Poor Quality Undersurface

Hello,
Our Anatomy department has been using an AirWolf 3D HD to print complex shapes in ABS plastic. Our current focus has been a model of a brain. Multiple images of the surface file rendered in MatterControl are shown below.
Attachment 1146Attachment 1147Attachment 1148



After a number of attempts, we have encountered consistent problems with the print. The top and sides of the object look great, capturing a lot of the details we want. An image of a partial print is shown below. Rectilnear supports with 3 mm spacing have been removed. The card next to the object is ~2 inches.
Attachment 1149

Here are the basic stats from the gcode file.

; layer_height = 0.2
; perimeters = 2
; top_solid_layers = 3
; bottom_solid_layers = 3
; fill_density = 0.3
; perimeter_speed = 5
; infill_speed = 10
; travel_speed = 50
; nozzle_diameter = 0.5
; filament_diameter = 2.9
; extrusion_multiplier = 1
; perimeters extrusion width = 0.85mm
; infill extrusion width = 1.02mm
; solid infill extrusion width = 0.85mm
; top infill extrusion width = 0.85mm
; support material extrusion width = 0.85mm
; first layer extrusion width = 0.84mm

The current problem is that the undersurface of the object looks ugly as sin.
Attachment 1150

There is a lot of drooping of the plastic, such that this portion of the model looks like it is made of spaghetti. The loops of filament lack the densely “stacked” look of the rest of the model. Varying the density of the support materials has little to no effect on the quality of the underside.

A lot of the structures we want to print are complex, such that we really can’t print them without prominent undersurfaces. Is there a way to deal with these really rough surfaces using variable speeds? Some other methods? Any assistance would be much appreciated.

Are you using cooling on the material as it exits the extruder? This can help tremendously with the "ropy" look.

Ooh! I don't think we've got the fan turned on during the process. Are there good suggestions for fan settings to avoid the ropey look?

Here's what we've got going so far...which I think indicates we haven't been running the fan at all.
Attachment 1153

What material are you printing with?

One thing you can try is to use thinner layers, that should make the steps on the overhangs less wide so you don't print so much in the air.
Besides that you can try using support structures or cutting your object in half and print the top and bottom separately and then glueing them together.

You can run some tests with a calibration object like this one: http://www.thingiverse.com/thing:58218
With my printer, using PLA, a 0.4mm nozzle, 0.25mm layers and plenty of cooling that test prints fine up to the 25 degrees overhang, 20 degrees have some defects and at 15 degrees I get a stringy surface.

We've been printing with ABS, although we've also got a supply of PLA. Honestly, whichever material would best deal with overhangs would be the ideal for us. I'll do some testing on the calibration object (thanks for the link), but I'm still uncertain as to what to do with the cooling fan settings.

Are there consistent settings that people use across all prints, or is this something that is varied depending on the object being printed?

This is why pro-level machines like the Stratasys use a soluble support material, which is deposited from another extruder wherever an overhanging surface is likely to need it. Otherwise, any overhang projecting at more than 45 degrees from the vertical is susceptible to problems like the ones you describe. One can print supports in the same material that is used for the print itself, but that makes for a rough surface underneath, and requires hand-carving to remove, which is imprecise and can be dangerous.

Andrew Werby
www.computersculpture.com