It is not a secret that if you want to be successful in any kind of 3D printing technology you must know some essential techniques. Knowing these key process related aspects helps you to 3D print desired objects quickly and at a very high success rate. Although, there are quite a few SLA 3D printing techniques for this, we will only focus on one and it is called attachment layer or raft. Although some people call it raft, we will stick to the term “attachment layer” as “raft” is commonly used in filament 3D printing.
Generally, attachment layer is a thin pad on which you lay down your 3D object. It can be incorporated during your CAD process or added with some SLA 3D printing software solutions. It is also worth mentioning that this SLA 3D printing technique is mostly relevant to “bottom-up” approach. This approach of resin 3D printing produces objects upside down.
There are quite a few observable failures that can happen if you do not use attachment layer or use incorrect one. The most common issue that we see is when 3D printed object separates from build plate and falls down. This can also have minor or significant damage to your VAT bottom coating (FEP film or PDMS). Another issue is when part of the model separates from build plate. In this case you will see part of the model still sticking to build plate and part of it hanging in the air. Last common problem is when model sticks to FEP film instead of sticking to build plate. That one might also be because of incorrect parameters of attachment layer.
Like most other 3D printing technologies SLA 3D printing produces objects layer-by-layer by selectively curing 3D resin. After each layer is cured, build plate moves up and separates newly cured layer from FEP film or PDMS silicon. Right after that, process continues with curing the very next layer and its separation from VAT bottom. This process continues until an entire object is produced.
The main problem with such “bottom-up” style resin 3D printing is previously mentioned layer separation action from FEP film or PDMS silicon. When more and more layers of a model are cured, separation of each subsequent layer puts higher and higher stress on all previous layers and their connections. It goes without saying that very first connection between your build plate and first layer has to withstand separation forces of an entire model! Therefore, it needs to stick to build plate extremely well.
So at this point it should be clear that very good adhesion between first layer and build plate is critical element in “bottom-up” style resin 3D printing. To improve this adherence, we have to increase surface area of the first layer. The larger the surface area is, the higher forces that keep it sticking to the build plate. So how do we increase that sticking force?
We cannot simply take first layers of a model and enlarge them to get better adhesion to build plate, because sometimes it can ruin an entire model. On the other hand, we can quite easily add extra layers to the model. To do that, we simply make a pad with your chosen software and place our model on top of it. See images below.
So here we have it: this extra pad is called attachment layer. Although, it is called “layer”, it does not necessarily mean that it must have a thickness of a single layer. Many people use various options, but we strongly recommend to make an attachment layer with a thickness of 1 to 4 normal layers. So if you print with 50 um Z resolution, your attachment layer should be 0.05 mm to 0.2 mm, and if you print at 100 um Z resolution, we recommend it between 0.1 mm and 0.2 mm.
There are pros and cons for keeping attachment layer a bit thicker or thinner. If you make it too thick, there is a risk of unwanted warpage of attachment layer, which will eventually cause decrease in forces and your model will fall down. However, it is usually way easier to remove 3D printed part from build plate when this pad is a bit thicker. On the other hand, by keeping its thickness as low as possible you will ease the stress of post-processing. It is a lot easier to remove and sand thin attachment layer than thick and bulky one. All in all, it is up to you to choose the best method for you.
When it comes to area of attachment layer, there is a rule of thumb that says that when looking from above at the model its area must be 10-20% larger than the entire visible area. This will ensure that attachment layer will have area that is larger than of any other layers.
Having well-designed attachment layer will ensure that your model will stick to build plate as firmly as possible. It will also help to smooth out potential irregularities on your build plate and, thus, produce flat foundation for subsequent layers.
It is important to mention that among community members attachment layers are typically treated as “bottom layers”. Therefore, they require extensive curing times to cure well and to properly stick to build plate. Recommended exposure times for bottom or attachment layers vary a lot. We strongly recommend to make it around 8-12 times longer than your normal layer cure time. Exact numbers should be figured out by experiments. You can also read more about cure times for bottom layers in our earlier article “Key things to know before calibrating resin 3D printer”.
Yes, the need for attachment layer does not change when using supports. After adding supports, do not forget to add attachment layer that covers full ground of your model. This will ensure proper base for support and will dramatically increase likelihood of success of your 3D print. See images below with an example.
Although, there are quite a few software solutions for DLP 3D printing that can add attachment layer, you must decide which one works best for you. One of the easiest solutions would be to use Creation Workshop software. You can find older, free version online. This software package allows you to add attachment layer with a single push of a button. You can scale it to your desired dimensions and easily export updated STL object.
You could also add attachment layer with your chosen CAD software. However, using CAD might not be a straightforward solution. It is very difficult to add additional elements to your model when you only have STL file. When you add supports, you will typically export your “supported” file as STL file. Therefore, adding additional components to that model with CAD software becomes quite complex.
I hope you get the picture, why attachment layer is so important in resin 3D printing now. Go and have some fun 3D printing without the fear of failures. And if you have some spare time, check out our other articles to learn even more:
Also, have a look at our SHOP. We have some great resins to choose from!