Water cooling system for laser cutter

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Water cooling system for laser cutter

Post by JamesCooper » Sun Apr 14, 2013 3:26 pm

Hi everybody,

This weekend I assembled the water cooling system for thesaxmachine's laser cutter. His laser cutter's laser tube uses a glass water jacket (another tube around the laser tube) with some hose and an aquarium pump, but didn't come with a proper reservoir or a radiator. This would work for a short time, but as the water collected heat from the laser, it too would heat up, and eventually it wouldn't be able to cool the laser properly. This is where a radiator comes in. It's job is to take air from the surrounding room, blowing it through many small fins in the radiator, which transfers the heat from the water to the air. As long as there is fresh, cool air in the room, this should be able to keep the water just a few degrees above room temperature indefinitely.

Unfortunately, there were a few hiccups. His bought-from-eBay laser cutter had a few interesting design decisions (besides just the lack of the essential radiator!). The diameter of the outside edge (O.D.) of the pump outlet appeared to be 1/2" and the laser tube's water jacket inlet and outlet appeared to be 1/4" O.D., but they were using vinyl tubing with an inner diameter (I.D.) of 3/16". This causes two problems. First, you need some way of getting the little tubing onto the big inlet/outlet, second, a small hose constricts the flow of water through it, meaning that, for a given strength of pump, we'll see less water flow than if we had used larger tubing. The manufacturers had made some sort of nylon adapter from the 1/2" pump outlet to 1/4" for the tubing, but it was still a stretch, and straight nylon doesn't provide much friction to keep the tubing from sliding off.

The other problem was that the radiator I purchased (a Swiftech MCR120-QP), didn't come with hose barbs, even though the product description showed that it would! :x This proved to be quite a problem because, due to silly historical reasons, computer water cooling components have standardized on the G1/4 pipe thread, otherwise known as British Standard Parallel Pipe thread. We don't use this at all in North America, so aside from stores that carry water cooling parts (none in Saskatoon), nobody carries adapters or fittings for G1/4 thread. :roll: Thankfully, another one of our members had some leftover hose barbs from his own water cooling system, but now the problem was that they were for 3/8" ID tubing. Finding an adapter between 3/8" and 3/16" tubing was quite a problem (to understand why, consider that the cross-sectional area of a circle is pi*(diameter/2)^2, so 3/8" has 4x the cross-sectional area of 3/16").

At first I couldn't find any combination of hose menders, hose barbs, or other fittings that would go between these two sizes, since a 3/8" ID hose would normally be used with a 3/8" pipe thread, whereas a 3/16" hose would usually be used with a 3/16" or 1/4" pipe thread (again, no sense using small hose with large pipe). I then decided to try making my own.
Hose adapter, cut-away
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Hose adapter, solid
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My hose adapter was made to go between 3/16" ID and 3/8" ID. The dimensions of the inner diameter of the adapter were taken from specs I found on another company's hose menders. Note how small the inner diameter is on this adapter: it's only 1/16"! Also, rather than use barbed ends, I added a lip. This is a common technique used on computer water cooling components. Although the hose doesn't tend to stay on by itself, if you use a hose clamp about 1/4" down from this lip, it presses the hose against the lip, creating a tight seal (note that the seal is between the hose and the lip, NOT the clamp itself). Part of the reason I did this was because I was worried that our 3D printer wouldn't have enough resolution to create a sharp edge for the barbs and also because it's difficult to build a steep, overhanging edge with a 3D printer without using some sort of temporary support that must be removed.
3D printing the hose adapter on the Solidoodle 3
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Here's the 3D printing in progress. As you can see, it's making a solid, consistent shape, with only a minimal amount of ripple. Unfortunately, I forgot to take a picture of the finished product. The 3/16" end wasn't quite as nice. It wasn't entirely straight and the layers each seemed to ooze out a bit, so there was significant ripple in the wall of the cylinder. So much so that you couldn't tell there was a lip on the end! Still, it was complete, relatively strong, and I could (with some effort) get air through the end. However, I eventually decided that the flow rate would be too low using this, so I went looking to build another one. And I snapped off the 3/16" end while trying to remove the 3/8" end from the hose. :cry:

