The outer surface of a print
If you are a frequent FDM 3D printer user you have probably already tried modifying the parameters called “number of shells / perimeters”, “top solid layers” or “bottom solid layers” in your slicer. These parameters represent “the outer surface” of your object. Currently, different slicers call each of these parameters different names. In the table below, we show a few examples of the different terminology used by various slicers to reference the outer surface and its components, as well as how 3D Matter refers to them for the purposes of this article:
Influence on visual quality
For most 3D printer users, the outer surface parameters matter from an aesthetics standpoint. You need them to make your print look good, but you also know that the more solid layers and perimeters you print, the longer it will take and the more material you will use. [2 perimeters / 2-4 top solid layers / 2-4 bottom solid layers] seem to be the default settings in a lot of slicers. And it is good to have them, because if you were to print without an outer surface, chances are you would not be very happy with the results:
|Testman printed at 30% infill, with 2 perimeters, 2 top and 2 bottom solid layers||Testman printed at 30% infill, with no perimeter, top or bottom solid layers|
The outer surface of an FDM 3D printed object is critical to a smooth finish.
Influence on mechanical performance
What we would like to discuss here is a bit less known: the influence of the outer surface on the mechanical performance of printed objects. During our research, we noticed that the thickness of the outer surface had a large impact on the mechanical performance of our tensile testing specimens, and we started thinking it could affect the replicability of our data. Here is an example test we conducted to compare a few configurations:
All other parameters being equal, and in particular always using a 30% infill, we printed Makerbot’s PLA on a Makergear M2 with the following parameters:
We then conducted some mechanical testing, and these are the results we obtained for the strength of each of these samples:
We can see that the thicker the outer surface, the stronger the specimens are: from 4 MPa with no perimeter, tops or bottoms to 18 MPa with 2 perimeters, 2 top and 2 bottom solid layers. Of course, part of it is due to the fact that the outer surface increases the amount of material extruded into the specimen. But even after adjusting for the weight of the specimens, we observe a similar trend: 1.2 MPa/g with no perimeter to 3.2 MPa/g with 2 perimeters, 2 top and 2 bottom solid layers.
The outer surface significantly affects mechanical performance, and should be given more attention.
Why it matters to 3D printer users
As 3D printing gets increasingly used for functional parts, 3D printing users will need mechanical data they can trust.
While an injection molded part is made of a homogenous, isotropic material, a 3D printed part is actually a structure, and as such the toolpath parameters make a big difference. We already pointed out the influence of infill percentage, layer height and infill pattern in a previous article. The outer surface is another key component of the toolpath, and is one more parameter that makes it difficult to get replicable, comparable data via mechanical testing.
All other things equal, if a testing lab always prints with the same number of perimeters, top and bottom solid layers, it will still obtain a decent relative measure between materials. However, as 3D Matter is now developing data that needs to be reliable in absolute value, we are setting a testing procedure that truly tests the parts’ properties.
The outer surface is a bit special because it makes the mechanical performance of the object geometry-dependent. Simply speaking, the outer surface’s influence depends on its thickness relative to the size of the object. The thicker the outer surface as a percentage of the total object’s volume, the greater the influence. For example, if you print a very small part with a 30% infill and two perimeters/tops/bottoms, it will be much stronger – per unit of volume – than a very big part with a 30% infill and two perimeters/tops/bottoms. The former will behave more like its outer surface, while the latter will behave more like its infill:
This is not only true for two different-sized objects, but it is also true for two different areas of the same object.
A way to remain geometry-independent for mechanical testing is to remove the outer surface from the equation. Therefore 3D Matter now mostly conducts mechanical testing on specimens printed with no perimeter, no top and no bottom solid layers.
However, to model the performance of a specific object, its outer surface needs to be included as well. So 3D Matter is also working on a procedure to model the outer surface separately.
 In this article, we have chosen to show the visually striking example of 30% infill, but even at 100% infill, we have measured that the outer surface has a big impact on mechanical performance, in particular for elongation at break