Mystery colour streaks - WILLING TO PAY FOR SOLUTION

Hi, I am desparate and willing to pay anyone that can help solve my printer problem for me.

S3D support and Craftunique are unable to isolate despite much problem solving.

PROBLEM:
Random and slightly diagonal lines/streaks across my PLA prints.
1.5yr old Craftbot3 (CoreXY design) with moderate usage.
Problem only started recently.
Started on head1 then eventually started on head2.
Layers are about 1-2seconds long

SOLUTIONS TRIED:
Rails - lubricated
Material - swapped with fresh reel
Speed - various
Extrusion - layers all look uniform and accurately laid, only colouring seems affected
Temp - not enough time for temperature to fluctuate so consistently within or between layers
Something loose - layers all look uniform and accurately laid
Stl file or slicer - printed previously without issue

It is only a problem with these parts, because they form part of a desklamp and have light shining through them.

Thanks

Its not very clear from the pictures.
What is the section of the part? If its thin walled or your material is semi transparent then this may be related to material/wall thickness. If so you could be very sensitive to factors that affect the material flow such as temperature, filament diameter and very small changes in z height (localized deformation in the z screws).
Is it repeatable? Z screws would be repeatable.

Try to do PID auto-tuning, seems like a temp problem, even if the LCD says you're at xxx temp all the time, it doesn't have to be true... Might also be affected by bed and hotend heating at the same time. Hotend not getting enough power during the bed heating.

BPB, I had exactly the same issue quite some time ago. It turned out that my PLA had picked up contamination on the spool, so that every few turns were either "clean" or "dirty, at a repeatable distance. That showed up as alternating dark and light bands on what were otherwise almost flawless prints.Like you, I was printing a translucent lampshade.

Your prints look like they are as good as can be, and it looks like you've addressed the issues one would think of. If your replacement spool was stored under the same conditions, that may be the problem? Is it the same manufacturer, material, and lot number?

A "band-aid" for the issue may be lightly sanding the finished part with 220 grit paper to even things out "optically", though this doesn't address the root cause.

Just an off-the-wall thought.

Edited 1 time(s). Last edit at 03/15/2020 04:20PM by rq3.

Thank you for all the responses
Apologies for my late reply

@MCcarman - Sorry i can't post the part, there is ip protection required on it. It is about 1.2mm thick, so it is thin walled (3 layers) and semi-transparent. But i was printing them before without any streaks. Streaks are not repeatable and are random. i also think it is a heat issue, and will be replacing the hotend.

@JaMiskater - Thanks, yes i suspect heat too. am replacing hotend in any case, so will see then.

@rq3 - Thanks, but i've tried a bunch of new spools now all stored under varying conditions. i don't think it is the material any longer. Will be sanding if all else fails

Fascinating pictures. It looks like it is layer spacing. The bright areas seem to be 2 close layers that blend into one, followed by a larger gap that throws a shadow. The darker areas seem to be more evenly spaced layers with each layer causing a shadow. I assume its a shadow anyway. The more shadows the darker it seems.
I doubt that's a z movement issue but changes in extrusion rate and temperature would be where I would look.