I made dozens of Montana Masks, here is what I learned
By: Robert Hirsch
Tags: Mask, 3D Printing, Covid 19

The 3D printable Montana Mask

My sister works as a nurse in a hospital in Los Angeles. Back in February we started talking about COVID-19, the reality of it, the possible effects and the preparedness of her hospital and hospitals around the nation. After talking with her, and the lack of urgency her hospital administration was displaying, it was clear to us that pretty quickly face masks and other PPE were going to face a supply and demand problem that would lead to price hikes and rationing. Worse, the situation was made much worse by state and federal governments interfering with supply chains, directing supplies to their reactionary whims and jacking up the prices. Meanwhile hospitals, deprived of their normal supply chains had to make do. Word, finally came in, my sister was going to have to suffice with one mask per day.

Could she use more if she supplied them? No. But at least she could use her own masks when not on duty, when in the street, going in stores, etc etc. So thanks to the wonderful contributions of people around the world, there are a huge variety of home made masks that you can make yourself, or you can get from people who are making hundreds of them. For me, my sister linked me to the Montana Mask project.

This article comes in 3 parts. Printing the Mask. Assembling the mask. Using the mask. Feel free to skip to the part you want.

Personal Limitations for Making Masks

Since I moved, I no longer have access to the best maker space in the country. Further, I live in Puerto Rico which had the strongest shutdown in the America, so Home Depot, as well as every hardware store, was not accessible. So I could only get materials through Amazon, which took an extra long time with limitations of what I could use (everyone was trying to get elastic cord for securing masks). So here was what I has to work with

The mask had to be able to be printed on my small 3d printer, the MonoPrice Mini Select V2.

The solid model of the file had to be complete with no modification, as my CAD software, can’t import STL files for modification (and I am jealous of people with better CAD that can).

All gaskets, headbands, filter material and accessories had to be able to be obtained through Amazon with a supplier willing to ship to Puerto Rico.

With this in mind let’s talk about printing.

Printing a Montana Mask

Just a few of the dozens I printed

Each unit has a mask and a filter holder. It was tempting to print masks first, then a bunch of filter holders. But I found that, because I was printing different sizes it was actually best to print them together. Further, its slightly faster to do it this way. I use Simplify 3d for slicing. Here is what my standard file looked like:

Setting up for a mask and filter holder print

Due to the size of the MPMSV2, there is limited space, so the mask has to be placed diagonally on the bed. I positioned the filter holder just above the mask bottom and used only supports that touched the bed. This worked great, and the supports that Simplify generated are so slim and easy to remove, I was able to get consistent parts print after print.

Some of the settings for the print were:

THREE top and bottom layers (important)

One perimeter shell

First layer height 200%, first layer width 90%, first layer speed 25 (this obviated the need for a raft

infill 25% (but I think less could work too)

I used two different processes, the only difference was that the filter holder process included support columns.

After this, it was pretty easy to print different sizes. I simply scaled the mask and the filter holder to 90% and 75% (for my youngest) and everything printed perfectly. However, I found that the 100% size was still a bit too small for my face and I really wanted a print at 105% size. However there was no face down orientation I could use that would fit the mask in this small printer. So I have to angle it upwards.

This is how to print a 105% sized Montana mask in a small Monoprice Mini.

Unfortunately the bottom surface of the mask isn’t as pristine and smooth as all the other masks. I could sand it down and paint expoxy resin on it…but who am I kidding? I’m not doing that for a mask.

The end result of printing:

The masks came out pretty good overall. There was some slight balling around the surface, a razor blade took care of most it it. So most of the masks were entirely usable. Some of the observations I noticed:

The 75% sized mask was way too small, that size might be fine for a 4 year old. I have a 10 year old. The 90% size works well for 10 and 13 year olds, at least my 10 and 13 year olds.

The filter holder fits perfectly every single time without any modification.

The three top and bottom layers are crtitical. Otherwise there are holes outside of the filter area. See?

If you dont print at least three top and bottom layers you get pinholes throughout

The eyelets for the elastics are simply too small. There was no reason they needed to be that small, and I see that other people who modified this design actually fixed that part. Here is one example:

From this article. Also, again, jealous that his CAD could modify the STL file.

