Effect of top thickness on print with bumps on top

Question

My son is printing a simple block with a small (2 mm high) bump on top - see image below of the cross section. The left shows the 3D model and the right shows where the plastic actually prints, using a grid infill at around 10 %, with a top height of 1.6 mm

As you can see, this is very weak, and the bump very easily snaps off, as it has very little plastic securing it to the layer below. When we increase the top/bottom thickness (in Creality Cloud) to 4 mm this issue goes away, and the bumps are well connected. But then the print takes a very long time.

Questions:

1. Is this the expected behavior of a slicer? Why does it not add material to firmly connect the bump to what is beneath it?

2. How exactly is the "top thickness" setting applied? E.g. if it's set to 2 mm, does that only apply to the layers comprising the topmost 2 mm of the model, or does it stipulate that there must be 2 mm of plastic at the 'top' of the model at all X, Y coordinates?

3. Is there a way to set the top thickness separately from the bottom thickness in Creality Cloud?

Answer 1

Modern versions of Cura have a feature called "skin edge support thickness" which will build extra perimeter-like layers under the edge of your "bump" down into the infill area, making it attach much better. I believe it defaults to on.

Your problem is probably that "Creality Cloud" is based on an ancient fork of Cura and does not do this, but you could check and see if they have anything similar available. The right solution, though, is to use real up-to-date slicing software like recent Cura (5.x at the time of this writing; I believe the feature was introduced near the end of the 4.x series), PrusaSlicer, or anything but the junk your printer vendor ships (or doesn't even ship but expects you to interface with via the cloud?)

Answer 2

Is it expected?

So you want to print a block with nubs.

Putting it into the slicer, and looking at the first layer of the top, you will see that there is a an empty hole that has the diameter of the nub's outer diameter minus the wall thickness, where only the infill is. A little bit further up, one layer over the top layer, it's easy to see that the whole nub is resting on just its wall, which is filled under it.

This means, the contact area between the nub and block is very limited: it's only the walls, and a little bit of squishy infill.

What are top layers?

The top layers are a number of layers that are directly under any surface that is defined as having a normal vector that is pointing "up". In our example, there are two: the circle from the nub, and the block without that circle.

Why does it work with super thick top layers?

Remember what I said earlier about contact area of the nub to the nub? If the nub is lower than the height of the number of top layers specified, the whole nub is solid, and any top layers that are still remaining get resolved to completing the top layer of the block. From the bottom there still is a bore, but the interface between the nub and the block is over the full area of the print. As adhesion goes with the contact area, that means the nub is more resiliant.

But as you print much more plastic, you also print much longer.

Is there another way?

In some slicers, you can modify the print behavior in specific parts of the print and define a different infill.

One such example is Ultimaker Cura, you could insert a (floating) cube at the interface between the block and nub, intersecting both for at least two layers, and declare that cube a print-setting modifier. Then you declare that cube to be printed with 100% infill, and you get the large interface area between the two parts and at the same time keep more of the model hollow than with many more top layers.