I got rid of my 3d printer a few months ago, donated it to some mad chopper bike makers who could make better use of it. Why? Because I no longer think it should be a part of my technological process at all. Because once I started making plastic parts by cutting, folding, and welding HDPE plastic, I decided this process is superior to 3d printing(I'm talking about the extruded printers like MakerBot here, not the high end industrial ones) in every possible way.
This Instructable is to build the simplest and most useful element of this system, the triangular tube. With this technology mastered, you can create water and air tight plumbing over a large range in scales extremely quickly, with arbitrary shape, with the ability to edit afterward as needed, all for free out of trash.
Tools and parts needed for this are:
HDPE liquid bottles like what milk comes in, along with some juice and bottled water brands
flat bladed tweezers(as opposed to pointy)
candle and lighter or matches
something sharp to score with like a fork
a straight edge such as a ruler
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Step 1: Harvest the HDPE
Find a bottle like the one shown in the picture which is storing milk. You want the milky colored plastic, recycle symbol "2". Cut out the largest flat sections you can based on the geometry of your bottle, this can be done with scissors, a knife or a box cutter or razor blade. Wash it! A word of warning here is if you're harvesting from milk or juice bottles(but especially milk), do it right when you finish the bottle, don't put it on a pile to deal with later, it will stink. Just get in the habit of every time you finish a bottle of milk you slice out the flat bits, throw the rest away to the recycle plant, and WASH your plastic feedstock, dry it, and set it aside somewhere.
Step 2: Trace Out Pattern
The pattern is as shown here, you just choose the dimensions you need for your application. I'd say with some work you could do anything from quarter inch to an inch and a half for the side length of the cross sectional triangle.
One dimension of your rectangle here is 3x the side length plus some for the weld tab and the other is the length of the finished tube. Dotted lines indicate folds, with the middle ones going one way and the weld tabs going the other way as noted on the image.
Step 3: Cut Out and Score
Use a fork or some other sharp thing and a straight edge such as a ruler or some PP plastic food enclosure trash to score fold lines, and cut the whole thing out with scissors.
Step 4: Bend and Fold, Bend and Fold
this involves a bit of fiddling, as you learn to either score deeper or bend harder with pliers or something. One of the great things about fabricating things this way with HDPE is a lot of times you don't even have to weld, you can just cut, fold, and bend using just scissors and a knife and pliers to make a ton of useful stuff. Even at room temperature this stuff is pretty workable.
Step 5: Weld
The nature of this weld joint is similar to that used in food technology such as the calzone or jiaozi. Bread dough is not so different from the state the HDPE will be in if you do this right, and the finished joint will look very much like the joint in these various bread-weld joints do after cooking.
Light the candle. Hold the joint above and just to the side of the flame, pushing the joint together with your fingers or the pliers, and wave it around to get even heating. If the plastic smokes or turns black or catches fire, you're too hot. What you are looking to see is the glass transition, which looks just like it sounds like: the HDPE actually looks like glass at this point, and works similarly to molten glass. It becomes clear! Be careful to not get any more of the plastic to go through the glass transition than you absolutely have to, since that will mess up your shape. This always happens, in my experience, you just have to keep it all under control a bit. When both sides are glassy, use the tweezers to gently smash it all together, and repeat all up and down the joint, let it cool, examine, fix holes, and repeat until there is a continuous weld joint down the whole length.
Step 6: Test and Deploy, Spread, Expand
If done correctly, this tube should be watertight! You can test it by using it as a straw. If it works as a straw it means there are no holes. If there are holes, just go fix them!
Can you use a desktop consumer 3d printer to make a watertight tube? Maybe, but it will often fail, and will require fiddling to get it to really hold water or air. In my experience it's a really bad idea to use anything from a commercial desktop printer in a place you actually want to not leak. If it doesn't leak this time, it will next time, and then you won't expect it. Another key point here is that in addition to being free added cost to use my own HDPE milk bottles, those bottles are regulated by FDA, which the spools of plastic you get from a 3d printer company are not. So when you make a fluidic system out of your own milk water or juice bottle, you have the whole regulatory power of the FDA backing the safety of your system also.
And how hard was this to design? You can "design" the part as you build it, just making it as needed by eye. This changes your part cycle time by as much as the desktop 3d printer does over using an outsourced machine shop to fab parts: it's huge. Instead of being forced to use computer with CAD model software just to make even the simplest rectangular element, the basic things I might want can be made with a pocket knife and a lighter. 3d printing is fabrication for rich people. One of the most expensive ways to buy plastic if you go to a large chain home improvement store is to buy a spool of 3d printer filament. Your cost per liter of plastic would be orders of magnitude lower to buy a whole plastic deck chair or picnic table. But what if you design everything in 3d CAD already? Surely 3d printers are good for prototyping then? Well if you're using really expensive CAD software, can still use it to design this way, and it can be a lot easier to do because it's so simple. Just make parts in your CAD software for triangular tubes, then scale, move, rotate, and copy and paste as needed and you can build a huge range of parts, all of which can carry fluids and gasses in a sealed passage if needed.
You can also prototype using paper. Fold and maybe tape the paper, see how it looks, and if you like it use the paper as your template for the plastic, tracing it out with a marker.