Hi, I found lots of banding when I print 2 pass on clear vinyl, it happens on lots of major brand's clear vinyl, like Oracal 3162 & 3165, 3M RJ-20 and many others as well. I asked the AVS and Art Mart, it happens on their RS-640 and VP-640 printer as well. It always shows the vacuum holes and marks on the print, and the banding line-up with all vacuum holes. It major happens on Darker Colour like Dark Blue, Dark Green, Dark Red, etc. I personally reckon that is printer's problem, the vacuum holes are too close to the feeding board and cause the issues. Please refer to the image.
Do they happen on your printer?
Any suggestion? I have been trying so many different methods.
Is this vertical banding? From the pictures it looks like it is, but its hard to tell the orientation of the print. But based on the picture of the red print showing the vacuum holes, i assume the banding is vertical.
In that case, are you printing with an Overprint? Can you let me know the exact mode you are using and the current heater settings. Also, which model do you have?
I've seen this problem before. It cannot be simply blamed on the design of the printer. If I understand SD's post correctly then by 2 pass you actually mean '2 overprint'.
The profiles you will be using will almost definitely have been produced at the setting of '1 overprint', or a single layer of ink. If you attempt to double the ink volume without using a profile to suit (or modifying the printers settings) you will likely encounter some challenges, regardless of which solvent printer you have.
If you are attempting to print for a backlit scenario, then you will need specific profiles for the job. If you cannot source these, or have them made for you, then you will need to look at countering the issues causing the unusual artefacts.
These artefacts are caused by the uncontrolled dot-gain from the extra ink volume. It is particularly worse where less heat is applied to the underside of the media (ie where holes are in the platen). The rest of the platen area is a fairly consistent temperature and is controlling the dot a little better.
Firstly, lower your vaccuum settings as low as you possibly can while maintaining a flat media. This will reduce the airflow through the holes and therefore reduce the cooling effect. Do a small test between each change you make so you can identify its effectiveness.
Next, Set your print direction to Uni-directional. Yes, I know it slows it down, but that's the idea, to assist the ink to dry to control the dot-gain.
After this you can try increasing the platen (or print) heater. Careful to do this in 3deg increments to monitor your media for any tunneling. You may find you need to push this to the maximum setting.
If this still doesn't yield a good result, then it is best to try again using a slower print mode if one is available. Say if you are using 'Standard', then try 'High Quality'.
It seems like you are most likely out of our ORACAL sales and support area since you are dealing with Artmart, however if you are still stuck, contact me via firstname.lastname@example.org and I might be able to source you a profile and set of parameters that will work for you.