1. Are Layer Lines on 3D Prints Mistakes or Part of the Method?
If you trace a 3D-printed object with your finger, you feel thin horizontal grooves spaced at regular intervals. It feels like running over tree rings viewed from the side. This is not because the printer is broken.
In the most common type of 3D printing called FDM (Fused Deposition Modeling), soft plastic is squeezed out in a thin line, and thin layers are stacked one by one to build the shape. Each new layer sits on top of the previous one. The edges of these layers remain as steps. These marks are called layer lines. Waseda University states that these lines "always occur in principle" for printed objects [1].
A research paper on FDM notes that the bumps and grooves created when stacking layers are considered the biggest challenge for structures made this way [2]. Therefore, layer lines are not occasional defects. As long as you use this method, they are something you must deal with.
2. Why Slanted and Curved 3D Prints Look Like Stairs
A vertical wall has small steps if you only look at the layer edges. The steps become obvious on slanted or curved surfaces. The actual roughness depends on other factors besides just the layer edges.
Imagine building a sloped ramp by stacking identical flat boards, shifting each one slightly forward. The edges of the boards create a staircase effect, leaving triangular gaps between the real slope and the stepped surface. When a 3D printer makes a round ball, it stacks these slice-like layers, so the same thing happens.
In research on converting 3D models for printing, a model is used to express the height of these steps based on layer thickness and the angle of the surface. According to this model, nearly horizontal surfaces have larger errors, while surfaces close to vertical have smaller errors [5]. This explains why the top of a ball or gentle slopes tend to feel rougher. However, this is a simplified model of the phenomenon and does not measure the actual surface roughness of the machine.
So, is making the layers thinner the only way to make the steps smaller?
3. Thinner Layers Make 3D Prints Smoother but Take More Time
The thickness of a single layer (called the layer pitch) is set in preparation software called a slicer. In FDM printing, a range of 0.1 to 0.5 mm is common, and some recent machines can go thinner than 0.1 mm. The finer you make it, the cleaner the finish looks, but it takes more time [3].
You can understand this by calculating the number of layers. If you make an object 20 mm tall with a layer thickness of 0.2 mm, you need 100 layers. If you change it to 0.1 mm, you need 200 layers. In this example, the number of layers doubles. Printing time gets longer, but the increase depends on the shape and settings. Manufacturer help pages explain that making layers thicker greatly reduces build time but makes the lines visible, while making them thin (like 0.10 mm) shows fine detail but extends the time [4].
There is also a limit to how thin you can go. The nozzle diameter (the width of the hole where plastic comes out) sets one limit on layer thinness. One manufacturer suggests keeping the layer height below 80% of the nozzle diameter. For a 0.4 mm nozzle, that means about 0.32 mm. This 80% rule is a guideline from this specific manufacturer; other companies may suggest different values, such as 75% [4].
In FDM printing, you must balance time and nozzle size to decide how thin the layers should be.
But what do you do with the steps that remain even after making layers thin?
4. Waseda University Research on Melting Away the Steps
There is a way to remove the remaining steps after printing. Researchers at Waseda University, led by Associate Professor Shinjiro Umezu, developed a method using a pen-shaped device that releases a small amount of solvent. This dissolves the raised parts of the surface and fills in the lower parts, smoothing out the layer lines [1].
The research paper shows roughness measurements (Rz) before and after smoothing for different layer thicknesses. For test pieces with 0.3 mm layers, the roughness dropped from about 95 to about 10 after finishing 9 times. For 0.2 mm layers, it went from about 65 to about 10 in 6 passes. For 0.1 mm layers, it went from about 30 to about 5 in 5 passes [2].
Comparing the numbers, just making the layer thinner from 0.3 mm to 0.1 mm reduced the initial roughness from 95 to 30, which is about one-third (95 divided by 30 is roughly 3.2). The number of smoothing passes also dropped from 9 to 5. In this experiment, making thin layers first and then smoothing them resulted in the lowest roughness.
However, solvents are chemicals that dissolve materials, so you must choose a solvent that matches your specific material [2]. This result comes from one study and does not apply to all materials or shapes.
5. Comparing 3D-Print Layers With a Stack of Paper
You can try this with paper. Take 10 sheets of copy paper and stack them, shifting the edges slightly to create a slope. When you run your finger over it, the edges of the paper catch on your skin like stairs. Next, use thinner paper. Adjust the number of sheets and the shift amount so the total height is the same, then make a slope and compare. The steps become finer, and the catching sensation decreases. You can feel that thinner layers mean smaller steps.
If your school, library, or community center has a 3D printer, ask to see an object. Run your finger over slanted surfaces and vertical surfaces to compare them.
If you want to read more, news reports from Waseda University are available in Japanese [1].