Updated on April 3, 2018
In this post we are going to discuss an issue of unwanted visible horizontal lines that appear on a 3D printed surface. Although, those horizontal lines in resin 3D prints are very annoying and disturbing, it is possible to name the main reasons why they appear that way and how to reduce the likelihood of such formation.
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.
1. Sudden changes of exposure time from one layer to another.
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.
2. Change in lifting speeds.
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. Therefore, that may cause stretches and deformations that are different from previous layers.
3. Unexpected stops and pauses of the print process.
If for some reason you decide to pause a 3D print, this will also vastly increase chances of visible horizontal 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.
4. Changes in model structure.
If you experience sudden changes of the model structure, it will also typically result in visible horizontal lines. For example, transition from hollow to solid part of the model may result in a line. Also transition from a small cross-section to a large cross section area could also cause a line to appear. 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. This variable shrinkage 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. In other words minimize transition 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.
5. Unstable position and foundation of the part.
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 horizontal lines in 3D printed surface. Such position changes will typically occur because of the following reasons.
- Lack of supports. Insufficiently supported areas will shift way easier and cause misalignment among layers.
- Insufficient exposures. This especially applies to base and support layers, because they have to withstand the highest number of separation cycles from FEP or PDMS (for bottom-up printers).
- Material properties. 3D printing resin may be too soft and flexible.
Robust and sturdy foundation with proper dense of supports will help to keep your part stable during the process. As a result, it will 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.
6. Resin in the tray is disturbed during exposure.
When trying to avoid surface marks and horizontal 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.
7. Durability of Z axis.
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.
8. Layer separation from FEP or PDMS.
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.
9. Material settlement (sedimentation).
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 horizontal lines on your 3D prints 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.
10. Incorrect printing parameters and general mistakes.
Apart from all previously mentioned issues that might cause surface marks and horizontal lines, you always have to keep in mind that there is some random factor. In most cases, horizontal lines and surface marks can be the result of completely wrong 3D 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. So if you really desire great surface quality, go for conservative printing settings and run extensive calibration procedures before printing sensitive 3D prints 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 horizontal lines.
If you need more help or you have found mistakes in this post, drop us as an email and we will respond to you in 12 hours or faster.
Don’t miss other AmeraLabs articles to find out more about calibration:
- Key things to know before calibrating resin 3D printer
- Impressive calibration part for resin 3D printers together with the guide on how to understand its features
- Attachment Layer in SLA 3D printing: what you need to know
- How to design parts for SLA 3D Printing
- 6 Key Principles of 3D Printing Supports that Work
- How to Make Jewellery with Silicone Mold
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