Designed during the Hack the crisis Sweden hackathon on april 5:th 2020. 7th place in the 'Save lives' category.
https://www.youtube.com/watch?v=iNOUVH5OP8w

Hi everyone,
This respirator mask was designed with the intention to provide an open mask design to the 3D printing community. Our goal was to make it actually useful and reasonably comfortable for the wearer while at the same time having a great seal to the face. The mask is also designed to accept a number of attachments such as valves, filters or filter adaptors.

Update 2020-05-05
Added adaptors for the Honeywell N99 80mm filter and 3M bayonet filter. Excellent design by user dantbant. Thank you for your contribution :)
Added STEP files for the Large filter attachment, twin valve exhale filter and all versions of tall mask bodies. STEP files.zip is updated.

Major update 2020-05-04
Lots of new additions in this update which required the file names and file structure to be overhauled. Parts are now organized into zip files, while some commonly used models are still outside the zip files for convenience.
New mask shape added called Tall (old called Standard). The mask itself is a bit narrower and taller. Should reduce pressure on the nose and be more comfortable. Due to the different shape, the exhale port has changed which requires model specific exhale valve options. These are of course provided in the corresponding ZIP file. If you prefer the old shape It is still available as 'Standard'.
There are 3 variants for each mask shape: without exhale port, with one exhale port and twin exhale ports.
Inhale filters are compatible with all mask versions, shapes and variants. Two types are available: Standard and large, the large version has the possibility to attach a valve to block the exhaled air to escape through the inhale filter material.
Exhale valves and filters, these are specific to the mask shape, corresponding parts are included in the "Mask body xxx" ZIP files. Two types are available: Single and twin valves. The twin valve version has a filtering option for the exhaled air, making sure the virus is not spread through the exhaled air. The twin valve can only be mounted to the twin exhale port mask body.

There will be a new youtube video coming shortly which will demonstrate all models and functions.

Update 2020-05-02
Added Large_filter_attachment_v1.1_BETA.zip
The valved inhale filter attachment offer several upgrades over the standard filter bracket. It offers twice the filter area for easier breathability, large inhale valve which keeps the filter material dry, and a better (and more forgiving) filter material mounting mechanism.
Compatible with both Shell v1.0 and Shell v1.1 versions.
Edit: The valve membrane should be printed by itself to reduce stringing and with cooling fan after the first 3 layers.

Update 2020-04-29
The filtering system for the exhaled air is ready for beta test. All files and information such as assembly and print orientation can be found in Twin_exhaust_filter_v1.1_BETA.zip. The assembly is snap-in-place, all parts should be printed without support at 0.2mm layer height as usual. This filtering system is best matched with a valved attachment on the inhale side, that way the exhaled air does not go through the inhale filter at all. However, as it is now, using this filter with the cloth filter attachment, only about half of the exhaled air exits through this filter. Next on the TODO list is a valved inhale cloth filter attachment.
Please feel free to try it out, I greatly appreciate all feedback :)
Edit: Only attaches to the twin exhaust shell Shell_TPU_v1.1_BETA_twin_exhaust

Update 2020-04-28
Just checking in. The filtering system for the exhaled air is still in development, there are still some work to be done. Hopefully I can release a beta version this week.

Update 2020-04-19
Added Shell_TPU_v1.1_BETA_Collection.zip containing 3 variants of an updated Shell design. Variants: No exhaust valve, one hole for valve (standard), twin holes for twin exhaust valves. There are also some printability updates, mainly for the strap slits. Please let me know if you find any issues with the design, otherwise I will bring them out of beta next week.
Added Shell_TPU_v1.0.stp STEP file for the v1.0 Shell. I will add the 1.1v once they are out of beta.

Update 2020-04-12
Added Headband_accessories_v1.0.zip containing STEP and Fusion 360 project file for headband accessories.
Added Exhaust_port_blocker_v1.0.stl if you wish to block the exhaust valve port. This forces the exhaled air to exit through the filter material. This is useful if you are worried about spreading the virus by your exhaled air. Be aware, you may shorten the lifespan of your filtering material. There's a TODO below which address this by also adding an option to filter the exhaled air.

Material choice

Good fit and seal has been achieved by printing the main body out of TPU and giving the main body a wide and flexible lip that seals tightly to the face.
Valves and straps are also printed out of TPU, however, I would recommend a store bought elastic ribbon or rubber to be used for the headbands.

Components should be printed with the materials that are displayed at the end of each file name. You can also look at the pictures, TPU parts are white, PETG are gray.
Please pay special attention when printing and mounting the valve membranes, make sure no unintended holes and gaps are present in the finished part. Also make sure that the smooth side of the valve membrane is facing the valve housing. This way the valve will seal better.

Fit and Comfort

Some people experience discomfort around the upper nose area when wearing the mask, to alleviate this, you may make two small cuts to the upper seal. This is illustrated in one of the product pictures.

Filter material

There are many choices for the filter material itself.
Here's a great article here describing the filtration capabilities of household items:
https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/
Seems like vacuum cleaner bags are best at filtering the air but hardest to breathe through, probably a mask attachment with a larger filter area will make breathing easier.
If you would like to develop your own attachment, template.stl and template.stp files are provided.

TODO

• Evaluate softer materials, especially for the straps
• Add to the supported particle filters adapters
• Develop an attachment with larger filter area for easy breathability (DONE)
• Different sizes, maybe child sizes?
• Add functionality to filter exhaled air, looks like the virus can spread even if one is asymptomatic (DONE)
• Exhaust port v1.1 as suggested by skl111
• Add second exhaust valve option (DONE)
• Add exhaust valve cover option (DONE)

All feedback is greatly appreciated, I will try to keep all parts up to date in accordance to community feedback.