DIY Particle Filter Mask Assembly System

Due to the COVID-19 crisis, many healthcare workers are facing a shortage of personal protective equipment (PPE). The DIY community has responded by developing many helpful solutions ranging from hand-sewn cloth surgical masks to 3D-printable face shields.

I’ve developed a simple, repeatable system for making disposable particle filter masks. These masks are made from 2+ layers of material taken from commonly available HVAC filters. The specific material I chose comes from 3M MPR 2200 pleated electrostatic filters. One filter yields several feet of material from which to make the masks. The masks contain a thin metal wire embedded in the base that can be bent to help them conform to difference face shapes.

The Mask Assembly System involves two 3D printed tools: an assembly jig and a flat pattern cutting template. These can be made on most hobby-grade 3D printers with sufficient build volume. I used a Prusa i3 MK3S. The STL files will be made available for anyone interested in making masks with this system. I printed the Jig with PLA at 15% infill, 0.2mm layer height, and 4 perimeters. I printed the Flat Pattern with PLA at 15% infill, 0.2mm layer height, and 2 perimeters. The Flat Pattern may need to be split into two halves for printing on smaller machines, then rejoined with CA adhesive. It can be printed as-is on the Prusa i3 MK3S.

With practice I can make one mask in about 8 minutes. I hope that others will take on the project, refine it, and donate many more masks to those in need!

Disclaimer:

I make no claims about the filtration performance nor the efficacy of these masks against infection from SAR-CoV2 or any disease. 3M and other manufacturers do not endorse this use of their HVAC filter products.

This project is intended to create emergency PPE like the cloth masks or PET bottle face shields made by others in the community – it is not certified or regulated in any way. Use at your own risk.

Supplies

In addition to the 3D printed tools, the following tools are required:

· Utility knife or X-ACTO knife with a new blade

· Scissors

· Hot glue gun

· Pliers capable of cutting 22 gauge steel wire

In addition to the electrostatic filter material, the following materials are required:

· 22 gauge 304 annealed stainless steel wire (McMaster-Carr 8860K62 or equivalent)

· (Wire up to 16 gauge is also acceptable)

· Hot melt adhesive sticks

· Fabric-covered elastic bands, shock cord, string, or other material for the straps

· Rubber gloves

· Safety glasses

Tips:

Use only the highest quality filters you can find. I used MPR 2200 because it was all that was available but there are other, potentially better filters that can be found.

Wear rubber gloves during the assembly because it’s more sanitary and helps protect your fingers from being burned by accidentally touching molten glue.

Wear safety glasses to protect your eyes when handling and cutting the wire.

When tensioning the wire around the groove, don’t pull too hard or the wire posts could snap off.

Clean up excess hot glue drips from your tools by pouring 70% isopropyl alcohol on it then just peel it away.

Keep a metal object such as a ruler handy to press against molten glue and help it cool faster. Don’t try using your finger to press on molten glue.

Step 1: Get Filter Material

Carefully disassemble a new, unused electrostatic filter and collect the material from it.

Step 2: Cut Flat Patterns

Stretch out the material so it stays flat on top of a suitable cutting surface, and use the Flat Pattern template to cut two pieces of material for each mask. I recommend cutting one piece with the pleat folds running horizontally and the other piece with the folds running vertically. This makes for a more rigid mask when the layers are built up.

Step 3: Wrap First Layer

Take the first piece of material and wrap it around the Jig making sure that the bottom edge contacts the ring at the base of the cone.

Step 4: Glue Side Seam

Use hot glue to join the side seam of the material.

Step 5: Glue Top Seam

Fold the top seam of the material so it conforms flat against the tip of the cone and glue that seam as well.

Step 6: Prepare Wire

Thread a piece of wire through one of the posts and wrap it around the post at least twice.

Step 7: Wrap Wire

Hold the filter material securely and wrap the wire around the cone so it presses the material tightly into the groove.

Step 8: Glue Wire Joint

Apply hot glue to the area where the two ends of the wire overlap, then allow the glue to harden.

Step 9: Trim Wire

Cut the two ends of the wire and ensure they are flush with the mask and no sharp ends are protruding.

Step 10: Glue Wire Ends

Apply hot glue to the ends of the wire to ensure they don’t bend away from the mask, then allow the glue to harden.

Step 11: Glue Wire Perimeter

Apply a continuous bead of hot glue all the way around the wire.

Step 12: Wrap Second Layer

Quickly wrap the second layer of material around the first while the bead of glue is still tacky.

Step 13: Glue Second Layer

Glue the side and top seams of second layer in the same way as the first layer.

Step 14: Add Extra Layers (Optional)

Apply additional layers at this point if desired.

Step 15: Trim Excess

Carefully remove the mask from the Jig. Use a small screwdriver and the 6 slots at the base of the cone to carefully pry the wire out of the groove. Then use a scissor to trim away the excess material below the wire. Keep about 1-2mm away from the wire so that only the fabric material touches the wearer’s face.

Step 16: Arrange Straps

Put the mask back on the Jig and use your choice of the 6 slots at the base of the cone to arrange the strap material. I use the 2, 4, 8, & 10 o’clock positions but you may use just the 3 and 9 o’clock positions.

I used string for this but it's not ideal. However, elastic strap material is in short supply right now as many people are also making DIY masks.