Updated on June 14, 2018
This time we will briefly discuss what raft is and why it’s so important if you want to successfully use your resin 3D printer. This article is dedicated to beginners of this 3D printing technology.
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.
What is raft in resin 3D printing?
Generally, raft is a thin pad on which you lay down your 3D object. This pad is made up of the very first few layers you 3D print. You can incorporate it during your CAD process or add 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.
Typical 3D printing problems when there is no or incorrect raft
There are quite a few observable failures that can happen if you do not use raft 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.
Underlying reasons for 3D printing raft
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. 3D printer continues this process 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 the printer completes more and more layers of a model, 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?
Adding raft and its key properties
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.
Raft thickness
So here we have it: this extra pad is called raft or 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 use a raft 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. We dive into details in this separate article: Why should you not use default 3D printing raft settings?
There are pros and cons for keeping raft a bit thicker or thinner. If you make it too thick, there is a risk of unwanted warpage of this 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 raft than thick and bulky one. All in all, it is up to you to choose the best method for you.
Raft size
When it comes to area of raft, 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.
Raft exposure time
It is important to mention that among community members rafts are typically treated as “bottom” or “base” 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 exposure 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”.
Do I need it when I am using supports?
Yes, the need for raft does not change when using supports. After adding supports, do not forget to add raft 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.
What software to use to add raft?
Although, there are quite a few software solutions for resin 3D printing that can add attachment layer, you must decide which one works best for you. Chitubox, Lichee, Creation Workshop – just a few of those worth mentioning. These software packages allow you to add raft 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 raft 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.
Happy printing!
I hope you get the picture, why raft 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:
- How to design parts for SLA 3D Printing
- 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
- 10 reasons why you get unexpected horizontal lines on your 3D printed parts
- Free NanoDLP Helper Spreadsheet
- 6 Key Principles of 3D Printing Supports that Work
Also, have a look at our SHOP. We have some great resins to choose from!