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I suggest that it doesn't matter. What does matter is that the mold has the ability to remove heat uniformly from both sides of the mold at the same time. Warpage is caused by one side of the molded part shrinking or crystallizing first, and then the other side continuing to shrink, usually after it is ejected from the mold. A method of obtaining this ability to remove heat uniformly is to design the water circuit to have lines of 7/16-inch diameter located two times their diameter from the surface of the molded part, and located three to five times diameter center to center from each other, and to allow for turbulent water flow by limiting the length of any given circuit to 53 inches long. That circuit must 23 psi and at a rate of 120 gal/ minute to be able to remove heat at a rate of 300 BTU / (hour/ft deg F), using 50F water. The key word here is uniformity.
-R. DeBeer, Montell Polyolefins, Lisle, IL, (630) 960-1181.
When trying to mold parts as flat as possible, is it better to have water lines running parallel ("A" side to "B" side) or perpendicular?
In reality, if you get proper water line placement regarding distance between lines and distance from the part, there will be little difference resulting from which way the lines run. Proper water line size/distance relationships will deliver a near uniform heat extraction. The general rule of thumb is 2.5 diameters between lines and distance from the part. If lines are too close to the part, it could cause nonuniformity in cooling, and changing the orientation between core and cavity might minimize any problems. I would suggest that the driving issue should be which way 1 can put the most efficient set of lines in the core or cavity having to work around ejector pins, drops, and other mold features.
Beyond getting good geometry of cooling lines, lots of cooling problems come from improper plumbing of cooling circuits. Just plugging all lines into a big manifold doesn't guarantee good cooling. If those individual circuits have widely varying pressure drops, most of the coolant flow will go to the low-pressure drop lines and the high-pressure drop lines will be starved. These lines can often be ones with baffles and bubblers because of the longer total flow length and possible restrictions from improperly sized baffles or bubblers. Proper cooling design to squeeze the highest efficiency out of a molding cycle is a lot more involved than most people wish.
-B. Sherman, Bluegrass Plastics Engineering Inc., Portage, MI, (616) 353-8412.
Crossing channels between core and cavity spreads cooling errors over the whole mold and reduces overall tendencies toward cooling-based molding faults. However, a good cooling system is designed to allow for accurate dimensional control of a product, and this can require channels that are split into several channels per half. Often, parallel channels are better in these cases. Such design allows water temperature increases to he controlled and, in special cases, varied temperature coolant to be used to specifically control the wall temperature of the model and, hence, the shape of the product.
Also, as for channel placement, the difference between the longest distance from channel center to part and the shortest distance should be 4 to 8 percent. This will provide ideal cooling as long as the distance between channels is not larger than about 70 mm. Channels further apart than 70 mm have a lot of difficulty maintaining anything like a reasonable temperature distribution in the tool. Tool life (strength, for example) and cooling efficiency are a tradeoff (as are ejection and good cooling). Cooling for short-term efficiency can be used in a tool that requires extremely long production runs, as the tool will eventually soak to a reasonably uniform temperature as long critical cooling areas are properly addressed.
However, no cooling system will work if the drillings are too large in diameter, because turbulent flow of the coolant is an absolute necessity to good cooling. Channels of 12 to 18 mm in diameter are usually ideal. Water flow rates must be high enough to provide turbulent flow (10 to 15 liters/minute recommended). If less than about 6 liters/minute flow rate is available, don't even bother trying to design a good cooling system, because you will never reliably achieve turbulent water flow or good heat transfer.
The temperature of the water can be anything from 41 to 149F. Completely different connections and plugs are required for anything above 158F and when oil is recommended. The actual temperature depends on the molding application. Also, I have a preference for insulating the clamp faces of the tool with at least 5 mm of insulating material to ensure that the only cooling that goes on is that which is designed. Cooling through the clamp platens can often skew molding conditions early in a long production run as the machine takes up to a day or two to equalize in
temperature.
-D.S. Henderson, Proen Design Australia Pty. Ltd., Melbourne, Victoria, Australia, +61 3 95002955.
Here is a wrinkle that has helped us in cases where there was not sufficient room in a mold to maintain that rule of thumb of 2.5 diameters between lines or to the cavity: Replace the mold alloy with one that offers greater fracture toughness and permits thinner wall sections. For instance, P21 has almost twice the fracture toughness of P20, and MAR 18 (250) has almost three times as much. That means you can safely lower section thickness between lines and cavities. In some cases, it also means that you can extract heat more quickly and, therefore, shorten the cooling cycle.
Generally, the tougher alloys are more expensive than P20, but they can sometimes help solve a space problem inside the mold or shorten the molding cycle. If you try to use this wrinkle, make sure to consider any difference in thermal conductivity between the tougher alloy and the alloy it
replaces.
-T. Gerson, F.T. Gerson Ltd., Toronto, ON, Canada, (416) 364-2457.
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