June 18, 2014

3D Printing - Step Two - The Model

The printer is now ready, what's next?

Part of my brief was to produce a spider like character called Stealth with 4 legs and wearing a baseball cap. I was given a simple model example and I started from scratch in Blender, I gave myself a 2 hour window to produce a prototype.

So you could just launch into knocking up a character and hope that it prints out... for example, you write a letter in Word and don't really care what printer it comes out on, they all produce the about the same stuff right? Well, that's fine for inkjets and laser printers, but it couldn't be further from the case for 3D printing.

Imagine the letter 'T'.

All additive based technology printers start printing at the bottom of the model. No problems there, but what happens when you get to the arms of the T. Selective Laser Sintering (SLS) has powder from the previous layer so no support is required in the model. Fused Filament Deposition (FFD) based printers cannot just print in mid air and the plastic coming out of the nozzle does not cool quick enough to support itself horizontally. Some form of Support is required. (In this case you could also turn the 'T' upside down and print... maybe a bad example, but you get my point)

Imagine the letter 'V'.

Starting at the bottom the sides move out pretty gently. Is supporting material required here? The answer is "Depends on the printer." The Ultimaker can print 30 degrees to the horizontal without support. A few other printers struggle.

Imagine the letter 'C'.

The bowl at the bottom is a lot less than 30 degrees, and the top right overhang starts in mid air, but there is a part of the letter directly under it...

Luckily all slicers (the software that prepares your model for printing) have some form of support mechanism that you remove from the finished print. Some are good. Some are bad. Out of the box, Cura (the software the Ultimaker uses) is BAD. With some tweaking to the gaps, supports are acceptable.

So what has all of this prepared us for in the realms of modelling a character for 3D printing. Well you need do do one of 2 things.
  • Develop your character and worry about the technology required to print it later - for example minions.
  • Develop your character taking into account the technology you will use.

So my spider.
  • The body is a flattened sphere. The underside will need support. If I make the sphere flat enough, the slicer can handle that.
  • The mouth. Keep it closed for now.It's just a feature on the body then so make sure it doesn't stick out too far.
  • Pincers can be made and the slicer can cope with supporting them.
  • Legs, these can be steep with flat bottoms for now.
  • Baseball hats have brims, I'll need to tip this back far enough back to not need support.

So, after my 2 hour design-fest version 1 ended up like this:

Primarily the issue with the print was due to adhesion problems with the glass and only a small bit of plastic touching it. Eventually it came away when the print head touched it and the print head basically extruded plastic randomly.

The fix is simple. Lower the model into the bed providing a flat bottom so a lot of the body touches the bed. This way I don't need any support for the legs either as they ar steep, and I can put the pincers on the print bed as well. Version 2 ended up like this:

Basically the legs fell over. So I would need to implement my knee joint sooner than anticipated. So chop off the legs and add a ball joint for the knees. We got a full body that looked like this:

Final tweaks to the body were to the mouth - there are still some overhang issues, to the eyes - the pupils need to stand out a little more, and the hat - needs tipping back some more.

Now I had to work on the knee joint. A scan round the web provided several inspirational moments, so I started with a simple 3 pin holdery do dah with the ball joint removed (plus some space). Initially there were several issues with this design, not least of which the PLA plastic was too brittle, but also the printer had issues printing such a small layer and so the pins were rough so the ball would not fit in the socket properly.

I made the gap between the ball and the socket bigger but the pins were still snapping most of the time.

I added some strength with a sheath, but there was still issues with the protruding pins.

I changed material to PLA/PHA and still the pins snapped.

I changed the design of the socket to be a sphere that wraps round the ball. This made the print less rough so once I got the right amount cut away, the socket was now too loose and the leg flapped around... decreased the joint gap... Ta da.

Here is a brief history of my knee joint journey, overall there were 11 iterations of the joint.

So the first full scale print. Well, the body worked fine I knew this, but 4 hours into the 14 hour print and the legs came free from the glass. No use I had to get the slicer to add some supports to keep the edges of the print from lifting... plus some gluestick for good measure - I was up against a time pressure by now. This is what I had 14 hours later:

Apologies for the colour changes, Camera phones are not all that great for product photography.

In Summary

I have basically been though the design of a model for the purposes of producing a final product and prototyped it, iterating though the major bugs. The next step would be to add some colour and send it to somewhere like Shapways to get it printed. BUT. Remember my word processing analogy at the top of this post. One of the cool things here is that the knee joints provide enough friction to hold the models weight, while being flexible enough not to break when clipped in place. Sending this to Shapeways will change these characteristics because the material and build process would be different. Iteration may still be required.

However, I went through 4 body iterations, and 11 knee joint iterations locally, so the cost and time of producing this product has been significantly reduced. Total time designing was about 16 hours, total time printing was about 35 hours, total plastic used was less than a single reel. Compare this with 15 trips to Shapeways at €8.50 per delivery, plus various quantities of plastic, and a 2 week turn-a-round for each iteration.

Coming up in the next episode...

I have 3 areas I want to take a look at.
  • An explanation of the bits that make the printer work. the terms and settings - way tecky, few photos... probably helpful but a little boring.
  • The more philosophical topic of what 3D printing holds for the future, and my thoughts on a lot of people saying that 3D printers will be in everyone's home in the future. This may be boring and argumentative.
  • Scanning and art - This will be a while as I need to meet up with a few people and get some scanning going.

So watch this space, comment if you have a preference or want to know more :)

Edit: Selection Made: Step 3, Lessons Learnt.

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