Blobs on knurling (zig-zag) walls

[alik]

Hi I am trying to print a mold using NinjaFlex filament. The inner walls of the mold have zig-zag shape. See the cross section below:

cross section of the model showing the knurling side wall

The problem is that no matter what I do I see plenty of blobs all over the zig-zag walls. The smooth outside wall is printed w/o problems. In the picture below the blobs form a vertical line because I set fixed layer start point. There are several such lines along the wall. If I choose random start point the blobs cover the entire wall randomly.

build-up on the zig-zag wall

I spent plenty of time playing with retraction and coasting settings and it had no effect. I realized that the blobs are formed because of the big number of sharp turns in the outline of the inner wall. The slicer calculates the extrusion based on the total length of the outline but due to the finite accuracy it produces a little bit extra extrusion in each sharp turn. After certain number of turns the nozzle collects so much extra materials that it creates the next blob.

The only fix that I could think of is to reduce the overall extrusion flow. But this will affect the smooth wall which will suffer lack of the extrusion. Is there a way to configure S3D so that I will reduce the extrusion only for the inner zig-zag wall? Are there other ways to handle this problem?

Thank you,

--Alexander

[rrdavis]

You could try enabling the "avoid crossing outline for travel movements" option on the Advanced tab. Be sure to set a high detour factor, and it might avoid some of the travel movements through the middle open area of the part.

[alik]

I figured how to handle this problem. I added the same zig-zag to the outer side:

knurling on both inner and outer sides

Now I am printing this part of the body with a separate process where the extrusion multiplier is set to 0.8. All the blobs are gone.

I think S3D should have another setting, something like "extrusion multiplier at sharp turns".