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 the misalignment of one layer with the next single or multiple layers. But why does this misalignment happens in the first place? There are quite a few reasons for that. So, let’s analyze them individually, together with potential solutions for minimizing each effect.
Please note that we are not talking about natural layer lines that may be visible throughout the whole surface of a 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 in 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 the attachment layer, foundation, body of supports, etc. These are critical to a successful printing process. However, modern slicers like Voxeldance Tango and Asiga Composer allow setting different exposure zones to the whole model. If you have to make changes to exposure times for the 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 costs unless it is absolutely necessary.
2. Change in lifting speeds.
The same applies to lifting speeds as to sudden changes to exposure times. SLA 3D printing does not tolerate unexpected changes to printing parameters at any time during the process. Consequently, when base layers and support layers have been completed and the printing process has stepped into the layers of the model, we do not recommend changing the lifting speeds. Changes in lift velocity will result in different mechanical stresses that subsequent layers must withstand. Therefore, that may cause stretches and deformations that are different from previous layers.
3. Unexpected stops and pauses in the print process.
If, for some reason, you decide to pause a 3D print, this will also vastly increase the chances of visible horizontal lines and marks. The main reason for this is the shrinkage of the 3D resin that you are using. Although some 3D printing materials are characterized by higher shrinkage properties than others, all of them more or less suffer from this problem. If you stop your 3D printing process, the shrinkage of the 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 the resin 3D printing process.
4. Changes in the model structure.
If you experience sudden changes in the model structure, it will also typically result in visible horizontal lines. For example, the transition from the hollow to the solid part of the model may result in a line. Also, a 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 in 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 a transition. In other words, minimize the transition from a small cross-section to a large cross-section area, from hollow to solid, and vice versa. This can be done by properly orienting parts 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 the position of the part will result in visible horizontal lines in the 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 must 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.
A robust and sturdy foundation with proper support density will help keep your part stable. 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 can greatly improve their rigidity. Lastly, you should also consider if the material that you are using is suitable for your print. Just changing your 3D printing resin to a bit harder or with lower shrinkage properties can vastly reduce the chances of deformation.
6. Resin in the tray is disturbed during exposure.
When trying to avoid surface marks and horizontal lines, an undisturbed resin in the tray is another key factor to consider. Any disturbances of resin will affect the 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 the consistency of the material. If you pour more of it into the resin tray, you can, for example, affect the homogeneity of pigment. Non-homogeneous pigment concentration will definitely affect light-blocking properties and, thus, could lead to loss of homogenous layer structure.
Another common culprit behind the appearance of horizontal lines in resin 3D prints is the use of a small heater inside the printer that blows directly at the print. In some printers, manufacturers place the heater beneath the printer cover, and at times, it may be oriented towards the resin vat. This can lead to challenges during printing. To mitigate this issue, it’s advisable to either disable the fan while printing or create a redirection mechanism to ensure the airflow is directed away from the printing area.
7. Durability of Z axis.
An unstable and wobbly Z axis can be a major source of various SLA 3D printing imperfections. If you have a 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 a wobbly Z axis. Observe how the Z axis moves and if the 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 a wobbling effect in the future.
Moreover, remember to keep your threaded rod clean and slightly lubricated. Any dust or dirt residue can significantly impact the fluid movement of the Z axis. Without the 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 have a somewhat firm bottom of resin tray coated with either PDMS or FEP film. Due to the quite firm bottom of the 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 (the side actually depends on the 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 the level of forces they withstand.
9. Material settlement (sedimentation).
3D printing resins consist of various components. When you mix them, most of them 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 differences 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 stirring and disturbing the resin can also increase the 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, resulting in a 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 the 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 to find the most suitable printing parameters. Finally, do not forget to shake your 3D resin bottle well because non-homogeneous material can also vastly increase the likelihood of horizontal lines.
What’s next?
If you need more help or have found mistakes in this post, email us, and we will respond as soon as possible.
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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