Since watching this surprisingly interesting video about corrugated cardboard, I’ve been thinking specifically about cardboard recycling. The essentials seem pretty straightforward: pulp the cardboard, and you get a fibrous mass you can then use to make something else — probably another cardboard sheet. (You may even have done this as a kid; I can remember taking some old newspaper, pulping it, and making it into new “paper” by drying it on a screen.)

This post is basically a series of questions and semi-connected thoughts, centered on the theme: I produce a fair amount of cardboard waste, which all goes into the recycling. Can I recycle it myself, into useful material for my hobby projects?

For this post, I’m thinking of something like a custom enclosure for an ESP8266 or Raspberry Pi-based project. A weatherproof/UV-resistant material would be nice, but I’m not considering that a hard requirement for this cardboard-manufacturing thought experiment. (After all, PLA plastic doesn’t really fulfill that requirement, either.)

First off, I know I could do some sort of pulp molding as described in eg. this Hackaday post. But most of the projects I’d want to use this material for are one-off projects, and having to print or otherwise make a mold defeats the purpose:

• Why wouldn’t I just manufacture the thing I needed, directly, from 3D-printed plastic or whatever material the mold is made from?
• For a one-off project, I would just end up throwing away the mold, which probably results in more material/money/energy wasted.

As far as I can tell, this leaves me with roughly 3 different options for manufacturing an object directly from cardboard: additive manufacturing, subtractive manufacturing, and what I’ll call “direct manual modeling,” which basically means “gluing cardboard pieces together.” Before I go through those options, though, let’s talk about ways to stick cardboard pulp together and give it strength and weatherproofing.

The most obvious thing I can think of is epoxy.¹ Epoxies are available with various properties, viscosities, cure times, amounts of UV resistance and weatherproofing, and curing agents (including UV-curable epoxy). It seems to me that you could choose an epoxy with suitable properties for a given project, mix it with finely shredded cardboard fibers, and then use that mix to make whatever shape you need. Think of it like a viscous, liquid, material that, once cured, is like a carbon fiber product: the finished product is “made” of epoxy, with internal carbon fiber providing strength. This is just … worse, because the cardboard bits are both smaller and weaker than carbon fiber.

You’d want to choose an epoxy that uses a curing agent appropriate to the project and manufacturing method. I’ll delve into my thoughts on that in my brainstorming on manufacturing techniques, below.

How much would these epoxy mixtures cost? Are they going to be more expensive than just buying 3D printer filament or resin? Am I going to end up making objects whose mass is basically epoxy, with some cardboard thrown in just to make me feel good? Is this worse for the environment than just recycling the cardboard and making the project enclosure from PLA/PETG?

Also: How much time and effort would it take to produce such a mixture? How finely would I need to shred the cardboard?

Moving on, my loosely-organized thoughts on how to make useful shapes from cardboard and epoxy. (I’ll also cover a couple additive-manufacturing-specific material ideas in that section.) It should be theoretically possible to produce items like electronics enclosures via any of these techniques, and with some easy finishing work it ought to be possible to add things like threaded inserts for screws.

Additive manufacturing:

One option for making objects is additive manufacturing, which basically means “3D printing.” With this technique, you can create fairly complex shapes in one manufacturing step which can’t be machined or molded in a single piece.

One approach, usable for additive manufacturing with a conventional FDM 3D printer, would be to embed fine cardboard fibers into a DIY plastic filament. I don’t think this approach sounds promising, for many reasons. You’d likely face frequent clogs, you’d have to make sure the cardboard withstands whatever temperatures you’re printing at, extruding consistent filament yourself is hard, and the bulk of the filament is going to be plastic you purchased instead of cardboard anyway. The filament would, at best, probably perform basically like standard 3D printer filament.

A similar but maybe less-bad approach, also suitable for additive manufacturing, would be to “enhance” the resin for an SLA (“resin”) 3D printer by mixing in fine cardboard fibers. This won’t produce nozzle clogs, since there is no nozzle, and it may be possible to make a nontrivial percentage of the manufactured object’s mass out of actual cardboard. The disadvantage here is that resin printers can generally print smaller objects than FDM printers.

