Initial Extruder Design

Extruder Design

As mentioned above, a primary concern for us is to keep things inexpensive and affordable. Hence we are developing a dispensing system based on using air pressure, as air compressors are commonly found in art studios for air brushing and related techniques. This is the approach of many of the groups cited above. Rather than simply using air pressure (below top), more instantaneous control is realized through a syringe feeding an auger screw, as shown in the schematic. (below, bottom)

 

Sophisticated versions of this concept are used widely in industry, but their cost is 2-3x that of the printer itself. Instead we utilize the poor man’s approach to auger based dispensing. The TS5000DMP sold by OK International is shown below along with a partially disassembled unit .

 

The “DMP” in the name stands for “disposable material path” and that is what the three plastic assemblies are in the picture below. They are 3 different Auger screw sizes that can be used in the TS5000. They are relatively inexpensive ($25 each), so they are disposable in the view of microelectronics manufacturers but not for starving artists.

 

The plastic parts of the DMP are intended for extruding polymers, and not hard and abrasive ceramics. Finely ground slip systems will help to extend useable lifetimes, but the plastic DMP will wear out over time and need to be replaced. Unfortunately, auger components compatible with ceramics cost thousands of dollars (e.g. see ceramic augers here ).

The key to adapting the expensive auger technology for inexpensive ceramic printing is the plastic DMP unit. A primary component of the slurry adapter is a housing to support the DMP unit. This is available at thingiverse as designed by Dries Verbruggen (below, top ), with a version we printed also shown (below, bottom).

The other needed component is the auger drive/stepper motor adapter (also available as part of thing:28018) to connect the stepper shaft to the square drive cutout on the auger.

We ordered all the syringe parts from OK International. They are the only place with the DMP assembly, so we bought everything else from them at the same time.

1) DMP8-10; 10 pack – 8 Pitch DMP with fixed black Luer collar. We went with the 8 pitch unit. The DMP parts are sold in a 10 pack ($250 – ouch!)

2) 73003RHB ; 700 Series, 30/55cc Receiver Head Assembly, 3FT Hose, $19.40

3) 7300007BLPK ; 700 Series Easy Flow Piston, 30/55cc, Blue, (qty=50), $25.47

4) 7550LL1NPK ; 700 series syringe barrel, 55cc, polypropylene (box of 50), $53.26

5) TN00DKIT ; Dispensing Tip Kit, $50.39 – this is quite useful to try a variety of tip sizes

6) TT20-DHUV-PK ; Taper Tip, 20 AWG x 1.25″ LG, Rigid, Pink, (qty=50) $21.02

7) TT18-DHUV-PK ; Taper Tip, 18 AWG x 1.25″ LG, Rigid, Green, (qty=50) $21.02

There are cheaper places for items 2-4, and you don’t need 50 syringes, but we kept things simple since we had to get the DMP from OK International. Items 5-7 are various tips to control the extruded diameter. We bought a variety for experimentation. Again, one can save money by being more selective. The total for the above was $440.56. Cost could be reduced by going with cheaper sources for all but the DMP.

A separate stepper motor was purchased for use in the slurry extruder ( Nema 17 (part 25253-01) motor for $16.94 from inventables.com). It did not have a flat on the shaft, so we ground one to match the coupler. It also lacked electrical connectors so we made a cable to connect the stepper with the Rambo control board used by the Rostock Max.

The effector plate design (below, top) was slightly modified by adding a couple of holes towards one corner for the ceramic extruder assembly mount. We went old school for this and just drilled in a couple of holes after sizing the layout. The picture (below, bottom) shows the motor and syringe mount attached to the effector via the standoffs.

The purpose of the mount standoffs is to minimize any reduction of the print radius that might result if the extruder assembly collided with the arms. With this location and standoff height we can keep the extrusion nozzle near the center (displaced slightly from the center location of the plastic extruder) and not affect the printer radius. With the magnetic effector mounts, it is easy to pop the plastic extruder assembly off and replace it with the ceramic unit. When this is done the extruder-appropriate firmware (plastic or ceramic) also needs to be loaded. The only modifications to the extruder from the Unfold design are the vertical standoffs and the plate with holes (one round, one square) to fix the syringe to the stepper as the proper distance for the DMP auger assembly. During printing operation, syringes can be replaced by just twisting off of the Luer lock on the DMP auger. The assembled unit is shown in the pictures below.

 

 

Some initial prints using the ceramic extruder are shown below. The red print surface is a piece of vinyl (the backside of a ceramist’s vinyl texture mat) that is placed over the glass and heated bed. Hence the print surface height for ceramics is slightly different that for plastics.

 

An 18 gauge needle (Tech-con 918025-TE ) with a 0.84 mm ID was used with a layer height of 0.5 mm. These parameters and the slip consistency/viscosity have yet to be optimized.

 

A view of a pyramid through the effector during printing showing the inner strut structure is shown below

The initial printing attempts used Von’s White Stoneware slip (thanks to Chad Hartwig for this) consisting of:

Edgar Plastic Kaolin    27

OM-4 Ball Clay        26

Custer Feldspar        17

Nephelene Syenite    4

Silica            26

Conclusions to date (August 9, 2013)

The extruder functions nicely but it is slow. To print large objects larger volumes of clay need to be extruded at a faster rate. One means of increasing the rate is to change the stepper motor. The motor presently being used is a typical 1.8o/step; 200 steps/revolution unit. To increase this by a factor of 2 we ordered a lower resolution, higher speed stepper motor. We located the Wantai 39BYGS202 (3.75o/step; 96 steps/revolution) at Green Energy Research for $17.55. The same motor is also available from Phidgets. Selecting a 6-pitch auger rather than the 8-pitch would also speed up delivery rate.

Neither of these steps will lead to the kind of rate we think is needed. Before we admit defeat and go augerless and rely only on air pressure, we will take a shot at building an extruder based on a drill bit. This concept is widely used for filament extruders (e.g. thing 83570 and thing:34653). An attempt at making a paste extruder has also been reported by a group at Bauhaus University Weimar. It is not clear if they ultimately had success by this route.

Initially we will base our design on a ¼ bit and a brass compression tube fitting right angle run tee. We will post an update as we progress.

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