Since I had realized the existing 3/16" tubing would fit on a 1/4" OD barb without much trouble, I looked at options for this. 1/4" is a much more common size and there are lots of fittings available. I bought a 1/4" hose mender and a 3/8" hose mender at Princess Auto, cut both in half, and proceeded to glue one 1/4" half to one 3/8" half with Gorilla Glue. Again, things didn't work out. Although it was easy enough to glue the ends together, after wetting, gluing, clamping, and waiting an hour as per the instructions, the ends fell apart almost immediately as I went to put the tubing on. The glue itself had turned into a soft, pliable foam, and had actually closed off most of the inside of the adapter. It wasn't strong at all! :x

Finally, I went look for some ABS glue (more suited to the plastic I was using) when I found the proper brass adapters for the hoses; the very things I had been looking for all along. :roll: I combined a 1/4" hose barb to 3/8" female pipe thread adapter with a 3/8" hose barb to 3/8" male pipe thread adapter. The result can be seen in the fully-assembled photo later on.

With a radiator and tubing finally in place, the next step was to close the system. The pump is submersible, designed to be used in outdoor water fountains, so it would need to sit in a tub of water. This tub would also act as a reservoir. The tubing and containers are actually slightly porous, so water will slowly evaporate from the system, though this process takes months. The system itself will also have air trapped in it that will need to be bled out and displaced. By having a reservoir with water in it, we won't have to worry about the water level falling too low and burning out the pump or the laser as the air is bled and the water replaces it. I decided to go with one of the clear plastic bins we were using to hold parts. I drilled two holes in the top, one for the 3/8" tube coming from the radiator to drain into the reservoir (avoiding the need to put an adapter on that back to 3/16") and another from the 3/16" line that would come out of the pump and into the laser. I also notched the lid in one corner using the band saw so that the power cord from the pump could come out without preventing the lid from latching. Unfortunately, the lid doesn't stay on very well, and it's not quite an air-tight seal, but it will collect considerably less dust and grime than an open container.

To keep the radiator and fan from falling over and becoming blocked, I attached them to the side of the reservoir using some stick-on cable anchors and cable ties. It's definitely a hack, but they are very flexible when it comes to fashioning methods to hold things together, as long as strength isn't a big issue. I also have bags of each, so I don't mind experimenting with them.

The fan itself is a 120mm computer fan that I pulled from an old machine. I cut off the standard PC fan molex connector and soldered on a 2.1mm DC barrel socket instead. I then found a 9V 1A power supply in a bin of spare parts that had a matching barrel plug. Although the fan is designed for 12V, it will run fine (though a bit slower) at 9V. It actually doesn't take much air flow through the radiator to keep it cool, since the radiator's design makes the heat transfer process very efficient. It runs slow, but silent, and there's a noticeable air flow coming through it.
Completed water system, reservoir and pump in front
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Completed water system, side view with reservoir and pump on left, radiator and fan on right
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Closeup of the radiator and fan with the attached fan grille
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After assembling all the parts, I added water to the reservoir and turned it on. It sounded awful at first, partly because the pump seems to have some issues (a notable tick and grind noise) and because it was mostly dry to start. But as the pump started to prime, water started to flow and I could quickly see the air bubbles in the line travel into the reservoir and bubble away to the surface, leaving very little air bubbles trapped in the system. I then shook and tapped the lines and radiator, trying to dislodge trapped air pockets. After about 10 minutes, I had removed all the trapped air I could and the system was running smoothly. Any air still trapped will likely work its way out during operation, but shouldn't affect the overall effectiveness of the system.

The last step now is to add some sort of biocide to the water to prevent it from growing slimy algae. I filled it with distilled water to minimize the amount of microorganisms and minerals in the water, but things always have a way of getting into the system. Adding a silver coil would be the simplest way to manage the problem. Some people add various additives to their water, such as ethylene glycol, but many of these are counterproductive. For example, ethylene glycol (better known as antifreeze) is used in car cooling systems because it lowers the freezing point and raises the boiling point. However, we don't expect our water to get anywhere near freezing or boiling, so this isn't needed. It's also reduces the water's ability to transfer and hold heat. Finally, it's toxic, so now you have to worry about that. So for now it's just distilled water, but we should add silver or some other biocide in the near future to prevent algae growth damaging the equipment.

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