3D Printing Resolution: Printing onto Plastic – Q&A

Posted By Sculpteo on Mar 19, 2015 | 0 comments

How do you take into account thermal shrinkage and printing resolution when you’re designing your 3D model? Matthew, process engineer at Sculpteo, explains the process in detail and give tips for your mechanical parts. For this  post we will focus on our plastic material which utilizes Selective Laser Sintering (SLS) technology.

•  Is 3D printing resolution a topic you’re following at Sculpteo?

Absolutely! As a lot of our clients produce mechanical parts, we have set up calibrating processes and quality control measures to ensure consistency and repeatability of a part. It’s imperative for direct digital manufacturing: when you’re producing hundreds or thousands of parts that will be assembled in a final product, you need to ensure that you are delivering exactly the same thing every time you make the piece. That being said, there are limitations to the technology, and there is a limitation between the dimensions of a CAD file and the dimensions of the final product.

• How does the laser sintering factor the printing resolution?

As you know, 3D printed polyamide is produced by SLS technology. During the 3D printing process, the laser melts consecutive layers of polyamide powder to solidify the shape of the object. So the laser is working in X and Y dimensions and the printer builds the Z dimension by adding thousands of layers. This means that we cannot expect the same tolerance in X and Y dimensions on one side, and in the Z dimension on the other side.

Let’s start with the Z dimension, which is the easiest. When you are creating your 3D file, you have to keep in mind that we cannot be more precise than the thickness of one layer. The layer thickness we offer at Sculpteo could be 60 microns, 100 microns or sometimes 150 microns.  So there’s no need to be more precise than the layer thickness. Let’s take an example: you are designing an object with a Z dimensions of 2.675 cm and you want to get it 3D printed on 100 microns printer. At Sculpteo, we will 3D print a Z dimension of 2.67 cm.

The X and Y dimensions are impacted by the diameter of the laser. Standard laser have a diameter of 400µ on the smallest SLS printers and 700µ for the bigger ones. As you can see, the size of the laser’s diameter is an important factor for respecting the dimensions!

The last factor is the thermal shrinkage.  The build chamber and the laser are heating the powder during the 3D printing process and then cooled down. That results in minor shrinkage of the final object. That retraction is an effect of the change in temperature. So for each printing batch, we have a set of cooling techniques which ensure the most accurate prints with respect to the 3D model.

• Which tolerance are you assuring at Sculpteo?

The polyamide objects we 3D print have a maximum precision of ± 0.3mm for pieces 99mm or smaller and a global precision of ± 0.3% for pieces 100mm or larger. For parts that are less than 1.5mm the tolerance could be higher due to the laser’s diameter.

•  I have printed an object 2 months ago and I need to reprint one : would it be exactly the same?

Not necessarily. If you plan to have several part printed, consider ordering them in a batch, you will have a better consistency on printing 10 X 1 item in a single job instead of printing 1 item in 10 different jobs.

•  How can I calculate in advance the maximum tolerance that would apply to my design?

I have created a form that will give you an idea of the maximum and the minimum dimensions of our SLS printers.

Here you can enter the dimension you want to check (for example 1.234):
mm
Will give you a dimension between:
mm and mm

I recommend to have a look on our design guidelines to prepare your model in any case.

• If I receive an object that doesn’t respect this tolerance, what should I do?

We apply different quality control methods on batch controlled items and single 3D prints. For single 3D prints, a part of our quality control consists in random picking of parts in our production workflow to validate that the tolerance is respected. But it’s not 100% and it happens that we send a part that is not respecting the tolerance. In this case, you need to send an email to [email protected] with a picture of the part and your order reference. If you can show the measure with a caliber, that quickens our the verification steps! In every case, we will reprint your object right away at no cost to you.