At best, a 3D printed part has strength equal to the strength of the underlying plastic material. Usually the strength is actually limited not by the plastic’s material properties, but by the strength between layers. Therefore, layer-to-layer (interlayer) adhesion is an extremely important element in the overall strength of a 3D printed part. CNC Kitchen on YouTube recognized this as well, and so he has created a wonderful video series around the idea. All of the graphs on this page come from his excellent work.
To get the strongest layer-to-layer adhesion in a 3D print, you can follow these tips:
- Layer Height: Layer height affects how forcefully the extruder must press molten plastic against the layer underneath the nozzle. The closer the nozzle is to the printed part, the stronger the interlayer adhesion, so lower layer heights result in strong prints – to a point. If your layer height is too low, you will run into extrusion problems, and perhaps underextrusion which will decimate your part’s strength! On a 0.4mm nozzle, the lowest most printers can easily print at is a layer height of around 0.12mm.
- Extrusion Width: According to CNCKitchen, the ideal extrusion width for strength is approximately 1.4x(nozzle diameter). For a typical 0.4mm nozzle, that means the strongest extrusion width is 0.56mm. You can see the material strength (red line) is never quite obtained, because the interlayer adhesion fails before the bulk material.
- Extrusion Temperature: The extrusion temperature can also affect layer-to-layer adhesion. The hotter the layer underneath is, and the hotter the new layer is, the stronger the adhesion between layers. This leads us to the next subject: Cooling!
- Cooling: However, test data actually shows that PLA (blue) and PETG (orange) become stronger in z (the direction affected by interlayer adhesion) if you lower the cooling speed! There are negatives related to lowering the cooling fan speed, but if you want a very strong part, it may just be the ticket.
- Controlled Extrusion: Underextrusion is the bane of strength. When there are gaps between material, it’s obviously going to lower the part’s strength. Make sure you have well controlled extrusion by following our other guide here.
- Bed Leveling: Getting a good first layer is absolutely crucial. If the first layer is off, then the actual layer height is off for the first few layers. Make sure that your bed is very well leveled to ensure high strength parts.
- Print Speed: Printing slower can increase the amount of time each layer has to bond with the layer below it, improving adhesion. However, printing too slowly is often the last thing we want to do, because it makes the print take even longer.
By following these tips, you can improve the interlayer adhesion of your 3D prints, resulting in stronger, more durable prints. If you have any questions or suggestions, please drop them in the comment section!