On Thu, 11 Feb 2016 20:19:49 +0000, dennis@home
wrote:

On 11/02/2016 11:05, T i m wrote:
On Thu, 11 Feb 2016 09:30:08 +0000, dennis@home
wrote:

snip

With FFF in plastic you may have to look at the design and change it to
take into account the weakness in the Z axis. If you can't then you need
to look elsewhere.

What weakness in the Z axis?

You can never get as good a bond when laying hot plastic ontop of cold
plastic as you get from the extrusion.

Maybe not, but it only needs to be 'good enough' for the required use.
Further, the already printed plastic isn't 'cold' as the whole job is
sat on a heated bed. The trick is to leave enough heat in the job to
keep the temperature differential between the plastic being 'hot
enough to extrude' and 'cool enough to maintain stability' once
extruded.

If you were to print a 25 x 2mm disk, kill the job and lift the job
off the bed whilst hot (with a blade), you could easily fold the disk
in half with your fingers.

With taller thinner jobs you often have to run cooling fans because
even though the heated bed can now be a reasonable distance away, the
rate at which you are laying filament on the top at 200 DegC means
the job can stay so hot it can move / distort, just from the 'drag' of
the filament.

Try printing a tall rod vs one printed on its side.

I have / do. ;-)

You will find one has a higher tensile strength than the other in the
long axis.

Of course. But let's say one is 90% of the other and the item 200%
stronger than it needs to be?


If the printer is configured and running properly then each new layer
should fully fuse itself to the previous layer? We extrude PLA at
around 200 DegC and using a heated bed (60 DegC).

Don't get me wrong, I have had faulty prints where the nozzle has say
partially blocked for a few seconds, the size / consistency of the
extrusion has been compromised and so a fault is created affecting the
Z axis but that's a 'manufacturing' not 'materials' fault.

I'll print a couple of solid beams and see how accurately I can test
them for bending / delaminating failure.

O, that's what i just suggested above.

I have printed many object that have reasonable length in the Z axis
(sometimes longer than the other two) and never had any obvious
weakness in that direction.

In fact we have dropped heavy assemblies, stepper motors and linear
rails that were joined together by plastic printed connectors and
components and anything thin that has ever broken has always broken
across the job with no regard to the thought that it was of
'laminated' construction.

e.g. The inter-layer bond is (to all intents and purposes) as strong
as the material itself.

Cheers, T i m

p.s. Do you have or use a 3D printer and if so, what model?