Strings inside of objects

Recently we have been encountering "strings" on the inside of hollow objects (think of a hollow cylinder). The strings appear in the same locations for each print, and are quite thick (not just wispy hairs) (see photo). So far as I know, things have been stable (printer, filament, etc), but something has changed. I increased the retraction from 1.1 to 2.2 but could see no improvement. Any hints as to how to diagnose and eliminate this problem? Are there FAQs or standard procedures out there of which I am not yet aware?

Thanks!

Jon

Edited 1 time(s). Last edit at 06/24/2016 12:40PM by jon_bondy.

Those look like printing perimeters in air with no support from below, nothing to do with retracts.

Edited 1 time(s). Last edit at 06/24/2016 04:22PM by Mikk36.

Thanks for the help. I can bridge about 1.5 inches with this printer, so bridging should not be so ugly. And this area was designed to be empty, so it is not bridging.

I agree with Mikk. Those "shelves" on the inside are completely unsupported (unless you add designed-in or generated support material).

When you're extruding into air, there's nothing to hold the plastic in the location where it's being deposited. You can design overhangs at printable angles (most printers will do 45 degrees from the vertical and potentially more depending on the configuration of the part) or bridge straight lines only starting on a horizontal span across a gap. Any other unsupported configuration will just spurt random plastic into the gap, just as you have there.

One thing you can do, if you don't want to add traditional support material and can't print it upside-down, is add a single solid layer on the first layer of the shelf. This will allow the printer to make a straight-line bridge on that layer to support the subsequent layers, and you can cut out the solid layer with drills, files, scissors, and anything else that seems convenient.

You might have your printer set up well for bridging, but printing unsupported curved perimeters is not the same as bridging. You just can't do that well.
You can easily spot those when you check the output in Slic3r before printing.

Thank you for all of your thoughts, but I have printed literally hundreds of these type of objects without a problem, until the past few months. My printer has been able to print this shape in the past, without support. I wish I knew what had changed.

Well, it looks like a lack of infill is causing that.
It would help if you showed us the gcode file at least.

Edited 1 time(s). Last edit at 06/25/2016 04:27PM by Mikk36.

Turns out that the original responses were pretty much right on. This is one of a family of 3D objects, and this particular object had a location where a bevel was "too horizontal". Once I adjusted the design so that the bevel was more vertical, it started to print properly.

Thank you for all of your help!