physical strength of bond/fusion between layers?

I am curious just how strong the bond is between the layers of plastic; is it a chemical bond, or a mechanical bond, or a mixture of both? Or is it for all intents and purposes, if done right, the same exact thing as a fully molded piece of plastic?
I am wondering, if one were to build a printer with a large print area (say, furniture-sized) and used it to print, say, a chair, would the bond between printed layers be strong enough to actually take the weight of a human sitting on it, or is the weakest link in the chain the bond between the layers?
I have seen the videos and articles about the guy who prints chairs but that's a larger format machine with a larger extruder; I am talking about taking a higher resolution/smaller nozzle reprap and just making it have larger reach in order to print large things, maybe a canoe or kayak.

My understanding is that the bond between layers is a diffusion weld. Its strength depends on the contact area and how long the interface between the two layers remains above the melting point.

Objects are weaker in the direction that pulls the layers apart. The chair does not have much force in that direction.

Here is a small object I made in ABS that could handle many kilograms in the weak direction: [hydraraptor.blogspot.co.uk]

If you print something the size of a Kayak with a high res nozzle it will take weeks and the whole object would need to be kept at a temperature just below the glass transition to prevent it warping. It will also cost a fortune unless you use granules.

---

[www.hydraraptor.blogspot.com]

@nophead: Are you still using those sky buckets?

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

Not for feeding machines as plastic comes on spools these days. It is still fastened to the ceiling and filled with short lengths of filament from the end of many rolls.

---

[www.hydraraptor.blogspot.com]

Ahhh... then all you need is RichRap's 3mm filament joiner and you are good to go!

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

Yes but they will all be slightly different diameters so I also need filament size sensor.

---

[www.hydraraptor.blogspot.com]

A new project for nophead!

If the distributors don't improve their filament quality you may have to build it!

"Build it and they will come..."

Wouldn't that be cool - fuse 2.75mm filament with 3.02mm filament and it extrudes the right amount of material...

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

... this could develop to a really complex task - simply for circular filament, but complicated for oval or misshaped

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I agree - I think it is easier to force the distributors to deliver high quality constant diameter round filament.

Quote
Albert Einstein
Everything should be made as simple as possible, but not simpler.

Quote
Albert Einstein
Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

One FDM object design guide I came across suggested that the strength in the weak direction (vertical) is about half of the strong directions. But in the end it's just something you have to take into account in the design.

It's also pretty easy to print objects that are much weaker in the weak direction than that. I've done this at least by using a cooling fan directed right below the nozzle. With PLA it didn't matter but with ABS, a pulley printed with the fan on could be easily snapped in two by hand, but printed without it the pulley was quite strong.