In last month’s column I referenced some new technologies for mold cooling and the positive impact they will have on the process. But there is still plenty going on with current cooling methods and how they are being underutilized in many cases.
For example, over the course of my career I have been part of many discussions about small cooling lines and bubblers and their tendency to plug up. This is something that you should not overlook in your water-treatment procedures. If your small water lines are plugging up, you obviously are having some sort of buildup in your other water lines too. This may not reduce your flow volume in gal/min (GPM), but will certainly have an impact on cooling and your ability to pull heat out of the plastic. So if buildup or plugging issues are a reason to not maximize your cooling and cycle times, you should focus on the root cause of the issue with the water treatment and not cut short your potential to reduce cycle times by exploring various cooling options. You always have to look at the big picture to understand the overall payback.
In last month’s column I also mentioned how important it is to consider the plastic material being used and to focus on cavity details that create steel conditions that will maintain heat, which contributes to quality issues and excessive cycle times. I stress this because the mold design cannot treat all materials the same with regard to cooling, steel/alloy selection, ejection, and lifter travel.
How small can you go with water lines? There is some new technology that uses liquid CO2 in lines as small as 0.060 in. to get into tight areas. I have not used this technology, but I can see where it could have a big impact in some situations. But in most cases, with a bit of creativity, we have been able to add water where others have said it was not an option. Let me give you some details on one such project.
On a small four-cavity tool, the part had a U-shaped notch in the center. The steel that cored out and created the notch was very thin—roughly 0.100 to 0.150 in. thick, 0.750 in. tall, and 0.750 in. wide. This steel section retained heat, not allowing the plastic to cool and requiring an excessively long cycle time of 40 sec. Any shorter cooling time would cause the hot plastic in the notch area to shrink smaller than the spec for the notch gap dimension. This steel was in the solid and adding water was not considered an option because the steel was so thin. A copper alloy material could have helped here, but to insert this area at such a late stage would have been very expensive. So we considered an option that most would have thrown out the window.
We EDM hole-popped some 0.060-in. water lines with a circuit that looped through all four cavities. This was one of those cases where you really don’t know what to expect until you try it. When we set the tool, water was just trickling out of the new line because it was so small. The results were amazing: This small amount of water flow through the thin steel had such a big cooling effect that even at a 17-sec cycle time (a 23-sec reduction) the notch gap on the parts were now too large! Our option was to run the small cooling line at a much higher water temperature to keep the part from cooling too quickly.
Though we were greatly surprised by the effect, when considering ratio of this “micro” water-line size to the steel thickness surrounding it, the water line was actually quite large. A 0.060-in. water line in a steel section 0.100-0.150 in. thick compares to using a 0.600-in. cooling line in a steel section 1-1.5 in. thick.
Out of curiosity, we then ran compressed air instead of water through the micro cooling line. This also had a big impact on cooling and the specified notch gap was still too large, though the dimensions among the four cavities varied, with the fourth cavity being slightly smaller because the air was heating up as it ran through the hot steel. The point is: Do not overlook options that may not have been the norm for years. Think outside of the box and re-invent.
Heat pipes or cooling tubes are also an option in some cases, but flowing water is always the best choice. I have proven this in trials—heat pipes definitely assist in cooling mold details, but never as much as actual water flow. With that said, I still use heat pipes in some situations because they are easily added in cases where adding water or a bubbler would require more work.
Bubblers are one of my favorite options for cooling in tight areas, so let’s look at them more closely. Bubblers are often overlooked as an easy solution to cool problem areas where normal water circuits are not an option. A bubbler only needs a drilled and tapped hole with clearance for the bubbler head and connections. There are also many options to implement bubblers internally without using the standard bubbler head. The water-line hole for a bubbler can be as small as 0.125 in.; the limitation is that the smallest bubbler tube available is 0.090 in. diam.
One aspect of bubblers that is commonly overlooked is the cross-sectional area of the ID of the tube and the area between the OD and the water-line diameter. This is one reason I always recommend using high-flow stainless-steel bubbler tubes versus the common brass bubbler tubes. For example, with a 0.125-in. diameter tube, the high-flow tube has an ID of 0.109 in.; a brass tube at best has an ID of 0.069 in. The high-flow tube thus has 250% more cross-sectional area, which translates to an increase of 250% in GPM. That’s why I never use standard brass tubes: They decrease your GPM potential.
You must also consider the cross-sectional area between the tube and the water-line diameter so that you are using the proper tube size. Some will assume that with a 0.250-in. hole you would use a 0.125-in. bubbler tube, thinking it’s half of the diameter. But you must consider the area. The area outside of the tube is 0.0368 in.2 and the ID area of the high-flow 0.125-in. tube is 0.009-in.2— that’s 400% less area or GPM. With a 0.125-in. brass tube, the area is only 0.0036 in.2, which would mean 1000% less GPM. So with a 0.250-in. diam. hole, use a 0.187-in. high-flow tube.
A 0.125-in. diam. hole with a 0.090 in. high-flow bubbler tube has greater area outside of the tube, at 0.0054 in.2, with the area of the ID of the tube being 0.0045 in.2. A 0.187-in. diam. hole with a 0.125-in. high-flow tube has area outside of the tube of 0.0153 in.2 and an ID area of 0.009 in.2.
I have rarely seen bubbler hole sizes and tube diameters properly sized to achieve maximum GPM. It is very simple using basic math to figure the areas of the ID and the OD of the tube and the hole. Just take the radius2 x 3.14 to figure the areas of the tube ID, OD, and the hole. For the area outside of the tube you subtract the tube OD area from the area of the hole.
On a tool running polypropylene with Ampco beryllium-alloy slides that cored out a large area, the cycle times were running higher than desired because of how hot the slides were. The slides did have water in them but not inside the cavity detail because it was assumed that water was not an option. We were able to drill three 0.125-in. diam. holes into the detail using three high-flow, 0.090-in. bubbler tubes. In this case, bubbler heads were not an option, so we installed an insert inside the slide to allow bubblers to be used. The slides went from being so hot that they would burn your fingers to staying cold. Cycle time was reduced by approximately 20 sec.
On another tool we were running glass-filled nylon on a round part with a 0.312-in. diam. hole through it that was approximately 5 in. long. A 0.156-in. hole with a 0.125-in. high-flow bubbler tube was used to cool the core. Even when running glass-filled nylon, the core stayed cool to the touch.
So the results you can achieve with basic cooling methods and a little outside-the-box thinking can be amazing. If you look at the big picture and the payback, the excuse of plugged-up water lines from untreated water can really cost you a lot of money.
ABOUT THE AUTHOR
Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Contact:firstname.lastname@example.org.