Alternatively, you could probably come up with a custom UV-curable cardboard-epoxy resin mixture that works well enough directly with an SLA/resin 3D printer. No idea if this would turn out to be cheaper than mixing into a 3D printer-ready resin.

Some questions with any of those options: how much cardboard could I realistically add to either material? Likely not much at all in a custom filament, and maybe a bit more in a resin, but I don’t know how much I could realistically add to a resin and still have it cure properly. Would the costs be basically the same as just manufacturing with standard 3D printing resin or filament?

Given my original idea of mixing fine cardboard shredding with epoxy resin to make a viscous epoxy-cardboard hybrid, I wonder if you could adapt a conventional FDM-style 3D printer to extrude extremely viscous, high-surface-tension cardboard-embedded resin instead of filament. You’d do that by replacing the filament spool with a resin tank, the extruder with a pump, and the hot end with a valve. Presumably you could place the valve very close to the extruder nozzle to help avoid excessive drips. But you’d still want to cure the mixture very quickly, allowing for layering as with a normal printer. You could maybe do that with an intense UV light placed immediately at the nozzle, facing the print bed; or by essentially enclosing the entire printer in a powerful UV curing chamber. In any case, though, this technique wouldn’t allow for anywhere near the fine layer heights and line widths that a filament-based printer produces, and overall print quality would be pretty bad. (Maybe you could take a print produced this way and clean it up afterwards? See “hybrid approaches,” below.)

Subtractive manufacturing:

“Subtractive manufacturing,” as I’m thinking here, basically means “start with a block and remove everything that’s not part of your object, probably with a little CNC machine.” To start, I’d need to produce blank blocks which could be machined into whatever shape I need. I can think of two ways to produce such a block.

First, I could just let a small vat of the epoxy/shredded-cardboard mixture cure, resulting in a solid material which could then be machined. That comes with all the questions outlined above (how much of the mixture’s mass can realistically be cardboard? does that amount of cardboard provide any useful material properties? how costly is it?). There’s also the question of curing strategy. For this method, it seems like a two-part epoxy mixture would make the most sense; UV wouldn’t reliably penetrate to the middle of a relatively large vat of the mixture.

Alternatively, I could layer cardboard pieces together with epoxy in between to get a block of the desired size. To ensure strength, especially with corrugated cardboard, I’d want to somehow fully saturate the resulting block with epoxy. (Potentially, it could even be saturated using the shredded-cardboard/epoxy mixture discussed earlier.) This could be achieved, I think, with a vacuum chamber or pressure pot. The same questions and curing techniques as above would apply. I imagine the resulting material would be quite strong, especially with corrugated cardboard.

(I do like the idea of using larger pieces of cardboard, particularly corrugated cardboard. I’ll mull this over a bit more in the “direct manual manufacturing” section below.)

One thing to note, though, is that depending on the shape of the starting material, you’re going to end up with a lot of little chips of waste. With metal, that’s not a huge issue, because it’s easily recyclable. But with a cured epoxy-cardboard mix, are those chips going to be usable? I suppose you could grind them into something fine enough to be combined into the mixture for a future project. (But, does that fine-grinding process affect the properties of the material at all? And again, would that material even have had any useful properties to begin with, when compared to plain old epoxy?)

Direct, manual modeling:

Could I just use recycled cardboard pieces directly to make things, gluing and waterproofing it with epoxy? Obviously this won’t allow for precision or complex shapes the way 3D printing or machining could, but it ought to be possible to make a simple, reasonably functional enclosure this way.

If I made something using this method, once the initial glue has cured, I’d consider saturating the cardboard with epoxy or with an epoxy/cardboard mixture, the same way I’d consider for assembling a machinable block from layers of cardboard (see discussion above). That could be accomplished by submerging the object in the epoxy mix and using a vacuum chamber or pressure pot. But: curing that would be complex. It couldn’t be cured with UV light, since the epoxy will be inside the corrugation or between cardboard layers. I think that means you’d need to pre-mix a liquid hardener with the epoxy resin, then submerge the object in it. That complicates things: you’d need to remove the object from the bath when the epoxy is viscous enough not to flow out of the cardboard, but before the epoxy hardens too much. You’d also end up wasting a large amount of epoxy.

