Generally speaking these lines are caused by misalignment of one layer with the next single or multiple layers. But why does this misalignment forms in the first place? There are quite a few reasons for that, so let’s analyze those one by one together with potential solutions of how to minimize each effect.
Please note, that we are not talking about natural layer lines that may be visible throughout the whole surface of 3D printed part. This article is about conspicuous and usually random misalignment between layers. Anti-aliasing and similar techniques, which are used to smoothen the surface, cannot completely solve these issues and are not covered here.
One of the most obvious reasons why surface lines appear is exposure changes throughout the print. Most of us are familiar with the concept of different exposure times for different zones of the print such as attachment layer, foundation and body of supports etc. These are critical to successful printing process, but if you have to make even slight changes to exposure times for model layers, then you can almost be sure that this will result in visible surface lines. Therefore, avoid exposure changes for model layers at all cost unless it is absolutely necessary.
The same applies to lifting speeds as to sudden changes to exposure times. All in all, SLA 3D printing does not tolerate unexpected changes to printing parameters at any time during the process. Consequently, when printing process has stepped into layers of the model (when base layers and support layers have been completed), it is also not recommended to change lifting speeds. Changes in lift velocity will result in different mechanical stresses that subsequent layers have to withstand and, thus, may cause stretches and deformations that are different from previous layers.
If for some reason you decide to pause a print, this will also vastly increase chances of visible lines and marks. The main reason for this is the shrinkage of 3D resin that you are using. Although, some 3D printing materials are characterized with higher shrinkage properties than others, all of them more or less suffer from this problem. If you stop your 3D printing process, shrinkage of already completed portion of the model will be disturbed. Therefore, it can shrink too much to align perfectly with subsequent layers of the model. So if it is not critical, do not disturb the rhythm of resin 3D printing process.
If you experience sudden changes of the model structure, such as from hollow to solid or from a small cross-section to a large cross section area, it will also typically result in visible surface lines. If you have such sudden transitions among layers, this will result in severe volumetric changes of layers. The volume of 3D resin in each layer highly affects the shrinkage of each cured layer. Uneven quantity of 3D printing material among layers results in variable shrinkage which is the main cause of such visible surface marks and lines.
As a matter of fact, the only reasonable way to avoid this cause is to minimize the likelihood of such transition, i.e. from small cross-section to a large cross section area, from hollow to solid and vice versa. This can be done by properly orienting part so that during the 3D printing process cross-sectional area of layers changes as gradually as possible.
If the part that you are printing does not have a solid foundation, it can be easily shifted during the printing. Even micro changes in position of the part will result in visible marks on the surface. Such position changes will typically occur because of the following reasons.
Robust and sturdy foundation with proper dense of supports will help to keep your part stable during the process and greatly reduce the negative impact of mechanical forces of layer separation from FEP film or PDMS. You should also pay thorough attention to exposure times for base and support layers, i.e. overexposing them a little bit can greatly improve their rigidity. Lastly, you should also consider if material that you are using is suitable for your print. Just by changing your 3D printing resin to a bit harder or with lower shrinkage properties can vastly reduce chances of deformation.
When trying to avoid surface marks and lines, undisturbed resin in the tray is another key factor to consider. Any disturbances of resin will affect established homogeneity of the resin at a given point in time. Just by mixing the resin or by pouring a bit more of it, you will change consistency of the material. If you pour more of it to the resin tray, you can, for example, affect homogeneity of pigment. Non-homogeneous pigment concentration will definitely affect light blocking properties and, thus, could lead to loss of homogenous layer structure.
Unstable and wobbly Z axis can be a major source of various SLA 3D printing imperfections. If you have wobbly Z axis, this can lead to surface marks that also feature cyclicality. So, if you notice that marks repeat periodically on the surface with equal spaces, that is a true example of wobbly Z axis. Observe how Z axis moves and if wobbling effect is apparent. If yes, try to change your Z axis threaded rod with a new one, for example, ACME trapezoidal thread. Try to install it as straight as possible to prevent wobbling effect in the future.
Moreover, remember to keep your threaded rod clean and slightly lubricated. Any dust or dirt residue can have a significant impact on fluid movement of Z axis. Without smooth movement of Z axis you cannot expect to print a part of the perfect surface.
During the printing most of the bottom-up style printers have to withstand layer separation. Some 3D printers actually have somewhat firm bottom of resin tray which is coated with either PDMS or FEP film. Due to quite firm bottom of resin tray, they usually exploit tilting action when separating layers from the bottom of the resin tray. This approach tends to create higher loads for one side of the model (side actually depends on tilting design and approach). Because of this the side of the model with higher loads will tend to shift and deform more than the opposite one and, therefore, might cause asymmetrical visible surface marks. If you want to avoid it, you have to consider these uneven forces and support each side of the model according to level of forces they withstand.
3D printing resins consist of various components. Most of which, when you mix them together, can stay homogenous, i.e. uniform. However, that is not the case with some specific components.
Pigments are solid particles that do not dissolve in the resin. Due to difference in density these tend to settle in 3D resin (that is why the label always states “Shake well before use”). Therefore, if you print very long prints with well pigmented resin, you can experience surface marks and lines due to pigment sedimentation.
If you like the resin you use, there is no easy way to avoid this fundamental issue. You can think that mixing during the printing might help, but remember that by stirring and disturbing the resin you can also increase chances of surface marks. One option is to use 3D printing resins that incorporate soluble dyes instead of pigments. These do not settle, because they dissolve in resin and result in uniform solution. Secondly, you can go for AmeraLabs 3D printing materials that have very carefully selected and balanced systems of pigments. Some of them do not settle for at least 1-2 days.
Apart from all previously mentioned issues that might cause surface marks and lines, you always have to keep in mind that there is some random factor. In most cases, lines and surface marks can be the result of completely wrong printing parameters. These can be too long exposure (cure time) for a single layer as well as too fast lift speeds after each layer is cured. Higher speeds mean higher delamination forces are exerted. So if you really desire great surface quality, go for conservative printing settings and run extensive calibration procedures before printing sensitive parts in order to find out the most suitable printing parameters. Finally, do not forget to shake your 3D resin bottle well, because non-homogeneous material can also vastly increase likelihood of surface marks.
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