Updated on September 2, 2021
This time we will take a look at one widely discussed issue of resin 3D printing called “blooming” and discuss how we could use light-off delay setting to avoid it. Although, this issue consistently happens with LCD/LED or so called mSLA 3D printers, it can also occur with genuine DLP 3D printers. This article should be interesting to both beginners and experienced 3D printing professionals as well. So, let’s dive in.
Recently, resin 3D printing community created the term “blooming”. It refers to the issue when after taking 3D printed object from the build plate, it has greasy surface that reminds of mucus or gel. You can usually wash away this gel, but even then you get uneven and rough surfaces. Probably one image is better than a thousand words.
After reading this article you will know why this issue happens and how you can use light-off delay and other techniques to avoid it.
Sources of the issue
Perhaps the most noticeable uniqueness of resin 3D printing is its materials. Liquid, photosensitive resins together with appropriate light sources are used to transform liquid material into solid. In the most popular bottom-up resin 3D printing method, a reservoir (also called VAT) is filled with liquid resin. 3D printer prints object upside down on a metal build plate. After each cured layer, build plate moves up, delaminates freshly cured layer from the bottom of resin tray and moves down again to cure subsequent layer. When all layers are completed, you have already 3D printed your object, which is now hanging upside down.
After 3D printer prints each individual layer, build plate has to go up and back down. During downward movement, previous layer and its cross-sectional area squeezes liquid 3D printing resin out of its way. When build plate gets down completely, only a small gap (depending on your layer thickness) filled with resin remains between the bottom surface of your resin VAT and previously cured layer. See image below.
The problem lays in squeezing motion of resin between previous layers and the bottom of the VAT. These liquid turbulences are the root cause of blooming effect.
However, that does not happen every time. That is because specific conditions must be in place to produce this undesired effect. “Blooming” is produced by premature start of new layer exposure. Simply saying, your 3D printer displays new layer on LCD screen or projector too early. So early that resin is still moving while being pushed out of the gap between the bottom of resin VAT and topmost surface of previous layer. If your resin moves when exposure of new layer starts, it will start to cure. Liquid resin will start to become gel, i.e. turn from liquid to solid, while still being squeezed. Thus, “blooming” is simply prematurely gelled material that your 3D printer pushed out of the gap between VAT bottom and previous layer. This often results in gel-like mucus on your object walls.
Next, we will run through all of the factors we were able to think of that have influence on “blooming” effect.
Factors that increase the odds of “blooming” effect
Material curing speed
Modern fast curing materials tend to have more severe issues of such premature curing. In short, if material is slower, it will not start to cure and transform into gel too quickly even while moving. This would somewhat prevent premature gelation of material. However, if you have high viscosity and relatively fast curing material, just imagine how this will end up! Even relatively slow materials became quite “fast” with the rise of monochrome LCD screens. With higher light output and shorter exposures such issues like “blooming” became more prevalent.
Cross-sectional area
High cross-sectional area of your layers means that 3D printer has more material to push out of the gap to prepare for the new layer. Simply saying, if you print non-hollowed models or models with very thick walls (more than 2.5 mm), you have to be very careful and adjust your printing settings appropriately in order to avoid “blooming” effect. While properly hollowed models will rarely face this problem.
Material viscosity
Material viscosity plays an important role here. More viscous materials tend to be a lot more difficult to squeeze out of thin gaps due to natural viscoelastic properties. One can image liquid honey and, for example, olive oil. An attempt to squeeze both of them between two metal plates should clearly indicate, which requires more effort. Simply saying, it takes more time to push out higher viscosity materials out of the gap and prepare for the new layer.
Temperature
Influence of temperature might be negligible here. But what works in winter, may not work in summer. Temperature changes can significantly affect the viscosity and cure speed of the materials.
Down movement / retract speed
Retraction speed is also important. This speed determines how fast the build plate moves from the highest position of the cycle to the lowest, i.e. the bottom of the VAT. When the build plate moves down, it pushes resin out of its way. The faster you go, the higher liquid turbulences will be.
How to solve “blooming” effect?
Since problem is complex, there are two main ways to overcome this issue. Let’s discuss them one by one.
Apply light-off delay setting
This is the most important setting to overcome “blooming” effect. Most 3D printers and slicers have this setting, you only have to find it. Some call it wait-time, some light-off delay. Essentially, this setting determines the delay of very next layer exposure when build plate is already at its lowest position. Right after build plate gets to the bottom, it stops, system waits for this pre-defined duration and then turns on light source and starts exposition of new layer. This delay brings more time to fully squeeze the resin out of the gap between previous layer and bottom of the VAT.
