Designing and Constructing Filter Masks

Anna Hickerson, PhD, Assistant Professor of Medical Device Engineering, KGI

Kevin Peter Hickerson, PhD, Nuclear Physicist

During the peak outbreak of the SARS-CoV-2 virus, the need for PPE to protect medical workers from contracting COVID-19, even in high resources countries, is severely elevated. The authors were requested by hospitals to help provide as much home or small-lab made PPE as possible using any rapid prototyping technology available. Existing methods include additive manufacturing (3D printing) of masks, but these have the problem of both a long time to construct each unit, as well as difficulty making a good seal. We present a method to rapidly design and manufacture masks using CAD models of a human head and vacuum forming mask shapes. The construction times are as much as 100 faster than direct 3D printing.

To create the mask, we needed a design for both the mask and a mold that is later used in a vacuum forming step. These designs were made using the OnShape cloud based CAD software.

To begin, we purchased a model of a human head that is commonly used in the gaming industry. This helped create a shape that can conform to a face comfortably. Future iterations can use other facial models for more variety of fit.

Splines were traced around the chin, cheeks, and nose bridge. The splines were used to create a wide surface in the shape of a ribbon that forms the seal between the mask and the wearer. With the seal surface in place, the rest of the mask was developed including two large holes for the filter material.

Once the mask was designed, a 3D printable mold for the mask was also designed to be used in the vacuum forming step. Rapid prototyping allows the mold design to be easily modified as more mask shapes are designed. And each mold can be used for hundreds of masks.

We printed the mold that we designed on the Ultimaker S5. We used polyester (CPE+) because its smooth surface and high melting point make it ideal for vacuum forming.

While a lot of people are doing a lot of good by 3D printing masks directly, you can see from this sped up video that something on the scale of the size a human head takes a long time to print. This mold, for example, takes over 13 hours to finish, even though most of it is empty space and support material.

The final printed mold can be used over and over.

To make each mask, we place the mold into the vacuum former. In this case, we used the Mayku Formbox. The vacuum former takes heated plastic and pulls a vacuum using a standard shop vacuum cleaner and pulls the soft plastic to create the form. Each time we make a form, we place a new sheet of plastic, and set it to the top where it begins to heat.

We use the time it take to heat up each plastic sheet to also remove the previous mask from the mold. This is done by cutting along a groove that we specifically designed into the mold. We also cut around the holes in the mold where the filter material will later be added. Once all of this is cut out, the mask can be removed from the mold to be placed back into the vacuum former to begin again.

The forming process takes about 2 minutes per mask.

We used HEPA rated vacuum bags for our filter material.

The bag is cut into flat sheets and placed on a heat resistant surface. A soldering iron set to 500°F is prepared to cut and fuse the layers. A 3D printed outline of the filter shape is placed on the filter material and the shape is traced with the soldering iron. This step takes approximately 10 seconds to complete per filter.

The filters are then attached to the mask using hot glue. A bead of glue is placed in the center of the mask and the alignment is checked. This process is done to many masks at once to allow time for the first bead of glue to cool and set. HEPA materials are rated to block 99.7% of particles 0.3 μm in size.

Glue is traced around the perimeter of the hole, and the filter is carefully pressed into the glue. The process is repeated on both sides.

Ribbons, or elastics bands, are pre-cut to length. Each ribbon is individually hot glued to the mask and set aside for the glue to cool and set. In the meantime, more masks and ribbons are glued. After the glue has cooled, each mask is tested to check that the ribbons are properly attached.

The masks are ready to be delivered!