Recycled Cardboard Plant pots (1)

Recycled Cardboard Plant pots (1)

We get a large number of parcels, and they all come with a lot of brown card which we just throw into the recycling. But at the same time, with our small vegetable patch getting better, it would be nice to have our own, home made recycled plant pots. I've seen people online do paper recycling, with various levels of success, and I've thought about some of the engineering.

The basic principles of paper and cards are that the individual plant fibres mesh together to make something stronger - as you chop up the paper to recycle it, the fibre lengths get shorter and less strong, and you counter this by making the paper either thicker, or corrugations etc. We don't necessarily need a lot of strength in a plant-pot, but we also want it to biodegrade in the soil, preferably after being used for a month or so. Ideally it would break apart and compost down at 6-7 months, which gives a plant a couple of months as a pot, then planted straight into the soil, pot and all.

So to make it, its a relatively simple procedure - first mix up existing paper (usually using water), any fillers, add a binding agent (which can be either a form of PVA, or something such as Rice or Corn starch), and then squash out the water and shape the object.

Minispacepig suggests adding coffee grounds to the mixture, which might help pad it out, but also add a form of fertiliser to the pot. We shall make some and see.

To shape it, we're going to use a mould and a press. The mould here is designed using parametrised inputs, allowing the shape, size, and holes to be controlled. The first generation design is here.

These are then 3d printed. I have some reservations on the strength, without more supporting being added to the walls and outside to constrain pressures, and thats something that I might try to add in the next generation - I'm almost certain there will be a second one (and beyond) because I think the water-drainage hole size and placement will be something to change - not to mention the method of our insertion of the pulp.

Post printing, they've turned out quite well. I've noticed that I made two design mistakes:

  • The outer wall diameter I set as being the plant pot width + wall thickness, but actually it should be 2* wall thickness. As the thickness is only a guess, we'll see if its strong enough or not.
  • With the plunger, when I added the thumb grips, the circle became "polygonated", and I didn't realise it actually had become polygons rather then a smooth circle and wasn't just a UX glitch of fusion - so the cylinder of the plunger isn't actually a smooth circle. This shouldn't impact its use, but is an annoyance.

Apart from this, the design is pretty good to the print:

  • The pegs to hold the side and base have come out nicely and fit well (the slack I added in is a perfect size).
  • The drainage holes all printed well without a ton of support (or any at all) - whether there is enough of them, or big or too big is yet to be found out.