This micro dot dispenser delivers 50 micro liters per button push. It was designed for the application of discrete known volumes of cyanoacrylate (CA) glue during medical device manufacture.
To ensure repeatable results during manufacture, we needed a dispenser that would deliver the same small quantity of glue each time. Although many commercial options do exist, they are often expensive and rely on pressurized air for actuation. I wanted to design a small hand held dispenser with a simple mechanism that could interface with a standard 2 ml syringe.
How I Designed this
First I used my calipers get the critical syringe dimensions and made a basic model, I knew that I wanted to interface with the finger tabs on the back of the syringe body so I made sure to get that profile exact.
Then I designed the plunger rail and an actuator rail to match, I made sure to space them in such a way that there would be sufficient room for the housing.
Next I sketched a profile for the housing and extruded it along the length of the syringe body.
Next I capped off the front of the housing and added an extra groove for the actuator rail. Also added two extrudes at the back and cut a slot in each, these guide the actuator rail during actuation.
I then made a cut out at the back of the housing to match the profile of the finger tabs on the syringe. I added two small circular extrudes to hold the syringe in place once it had been pushed into the housing. I added some chamfers for ease of use and some fillets for aesthetics.
I made some changes to the actuator rack, including a cylindrical extrude at the front which I used to center the spring, and a beam to attach the push button to.
A cross section of the final assembly is shown below. When assembling the dispenser, first drop a 4 mm OD spring into the housing, there are cylindrical extrudes on both the housing and the actuator rail which fit into the center of the spring (I used one from an old pen). Then the actuator rail (red) can be slid in from the back until the tabs at the back click into the slots of the housing. Finally the push button can be pushed onto the the actuator rail. After that, just insert the syringe and make sure the plunger mates with the plunger rail.
I was really happy with how this dispenser turned out, the fits and tolerances worked perfectly, and the over all feel was great. There were some issues with dispensing though, and I would include a ratchet on version 2 to prevent the piston from pushing back, also the CA I was using was not very viscous so there was a small amount of leakage after each application. The next iteration should also include some kind of draw back mechanism, i'e two steps forward one step back, this would hopefully produce a nice clean drop with no leakage afterwards.
For these parts I used formlabs Grey Resin V3, it has a great surface finish and I've found the dimensional accuracy to be pretty high. I used 0.6 mm support points with a density setting of 1. There were some internal supports on the housing but they were easy to remove and I filed the inside with a round file before curing.