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The Troubleshooter: It's all about water


Quick disconnects limit the effective size of the water circuit to 1/8 inch overall. Use a 3/8-inch oversized quick disconnect to help prevent cooling loss. Plugs can be installed in existing threads or mounted flush.
This article continues the Troubleshooter’s Rules of Thumb series. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

Waterlines are so important and most of us pretty much take them for granted. When I am troubleshooting a problem and straighten out the runners, gates, and venting issues and find we still have problems with warpage or slow cycle times, then I take a look at the water system. I look for restriction to water flow or lack of pumping pressure.

Making a Connection

I have a couple of rules that apply to waterlines. I don’t like to use jumpers and I don’t like quick disconnects. Let’s take the jumper issue first. I do not want more than a 5 deg F increase in water temperature between the inlet and outlet ends of the water circuit. Any more than 5 deg F tells me that with one waterline we are trying to cool more steel than we should.

Also, jumpers can collapse and close off the water flow on the back side of the mold for a number of reasons: they get too old, or are bent over by the safety door on the back side of the machine, or aren’t wire reinforced to prevent them from collapsing from the hot water running through them. Another issue with the rubber hoses and hot water is that the rubber on the inside of the hoses can become degraded and let little pieces of rubber flake off and plug up bubblers or the small-diameter sections of the waterlines.

Second, I do not like quick disconnects because the inside diameter of some quick disconnects can be as small as 1/8 inch, which causes the effective size of the water circuit to become 1/8 inch overall. This small detail reduces the effectiveness of a water circuit dramatically and most of us are not paying close enough attention to catch it every time.

When we start out with 7/16-inch water channels in the molds and ½-inch hoses connecting these waterlines to a water pumping source, reducing all this down to an 1/8-inch effective diameter because of the quick disconnects doesn’t make sense to me. You could go to oversize quick disconnects, which will allow you to have a 3/8-inch inside diameter and you won’t have nearly as much cooling loss as when you use the smaller ones.

Pressure Pointers

Pumping pressure is another issue with proper mold cooling procedures. The old water control units used to have a ½-hp pump motor pumping water through the waterlines. If we bought a “muscle unit” that had a 1½-hp pump, it would do a much better job of cooling the molds than the old ½-hp units.

Today we can buy water control units with bigger pump motors. I saw some units with a 5-hp pump motor the other day that were doing a great job of cooling the mold on a polypropylene part, and it was providing a decent cycle time for the molder.

All we are trying to do by increasing the pumping pressure is to make the water flow through the waterlines turbulent instead of laminar. Turbulent water flow removes heat at a rate four times faster than laminar flow. It is especially important to achieve turbulent flow in molds that are running olefin materials, because we usually run 50 to 75F mold temperatures for olefins. Most of the other materials that we run need hotter molds so we don’t notice the lack of turbulent flow quite as quickly. We usually notice the lack of turbulent flow when we see the slower-than-quoted cycle times.

One issue that occasionally pops up is we sometimes feel we aren’t getting good cooling because we don’t have anything in the water channel to stir up the water and help it become a turbulent flow system. Some molders are experimenting with putting spiral flow inserts into the water channel to correct this problem. Even using a pulsing method of running water through the channels will do a pretty good job of removing the heat from the mold steel. Pulsing means the water is cycled in and out of the water channel in a push-pull motion instead of just being pumped straight through the mold.

I won’t be surprised if one of these days we see toolmakers drill the mold water channels oversized so they can insert tubes into the waterlines. These tubes will be thin-walled, with projections to the center of the water channel. These projections will disrupt the flow of water and allow it to become turbulent, and therefore better at removing heat from the mold steel than just a straight flow of water.

Tim, my Boston toolmaker friend, puts water in the bottom support plate, the A and B plates, and the top clamp plate. He says it helps keep the mold from transferring heat to the stationary platen and that this method works especially well with hot runners. His motto is, “Make the mold work for you, don’t you work for it.”

How Cold is Too Cold?

Another water problem I run into is when a chiller is used to cool the mold. I like the extra pumping pressure we get from using the chiller, but I don’t drop the temperature down anywhere close to the freezing temperature of water. Sure we can put ethyl glycol in the water at a 50/50 mix and keep the water from freezing, but the mold can still end up rusted. The reason is that condensation occurs when a mold is shut down while ice-cold water is running through the waterlines during high-humidity conditions; and condensation causes rust. I prefer to use the extra pumping pressure of the chiller and run at about 40F to extract the heat out of the mold steel instead of just pumping cold water through the mold. This eliminates the possibility of rusty molds.

Another molder I work with has installed GPM (gallons per minute) gauges in his waterlines to be sure he doesn’t have any restrictions to water flow that will affect his cooling rates. That’s about it for me on water. By paying attention to just five or six areas in the water system we can deal with both warpage and slow cycles. You can see or measure the warpage problems and you can check the cycle time by multiplying the thick section of your part by 250 to see if your cycle time comes close to this target cycle calculation.

So the next time you see a jumble of waterlines on the backside of your molding machine or frost on the waterline quick disconnects, think of these rules of thumb.

TAGS: Materials
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