The Polypropylene (PP) is a new material for SLS®. This polymer is already used in about 1/3 of all plastic injected parts in the world so it has the potential to become the new standard in Additive Manufacturing (AM) applications as well.

At BASF Forward AM, it was clear that they needed to work on a PP powder that would fit the 3D printing needs and applications – to make the convincing material benefits of PP accessible to customers in AM and to jointly drive its industrial adaption. The PP development for powder bed fusion started in 2016 and the first product has been on the market since 2018 to become more and more adopted by the users.

Passionate by 3D printing and powder bed fusion, Dr. Stefan Josupeit, Head of Business Line Powder Bed Fusion at BASF Forward AM, gives insights on the challenges and opportunities of this advanced material

What were the main challenges of developing PP powder?

“A good powder bed fusion material has to be tailored for the SLS® process. We did work on the best formulation to combine great processability, thermal stability and printed part performance. We also took particular attention to the particle properties. But I think the best process performance can only be achieved by a close collaboration between the material supplier and the machine manufacturer to really provide an overall system solution with best compatibility and user experience. Our team works closely with Prodways Tech engineers to adapt the Polypropylene for SLS production® and launch the PP 1200. Thanks to the recoating technology, the Prodways printers’ reliability and thermal control, the PP 1200 is very easy to process on ProMaker P series printers.

Moreover, Prodways made an amazing development with the new 3D printer ProMaker P1000 S that perfectly fits several innovative materials, including PP 1200.”

What are the Polypropylene specific capabilities?

Polypropylene is characterized by a low density, a good chemical resistance and a high ductility.

A lower density also means a lower cost because you need less material for the same printed part volume. It is interesting for lightweight parts aiming at energy saving. And more applications are possible, especially if you consider the compatibility of the material with chemicals. The high ductility allows for the printing of very flexible parts. And finally, it is a very stable material in terms of aging so that the material can be used with a quite low refresh rate, saving resources and energy. For PP 1200 the refresh rate is 50%, but there is no limit of cycles.

Also, as a well-known plastic, the PP can be recycled which is more and more important for sustainable production. And as PP is used in many industries, of course, you could easily think about a second life of printed parts.
Finally, by using PP, you can substantially enlarge the application scope at more affordable cost compared to polyamides for example.”

Is PP a disruptive material in comparison with PA11 or 12?
I would say that we need to look from two different viewpoints.
Let’s first start with the viewpoint of the traditional injection molding plastics industry. PP is a very commonly used material, so the answer would be no. But this can be a huge advantage for the customers, even if it does not sound disruptive – because they know the material very well and they can continue working with the same base material type.

On the other hand, looking through the glasses of Additive Manufacturing and powder bed fusion, PP is something super disruptive – because the market is mainly using PA12, which is a good material but very specific to the “3D world”, and for many applications not the best choice.
If we directly compare printed PP to PA11 or PA12, most of the key mechanical properties like stiffness and ductility are on a similar level, while tensile strength is a bit lower due to the nature of the material. In the end, we need to look into the individual part quality requirements anyway, which have to match the material performance. This could include for example chemical resistance, in which PP outperforms many polyamides.”

What is the main difference between parts made in AM Polypropylene and injected Polypropylene?

“I think it’s very important to address the question because it’s important to manage expectations. As I said PP is a well-known plastic and it is the same material for both processes. But, due to the specific 3D manufacturing process, the heating & cooling rates and so on, the properties of the printed parts can be different even if the same base material is used. However, this does not mean that the parts are worse, they are just a bit different. This not only refers to PP, but to any powder bed fusion material. It just needs to be considered in application development, in which we support our customers very closely.

For example, printed parts often have a limited ductility. Our PP’s ductility still high considering the 3D printing process, but lower than what you might expect from injection molding. On the other hand, the strength and stiffness are higher compared to injection molding. Again, these trends are more or less similar comparing injection molded with 3D printed parts in general.

However, we have to be aware that in injection molding there are hundreds, maybe thousands of different PP grades for certain applications from super tough to super stiff, while we are talking about one grade in powder bed fusion, so we might need to adapt designs and requirements. That’s why it would never be a blind substitution.”

What kind of applications is the most suitable for PP?

There is a broad scope of applications, for example industrial parts in contact with corrosive media such as fluid tanks and guides, but it can be any kind of manufacturing series parts where PP is the optimum choice. Other examples of applications are parts that are somehow in contact to the outer skin, for example medical parts in the orthotics or the prosthetics sector. I would like to highlight that there is already a skin contact statement available. Many of our customers in addition appreciate the recycling capability of printed parts, as PP has already a big share in plastic recycling streams.

Last but not least, the material can be perfectly used for sustainable and cost-effective functional prototyping without need of a material switch between the prototype and the finally injected part.

These are only examples. We are always happy to look into new applications, because finally we’re in the 3D World and we never know which new application will take off tomorrow.”

We know that PP is common in food packaging application, do you think of AM PP with food grade?

“The current PP is not a food contact grade, unfortunately. But we do work on an next generation PP material with a food contact suitable formula. The idea with this new PP is not only to enter food contact applications, but also to enable use for cosmetic packaging, which is more and more requested.”

What are the latest developments planned & innovation in terms of post processing?

“You can apply the same standard mechanical post-processing methods like mass grinding or sandblasting to obtain already a nice surface finish compared to “traditional” AM materials. And if you like to really have smooth and aesthetically convincing parts, you could apply vapor smoothing. We are collaborating with companies that offer this technology adapted to PP. With Vapor smoothed, PP parts look really amazing! For thin-walled parts, we can even achieve a slight translucency of parts, so you can look inside or through, which is something completely unique for powder bed fusion materials.

We can also look into coating technologies, and we know that this is important especially for the consumer segment where the parts’ aesthetics play a major role. What we observed with PP is that it’s a bit trickier to coat due to the chemical nature. So we developed a coating solution including a primer which can be applied to the part first and then you can apply any top coat afterwards.
At BASF Forward AM, we also offer our own coating solutions tailored to the 3D printed parts. We do have a range of flexible coatings that can be even ductile, so you can bend the part and squeeze it and the coating will not break.”
www.prodways.com

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