Use 3d Printing to Cast Climbing Holds (or Anything Else)

When my kids got interested in rock climbing my natural first thought was to build them a climbing wall. Okay, I like climbing as much as they do, probably more, but the kids always make a good excuse. "But sweetie, I bought the cnc vertical mill for the kids." As an avid 3d printer I thought why not 3d print some climbing holds?

At first blush 3d printing seems ideal for the job, but on closer investigation it presents some problems. The first is strength. 3d printed parts can be weak especially when stressed across the layer lines. They can be made stronger by making them denser up to the point that they are 100% solid, but this adds dramatically to the cost and print time. A fist sized climbing hold printed at 100% infill would take between 12 and 24 hours to print. This is compounded by the tendency of large solid prints to fail by peeling off the build plate as they are being printed. It typically wastes a lot of material which brings us to another problem. 3d "ink" is expensive. Most of my machines use plastic filament which costs a minimum of $20-30 per kilo. So we are looking at $5-$10 a shot to try to print something which probably won't work and may not be strong enough if it does. Given that a climbing wall requires hundreds of holds this was beginning to look like a real non-starter.

With a little research I was able to discover that most climbing holds begin with a master carved either by hand or using a cnc mill. The master is used to make a silicone rubber mold from which the holds are cast using polyurethane resin. Silicone is rather expensive and since a mold needs to completely cover the master it requires a fair bit of material. This approach works well for a factory because they can pull hundreds of casts from a single mold so the cost is insignificant. I wanted to make enough holds for a whole climbing wall but it would be rather boring if they were all identical. Silicone molds would be rather expensive if I only used each one a few times.

3d printing is obviously a good way to produce the master, but what if I could skip that step and directly 3d print the mold instead? That would save the time and expense of moldmaking. TPU or thermoplastic polyurethane is a printer filament which is amazingly flexible. People use it to print everything from shoes to custom rubberbands (because sometimes a stock rubber band just won't do). Depending on the design of your extruder it will be either easy to print or it will leave you tearing your hair out. Luckily for you TPU would be the ideal material to print yourself some new hair. While not nearly as flexible as silicone, TPU is flexible enough that parts can be de-molded once they have cured.

It is best to print the mold as thin as possible because that will make it the most flexible. It also saves print time and material. In a couple of hours I can print a mold that will produce dozens of copies of a hold. The cost of the TPU mold is maybe a dollar which is much better than $10-$20 for a silicone mold. It is neither as durable or as easy to use as a silicone mold, but these short-comings are more than made up for by the economy and convenience of printing the mold directly.

Look through the offerings of any climbing hold company (and there are lots these days) and you will see the amazing variety and artistry which are expressed by today's hold shapers. A little experience at your local climbing gym will give you some idea of how various shapes work and how you want to shape your holds. There are lots of modelling/sculpting programs out there that you could use. Meshmixer and Blender are two free ones which are a good place to start. I use a sculpting program called 3dCoat which is inexpensive, amazingly powerful and fairly intuitive once you get past its interface quirks. You can sculpt anything you can imagine. You can import other 3d models or 3d scans and incorporate them in your model. You can create any texture you can imagine and many you probably couldn't. If you work with 3dcoat in voxel mode you don't run into any of the annoying limitations that plague most 3d modelling programs. It just works.

I have designed holds from scratch. I have made holds from other 3d models like the tiki hold. I made a hold from a 3d scan of my daughter's face. I made a hold which is an exact copy of Yosemite Valley 6 inches long. Anything is possible.

So you have sculpted this awesomely sick hold which looks like a gob of alien snot now you need to turn it into a mold. Using your modeling software create a shape a little bigger than your hold and boolean the hold out of it. This is easy and very reliable with 3d coat, not always so with other programs. When you are done you end up with a hollow mold the shape of your hold. But wait, you need some way to attach the hold to the wall. . .

You could drill the mounting hole and countersink after you cast the hold, and if you have this thing where you really like drilling things you might want to. This is a safe place. We won't judge. But most of us will want to cast the mounting hole in place. It is possible to incorporate the hole into the mold as one piece, but you end up with a long thin printed post in the middle of the mold which makes it difficult to unmold and tends to tear off the first time you use it.

