This tank is large enough to support the ammenities, such as a heater and an aerator. It is large enough for almost any home printed circuit board manufacturing job. The materials list includes only commonly available items. It holds a maximum of 2.45 US gallons (9.23 liters) of etchant. Acrylic and HDPE can both handle any of the etchants, in the strengths at which they would be used.
Parts List
2 ea. Sheet acrylic, 14" x 11" x 0.92", although other thicknesses will work fine.
1 ea. Sheet HDPE 4" x 14" x 3/8".
2 ea. Sheet HDPE 4" x 12.5" x 3/8".
One tube of silicone sealer. Clear or white may be preferred for asthetics. I used fish tank black for the photos.
A piece of 3/8" schedule 80 PVC pipe 14" long to hang the printed circuit board on.
26 ea. #6 x 1/2 in. Stainless-Steel Pan-Head Phillips Sheet Metal Screws.
4 ea.#8 x 1 in. Stainless-Steel Flat-Head Phillips Sheet Metal Screws.
2 part epoxy to coat the screw heads for protection.
Preparation
The cutting board needs to be sawn to make the pieces. There is an inch of waste all of the way around, due to a groove near the edge, leaving 18" x 13" of clear material in the center. Look for the mold mark on one edge. Rip that edge clean, then rip three 4" wide strips lengthwise from the cutting board. Keep in mind this stuff cuts like air. It goes right through a table saw. Also keep in mind that the cutting boards are injection molded, and have stress which will be relieved when you make a cut. Most of the stress is on the waste you rip first (where the mold's material entry point is). You can see the warp in the photo below. The top strip was cut first. The other strips do not show the stress relief that the first waste cut shows.
There are better cutting boards, 20" x 15" x 1/2" for around $16.00. You will have to adjust the dimensions a little, but it might be worth it to get the 1/2" thickness. You can also get the stuff cut to size from The Cutting Board Factory. You can email their sales people and ask for a quote. They quoted me $4.00 each for 12" x 4" x 1/2". There is a minimum order of $15.00.
Cut one piece 14" long. Cut two pieces 12.5" long. In each of these two, drill a 5/8" hole centered on the width, 12" from the bottom end. I used a Forstner bit. Cut the piece down to 12", cutting the hole in half. You should now have two pieces each having a semi-circular indentation on one end. The indentation is to hold and center the rod that the PCB will hang from.
Drill two holes in each end of the 14" bottom for screws that will go up into the ends. The holes should be 3/16" in from each end, 1" in from the edge, and need to be countersunk or counter bored. You must drill pilot holes in the ends that are the appropriate size for the screws used. HDPE does not compress like wood, so you can't drill under sized holes. It helps if you can clamp the end in a vice and use the bottom as a guide.
Run a 1/8" bead of silicone sealer the length of the bottom of the end piece (see drawing below). When you drive the screws in the HDPE, there is going to be silicone everywhere. Assemble the HDPE parts on a piece of waxed paper or paper towel. Drive the screws in carefully and stop when they seat well. Wipe the excess off of the outside and the edges, leaving the bead on the inside as it formed. The only thing holding this together is the pair of screws. The silicone adds nothing structurally. It does not bind to the HDPE at all. In fact, they line the silicone tubes with HDPE to prevent it from sticking to the tube.
When the corner is tight it should look like this...
And the completed HDPE assembly...
The acrylic should already be the size you need, but if you need to trim it down, score it with a knife and straightedge, break it, and sand the edges lightly to break the sharp edge.
Drill the holes in the acrylic. Use a drill intended for acrylics. I had to order mine from a plastics supplier, because I couldn't find them locally. The holes must be oversized because acrylic has a high coefficient of thermal expansion. It will move around as the temperature changes. To help it deal with the movement, we use screws in oversized holes to hold the acrylic against a silicone sealer gasket to allow for expansion. Although the sealer will bind a bit to the acrylic, it will not bind at all to the HDPE. If we tighten the screws all of the way, the silicone will be squeezed out almost completely, and there will be little or no give for expansion. If we stop just as the silicone starts to ooze out, the silicone will stay in the joint and act as a gasket.
A drawing that illustrates how the joint between the HDPE and the acrylic is made.
It should not look like this. This is over tightened by about 1/4 turn. It still seals, but there is a chance the silicone will not handle expansion well.
It should have a bead like this on every edge on the inside. If it doesn't, you'll need to add a fillet manually. The reason the tank is 4" wide is so you can get your hand in to fix this very thing, should it need fixing.
Assembly Summary
Follow these steps to assemble the HDPE tank parts you prepared above:
Apply a 1/8" bead of silicone sealer to the bottom edge of one end piece. (larger if your HDPE is 1/2" thick)
Join the end to the bottom using two 1" long screws.
Repeat for the other end piece.
You should now have a U-shaped HDPE assembly.
Follow these steps to add the acrylic sides to the HDPE parts:
Lay a pre-drilled acrylic side panel on the HDPE assembly and mark the holes.
Put a piece of tape on the HDPE end pieces to mark the top of the acrylic.
Remove the acrylic panel and drill the holes in the HDPE using the appropriate drill bit.
Drive the #6 x 1/2" screws into the holes in the HDPE, stopping about 1/8" from the end.
Remove the screws.
Repeat the above steps for the other side.
Finish with these steps:
Apply a uniform 1/8" bead of silicone sealer to the HDPE as shown above. Stop at the tape.
Leave it to cure for 15 minutes.
Strip the protective cover off of both sides of the acrylic panel.
Carefully place the acrylic side panel back on the HDPE assembly, lining up the screw holes.
Drive the #6 x 1/2" screws through the acrylic into the HDPE, but stop when the silicone oozes out - do not sqeeze it all out..
Scrape excess silicone sealer flush with the outside edge, but leave any squeeze-out on the inside.
Repeat the above steps for the other side.
Leave the assembly to cure 24 hours.
Testing
Don't test it on etchant. Test it on water with Koolaid or food coloring in it. Look at all of the seams to verify there are no leaks. If there are leaks, rinse out the tank and tighten all of the screws on the leaky side one quarter turn and test it again. If there are no leaks, rinse it out and coat the screw heads with epoxy to keep them from being eaten by the fumes from the etchant.
Use
The tank should be rinsed out before and after each use, and stored in a bag or box to keep dust and other contaiminates from settling inside. Set the tank on a level surface, fill to 1" from the top (of the sides) with etchant. Using fishing line, make loops long enough to reach from the 1/4" plastic pipe, to the top of the printed circuit board when it is in place in the etchant, and back to the plastic pipe. Drill two holes in the circuit board, one in each upper corner, and run the fishing line through them. The plastic pipe is then placed across the tank in the two notches.
