3D Printing - How Just One Setting Can Make Weighty a Difference

I am currently printing a prototype of the Grumman Mallard. I have printed a lot of planes now and have seen many of the problems that can arise. Like most of us who have taken the dive into 3D printing I find myself tinkering with settings here and there to improve the prints that come of the bed.

A problem I see regularly is under extrusion of infill that of the infill being caught by the print nozzle on its travel during layer change, often this is accompanied by an audible click.

In this photo you may be able to see the under extrusion of the infill, and on the far right 'broken infill' where the nozzle has travelled.

Using the Z-Hop when retracted function I thought I might be able to rectify this. The function causes the Z axis to move during the travel so that it doesn't his the model. Great! It adds a little print time but what's not to like!

The part on the left is with the Z-Hop at retraction setting and the part on the right is without. You can see that the infill is more visible on the surface of the left hand part. There is a bigger problem however.

WHAT?! The Z-Hop part almost 20% heavier? I haven't changed any other settings, nothing to do with flow, infill percentage, wall thickness. Just told it to make a z-axis movement before travelling.

So what's happening here? It turns out that is is connected to another setting, the retraction extra prime amount that is used to make sure there is no under extrusion at the beginning of a new layer. The slicer is programmed to retract filament at layer change, and then prime the nozzle with filament at the end of a layer change. The Z-Hop is essentially seen as a layer change so has the additional retractions and more importantly the extra prime amounts.That is why the infill is much more visible in the heavier print. The extra weight is due to all this extra filament and the properly extruded infill.

So the outcome of this? Accept slight messy infill, and don't use the Z-Hop when retracted setting if possible. The two parts are pretty equal in strength and the lighter part is certainly strong enough for an RC plane. The bottom line is, make sure the wall of your prints are good, without any under extrusion but don't worry too much mid print if the the inside of your model doesn't look perfect.

Jon