Making Coroplast Boxes

Coroplast is the material we see in yards during election time.  Sheet corrugated plastic, it has a double wall with the corrugations inside.  Strong, lightweight and fairly inexpensive at around $11 per 4' x 8' sheet in my area.  If you can't find it locally there are online sources.  Coroplast will degrade over time in direct sunlight unless painted and become brittle so keep this in mind when attempting to use salvaged pieces and it's also a good idea to work with any coroplast in a warm area so the plastic will be pliable when forming. I advise to not use painted coroplast if making composting worm bins, at least for the inside of the box.  It's ok though to use colored coroplast sheets that are made as such far as worms go and probably best to use a dark, opaque color such as green for worm benefit since they are sensitive to light but it will still degrade if left in direct sunlight. Krylon Fusion paint is made for plastics and would be good to use for painting the outside of wormery boxes if they are going to be exposed to direct sunlight.  Wormeries need to be shaded if hot but there may be times, such as a greenhouse during winter that sunlight may be of benefit.

I like coroplast because it gives me the option to build boxes to my specifications and not have to rely on finding the "just right" plastic tub.  Sadly I could not find anywhere online showing how to actually do this so here we go with a custom coroplast box tutorial...

Any size you need can be made using this method. For tutorial purposes we are going to build a box with a 2' x 2' square bottom with 1' tall sides.  We start with a coroplast sheet section exactly 4' x 4'.

A straight line is marked corner to corner.  Measure 1' in from each side and mark to the lines.  We now have a 2' x 2' box bottom marked.

Next we mark in four places, shown in red below, to cut all the way through for side flaps which will form the walls and secure the sides together.  These are the only places we will cut completely through the sheet.  All other marks will only be cut through one layer and folded over to create corner seams.

If you're like me and want a perfect box which is easy to form and will have straight sides, pay close attention to the cuts which will form the folding side flaps or rather the line between "A" & "B". You will notice they do not exactly line up with the lines drawn for the box bottom.  This is because when we have all the cuts made and flap "C" is folded to form a wall, we will have the thickness of the coroplast sheet sticking out.  So this flap line is cut to compensate for the thickness of the sheet and will wrap around the side forming a flush corner to the side walls, keeping the box wall straight.  If we don't do this, the box will have slightly bowed sides instead of  perfectly straight.  I have no way of knowing what thickness the particular sheet will be that you get so I have not added precise measurements. Not all sheet is the same thickness so you will have to compensate.

Use a sharp box blade or exacto knife, take your time and make straight cuts along all the seam lines to be folded, taking care to only cut through one side of the sheet.  Some cuts will be easy since you will be cutting along with the direction of corrugations (flutes) and some will cut across them but it's fairly easy with a sharp blade.  It is not necessary to cut completely through the flutes when cutting across them but do cut most of the way.  The cut flutes will completely separate when folding the flaps.  

After making all the cuts, flip the sheet over and completely fold over, pre-bending all the flaps one at a time to form corners and bottom seams.  This is where solar degraded or very cold coroplast may snap and break.  By doing this, the corners and walls will then form easily, making a straight, perfect box.  The four flaps marked as "A" above fold around and attach to walls "C".  We can use double-sided adhesive tape, plastic glue or whatever to secure the flaps and it will not be in contact with any worms or otherwise critters we have in the box.  Another option some might consider is heating a small metal rod with a torch to poke and melt holes about 1/2 inch apart all the way through the flaps and sidewalls which will in itself semi-secure the coroplast together, then installing small zip ties through the holes to make a permanent join.

Hope this proves helpful to someone.  I'm fairly certain you can't find a plastic box this size for the price that will be any better. One could get a 2' x 6' box with 1' tall walls for the cost of one sheet and I can pretty well guarantee you can't beat that. I think they will hold up well enough, unless perhaps one is planning on allowing them to fill completely with substrate which will probably cause the sides to bow out. Some form of top edge support would probably be needed.  I don't plan to let mine get that full.

Also think these would be great for burying out in the yard for designated flower beds or in-ground wormeries that would keep weeds or some creeping grasses from invading like my yard is full of haha!  The corners are not water tight so they would drain unless one makes them so with something like 100% pure "Silicone 1" which is safe for use in aquariums.  All other silicones will kill fish, worms or whatever if it's treated with anti-mold chemicals, a good to know bonus factoid!

I will use this same method of cutting/folding coroplast strips to make enclosure walls for Clean Bug System of raising mealworms which will fasten to screen bottoms with silicone or melting/fusing permanently to the screen (another article coming soon).  

Have fun, save money and don't cut yourself! :)