Introduction: Climbing Holds From Milk Jug Recycling

About: Engineer by day, transformer of junk into non-junk by...well that's usually by day too. Living by Colossians 3:17

I've seen quite a few posts and videos around about recycling common plastics by melting down HDPE (High Density Polyethylene) plastic into new shapes, and I've been wanting to make myself a rock climbing wall. Logically, why not try both at the same time? I also tried to find ways to use they fewest/most commonly available tools so you won't need a lot of fancy equipment for this project.

The best part of this project is that it is 99% free and using materials people would otherwise throw in the landfill.


HDPE type plastic items

Soap and water

Acetone/nail polish remover (optional for cleaning)

Parchment paper

1-2 inch 1/4"-20 bolts- 2 per hold

Nuts and washers to fit the bolts

A vertical surface to cover (Optional: Plywood sheets/scraps)


Strong scissors/tin snips

Toaster oven or oven

Metal pan

Drill press/Power drill

1/4" drill bit

5/8" paddle bit

Saw (optional for faster cutting)

Power Drill (for mounting plywood)


Cutting things

Squishing hot stuff

Rough hand shaping (similar to modeling clay)

Safety Precautions:

Heat resistant gloves

Ventilation (recommended)

Common sense, you will be handling very hot material

Step 1: Be a Hoarder

First things first. Gather up a big ol' pile of items made from HDPE plastic. The most common things will probably be milk jugs and plastic buckets, but you can also use water bottle tops and the snap ring below them (though not the bottle itself), many medicine bottles, shampoo and detergent bottles, ice cream buckets, Folgers type coffee cans, yogurt and margarine containers, some snap-on 6-pack holders, 55 gallon plastic drums, plastic "tile" drain pipe, anything that's stamped with that special HDPE marking of the recycling triangle with a 2 inside it and/or the letters HDPE (pictured above). If you don't know what type of plastic something is made of, be sure to find the #2 recycling mark before using it. HDPE is safe to heat but some other plastics can burn or release toxic fumes when heated and ya don't want that. Polypropylene (PP, #5) and Low Density Polyethylene (LDPE, #4) are also safe to be used and work similarly, but melt at different temperatures. For this project let's stick with HDPE as it's the easiest to work with.

If you call around a bit you can get a lot of material pretty quickly. Restaurants (Good results at Wendy's and Big Boy), Sam's Club, donut shops and grocery stores with delis are great places to get large amounts buckets from, and most will save them for you if you ask. Even the local pie shop had buckets from pickles (at a pie place?) that they let me nab. The local laundromat had a bunch of empty detergent bottles they were happy to be rid of. Sometimes you have to call and ask the manager to save things up for a time before stop by for a pick up.

Put a box on the counter/porch/garage to toss your empty milk jugs and bottle caps into, and ask the friendly neighbors to do the same. Steal (with permission) their empty kitty litter containers. Ask the boss if you can have the empty coffee cans from work. You could also go dumpster diving, though you probably won't be quite that desperate. Don't bother with anywhere that will charge you to take buckets or jugs, check around for free ones and you'll soon have more than you know what to do with.

I also recommend checking around on and making a post on Craigslist and similar sites. The local Free Stuff Facebook pages are also a great source. Within a few hours of making two posts I had close to 100 milk jugs, nearly a dozen detergent jugs and a handful of coffee cans from generous people willing to donate them.

You can sort them out by color and/or type of HDPE or leave them in a piled mass in the living room. Be sure to at least lightly wash or rinse containers if you'll be leaving them set for a while, else they can start to stink (especially milk). The dishwasher comes in handy if you have a large amount to clean at a time.

You can keep collecting while starting on the next steps.

Step 2: Know Your Type

Once you think you have enough junk gathered up to move on, or when your pile starts overflowing from the garage, you can start sorting, cutting and cleaning all your plastic. There are actually two different types of HDPE with slightly different properties, and knowing the difference can save a lot of cutting and possibly end up with a higher quality end product. There aren't any "official" names for the types, so I refer to them by their properties.

The first type is the blow-molded HDPE, and is what most HDPE jugs and bottles will be made from. This type will somewhat hold its shape and doesn't get quite as sticky when melted. These type need to be cut into smaller pieces because they can trap air bubbles below them which would weaken your recycled plastic. It is slightly easier for hand shaping when hot but not as good for molds because it traps air bubbles and doesn't flow into shape. I will refer to it as non-flowing or non-porous HDPE for this Instructable.