Alternatively, I could use the “immersion in pre-mixed epoxy/hardener under vacuum” strategy to pre-prepare epoxy-saturated sheets of cardboard before using them to make something. This would at least result in less waste.

Perhaps there are curing agents that can be applied in gas form², so you could saturate the object with epoxy resin under vacuum, and then put it in a pressure pot filled with a high-pressure gaseous curing agent?

It wouldn’t be practical to make a very precise or complex object this way, but I think it could work. Cardboard chunks could be glued in place and then later drilled out for threaded inserts, and it would be easy to add epoxy-saturated cardboard rectangles to form things like a lip for a lid to sit on.

Hybrid approaches:

It may be possible to combine these techniques to get the “best” of several worlds.

Starting with a model produced by an additive process, you could then put it through a subtractive process (a tiny CNC machine, again) to cleanup rough edges and drips, add or clean out holes for threaded inserts, and clean up tolerances where different pieces need to fit together.

This could be slightly annoying, though, because you’d need to generate a tool path that starts a little ways out from the actual part, even though imperfections are likely to be localized. The machine will, I imagine, spend some amount of time cutting empty air.

Or, starting with a cured, hand-made object, you could probably use a tiny CNC machine to clean it up, removing gross imperfections, allowing for threaded inserts, and ensuring parts can fit together. Heck, you could even use a Dremel tool in this case, since anything you made by hand from epoxy and cardboard isn’t going to be particularly precise anyway. (Doing this by hand also means you don’t need to have an accurate 3D model of your non-precision, handmade part, which I think you’d need to generate a tool path for a CNC machine-based cleanup.)

Alternatively, perhaps I should consider something like the cardboard molding process I linked at the beginning of this post. I could produce a molds for generic enclosures in a few “standard” sizes, and then use them to produce relatively high-quality, generic cardboard parts. Those parts could them be customized for a given project, either via an automated process or by hand. (That would mainly include adding holes for threaded inserts, cables to come into the box, etc.) There’s the question of where weatherproofing comes into this particular process. I could envision several options:

• The cardboard pulp used for molding could actually be a cardboard/epoxy mixture, assuming such a mixture can be cleanly removed from a mold once it’s at least somewhat hardened.
• Depending on the pressure used to mold the object, perhaps it could be saturated with epoxy in a vacuum chamber or pressure pot.
• Or, the object might be made solely of cardboard. After customizing it, you could manually paint all exposed surfaces with one or several layers of epoxy (and then perform some final cleanup for eg. threaded inserts).

One other “hybrid” idea: you could make a very rough and disposable ad-hoc mold for a specific, maybe using yet more cardboard, and fill it with the epoxy/cardboard mixture. Once it hardens, it can be shaped and cleaned up with CNC or by hand. However, this seems like a complex process: compared to conventional 3D printing, this adds a molding and a cleanup step. This would probably only be worth it if you find a high-performance cardboard/epoxy mix that can’t be used with some sort of 3D printer and you don’t want to use it to cast a large blank block (for cost reasons, I guess?).

Finally, I’ll just note that, same as with the “direct modeling” approach above, the question of completely, properly curing the epoxy may be more difficult for these objects. This is because of the potential for odd shapes, and the constraints that epoxy must saturate the object’s cardboard while minimizing wasted materials.

Conclusions (or not)

🤷🏻‍♂️

Would any of these techniques actually make sense? Could they produce useful materials, and could those materials be used in place of plastic for one-off projects? Did I just reinvent 3D printing, but worse? Is this all a bad idea and I should just keep throwing all my cardboard in the recycling bin? Am I totally clueless about how epoxy works? Am I assuming cardboard pulp would be more of a useful additive than it actually is?

Are there other use cases for DIY cardboard recycling?

I’d love to experiment with these options and answer the various questions I’ve mused about here, but I don’t quite have the time to do that right now. It’s certainly something I’m going to keep in mind as a possibility for future projects.

In the meantime, can you point me to any reading or videos on this subject? I surely can’t be the first person to have these questions. Find my contact info here.

Updates

June 9, 2022

Via this Hackaday post, tonight I learned about the Cardboard Ninja YouTube channel, which presents some projects that recycle cardboard boxes in a much more straightforward manner: cutting the boxes up and reassembling the pieces into shelves, cat houses, and more.