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November 2, 1999

7 Min Read
Waterlines 101: The basics

Editor’s note: Consultant Bill Tobin of WJT Assoc. spends his time helping molders diagnose molding problems, and offers his comments on some basics of cooling your molds.

It isn’t rocket science to understand that a mold is really a heat exchanger. The heat you put into the plastic to melt it must be removed enough not to shrink or warp a part. Most people put in 7/16-inch-diameter waterlines. Why? Two rules of thumb:

  • The drill size for a 1/4-inch pipe tap is 7/16 inch, which allows you to put in the quick disconnect nipple without having to redrill the hole in order to tap the threads. Waterlines control heat within three diameters of themselves. The 7/16-inch size is a happy medium between many small lines and the fact that larger lines don’t particularly increase the heat exchange benefit. With a minimum of 1.5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation.

Yet, simply having the right diameter waterline is only one of the three elements of good cooling. Assume you have a thin tube-shaped part, about 3 inches long. One end has an ID of 1/4 inch and the lower end is about 3/4 inch in diameter. A good tool designer, moldmaker, or molder would immediately say these cores should probably be made of P-20 or beryllium copper. (While on its face many people would reject beryllium copper because of its insufficient hardness, recent developments have allowed this metal to be almost as hard as P-20.)

Cooling the Core
With the core cut to configuration, we now need to cool it (our second element). Conventional cooling choices are either a baffle or a bubbler. Here is where most people tend to fall into a trap. If the water is directed up the tube with the bubbler or on one side with a baffle we still have to maintain the cooling characteristics with turbulent flow. We must be sure our bubbler or baffle is not so constrained that the entire system is strangled and turbulent flow stops.

The next concern with this insert is proximity. If we can’t get the water close enough to the heat, it won’t make any difference if it’s turbulent or not. This is especially a problem with thin protruding cores. If you can’t get the waterline near the source of heat, bring the heat to the waterline. This is done with the use of gas pins—hollow pins that contain a low-boiling liquid (sometimes Freon). One tip of the pin is heated, and it immediately transmits the heat to the other end of the pin, which should be located in the middle of the waterline.

Managing Heat
Two other important factors are where to put waterlines, and how to hook them up. Nearly everyone cools the cores and cavities, but the most common mistake is not getting sufficient cooling to the runners, sucker pins, stripper plates, and (most importantly) the sprue—whether it is a hot runner or conventional design. Anywhere there is heat it should be managed. If you have a stripper plate on the ejector side with sucker pins holding the runner and it is not cooled, it’s just a matter of time before (1) it heats up and starts galling the pins because of thermal expansion, or (2) the sucker pins are so hot and the plate is so warm that the runner will not stick to the sucker pins and will come off the plate like a wet noodle.

Internal looping should be serial. The water circuit should be able to enter the mold and find its way out again without splitting into separate paths. When water circuits are split internally in the mold, turbulent flow can be measured on the in and out, but how do you know this is what’s happening in the mold? Water will always take the path of least resistance. Therefore, if there is a parallel circuit but one leg is constricted and the other isn’t, the water will have a tendency not to flow into the restricted portion.

Parallel circuits, which are a form of an internal manifold, should be avoided at all costs. The machine manifolds work because they are usually fed by 2-inch or larger lines. So long as the plant water can maintain the pressure, the parallel machine manifold will deliver pressure to each line equally. Bringing a waterline to the mold and then making a small manifold or splitter to allow that one line to be split into multiple lines will only work if the pressure and flow out of your mini-manifold is equal to the pressure of the line in.

While this is possible, it is also difficult. If you must have a manifold on the mold, put it on externally with large pipes as the inlet, and split it into smaller lines to the mold so that the circuits can be balanced before the handles of the valves are taken off and welded into place.

Waterlines and water circulation in a mold are easy. However, simple oversights like the ones mentioned here can turn the best mold into a loser.

Put that intern to workIf you’re one who believes in summer interns, here are a few projects that might help you run your business better. Interns are good at doing those pesky little jobs you’ve wanted done but never had the time to do. The intern also learns something important, such as the fact that two plus two is never four. In fact, two plus two almost always equals approximately 3.957 because of downtime and scrap. Here are some projects.

Look at your machines and molds. It is interesting to note that most machines come with undersized manifolds, which usually require looping. There is even a philosophy in some shops where, if the manifolds have only four inlets and outlets per side, the tool will be specified as built with no more than that number. This constriction brings some interesting challenges to the moldbuilder. And for the molder, mold setup often ends up looking like a can of worms with external splitting manifolds, jumpers, and waterlines all over the mold.

Have your intern do some interviews with the setup crew. Look specifically for how much time is wasted and how often the mold setup is delayed looking for the right length waterline. Either the techs cannot find the right length or the current hose is worn or leaking and therefore useless. Find the average time per setup. Find the cost in lost production per hour. Multiply these two numbers together. This is the average loss of money per job. Have the intern find out how many times molds have been hung in the past year and multiply that figure by the average loss. Many will find this number is a semifantastic amount of lost money and machine time.

Now have your intern do a simple study. Usually waterlines can be classified into four categories: long, medium, short, and jumpers. Have the intern measure as many lines as possible and these categories and lengths will become glaringly apparent. Here’s the tricky part. Color-code your hoses. For example, red is long, blue is medium, black is short. Any worn or leaking hose is cut down to a good section and is recycled into a jumper.

Setup sheets can now become easier by identifying the number of long, medium, short, and jumpers for each mold. These are now color-coded and easier to locate. If a particular length is not available, it can be taken off another press that isn’t using it. Also, keep in mind that waterline hose is embarrassingly cheap compared to the downtime.

A second project is another exercise in dispelling silliness. Spend a few hundred dollars and buy a set of manifolds with more outlets. With a mold in the press that is looped, have the intern measure the flow of the water in gal/min, and note the scrap and cycle time for a typical run. Install the new manifolds without looping. Instead, hook up as many direct circuits as possible to the manifold. Take the same measurements the next time this same mold is run. The results will be dramatic. The scrap will go down, and the cycle time will shorten. Technicians will learn that any time there is a loop where there could be a direct circuit, a penalty will be paid in terms of productivity.

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