0.5 sec to 1 sec of light-off delay is generally enough to fully overcome blooming.
But you must be careful, as different resin 3D printers treat this setting differently. Some 3D printers actually require you to input just pure light-off delay time, like 1 second, but there is another side of the story here. When using other 3D printers, you will need to enter the duration of a complete layer preparation cycle. I know it sounds confusing and, to be honest, it is! Let me explain it in more detail.
Simply saying it is the time between exposure finish of one layer and exposure start of the next one. The problem here is that determination of this setting might be very tricky and this value can change dramatically if you change some other settings (like motor movement speed or lifting heights). There is no other way to measure it apart from empirical one. First, you have to set you desired motor speeds, lifting heights and then do a “dry run” with your 3D printer, i.e. 3D print without resin. Measure the complete duration of motor movement cycle with chronometer and add roughly 1 second. That is your new light-off delay. Enter it to the software and double check with another “dry run”. If you see that after motor has finished its movement there is approximately 1 second pause before light source is turned on to start cure of a new layer, you are good to go.
Reduce cross-sectional area as much as possible
This is quite worn out recommendation, but it applies to many resin 3D printing issues. We covered how it creates higher suction forces in our previous article about 3D printing supports. If you print non-hollowed models with large cross-sectional area of layers, you will have difficulties with “blooming” effect too. Large area will result in severe liquid turbulences and difficulties in pushing it out of the gap between VAT bottom and previous layer. Simply saying, it will require more time and, thus, increase your waiting and printing time. Therefore, if possible, reduce cross-sectional area of your prints as much as possible. You can do that by properly hollowing and orienting your models.
3D printing experiment
Let’s make a quick experiment that would help to understand how “blooming” effect is present with various light-off delay and retract speed settings. We did not mean to make this trial extremely scientific. Its purpose is to show this phenomenon in a simple manner and also how some specific 3D printing settings affect results.
For this experiment, we 3D printed a cube with dimensions of 30 mm x 30 mm. We did not hollow this cube on purpose to exaggerate the effect for visual purposes. We 3D printed with TGM-7 resin on Phrozen Sonic Mini 4K at 50 um layers and 2.5 sec exposures. After printing objects were cleaned with isopropyl alcohol and post cured for 30 minutes. Our 3D printing room has a controlled environment with an ambient temperature of 25 °C. Let’s review the results.
0 sec and 1 sec light-off delay. 160 mm/min retract speed.
It is clearly visible that absence of light-off delay produces significant surface “blooming” defects. On the other hand, 1 second light-off delay completely eliminated “blooming”.
Let’s take a look at how severe “blooming” effect is with various light-off delay settings.
0 sec and 0.25 sec light-off delay. 160 mm/min retract speed.
As previously mentioned surface damage is clearly visible with 0 sec light-off delay. But even with 0.25 sec light-off delay there is a significant improvement, although, some defects are still seen.
0.5 sec and 0.75 sec light-off delay. 160 mm/min retract speed.
At 0.5 sec light-off delay, scars on the surface are still barely present, however, at 0.75 sec – almost no “blooming” effect is present.
1 sec and 1.25 sec light-off delay. 160 mm/min retract speed.
Ignore the fact that there is just a half of the cube. Our bad… But results are clearly visible – with 1 sec and more of light-off delay, there is a negligible effect of “blooming”.
0 sec light-off delay. 20 mm/min retract speed.
By the way, we also ran a quick experiment to try if drastically reduced retract speed joined with 0 sec light-off delay can improve surface quality and reduce “blooming”. Result are below.
As you can probably see, very low retract speed doesn’t help. Surface damage is still visible.
Conclusion
Experiment clearly showed that even very short light-off delay times can significantly improve the quality 3D prints. Meanwhile durations above 1 sec seemed to be unnecessary, but can be considered in some cases with large cross-sectional areas. Last, but not least, reducing retract speed does not replace light-off delay and does not completely solve “blooming” effect. Therefore, you need at least short light-off delay to get decent surface quality.
Other articles from AmeraLabs
- Impressive test print for resin 3D printers together with the guide on how to understand its features
- Experiment: 3D Printing Supports that Work
- Why is it so important to shake the resin bottle?
- Why should you not use default 3D printing raft settings?
- How to design parts for SLA 3D Printing
- The Complete Resin 3D Printing Settings Guide for Beginners
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