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This Instructable will show you how to design and print something simple and useful: your own sandals (also known as flip flop)!

The size of sandal you can print is a function of your printer and your willingness to spend time on CAD and assembly.  If you are limited to a 4 inch by 4 inch print bed, such as on a Makerbot Cupcake, then you can print sandals for a 5 year old if you do it in two parts.  If you have a Makerbot Replicator or a Printrbot+, on the other hand, you can print full sized adult sandals in two parts.

What you'll need:
  • Paper
  • Pencil
  • Ruler
  • Scissors
  • A CAD program such as Google Sketch-up & basic knowledge on how to use it
  • A 3D printer that uses ABS plastic or similar robust material and knowledge about how to print
  • 9/16" nylon webbing or similar (3 feet should be enough to give you one free mistake)
  • Small file or sandpaper
  • Acetone or similar ABS solvent + eye dropper or q-tip
  • Hot glue
  • 2 small nails
  • wire cutter or hack saw for cutting nails (optional)

Step 1: Size the Sandal and Design the Base

Before you start, you need to figure out what size sandal you are going to make.  If you are determined, you can print an adult sized sandal on a small bed printer by putting together 6-8 separate pieces, but to avoid over-complicating things, I suggest limiting yourself to whatever you can print in two pieces, and that is what this Instructable will describe.   For example, if you only have a 4"x4" print bed, then you should limit yourself to something you can make that is approximately 4"x8" or smaller.  That's what I started with, but it was enough to create shoes for my daughter who is nearly 5 . 

Once you've made sure you can actually print a sandal of a reasonable size, the first step is to have the person who will ultimately end up wearing the sandals stand on a piece of paper, so you or a helper can trace their feet (see image 1).  Make sure that you also trace between the big toe and 2nd toe, since that is where the strap will go (image 2).

After you've traced both feet, get out your ruler and draw rectangle a little bit (say 1 cm) outside your foot outline.  You want the sandal to have a little space around the foot.  Now convert the rectangle into a polygon by cutting off corners with the ruler until you have a shape that looks more sandal-like.  Six sides should be enough for something that looks reasonably good (image 3).

When you have something that you are happy with, double check that the same measurements work on the other foot.  If you find that one foot is smaller than the other, use the dimensions for the larger foot.  When you are done, you should have a simple shape that will serve as the outline for both the left and right sandals and the measurements you need to create in your CAD program.

Step 2: Check Your Work With a Paper Model

Before we make the CAD model, we're going to check our work with a paper model.  

Cut out a piece of paper in the shape of your sandal (image 1).  Have your soon-to-be-sandal-owner stand on the cut-out and make sure that it look right (image 2).  Flip the cut out and check the other foot, too.  Note there should be some room on the right and left side of the middle of the foot for the sandal straps.  

Cut a slit where you marked between the big and 2nd toe, and put the strap through (image 3).  Make sure that when the sandal's owner stands on your paper model, the strap is in the right spot.   If something is wrong, redo the previous step, make a new paper model, and try again.

Once the size is right, we're ready to find where the strap will go on the side of the foot.  If you are printing a 2-piece sandal, draw a line across the middle of your paper model to estimate where the boundary will be between the two pieces.  This is important for the next step.

Have the sandal owner place their foot on the model, with the strap between their toes again.  Drape the strap over the foot and bring it down to the side of the foot, where it looks like it should attach to the sandal.  If you are printing a 2-piece sandal, it will be stronger if you can make sure this spot is entirely on one piece or the other.  Mark your model to indicate where the strap should go.  Measure how long the strap is where it meets the sandal (i.e., how long the slit for the side strap will have to be), and how far from the strap to the end of the sandal (image 4).  Repeat on the other side of the foot.  Note that the strap will look best if it comes in at an angle, so the side strap slits will be longer than the toe slit.

Step 3: Time for CAD - the Basic Shape

Now that you've checked your sandal shape with a paper model, we're ready to recreate it in your CAD program of choice.  You can use something like Autocad 123D or Google Sketch-up.  All of the CAD work is limited to beginner-level skills, but if you are unsure of how to do something, you can usually find tutorial videos on on the Internet.

First we will recreate our sandal shape in the CAD program (image 1 below).  I find it easiest to use the ruler tool to mark out the dimensions and then connect the dots with the line tool.  Remember your measurements will probably be different than mine! 

For a nicer look, round out the corners of your polygon by using the curve or circle tool (image 2).  You can do this by making a curve or circle that is tangent to both edges leading into the corner.  I used circles of radius 5mm.   If you use circles, remember to delete the parts that don't contribute to the edge, as well as the old bits of the line that used to be the corner.   When you are done you should have a closed polygon with rounded corners (image 3).

Next, extrude the shape up.  I used a thickness of 5mm (image 4). 

