T-REx Pen

Transparent Rapid Extrusion Pen (free hand stereo-lithography)

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This project is about combining innovations in cheap computing with free hand rapid prototyping to create something new: a way of quickly and precisely creating 3d artistic objects out of transparent and flexible materials.

If you are a maker then you have doubtless heard of free hand rapid prototyping projects like the 3Doodler and Lix. The problem with these products is that it is difficult to get nice smooth flowing structures in 3D. While the Lix greatly improved the form factor, it also carries a more premium price tag and still has the same problem that if the speed you move your hand through space is too fast, the part will sag or become too thin because the plastic hasn't had time to cool or the pen didn't output material at the right rate. Additionally, both pens need to be continually refilled with filament rods to continue operation. This distracts from the creative process.

Enter the Transparent Rapid Extrusion Pen. The T-REx Pen solves the above problems by moving away from FDM style rapid prototyping and into the world of computer assisted free hand stereolithography. The T-REx Pen uses a Raspberry Pi Zero and two webcams to precisely track the position and angle of the pen in 3-space. At the tip of the pin there is a ring of UV LEDs to activate and nearly instantly harden the resin. A thin tube pumps resin from a reservoir to the pen by means of a peristaltic pump controlled by the Raspberry Pi. A button on the side of the pen activates drawing functionality. When drawing, the Raspberry Pi senses the rate the pen is moving and adjusts resin output speed and curing LED brightness accordingly. In this way, no matter how fast or how slow you choose to draw, the pen will keep up with you (within maximal curing speed and assuming no acceleration is faster than the camera can handle of course). All you have to do is press the button and draw at your own pace. Because no resin or motors are in the pen body itself, the drawing instrument can be kept sleek, thin, and well balanced. Also, because the material the T-REx draws with is resin, smooth transparent structures can be created with hardly noticeable seams. Because a computer is watching what is happening, potentially it could auto turn deactivate the pump whenever you leave the region of a predefined model or provide tactile feedback when you approach the boundary. That is an idea for the future though as the initial prototype concept still needs to be proven.

If I win a Raspberry Pi Zero I will personally develop the project with regular updates. This project has a target budget of $60 and I plan to open source everything at the end.

  • 1 × Raspberry Pi Zero ($5.00) If I can get my grubby hands on one before christmas
  • 1 × Stepper Motor Controller Breakout ($22.50) Adafruit DC & Stepper Motor HAT for Raspberry Pi - Mini Kit. Have an equivalent breakout for arduino and might be able to make it work with pi but this is more convenient
  • 1 × Stepper motor ($14.00) Stepper motor - NEMA-17 size - 200 steps/rev, 12V 350mA
  • 1 × 3D printer for structural components and pump or could machine them (your choice, ~$4 plastic cost)
  • 1 × Aluminum chunk (free) To be made into pen body. Found for free in machine shop, could be wood from a stick in the yard in principle

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  • Ordering things

    Amos Manneschmidt03/07/2016 at 22:56 0 comments

    A Raspberry Pi Zero and some UV LEDs are now on order. I will commence proof of concept tests with a syringe, LEDs, and some resin when they arrive. The motion tracking part will be developed separately as well as the pump. Currently waiting on the arrival of my 3D printer (Peachy) to construct the pump. The pump I've decided needs to be made out of two tracks of tubing with 3 rollers each, positioned 90 degrees out of phase so as to smooth out the natural pulsations one typically finds with peristaltic pumps. The body of this I plan to 3D print but if my printer takes too long to ship I will revert to machining it out of wood (to keep costs down). If pulsations are still too strong after this I will add a chamber with a permanently trapped air bubble of adjustable size and this will make it even smoother at the cost of adding a minuscule lag in changes to flow rate.


    I suppose I should have done more research before starting this project but it appears that there already is a UV pen called the CreoPop. That said, I believe this project is still very justified as the CreoPop appears to have the same refilling problem that the Lix and 3Doodler have as well as a similar (slow) draw speed. I suspect that they are using LEDs whose peak wavelength doesn't quite match the peak curing wavelength. Additionally I plan to use more LEDs to cure faster. If the resin still doesn't cure fast enough at max power, there are very catalytic accelerators that can speed it up more. See a demonstration of CreoPop below.

    Ideally I would like the maximal draw speed to be about how it appears in this time lapse at the 5 second mark. Because this is a time lapse, the T-REx curing speed needs to be about 4 times as fast. UV resins undergo exothermic reactions so speeding up this process means that I definitely cannot claim that my resin will be "cool." In fact it likely will be quite hot. Stay tuned for more updates.

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