It presents a major technical problem : how to reproduce the screw pitch precisely enough so that it can match the screw pitch of the vial?
Thus we had to determine the dimension of the screw pitch. It has been performed by reverse engineering. We have cut a cap in four, being careful not to crush the shape of the screw pitch. Then we measured the different dimensions with a caliper.
Here is the industrial drawing of the whole part with a thickness of 0.5mm, edited with LibreCAD :
Industrial drawing of the double cap, with a zoom on the part of the connector (thickness 0.5mm)
The next step was modeling the cap, with Blender. We started designing the screw profile and we extruded it in a spiral way. Then we modeled the rest of the cap around the screw pitch. We got one cap we can duplicate by mirror. The last thing to add was the connector between the two caps, respecting the same dimensions previously defined.
This is a view in section of the modeled double cap rendered from blender, where we see the screw pitch and the connector :
Here is also the .stl file of the whole connector, with a thickness of 0.5mm : Doublecap_0.5mm
Then we printed it in 3D with the help of Sam at the Hackuarium. This time we used ABS and here are the characteristics of the printing :
- Temperature : 235°C (for ABS)
- Material : ABS filament (1.75mm)
- 5mm nozzle
- 190*190 bed (temperature 70°C)
Printing of the whole double cap (step of the connector printing)
The major fear of printing this piece was the basis of the connector part. It is printed vertically and the 3D printer must create it without support. Finally, it was not a problem. The major problem came from the temperature of the piece. Because of the direction of the printing, the first layers are in contact of the bed at a temperature of 70°C, but the upper layers are in contact to air and the ABS tend to retract. So the diameter of the whole piece reduces with the height. Thus it is more difficult to screw the vial on the side with the connector, which becomes more complicated when the vial is protected by a seal. So the solution was to increase the scale of the printed piece by a few millimeters.
The positive part of this printing session is that the screw pitch matches the vial one. Finally we succeeded and we get a double cap with two sides that we can screw on the vial.
So we can test it in real conditions. As expected, the seal is pierced by the connector after starting to screw and the two vials communicate by the connector. The double cap assures to return the device in a single piece to the lab to destroy the GMO present inside.
Here is the result we get :
The next goals are :
- Find a solution to seal the vial
- Improve the double cap, by printing it more precisely and adding to it rubber gasket to make it more hermetic.