A better solution is to print a socket at the bottom of your mold which will accept a piece of round rod or a bolt. The socket forms the countersink for the bolt head while the rod forms the bolt hole. Smooth round rod is preferable because it is easy to remove. A bolt has the annoying habit of casting its own set of screw threads which can make it difficult to remove. If you get to it while the plastic is still soft it comes out okay, but you have only minutes to do this before it becomes a wrestling match.

Now that you have the model finished simply print it in TPU. Print as few shells and as little infill as you can while still having a decent print as this will make it easier to unmold. All of my molds leak a little, but that's okay. The resin seeps into the mold and seals it the first time you use it. It is fairly viscous and hardens very quickly so it doesn't leak a lot. Still I wouldn't recommend doing this kind of work on top of Aunt Ermintrude's Credenza unless it is really ugly and you need an excuse to get rid of it.

I made some holds using a fiber reinforced high strength industrial grout. They look and feel like real stone which is nice, and it is very cheap, but its lack of strength in tension limits your design options. Thin sections can be too fragile, but it is quite economical for big chunky holds. A better material is polyurethane resin. This is what the pros use. It comes in a 2 part system where you mix equal volumes of A and B, add some pigment and filler if you like and pour it into your mold. I use a product called RC-3 from Alumilite. This stuff sets amazingly fast. You have about a minute to get it mixed and poured. If you want a uniform color it is helpful to completely mix the pigment into the A side first before you start the clock ticking.

But wait, before you pour into your new mold lets think about this. Your mold is made of TPU, the PU stands for. . . polyurethane. And you are about to pour it full of. . . polyurethane! Are you mad? What kind of sick joke is this? Relaaax dude. Just chiiiill. Everything will be cooool. We'll get by with a little help from our friend Stoner. . .

Okay, seriously, who in their right mind would pack an aerosol can full of evil smelling chemicals and market it under the name "Stoner"? What were they thinking? What is their secret agenda? I can't imagine, but it is the best mold release on the market. One good spray in your mold and polyurethane will not stick to polyurethane. The day is saved!

Another thing which is good to do at this point is to drop a washer onto the rod which forms the hole. This will embed it at the bottom of the countersink where it will live out its days happily spreading the load from the bolt head. This is what washers long to do. Little #4 washers dream that someday they will grow up to be big strong 3/8" washers and that they will get a job this cool.

Now pour your mixed up resin into the mold. Or mix up another batch if the first one hardened while you were reading silly instructables. This stuff doesn't put out much in the way of fumes, but as with all class A sinister goo you should familiarize yourself with all the suggested safety precautions for the product you are using. I like to read the MSDS for any sinister goo I work with.

Try to get the resin level with the top of the mold, but don't sweat it as long as you are close. Curse you meniscus!

You really don't have long to wait at this point. By the time you get the residue of the evil goo off your hands (seriously wear some gloves) your hold will be ready to unmold. The resin goes from translucent to opaque as it cures and a few minutes later it is hard enough to unmold. I suggest wearing gloves for this part too because the goo and the mold may still be sticky in places. Start by taking the rod or bolt out and then gently flex the mold around the edges. At this point you may wish you had started with a simpler design with fewer overhangs, but be patient and persistent, or start again with a simpler design with fewer overhangs.

At this point your hold is out of the mold and ready for final finishing. For starters the back is probably not perfectly flat (see what you've done meniscus). But a little sanding can flatten it perfectly. While you are at it check your hold for any other sharp edges or rough spots. While the the resin is young and impressionable it is easy to shape with a little sanding, but as it ages it gets grouchy and set in its ways. It may even start to vote republican so be careful.

Now you have a climbing hold just as good as the ones you can buy, but many times cheaper and totally custom. You can print dozens of TPU molds for the cost of a single silicone mold. And you can cast dozens of holds in the time it would take to print one directly.

If you want to learn more about this project, we talked about it on the 3dPrintingToday Podcast, available on iTunes and Stitcher Radio.

Have fun and let me know what you come up with.