide heating during winter in the plant and at summer you reject this heat out.
 
Next is to estimate the quantity of water needed to cool the injection mold. This depends on the nature of the process and the quantity of plastic processed per hour.
With regards to ZERO sink-marks, however, this is a Holy Grail that is practically impossible to achieve with basic injection molding, as soon as you locally thicken the general wall section by adding ribs or any other protrusions on the opposite surface. However, following the well-proven guidelines on ratio of rib-to-base wall thickness will always help minimize the physical dimensions of sink-marks. Conversely, the higher the gloss level, the more you will see sink-marks, for any given depth. On a high gloss surface, even a dip of a few micro-meters will be seen under certain light conditions. The nearer to feed points, the more you will be able to make the packing pressure do its job of "inflating" the slightly thicker section as volumetric shrinkage pulls it inwards. You can see often this effect very clearly along the length of a rib.

Another means, other than packing pressure, of resisting the pull caused by volumetric shrinkage is to use fluid-assisted molding (gas or water). Great results can be achieved compared to conventional injection molding. Also, plastic parts produced using Mucell technology benefit from microcellular pockets of inert gas to maintain the internal pressure needed to resist the localized sinking. Use of the Rapid Temperature Cycling technology can even "iron out" the otherwise "grainy" surface caused by bubble collapse, resulting in superb gloss (if the material is intrinsically glossy, that is).
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