Assembling the Montana Mask

There are three things that need to be done to prepare a mask for use. First, filter material needs to be obtained. Second, gasket material needs to be obtained and applied properly. Finally, the method by which to affix the mask to the head needs to be decided upon and implemented.

The Filter

There are a variety of choices for the filter material. You can cut up an N95 mask. You can cut up a surgical mask. Here is a summary of two dozen materials you could use for filtering. Bandanas are basically useless. I chose to use a car cabin air filter, and cut it up. I mainly chose this because it is designed for a low pressure drop (unlike a vaccuum bag), and it is not made of paper or cotton which are hydrophillic materials (water loving). A hydrophilic filter will get harder and harder to breath through as it gets wet from your moist breath. This should give me 99% protection in the 1 micron range and somewhere in the 70% range for 0.3 micron. Its no N95, but its far better than what I see most people using. Here is me installing the filter material.

How to install the filter

The Gasket

The image of the mask at the start of this article shows the gasket made of closed cell foam. That’s fine. But I wanted something a bit more flexible and that would perhaps comform better to provide a better seal. I found some weatherstipping on amazon. It features a pressure sensitive adhesive, is very conformable, and quite cheap. It ended up being perfect.

The gasket is installed starting from the bottom (trust me), and you slowly run along the edge of the mask pressing the adhesive as you go along. At the end, you cut it off slightly longer than the remaining space and sort of smoosh it in. This makes it so that there is no gap at the bottom. Here is an example:

This gasket is quite comfortable.

At this point you can try out the mask for fit and to see if there are gaps when you breath. MY wife found that the bridge of the nose of the mask was way too tight for her. There is an great solution for this and I can’t beleive how well it worked.

Boil some water. Remove the gasket (1.2 minutes of hard work destroyed!). Dip the nose of the mask into the boiling water for 10 seconds. Take it out and spread it apart as neeeded. Careful, it is more malleable than you would expect. Here you can see a mask that has been modified this way (the bottom one).

The masks can be hand modified

Head Mounting

The most obvious way to wear the mask would be to put some elastic across the eyelets on the side and pull it behind the ears. Wearing a normal N95 that way can start making the ears hurt. This mask is much heavier (not heavy, just heavier) so I don’t think that this is a good long term method.

One method that worked really well was to sew 2 buttons on to a hat. Then attach elastic hair bands to the mask at each eyelet and imply pull them over the botton.

As long as you wear a hat, this is perfectly comfortable

I did a similar tactic by sewing some velcro to the to of a hat. The hairbands were still inserted into the eyelets (this time only using 2 hairbands instead of 4) and now I could simply attach the mating velcro straps to the hat.

This is actually my preferred embodiment as I always wear a hat

And here is one using thick velcro straps to secure to the head without a hat. The velcro doesn’t need to be near this wide

There are obviously many more possiblities including printing something for the back of the head to hold the hair bands. But all of these were comfortable and would let me wear the mask for hours.

Using the mask

The first thing to mention is that there was no way my wife was going to wear a boring cold mask like this. PLA is a great material and it holds paint very well. Further, if you let the filter degas for a day or so, you can draw on the mask area with a Sharpie. So, this is my wifes mask:

Make your mask yours.

When to use the mask

I have used the mask a lot recently. I like that it isn’t right up against my face. There is no fabric to get in my mouth, no moisture that builds up. The opening is truly big enough to allow full breathing. I have worn it for hours (the hat style).

I want to warn against something pretty important. Do not wear this mask while exercising. Some places might require it, some runners and cyclists might prefer it, but this is not a mask for that. It’s not a comfort thing.

There are a number of studies that look at mask dead space (the space between your face and the mask) relative to tidal volume (the amount you breath in and out). As the dead space gets larger, the more CO2 remains the next time you breath in. For walking and standing, this is no issue fo rthis mask. But for biking and strenuous activity, I found myself feeling bad after a while. For exercise, if you must use a mask, I’d use something with far smaller dead space, such as cloth masks or surgical masks.

All in all, these masks work well and print easily. They are simply to put together and getting it to fit faces of all sizes is pretty easy. Hope this has been helpful.