It happens sometimes you are ready for a wild session of DIY... and because of an unfortunate deadly global pandemic you find yourself without a decent dust mask. You are quarantined. All the shops are closed.

Well, can this stop us? Of course it can't! We are going to build one from scratch and from scrap, using just a handful of materials:

soda bottle (2L Pepsi in my case)
6 mm PVC tube
cosmetic cotton discs
rubber band or string
3D printed parts (see attached STL files)

And just a couple of tools:

3D printer
scissors
scalpel or sharp knife
glue

The idea is to use the top of the bottle as a dust mask and design a 3D printed screw-on filter. But... what kind of thread do soda bottles have? After asking the question to our Lord and Savior (Google) an answer came through: the threads are standardized by the International Society of Beverage Technologists (ISBT), and a list of lots of thread specs is available at ISBT's page. Warning: the thread specs are extremely detailed!

The thread on my bottle is called PCO1810 (also PCO28) and is the older style. Outside diameter is 27.43 mm and pitch is 3.18 mm. It could be you have a bottle with a more recent thread called PCO1881 (lightweight and lower profile). If this is the case outside diameter is 27.4 mm and pitch is 2.7 mm.

Based on these, I was able to design the part I needed and got a really nice fit on the thread. Although the thread profile is specified in detail in the PDFs (root and tip radius etc), I just used a trapezoidal thread and it fitted fine.

Enough talking now! Down to business!

Step 1: Let's Build It!

Building the mask is easily done in few steps:

3D print the required parts (adapter, cap and holder)
glue the adapter on the cap and wait for the glue to dry
position the cotton disc in the holder and secure it on the adapter (it's a bayonet mount)
screw the assembly on the plastic bottle (you can cut the bottle to adapt it to the shape of your face by trial and error)
cut the PVC tube to match the length of the free edge of the bottle
cut the PVC tube along the axis, and fit it on the bottle edge. In this way you'll avoid cutting your face. Hopefully... :)
pierce some holes near the edge of the bottle, insert the string or (better) the rubber band and tie a knot to secure it in place.

...aaaaand, it's done! :)

It’s important to remember that the majority of us aren’t medical professionals, and that even the most well-meaning efforts can end up making things worse if they aren’t done correctly. Printing out an untested design in a material that was never intended for this sort of application could end up being more dangerous than doing nothing at all. A professional and certified dust mask will always be better than a self made one.

That being said...

...find a head and to fit it on!

Should you find this design useful please feel free to share it, modify it and improve it. Some modifications I'd like to try are: