Zipping Together Two Mummy Bags

My boyfriend I are avid backpackers and do a lot of camping trips in the spring and summer time. We have increased our inventory of gear since we first started dating (I had most of the gear at the beginning of the relationship, now we are about even). He purchased a new sleeping bag last year (the REI Helio down, right zip) and I knew at some point I would need a new sleeping bag as well. I had been using an REI Radiator sleeping bag leftover from my childhood backpacking trips with my family- literally, this thing is 20 years old. It held up amazingly, though! Check out this picture from circa 1997 and one my boyfriend snapped today- same bag:

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You can see I’m on top of two other sleeping bags, which leads me into the fabrication topic of the day. Many sleeping bags cab zip together, which is really great as you can cuddle and steal warmth from a significant other. In order to zip together, two mummy bags must each be one right-zip and one left-zip. They also need to be of the same zipper manufacturer. I knew that when I would purchase my next sleeping bag, I would want it to zip together with my boyfriend’s Helio bag.

I went to REI to try to find an REI brand sleeping bag that could potentially zip with the Helio. I spoke with a very helpful REI rep, who recommended the REI Flash sleeping bag. Since my boyfriend is very tall and has a tall sized sleeping bag, the rep also recommended going for a regular size men’s bag as it was left zipping and might fit a little better next to a tall bag. However, he wasn’t positive the Helio and Flash would zip together and unfortunately, they had none left in stock. The bag had all the specs I wanted (correct temperate range, great weight for backpacking), so I decided to take the risk and order the bag online. However, when I received the Flash, we immediately realized it would be impossible to zip the bags together for two reasons:

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Firstly, both of the zipper fasteners were on their respective bag’s top zipper. One would have to be on the bottom and one would have to be on the top in order to zip properly. You could potentially flip one of the bags over, but then the head part of the mummy bag would cover your face- not ideal.

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Secondly, even if the zipper fasteners were on their correct side, you couldn’t attach the fastener to the zipper due to a blocker (item circled).

I realize the reason this all probably happened was because both of the bags are men’s sleeping bags and REI only guarantees that their REI brand men and women’s bags will zip together. I was so happy with the Flash bag that I didn’t want to return it and go through the trouble of finding another great sleeping bag that could zip with the Helio. It was time to figure out a possible solution using 3D printing!

I took the measurements of the zippers using a caliper and snapped a couple photos from plan and elevation views. I then brought the images and measurements into Rhino and modeled the zipper. I modeled two zippers, one the same height as the bag’s current zipper and one a millimeter taller in order for the blocker piece to fit through it. Check it out:

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Two poly-surface zippers with different heights and two meshes, exported as an STL to be printed.

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Ready to go!

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I printed two of each of the two-sized zippers on the Mojo 3D printer at Techshop (printing away in the image above). Once the support came off, it was time to test them:

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And it works! This video shows just one zip fastener, but in reality each zipper on the bag, bottom and top, has two zip fasteners, like so:

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Our two mummy bags finally zip together! We did run into one problem though: even when the bags are zipped together, the Helio bag still has an open foot area at the bottom as the starting point of the Flash’s zipper is higher than the Helio, like so:

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You could potentially start them at the same point at the bottom, but then the Flash’s head would be much shorter than the Helio. Either my boyfriend’s feet will be cold, or my head will be at his chest- so I think I’ll be buying him a bunch of wool socks 😛 Jokes aside, I am still trying to come up with a solution for this, and will update my blog once I can come up with something viable. In the meantime, happy camping!

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