Z resonance

Couple questions. 1. I am assuming you are printing PLA. What brand and at what temp are you extruding? 2. What is your print speed?

My immediate thought is that your extrusion temp is too high and even with fan cooling the filament is sagging into the center of the model since you stated this does not happen if you print an infilled model.

Ultimachine 4043 @ 196c (I have since lowered this to 190.)
85mm/sec the stock settings config for slic3r by Gordon

The upper picture individual layer cross section is shaped like a "C" and is about 2mm thick as is the bottom of the bottom picture. The top section of the bottom picture is solid and about 5mm thick. My fan is an 80mm mounted on the led strip on the front. I need to design a new fan shroud as the one gordon designed doesnt work for the e3d v6 extruder. Do you think that fan is the issue?

The print failures are evenly spaced so I would assume that whatever is causing them occurs in a cycle. The 5mm part does not seem to have any problems which makes it harder to diagnose. Maybe the 5mm thickness creates enough of a support barrier to prevent sagging. Or the print time for those layers is longer, which allows the PLA to cool before the next layer is applied. I can't pinpoint anything so I toss out these crazy ideas.

1. Your reel is binding at some point in the rotation. The extruder doesn't get enough filament as it struggles to pull the reel free but once it does, the reel rotates around easily until it cycles to the binding spot again.
2. Your filament is damaged in some way at one spot in the reel. Every time the reel gets to that part in the loop, the problem occurs then goes away.
3. Your extruder clamping gear is stripped or packed with filament grind in one spot. When it rotates around to that spot the filament slips a bit resulting in a lower federate. (This is unlikely because the gear is small so the spacing of the problem would be much closer.)

I can't think of why the fan would cause this problem, unless it is turning off at evenly spaced time intervals. I suppose if you had an intermitted short in the fan wiring, it could heat up, break the circuit, cause the filament to overheat and sag, cool off, reconnect the circuit and cool the filament properly until the fan heated up again.

I've got an E3d also and need to update my fan shroud. If you happen to design a new shroud consider posting it.

Chris

Figured it out. The bottoms of my z rods were hitting the edge of the holes of the lower left and lower right z bracket from the 1.5 set of parts. My Z rods also have a slight bend in the bottom which is what is causing it to hit.