The other type is injection-molded HDPE, and is what most large buckets and bins are as well as (often) their lids, and also milk jug lids. This type will "flow" when melted, meaning it will take the shape of molds much better without pressure. It also is quite sticky in its molten form but still releases and pulls off easily when cooled. This type works better for molds because it will flow into small areas, but not as easy (though still usable) for hand shaping due to its high stickiness. These can be left in larger chunks, as it will allow air bubbles to pass through while molten. I will refer to it as flowing or porous HDPE for this Instructable.

If you're not sure which type it is, there's a few ways to find out. Usually the easiest way is to look on the bottom of the container. If it has one or a series of circular imperfections on it, it is most likely the flowing type. If there is a single ridge-like line across, it is most likely non-flowing. You can also snip off a small piece and melt it on a pan in the oven. If you can still somewhat tell what shape it was after it's melted, it is non-flowing type; if it becomes a pile of goo similar to when you accidentally squirt toothpaste onto the sink, it will be flowing type.

You can mix the two types together in melts and molds; I tend to put layers of flow and non-flow to get the flowing to fill in the cracks of the non-flowing. You can also create neat patterns by mixing and twisting colors together.

Step 3: Mr. Clean and Process

Start with a preliminary rinse/light wash. if you already did this before storing them it is still good to do it again. Remove labels and the glue beneath them if you can, and make sure to remove any lining inside of lids as well- typically easy to do by sticking a knife blade under the edge of the lining and prying up. If it takes too much work to remove something, cut around it and toss that section in the recycling. If you have a few good sources of HDPE items it often isn't worth the effort of removing labels and the residues under them unless it is a special color that you can't find much of. If you really want to keep a piece that has a stubborn label, rub it with acetone (nail polish remover) in a well ventilated area.

Cut your containers into manageable strips and chunks, and wash them well in soap and warm water. Any dirt and gunk left on your plastic will become contamination and weak spots. Set it all aside and let fully dry.

Cut your clean plastic again into meltable size bits. Flowing can be as big as can fit into your melting pan, non-flowing should be smaller size (I go with about 3" wide max) and shaped as to not trap air bubbles beneath them. A strong pair of scissors cuts milk jug material easily, or tin snips cut it like butter through a hot knife. Buckets and thicker material will need tin snips or a saw. You can toss the thinner-walled containers in the blender to save on wrist strain. Just be sure to have a dedicated blender (you can often find them cheap) so that your wife/mother doesn't get mad at you when she finds bits of plastic in her next smoothie.

I'm currently working on making a cheap-but-sturdy shredder to use in the future. Shredding isn't really necessary but can make small pieces with much less effort than hand cutting.

UPDATE: I did tests between large and small chunks and there were a few main differences.

  • The smaller the pieces the more splotchy and less blocky or stripey it looks.
  • Wider pieces of flowing type deformed more if not under pressure while cooling.
  • Wider pieces of non-flowing material had no significant structural difference in final product from smaller pieces

Step 4: Have a Melt Down


HDPE does not release toxic fumes when heated. I would still recommend doing this in a well ventilated area in case of accidental contaminants that may burn and stink. There often is a slight sweet smell while melting, this is normal. If you get a strong unpleasant odor either something in your oven is burning or there is non-HDPE plastic in your mix.

I would recommend not using the kitchen oven for this. Even though it doesn't release any fumes, some people may yell at you to quit contaminating her... er, their oven with your "stupid projects"(harrumph).

Set your oven or toaster oven to 300-350° and grab your melting pan, parchment paper and plastic bits in whichever color(s) you desire. The parchment paper prevents molten plastic from sticking to the pan, put a sheet of it on your pan and spread a small handful of plastic on top of that.

Put it in the oven for about 10 minutes, then check it. If the plastic is squishy looking and losing its firm shape, push any plastic outriders from the sides of the pan back into the center mass and add another layer of plastic bits. Note that semi-opaque bottles such as milk jugs will turn fully clear when heated, you can use this to judge the timing.

Repeat this process until you have the desired amount of melted gloop, then let it heat for another 15 minutes or so to get a big gooey mass. You can compress and squish the plastic into somewhat of your final shape between heatings; be very careful, this stuff is very hot and will stick to your skin like napalm. If molten plastic makes contact with your skin, quickly douse with cold water.

While making my holds, I lacked a plentiful supply of colored bits, my main source being semi-opaque jugs and grey and white buckets. I found that using the plentiful bland colored plastic for the main body and adding bits of color on the outsides during the last heating worked well and added neat patterns to the holds.

You can press and clamp this molten gummy sludge into forms and molds, but for these climbing holds it was easier to form them by hand.

Step 5: Handle the Holds

I don't have a ton of pictures of the shaping part because of a combination of gloves and hot plastic. This step has to be done fairly quickly before your material cools and hardens.

