Evaluation of members on the printability, utility, level of detail, etc.
3D model description
This is a motorized solder paste dispenser for soldering SMT boards that was inspired by this thing http://www.thingiverse.com/thing:384680. The home directory of this project is on github here https://github.com/zapta/misc/tree/master/paste_injector and it include hardware and firmware design of a compatible controller board that is Arduino compatible (it looks to the Arduino IDE as an Arduino Pro Mini).
I printed successfully using both PLA and ABS and standard slicing parameters (3 layer shells, 20% infill).
Hints, in assorted order:
When printing the motor axis coupler, make sure to have your extruder temperature high enough for a good inter layer bonding.
The step motor listed in the BOM is available on eBay from many sellers. They typically come with a ULN2003 driver PCB which is not needed here.
When mounting the coupler on the motor, try to minimize the free play along the motor axis. I used a a sharp knife to pull out the motor axis while pushing the coupler toward the motor before tightening the coupler screws.
You can insert the threaded inserts using any standard solder iron that fit into their holes. Preferably you will get tapered inserts similar to the ones specified in the BOM. If not, tapper them using a file or a Dremel tool. This is especially important for the insert that goes into the plunger rod. Otherwise, the internal hole hole will get clogged and the M4 threaded rod will not fit in.
When inserting the threaded inserts, don't push them too hard, let the heat do most of the work. This will result in better adhesion.
I programed the controller board using an AVRISP MKII but you can use whatever works for you. Github has the binary image file that include the Arduino bootloader and a script that programs the AVR fuses.
If you want to hack the firmware of the controller, solder a 6 pin header at the pads marked with FTDI and use an adapter such as FTDI Basic to hock the controller you your computer with the Arduino IDE. If you come with interesting firmware updates (e.g. having the red LED blinks proportionally to the speed), send me pull requests on github.
Use thin and sticky solder paste. For example Chip Quik T5. If the solder paste doesn't stick well to the PCB, dilute it as needed with IPA.
Avoid air bubbles in the syringe. Air bubbles makes the rate flow control more difficult.
The potentiometer controls the speed of the PRESS button only. I got the best results buy setting it on low and achieving a very slow rate of flow and then touching the PCB pads at a fixed rate to achieve consistent drops. This requires some practice.
The potentiometer is linear but the firmware does logarithmic mapping to allow very fine control at low speeds.
This design is specific for the BD syringe listed in the BOM. Most likely others models of syringes will not work. It requires specific dimension and rubber plunger dimensions and was design to achieve high friction between the rubber seal and the syringe's cylinder to avoid counter rotation. If you notice excessive rotation on the plastic plunger, clean the rubber seal with IPA to increase friction.
The controller board has a micro USB connector and operates from a standard USB charger.
I got good results with blunt bend 22GA Luer Lock needle. I got mine from ebay. If you want to experiment, buy an assortment of needles and see which one works for you.
After attaching the bent needed, you can orient the needle in the preferred direction by loosing the two syringe holder screws and rotating the syringe.
I use the dispenser with two hands. Right hand holds the syringe and controls the needle while the left hand presses the PUSH button as needed.
I attached the PCB to the motor mount using a Scotch Exterior Mounting Tape. This is a thick, heavy duty, double side tape. You don't need to cover the entire PCB, just a few small pieces will do. Make sure you have tape under the two buttons for extra support.
The plunger model has a small conical piece that fits at the bottom. Just glue it there before inserting it into the rubber seal. It was separated into two pieces to allow printing without material support and it eliminates the air pocket below the rubber seal (remember, air bubbles are bad).
Solder paste last longer when stored in a cool place. I place my dispenser in a plastic bag and put in the fridge (not the freezer). If you don't want to place the motor and electronics in the fridge, not problem, remove the two screws of the syringe holder, and turn the syringe to screw it out of the motor mount (don't pull the plunger out, this may insert air to the solder paste chamber).
When storing the solder paste, it's a good practice to remove the needle and close it with a Luer Lock cap, you can buy them everywhere, including amazon and ebay.
If you keep using the same needle (I do), make sure to clean it after each usage. I flush it using a syringe filled with IPA.
The PCB currently currently on github (as of Nov 2015) uses a 'reversed' micro USB connector. This is for no specific reason, just because I have them from a previous project. Make sure to get the connector listed in the BOM or feel free to modify the PCB for your preferred micro USB connector.
If you are looking for PCB manufactures for your controller board you want to check prices at http://pcbshopper.com . This is 30x40mm two layer board.
I glued the two washers to the syringe holder using super glue. This way they don't fall off when removing the screws. Not a big deal though.
The 3D models were design with Openscad. This is an open source 3D programming package. If you want to tweak any dimension, load the provided .scad file in Openscad and regenerate the .stl files.
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