A Home Photoetching Operation
Chapter 3

Tank 4a

Years ago I received a 500-watt titanium cartridge heater in exchange for some work I did. I thought it might be a nice idea to make a separate, insulated reservoir for the etchant incorporating the heater, and simplify the tank itself. The tank would only need a sloped drainage bottom.

I also wanted to bring as much of the pressurized piping as possible inside the tank wall, where a failure would be contained. At the hardware store, I discovered a flexible underground sprinkler pipe riser that could be cut to length, and used two of them to make the internal connections. Only a short piece of flexible hose remained outside the tank, and it was trapped between the pump outlet port and a barb fitting in the tank wall, so even if a hose clamp failed the hose would not slip off the barbs.

I cut the sump out of Tank 4 and fabricated and installed a new, shallow-V-shaped bottom plate. Unfortunately, that's as far as Tank 4a went. Upon further thought, the logistics weren't as clear-cut as I had initially thought. Due to its construction, the heater had to be installed horizontally in the wall of the reservoir, which meant the reservoir itself was going to be basically the same size and proportions as the original sump. And there were actually more tubing connections to be made than in the original configuration.

I should have kept the sump, but the upshot is that I had effectively trashed Tank IV.

Tank 5

Tank 5 is basically a redo of Tank 4 incorporating the lessons I had learned from the earlier tank. The main differences are: seamless bottom plate, larger titanium heater, motorized rotator for the workpiece, big strainer on the pump inlet, and an AC junction box with switches for the pump, heater, and rotator. I reused the pump, nozzles, plumbing, and top cover from Tank 4a. I have a bunch of castors given to me by a co-worker, and made a nice sturdy rollable stand using PVC pipe.

A rear view of the tank before I installed the rotator. From top to bottom are the pump with inlet strainer, the cable coming from the heater's temperature sensor, the clamping plate for the heater. I started out with only the four side castors, but added the front and back castors when I discovered the tank could relatively easily tilt fore and aft. If I could have found 3-way elbows for the corners of the frame, I could have gotten away with just four castors in the corners. The castors themselves are mounted in pipe caps, which I drilled out for the threaded shanks of the castors, installed with large washers to spread the load. The caps are just pressed into the pipe tees.

The bottom plate is a single piece of 3/8" PVC sheet, which I heat-bent using a borrowed strip heater on the bottom surface, supplemented by a hot-air gun on the top surface because of the thickness. I laid out the flat pattern to locate the bends required for the drainage slopes and sloped sump bottom in CAD. I pre-cut the wall to shape, made the first bend in the bottom plate and then used the tank itself as the form to ensure each subsequent bend was fitted correctly. I glued the bottom plate in place using Devcon Plastic Welder, a two-part adhesive that includes solvents to strengthen its bond with plastic. Time will tell whether this is better than the more conventional PVC cement.

Inside the tank you can see the Cobra Connector flexible hoses, the nozzles, the workpiece-holding "spider", the titanium heater (both metal tubular shapes), the gray PVC tube holding the heater's temperature sensor, and the inlet to the pump.

The temperature sensor along with its cable, is in a heat-sealed Teflon sleeve (it came that way). I threaded the sleeve through the 1/4" PVC tube, and then heat-bent the tube around it. I glued the tube into the tank wall well above the sump's liquid level.

I mounted the nozzles in tees with threaded reducing bushings. I bored out the opposite end of the tees, and fastened them to the wall using end plugs placed through holes I drilled in the walls, all well-glued with PVC pipe cement.

The Cobra Connectors come in large length increments, but can be cut down to the exact length you need. Pull the blue housing back from the connector enough to expose the inner PVC tubing, pull the end bushing out of the inner tubing, shorten the sleeve and tubing by the same length, and reassemble.

I fitted a new, larger-capacity strainer to the pump. The strainer came with a stainless-steel screen, which I replaced with a polypropylene screen which I cut to size and welded the ends together into a loop by melting with a soldering iron. There is just enough tubing to lead from the pump outlet to the barb fitting (which I machined from PVC rod) in the tank wall, and the tubing is secured with stainless steel hose clamps. The pump itself is mounted on standoffs I machined from more PVC rod and glued onto the tank wall.

I machined a scrap of the 3/8" PVC plate into a "saddle" fitting the tank wall, also machined a cutout for the heater to pass through, and glued it well in place. After the glue was set, I continued the hole through the tank wall using holesaws and hand files. Another scrap provided the clamping plate, and four lengths of all-thread screw into the saddle but not through the tank wall. The gasket is a piece of silicone rubber sheet.

The rotator is a 115VAC, 3 rpm gearmotor, mounted on yet more 3/8" PVC (I had to buy a whole sheet of it, so I'm using it for everything I can!) It drives the spider shaft through a random pair of timing belt pulleys and a short belt. The motor pulley is set-screwed in place, but the driven pulley is not fastened to the spider shaft. I found some kind of rubber bushing or seal or boot in my random-parts box and superglued it to the driven pulley. The shaft is a push fit in the rubber piece, and I can slide the shaft through the rubber to center the workpiece between the nozzles. There is plenty enough friction to reliably drive the spider. Just under the spider feed-through you can see the head of the pipe plug which holds the nozzle tee in place inside the tank. The splash shield for the rotator is a piece of the thin-wall PVC pipe I had bought for the Tank 4a remote sump, which I cut, softened using the heat gun, and hand-formed around a form I made from some 2x4's glued together and bandsawed to shape.

The control panel and heater controller are mounted on another cutoff of the 3/8" PVC. I provided switches for the pump, heater and rotator. The mounting plate has an overhang to prevent any etchant drips from falling on the electrical parts. The sleeved, gray cable at the upper left is for the temperature sensor, which is hard-wired to the controller. The pump and heater plug into a regular wall socket, and I hard-wired the rotator cable (which just passes through a rubber bushing in the junction box wall) to its switch.

Components

• Flowjet inlet strainers
  I used the 01740-012, but the 01720-112 has a polypropylene screen already.
• Cobra Connector
  The flexible tubing I used inside Tank 5.
• Devcon Plastic Welder
  The adhesive I used to fabricate Tank 5.
• CLEPCO heaters (This is an "orphan page" and not linked from CLEPCO's homepage.)
  My heater is a QDTY5 500-watt titanium heater with digital control. CLEPCO may not make this model any more.
• Gearmotor
  The Crouzet 3 rpm AC gearmotor for rotating the spider.