While your goo is still hot, pull it out of the oven and peel off the parchment paper (may destroy the paper in the process, that's ok). If it sticks to the paper too much, let it cool for 15-30 seconds and it will peel of much easier. Plop the glop on a layer of parchment paper lain on a flat surface such as the kitchen table or another pan. Use your GLOVED hands to mold the goo into the shapes you want for a climbing hold. Make sure to plan in areas to drill holes for mounting bolts and press it down on your work surface to make a flat back to mount on the wall. You can also use metal objects to aid in shaping, such as the bottom of a metal pan to make a flat side of the hold or aluminum cans and pipes to make indented holes. Try to round over edges into comfortable shapes to hold when possible.

HDPE will try to shrink together as it cools, so keep pressing into the shapes you want for a couple minutes until the outside shell cools slightly and no longer creeps back when released. Even after this it will still be hot to the touch, so set it aside for a few hours to cool (or toss it in the freezer if you're impatient). I let them all cool at least overnight before working them to ensure the centers were fully solid.

Trim up the edges with a knife or saw. Planing or sanding (produces a lot of fine plastic dust so use a mask) the bumpy back side flat helps the holds sit better against the wall, though they still work if left rough. Sanding also brings out the designs even more. The material can be easily shaped with woodworking tools when cool. Machined surfaces are typically very glossy and even.

Use a torch flame and lightly run over the holds for a smoother, shinier finish and to make small adjustments in shape such as rounding sharp corners.

I chose to make most of my holds similar in shape to commercial ones (kinda), but they can be made almost any shape you can imagine. Have fun with it! Flowing type HDPE can also be melted into a mold for special, more accurate shapes.

Step 6: Hole'd On

Drill thru holes for your mounting bolts. The best option is to use a drill press in order to be fully perpendicular to the direction the hold will mount, but a hand drill would work too if you're careful. Drill in short pecks so the friction doesn't melt the plastic and gum up your bit. Use the bit size that you need for the bolts you use; a 1/4" bit for standard 1/4"-20 bolts. Be prepared for lots of plastic flakes that will make a big mess.

Make sure to clamp the hold (or hold the hold very tightly) so the drill doesn't wiggle it around or grab and throw it. They make quite solid projectiles when they fly across the shop, I may or may not speak from experience.

Counterbore the holes down to about a 3/4" thickness with a 11/16" bit to make a flat area for a washer and the bolt head. Instead of buying a brand new counterbore bit for $10-20, a cheap paddle bit (less than $5 at Home Depot) will work just as well and be more useful for other projects.

Slip a washer into each counterbore hole and insert the bolts. If the fit is just right the bolt should lightly thread into the plastic with a screwdriver, you don't want them to wiggle too much.

Typically you will want two mounting bolts per hold, to make them stable and keep from rotating while holding the weight of a climber. For larger or very wide holds more may be needed. Small finger/toe holds may need just one.

Any waste chips and messed up holds can be remelted down in the next batch.

Of course, you need to give your holds some names too.

Step 7: Time to Screw Up!

Drill some holes and mount your new holds to a wall!

Have some fun with the location. On the underside of the stairs would make for a more challenging climb, around corners, build a frame of plywood and 2x4 scraps to mount onto, or screw the holds into a tree or pole for the kids to climb up to their tree fort.

I used a wall of the barn that's been needing to be covered for a few months. Drill holes with your ¼" bit through ½" (or thicker) plyboard in pattern to match the holes of your holds, insert the bolt through the hold and plyboard, and tighten a locking nut onto the other end. It's plenty sturdy enough for me to climb on without issue.

Be safe and add padding on the floor or a sturdy belay system.

Step 8: What's Next?

Other ways to modify or continue this project

HDPE is a great material that could be used for plenty of more projects, it is very easily workable and can be melted into molds or machined into shapes after pressing and cooling. Here's some of the ideas I've had, feel free to comment your own.

  • Shoving bolts into a semi-molten HDPE in a round mold to make drawer knobs
  • Put colored LEDs inside of semi-clear holds for night climbing
  • Bigger better climbing wall!
  • Melt sand/abrasive into the surface of the holds for more grip
  • Similar project adapted for Polypropylene and/or LDPE

Ideas, comments and questions are always welcome!

All in all, this project took a bit over a month's worth of weekends and the free evening to finish (and it isn't quiiiite finished as much as I'd like it to be) and I'm pretty proud of how it turned out. Don't be discouraged if it doesn't come out perfectly the first time, my first one I used wax paper instead of parchment melted to the plastic and wrecked the whole batch. I hope this inspires someone else to do their own project, please share similar ventures of your own in the comments, and keep on makin'!

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