To visualize part of Nmech's answer: in the image, the washer actually greatly increases the contact area of the bolt head.
The bolt head looks pretty big:
But most of that is the shaft, which obviously does not spread out load on the material. So the actual contact area looks like this:
Comparatively, the bolt head on the washer looks like this:
That's a ...
I will expand on DKNguyen answer, because to my knowledge also the two reasons are:
reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation)
change the joint tightening characteristics (see joint diagram).
reduce contact stresses on surfaces.
The basic idea is that since contact stress is defined as:
$$\sigma = \...
Except for special applications, most washers are made of dead soft steel, which deforms under the compressive load imposed by a tightened bolt head. As the washer smooshes, it minimizes stress concentrations caused by bumps under the bolt head and surface flaws in the part the bolt is running through.
It is for spreading out the stress.
But it is also for giving the bolt a bearing surface to turn on. The washer always goes on the side (nut or bolt) that is being turned. It prevents it from marring up the work surface and also changes the tightening characteristics. I don't know the specifics of that though but that's what I was told by a toolmaker. Always ...
A google query “hdpe book binder screw sets” revealed a product made of hdpe which consists of a screw and matching sleeve. This item shows that hdpe can be used as a screw or bolt, with tapped threads in hdpe as well.
Finding a product which has appropriate dimensions for my project will take additional work, but it’s good to see that hdpe can be used for ...
Two pieces HDPE can be welded together with heat. You can use filler rod for this but it is also possible to do this with just heat. Additionally you can ultrasonic weld HDPE but this itself is a bit tricky in my experience. Any one of these should be sufficient for your application.
You can obviously try to glue HDPE, its possible. But the glue itself ...
You should research things yourself instead of "hearing" that things don't work.
Superglue (CA glue)
Cyanoacrylates Cyanoacrylate adhesives can be used along with primer
Permabond POP. Cyanacrylates set almost ...