Step 4: Making the Strap Slits

Using a ruler or calipers, measure the width and the thickness of your webbing (images 1 & 2).

On the top of your sandal model, measure out the line that should go between the big and 2nd toe (image 3).  Make a rectangle that ends at the deepest point between the toes.  The width should be 1-2 mm or so more than twice the thickness of your webbing strap, and the length should be the width of your webbing.  In my case, the webbing was about 4mm doubled and 17mm wide.  As a result, my toe strap rectangle was 6 mm x 18mm.  Extrude away the rectangle through the body of the model to create a slit for the webbing (image 4).

Now measure for the side strap rectangles (image 5).  I put them 3mm in from the edge.  Because my webbing was about 2mm thick, my slits were 3.5mm x 25mm (25mm, because that was the width of the webbing when it came in at an angle, per step 2).

Extrude away these rectangles through the body of your model to create holes.  You should now have three slits (image 5).

Flip your model so you are looking at the bottom, and the top of the sandal is face-down. 

We're going to extend the toe slit a little on the bottom only.  Make rectangles at each end of the toe slit that are the width of your slit and 5mm long (top of image 7).  Extrude away at least 1mm more than the thickness of your webbing (e.g., 3mm, in my case).  See the top of image 8.

We're also going to make the side slits a little wider.  Draw rectangles on the inside-edge of the side slits that  are the length of the slit and 1mm more than the thickness of your webbing (I used 3mm) wide.  See the bottom of image 7.  Once again, extrude away at least 1mm more than the thickness of your webbing (e.g. 3mm in my case).  See the bottom of images 8.

Step 5: If Needed, Split Your Model in Two

Skip this step UNLESS you are printing a 2-piece sandal.

Since you are printing a 2-piece sandal, we now need to split the model into two parts.  We want the model to fit together strongly, so while you can do a straight joint, I designed mine with something that looks more like a puzzle piece joint.

Measure to the middle of your model.  Either draw a line across, or if you want to make a puzzle fitting, draw a puzzle pattern similar to the one I used below (image 1).    Use the line tool to connect the dots (image 2).  Make sure that the total size of each part will fit within the boundaries of your print platform. 

Once you have something that looks good, copy your entire model so you have two versions (image 3).  Make sure you save before this next part, so you can always go back if you mess up.

Now we will delete one half of each copy, so we end up with two different, unconnected parts.  Depending on your CAD program, you can either "extrude away" or select and delete alternating halves on each model.  It may help to first delete the slits (image 4).  Then extrude away or delete the model half missing the slits.  After doing so, you still may have some extra lines that you have to manually select and delete (image 5). 

When you're done cleaning up the unneeded lines, you should check the edges of each half to make sure you didn't accidentally create any holes.  If you did, fix them (image 6) by drawing a rectangle over the opening. 

[OPTIONAL:  If you know your prints turn out slightly larger than what you specified in your CAD program from past experience, it may help to reduce the size of one of the puzzle fittings so that if you were to put the two halves of your CAD model together, there would be a slight gap.  To do this you can select each face of the puzzle edge and "extrude away" .1mm or so.]

When you are done with this step, you should have two parts, each of which is one half of your sandal (image 7).  If you make each into a component, you should be able to move them to make sure they fit together nicely (possibly, with a small gap if you followed the optional step above).

Step 6: Tread

Change your view so you are looking at the bottom of the sandal.  ABS is slippery on wood floors and similar surfaces, so we need to put some tread on the sandal.  We'll do this using some kind of rubbery substance (I used hot glue, but anything rubbery that adheres to ABS should work).  To give the rubbery bit a place to stick, we need to put some kind of tread on the bottom of the sandal.  You can use whatever pattern you like, but I made a zigzag pattern for mine.

Start by making a zigzag line across the bottom.  Copy and paste it a few mm away so you can create a channel (image 1).  Repeat over the bottom surface of the sandal, extending and trimming the zigzag pattern as needed.  When you are done, extrude away 1mm.  In the end, you should have a pattern like in image 2. 

If you are doing a 2 piece sandal, repeat on the other half.

Step 7: Time to Print!

Now we're ready to export the model(s) to STL and then print!   Many programs like Autocad 123D can Save As or export directly to STL.  Sketchup requires a plug-in.  If you've done a two-piece sandal, export each part separately and save it as its own file.

After exporting your STL files, open them in your 3D printing program (e.g. ReplicatorG).  The top of the sandal should be face down, so there are no overhangs (image 1).  If that is not the case, flip your model.  For example, in ReplicatorG, click "Rotate" and then two clicks of the "+X" button should flip the model over.  Then do "Lay Flat". 

Settings-wise, the default will probably work for most machines, but just in case, you'll want at least two solid layers before any infill.  I used an infill of 35% and a hexagonal pattern. 

