frame sheet: hdpe -vs- abs -vs- umhw -vs- ?

[tom c]

I’ve completed my first backpack (woot!) and am going to head to an industrial plastics supplier to buy material for a framesheet (eg; they have just about everything, definitely hdpe, umhw, abs, acrylic …)

What’s the best framesheet material and thickness?

I’ve read around ~1mm of hdpe or abs works decently; which holds up better?  From other projects, it’s when a ‘crease’ develops in the sheet that it drops significantly in strength / comfort; which material resists this best?

Thanks!

[Lester Moore]

Congrats on the MYOG backpack Tom – would love to see some photos. Any reason for using a frame sheet rather than stays? Aluminum stays would be more rigid and much less prone to deformation than a thin polymer sheet. I’ve played around with frame sheets made from hdpe storage bin side walls, but they end up being heavier than a U shaped piece of 1/4″ diameter aluminum rod.

[Link .]

you can buy one that Gossamer Gear uses for their packs

REMOVABLE (ALUMINUM) BACKPACK STAY

Or the style that ULA uses if you want to keep it simple and buy from a pack maker

CIRCUIT ALUMINUM STAY

 

[tom c]

I’m actually make a child-carrier insert for a seek-outside load-carrier.  So not quite as awesome as an actual MYOG pack (but was trying to keep the thread simple).

The seat is a brazed truss structure that provides the seat, load-bearing for the bag, and ~7″ separation for there to be room for the kid.  The “frame sheet” is actually just protecting the xpac from the metal truss, and providing a flat back for the kiddo (where the bag would otherwise barrel).

Weight for the lower half is less than a pound (brazed .030″ steel).  v2 could probably get down to 10oz or so.  But, my ‘weight budget’ was a 9lb osprey child pack (!!), so I’ll be around 6lb, and i get a full bag for the bulky stuff (wife can take smaller heavy items).

 

[tom c]

I made a run to the vendor and bought some acrylic and polypropylene corrugated plastics.

HDPE seemed like the best option for solid sheet, but the 1/32″ was _way_ too flimsy, and the 1/16″ seemed too much / too heavy.

will update when i finish it this weekend.

[Craig B]

Haha!  A welded steel frame!  That’s old school ;-)  I just got some samples of thin plastic sheet for my own pack project, and polypropylene is the least dense (15-20% lighter for the thickness).  At 1/16″, it’s pretty solid, but you could probably get away with 0.05″ or 0.04″ for your application.  It’s a bit softer than HDPE, but still pretty stiff in those thicknesses.

[David Thomas]

I was thinking corrugated plastic would be far stiffer for the weight. And if it buckled too easily in one dimension, hot-glue another layer in the opposite directions. Maybe double thick everywhere but just some perpendicular strips would add a lot of strength in the perpendicular direction.

Or consider hot-gluing (or Gorilla Gluing) two corrugated sheets on a bias – one 20 degrees left of vertical and the other 20 degrees right of vertical – most of the stiffness and strength would be vertical but with a lot more resistance to buckling on the horizontal

But don’t buy it. It’s all over as campaign signs and “Lose 30 pounds in 30 days” advertisements illegal hung on power poles.  I’m the president of an electric utility and you have my permission and encouragement to remove them

In my area, the expectation is that all campaign signs start coming down the evening the polls close and is completed the next day. Anything up after that is fair game.

[tom c]

David – thanks for the suggestion on campaign signs. I’m all for reuse but honestly I don’t see much of those around here at least not of corrugated polypropylene.  A whole 4×8 foot sheet was about $15.

The corrugated polypropylene has actually proven to be stiffer and less prone to collapse than I had expected.  It’s certainly more durable in the long Dimension but it doesn’t fold as easily along the corrugations as normal cardboard does… Which is nice for my application.

[Ben H.]

Having used a couple child carriers like that, I have to say, that looks spectacular.  Very well done Tom!

Are you getting a strap system that goes over the shoulders of the child?  These things can be kind of tippy.

[tom c]

@Ben – the Seek Outside packs have a ‘Y’ top strap over the roll top that served pretty well as a harness.  No great pictures but this shows it:

[tom c]

the pack worked fantastically, btw; I finished it with some fleece over straps for comfort, and a seat of corrogated cardboard + contoured 1/2″ closed cell form, 1″ of open cell foam, sewing in a soft nylon.

[Author]

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