Tuesday, July 30, 2013

3D Printer Extruder Design (Part 2) - K-Head

As discussed before, the key to extruder design is in the thermo control. As I was trying to get the QU-BD extruder to work, I have kept on making improvements and changes to it. By the time I had finally had it working reliably, it is already a complete different design.

Here are some picture of the QU-BD extruder and how they look on my printer. It is actually behind a wood block that holds it on the default tool holder that comes with the ShapeOko CNC, so it is not clearly visible.



Here is the final working extruder on my printer. For the fun of it, I would call it K-head. It is held by an aluminum holder. Also, more details can be seen with the fan off on the second picture.


Here are a few most important improvements I have made to it:

1. The hot-end tube


The original hot-end of QU-BD uses a stainless steel tube. This is actually one of their selling point: a complete metal extruder. However, I find out that even though stainless steel has pretty low thermo conductivity, that is still way to much for a hot-end. Indeed, the majority of the filament jam is caused by too much heat is going up the hot-end tube. I have tried to put extra heat sinks on the tube, has drill out the inside to a larger size, and tried to print at high speed trying to get cooler filament into the melting portion. None works until I changed the hot-end tube to a different material.

The tube you see here in the final working K head is made of PEEK. Indeed, it is called "Self-lubricating Carbon-Filled PEEK" (1/4" D, 1 ft, 1595A11, for under $8.75). Because I don't own a lathe, I like it very much the factor that PEEK is much easier to drill and tap than stainless steel.

With the new PEEK tube, I can touch the aluminum holding plate and only feel warmth even when the hot-end is heated to and held at 210C for a long time. The PEEK tube not only have a low thermo conductivity, it is also self-lubricating which makes jamming very unlikely.

2. Thermo shield of the heating block

This is with the original QU-BD but it is not shown in their instructions. It took me a long time to realize what the white piece is for (it is a ceramic heat shield tape). And doing that greatly cuts down heat radiation loss, and makes the hot-end heating and temperature much more stable.

3. Aluminum filament tensioner

The original QU-BD uses a HDPE block with a set screw to hold the filament to the driving wheel. I found that hardly working. Maybe I am too cheap, and the filament I bought (from Amazon) has poor uniformity of diameter. There are a lot of designs on the Thingiverse on the tensioner for direct drive cold-ends, but it takes a working printer to print them. Even when I finally to get my printer work just enough to print a piece with PLA, I found out that it did not hold well as the motor is getting hot from driving current. So I finally just decided to make it in aluminum so it works under any temperature.

A small trick also used here which I learnt from one of the Thingiverse designs. The trick is to use binder clip instead of steel springs to supply the tension. Not only the binder clip is easily strong enough, it can also be held in place very easily by drilling small holes in the holder. The binder clips are very easy to find, which is another advantage too.

So here is the K-head that is finally working reliably for me. Through the process, I had a lot first hand experience about the extruders, particularly what works and what does not. The QU-BD extruder is really not a good design. It is not that theirs is not working. It might be, but with that design, there is a very narrow band of setting that it might work (specific filament, specific stepper motor, specific temperature, specific print speed, etc.) But real world reality is never always in the ideal case, and anything different will throw off the balance. In that sense, we can clearly see that the QU-BD is not a good extruder design.

My K-Head is a lot more reliable, and works in a wide range of settings. I will see if I can keep on making it better. For now, I will go print some fun stuff.

Friday, July 26, 2013

3D Printer Extruder Design (Part 1)

The extruder is the core of a 3D printer, because besides the extruder all that is left of 3D printer is a Cartesian Robot (i.e., some machine that moves in X/Y/Z space).

In the process of building my 3D printer, I found out that most of the time is spent trying to get the extruder to work (maybe that is because I have the Cartesion robot part working already from the hobby CNC). I started with a QU-BD extruder kit and thought it should be a straight forward process. But it is anything but. In the process of getting it to work, I have been replacing this and that of the extruder. And the end result is something that is completely different and new.

Below is a picture of QU-BD extruder. It looks nice, but as discussed below per my experience, is not a good extruder design.



The key issue of designing a good working extruder is thermo control, i.e., to control the heat. The basic of extruder is very simple. There are two parts: a cold-end and a hot-end. The cold-end is the stepper motor and corresponding mechanics that drives the filament. The hot-end is basically a tube, with filament coming in on one side, molten plastic going out on the other. The side on which the molten plastic goes out is fitted with a nozzle, so the plastic is squeeze to form the printed parts. But how to squeeze the molten plastic out? It is driven by pushing on the incoming filament by the cold-end.

Below is a diagram for the hot-end of J-Head, a popular extruder used in many 3D printers.



Because the tube is heated, so the incoming filament is getting heated while it is going down from top. As it is heated, it gets soften and also expand in size. Eventually, the filament is melt into molten plastic to be squeezed out. In physics and material science, this is called Glass Transition. Here lies the key of the extruder thermo design. If the filament is getting close to the glass transition too high on the tube, pushing on the filament will expand on the soften filament and push outward toward the wall of the tube. This creates a plug and prevented the force from doing downward to squeeze the molten plastic out. Thus created a jam.

To avoid these jam from happening, there are a few factors in play:

  1. Extruder temperature. Hotter set temperature would make the tube hotter, but it also makes molten plastic easier to be squeezed out.
  2. Thermo conductivity of the tube. More conductive material makes more section of the tube hotter.
  3. Feeding speed. Fast feeding can make filament going through the tube cooler.
  4. Heat shield and insulation. Radiation of the heat also plays an important role in thermo control.
  5. Other heat sources. The stepper motor can get really hot when run at high current.
  6. Cooling. Active cooling with a blowing fan.

A working extruder will find one of such point of above combinations for the extrusion to work continuously. However, a well designed extruder will have a wider range of combination and conditions for it to work well. In my experience, QU-BD extruder emphasizes too much on the all metal and turned out not be a good design. I will discuss in my next post what would be a better design of an extruder.



Wednesday, July 3, 2013

Converting CNC to a 3D Printer

I have finally got time to convert my CNC into a 3D Printer.

The process looks relatively simple on the paper: just replace the cutting tool head with an extruder, and optionally add a heated print bed. The extruder is the part that extrudes melted plastic to form the printed object. The heated bed is needed to eliminate warping of the printed object. Without the heated bed, the bottom layers would already be cooled when the top layers are printed, resulting in warping and lifting of corners away from the bed.

First there is a picture of the completed conversion, and a few printed object in the foreground. The extruder is behind the wooden block which holds it in place. The heated bed is the red piece on the bottom, and on top of it is a aluminum sheet with blue painters tape.


In reality, it is a great learning process, and it is not at all easy. This is my first printer, so I have a lot to learn about the extruder (which is the most critical part of a 3D printer). To get it to extrude smoothly is a huge battle, because I have no experience on where to look when things do not work (and it still does not work till today). But after a long struggle, and a lot of trial and error, I can say that I have a much better idea on the issues of 3D printer now, and how to solve them. I will talk about them in the next few days when I am still waiting for some parts to arrive in mail.

Here are a useful link about 3D printing.

On their website, they said that this is a "first general-purpose self-replicating manufacturing machine". I disagree with that statement. The machine only prints the plastic parts of the frame. It does not make any of the metal rods, the nuts, the bearing, let alone the electronics. That statement of "self-replicating" is a marketing gimmick. However, aside from that, there are tons of useful information on it, and a lot of smart people are contributing. If you need a place to learn about the in and outs of 3D printing, that is the best place to start.