Print it!  If you are doing a two-piece sandal, print both pieces (image 2). 

At this point we've only done one foot, but the second is simply a mirror of the first, so you don't actually have to CAD it up.  Your CAD or 3D printing program (e.g. ReplicatorG) should be able to mirror the parts for you.   For example, in ReplicatorG, while viewing your model, push the "Mirror" button, and then "Reflect in X" (image 3).  If that doesn't do what you expected, try undoing that and mirroring it in a different axis (your model may be rotated relative to mine).  Save the resulting file with a new name (and remember to do this for each part if you are printing a two-part sandal).  Print the other shoe.

Step 8: Join the Two Halves, If Necessary

SKIP THIS STEP UNLESS YOU ARE MAKING A 2-PIECE SANDAL

If you've done a two-piece sandal, before we continue on to the last step, it's time to chemically weld the two halves together.  First, we need to make sure that the two pieces actually fit together.  If they don't fit (and they probably won't), put the pieces one on top of the other as though you were going to put them together and see where there is overlap.  Use a small  flat file or sandpaper to take off plastic at the joint, a little bit at a time, until you have a snug fit (image 1).  I have found that filing the sides flat on both pieces is usually enough to make them fit.  Make sure you don't take too much off, as you want the fit to be snug.

When the two piece can be placed together, we will chemically weld them together, using acetone or a similar ABS solvent.   I used acetone because it is cheap and available in pure or nearly pure form from drug stores and hardware stores.   Note that nail polish remover with just a little acetone in it won't do the job - you need pure or something close to it.

REMEMBER THESE CHEMICALS ARE DANGEROUS SO USE CAUTION AND WORK IN A WELL VENTILATED AREA.

Prepare your work-space by getting something flat that the plastic will not stick to, such as glass or wax paper on a smooth surface, along with your acetone and an eye-dropper or something similar (image 2).

Put the two sandal pieces together so they are flush.  Using an eye dropper or similar mechanism, put a small amount of the solvent all along the joint and flex it to make sure the solvent works its way into the cracks (image 3).  Flip the piece and do the same thing.  The solvent should melt the plastic and it the edges should get mushy.   If you apply too much, wipe away the excess with a paper towel.  Make sure the pieces are flush.  Push them together for a minute so the plastic joins together and then place the parts on some wax paper or another flat surface (that won't be ruined or melted by the solvent) to dry.  I used a piece of mirror glass as shown in image 2.  If the pieces don't lay flat, you may need to weight them down, in which case put some wax paper on top and then the weight.

Wait for the piece to dry.  I recommend waiting overnight, just to be safe.

When you think the weld is done, flex the parts gently to see if the weld holds.  If it doesn't, try again.  Done right, the pieces should be firmly connected, as though they were made out of a single print.

Step 9: Adding the Straps & Tread

Fold the webbing in half so the two ends are together.  Pass them up through the bottom of the sandal at the toe slit, but leave a loop hanging below the sandal.  Take a small nail (or something comparably small and strong) and place it in the loop (image 1).  Cut it to the length of the extended toe slit on the bottom of the sandal (image 2).  It also helps if you snip off or fold over part of the nail head so it sits down into the hole better. 

Next, pass the ends of the webbing strap through the side slits.  You may have to adjust the strap so you only have lots of extra on one side to minimize waste when you trim the webbing.   When you have it close, you can pull the webbing taught at the toe slit as in image 2.

Have the sandal owner place their foot on the shoe and tighten the strap until they are comfortable, but able to hold the sandal on.  Once everything is positioned right, mark where the webbing should be cut (image 3).  Repeat on the other side.  Pull the webbing out of the side slits and trim it with scissors.  BEING CAREFUL NOT TO SET IT ON FIRE, melt the edges of the webbing with a candle or lighter just enough to prevent fraying. 

Put the webbing back through the side straps.  Finally, put hot glue all around the strap end, which should be just on the inside of the strap hole (i.e., not sticking out of the bottom of the sandal) (image 4).   Hot-glue the nail in place (top of image 5).  After the glue cools, test your work.  The straps shouldn't move when tugged.

The very last step is to add the tread.  ABS can be slippery on wood or similar floors, so it's important to have something that gives the sandal a little grip.  I used hot-glue, but anything rubbery that sticks to ABS will work.  Simply follow the tread pattern you made with hot-glue, and let it cool (image 5). 

CONGRATULATIONS!  You should now have a fully functioning sandal (image 6 & 7).

To keep it simple and easy, this is where we'll end.  However, you can keep refining your sandals by adding arch supports or additional "comfort" layers using silicone or leather or other materials.     
Cool . good job?
That is pretty darn cool! I really need to learn to design something for a 3D printer!

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