I've taken to preparing sheet PVC in bulk. It's much more convenient than thermoforming each little piece of pipe as-needed, as flat sheets are usually the shape that I want to start from when making each PVC part.
Here's how.
The first order of business is finding the pipe, and fortunately that's easy. Larger diameter is better, as larger pipes will allow you to make longer sheets. Pressure rating is not important, except for the fact that pressure-rated pipes tend to be thicker. I've been using pieces taken from a drain pipe (which has a pressure rating of basically nothing) for years, and it's plenty thick enough - when I do want thicker sheets, I can just devcon multiple layers together. There's nothing particularly special about this pipe - I just walked into the nearest plumbing supply store and asked for the largest drain pipe they had.
Cutting the pipe into manageable pieces can be more difficult, however. This pipe is way too big for a standard pipe cutter and a bit too big for my miter box. A simple hand saw could have been used to chop it into chunks - but I wanted edges that were closer to square, so as to make rectangular sheets.
I do this by tightly wrapping the pipe in paper, and using that as a guide for the saw. Ideally, the paper should form a sleeve around the pipe that is tight enough that it does not slide easily, yet loose enough that it can slide down after each segment has been cut off. I make the pipe segments just short enough that they can fit into my miter box sideways, as the length determines the width of the finished sheets. Using paper as a guide for a saw does fray the edges of the sheet, and the resultant cut is neither perfectly straight nor perfectly square, but it's close enough.
The miter box does see use for one purpose, though - it's a convenient way to cut slits into each pipe segment to turn them into rectangular sheets.
Cutting the pipe into pieces by hand is by far the most time and labor intensive part of the whole process. Once that's done, the rest is easy. If your saw leaves burrs, you'll want to remove them, as they overheat and burn easily.
There are a variety of ways that people heat PVC for thermoforming, and there's one that I'll mention now only to say that I don't use it - a heat gun. Heat guns are great for heating only part of a piece and for heating pieces of awkward sizes and shapes. However, it can be difficult to not accidentally leave cold spots and it is easy to overheat parts of the sheet. The main reason that I don't use a heat gun is that I don't have one, because I've never needed one. I'll get to what I do use in a moment - but for now, it suffices to say that it works well enough that I've yet to feel the need to get a new tool.
So, go ahead and use a heat gun if you happen to already have one, or if my method is not suitable for some reason. They're good tools, aside from the fact that they don't have much in the way of other uses than this one in this hobby, and that they require some care to use.
Toruk does use a heat gun, though, so I'll quote what he said on the old HvZ forums about flattening PVC with one: "Heat up your slit pipe EVENLY. Keep it rotating and keep the heat source moving. For this 1.5KW heat gun, I am keeping the nozzle about 1/2" to 1" away from the pipe and taking about a minute to cook the average pipe chunk. Use a tool to prevent the pipe from blowing away and for controlling its rotation. Prod it occasionally. When it is at the correct temperature, it will become flexible. Do not overheat. Pay particular attention to edges; don't let the area around the split get browned around the edges, but get it a little hotter to counteract increased cooling rate."
There's another method that's worth mentioning, this time to point out that I don't recommend it: combustion, as in a gas stove or propane torch. Remember how I said that it's easy to leave cold spots or overheat parts of a project while using a heat gun? Imagine that, except the tentacles have spikes. The heat produced by fire is very intense and often very localized. You can scorch PVC quite badly this way if you aren't careful, which is not only bad because it degrades you materials but also releases carcinogenic fumes.
Of course, you use what you have - but if for some reason all that you have is fire then proceed with caution! Work in a well-ventilated area. Browning indicates overheating leading to structural degradation and is a warning sign that you are going to start making fumes. If you get smoke, fumes, or even a strong smell from your PVC, back off, you've gone too far.
Now, what do I use for flattening pipes into sheets? Well, to start, boiling water. Slit pipes can weave together in such a way that two can easily fit into one pot, so I boil them up two at a time. The lid is there to hold some of the steam in, and is raised because the pipes are a bit too long to fit fully into this pot. That's not a problem, though, as enough heat travels upwards to allow the whole pipe to bend.
PVC does not have a single fixed temperature at which it becomes possible to themoform it. At 100 degrees Celsius, it becomes soft enough to be crudely molded into rough shapes, and that's enough to squish it approximately flat, like this:
PVC has a low enough thermal conductivity that, if handled gingerly, it can be handled with bare hands at this temperature. However, boiling water near fingers is dangerous. I haul heated sheets out of the pot with chopsticks before crushing them by hand. If you're looking for hand protection, you'll want water resistant gloves - layering a good set of work gloves over winter gloves works fairly well.
This is a good point to double-check that all of the burrs are gone.
The advantage of squishing sheets approximately flat is that this allows them to fit in . . .
. . . a toaster oven!
Toaster ovens are nice. They heat things (mostly) evenly up to a (roughly) pre-set temperature, and are easily large enough for most nerf-related purposes. They are not, however, precision instruments. The temperature on the dial is not a reliable indicator of the maximum temperature that the PVC can reach, as toaster ovens heat by radiation as well as convection. It is still possible to burn your plastic this way. That's why you see a flattened old aluminum pie plate in the oven here, rather than the solid tray that came with the toaster - it'll limit contamination of the oven in the event that something does get scorched.
The oven is set here for 375F. 400F is generally regarded as the maximum useful processing temperature for industrial PVC thermoforming. However, the appropriate temperature setting for your oven may vary. I recommend starting at 300F (150C) and adjusting upwards from there. The depth of your oven may also limit the width of sheet that you can make in it.
Once again, you can handle the plastic without gloves, but you might prefer to wear 'em anyways. I wear a glove on one hand, and use only that hand for reaching into the oven, in case I accidentally touch the inside.
One 'gotcha' inherent to this method of heating plastic is that PVC 'remembers' the shape that it was initially extruded in and will attempt to return to that shape when heated. This can cause sheets to curl and come in contact with the protective grills on the hating elements, burning them. The sheet in the picture above did end us a little singed because I spent too long fiddling with the camera. When your sheets curl enough to approach the heating element, that's a sign that they're hot enough to be flattened. This only takes about a minute - don't leave PVC in the oven unattended.
There are two sheets of scrap fiber board here, and the PVC is squished flat between them. You can use any sort of scrap wood, so long as you don't mind whatever texture it has being pressed into the sheets. Metal sheeting would be ideal from a thermal conductivity perspective, but wood works well enough. Beware that if you use plastic sheets to crush you PVC flat, the heat from your PVC may warp them. This picture was taken on my first round of flattening PVC (the pot of water was left over from the boiling stage), and I later discovered a better technique: park the chair in front of the toaster oven, and sit on it while heating the PVC. Each sheet will cool in the time that it takes for the next one to heat.
Well, there you have it. This is enough PVC sheeting for several years worth of small fixes or a few larger projects, and with this method it's easy to churn it out in an evening.
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