If 3D printing wants to establish itself as a fixture among manufacturing technologies in the world of plastics, it won’t be able to avoid polypropylene (PP). PP is one of the most widely used plastics, especially in the packaging, electrical and automotive industries.

PP is mainly processed into products by means of injection molding. As a 3D printing material, PP is not yet so widespread, especially in comparison with the widespread use of, for example, PA12 as a 3D printing material. The reasons for this lie in the properties of polypropylene and the challenges associated with the respective 3D printing technology. In the following, we would like to take a closer look at 3D printing of polypropylene and show how the material can nevertheless also be processed additively.

What is PP (polypropylene)?

Polypropylene, like most other plastics, is a thermoplastic. This means that the material can be melted and shaped when heat is applied. Even after cooling, PP can be melted again and reshaped. This is a property that is absent in duraplasts or elastomers, for example, because the molecular chains change too much.

Both the chemical and mechanical properties of polypropylene make it a very popular material in a wide variety of sectors. In the packaging industry, polypropylene comes into its own with its chemical resistance to solvents and greases, as well as most acids and alkalis, and its hydrophobicity. Another industry sector in which polypropylene is used is the automotive industry. It benefits from the material’s light weight and high toughness and impact strength. For example, PP is very difficult to break and does not usually cause sharp edges when it does break.

But like any medal, PP has a second side. The surface of the thermoplastic is extremely inert, so it can only be bonded with appropriate surface treatment. 3D printing polypropylene requires extremely precise temperature management. As a thermoplastic, it is particularly suitable for filament or powder bed-based 3D printing technologies.

What are the challenges of 3D printing polypropylene?

The property profile of PP makes additive processing challenging. Due to the material’s hydrophobicity, for example, it is more difficult to use with 3D printing technologies that use a water-based ink for sintering. Moreover, as a thermoplastic, PP is highly heat sensitive and thus requires precise thermal management as a 3D printing material to ensure stable layer bonding and control warpage of 3D-printed components. The crystallite melting range of PP is approximately 160-165° C.

The low surface energy of the plastic makes it difficult to process using the classic binder-jetting 3D printing process, in which a powder material is bonded with a complementary binder. Instead, all those 3D printing technologies are required that work by means of energy input in the form of heat to generate a layered composite.

Translated with www.DeepL.com/Translator (free version)

Which 3D printers can process PP?

As a 3D printing material, PP is often processed as a filament with the FDM 3D printing process or as a powder with the SLS 3D printing process or the HSS process. In addition, it is possible to process PP as a composite material, for example by adding glass fibers to the filament in order to increase the mechanical strength.

In FDM 3D printing, the PP printing material is extruded as filament through a heated nozzle on a build platform. The nozzle controls the melting of the material and must therefore be compatible with the sensitive melting behavior of polypropylene. To fix the components on the build platform, special films can be laid out on it. This guarantees stable 3D printing.

In SLS and HSS, PP is used as a 3D printing material in powder form. In the SLS process, the powder is applied to the build platform by means of a recoater and sintered layer by layer by laser energy. The HSS process works in a similar way. Here, too, the powder is applied to a build platform. Then, those areas of the bed where the component is to be formed are colored with an infrared light-absorbing ink. To melt the powder, the build area is irradiated with an IR lamp. The inked areas absorb the energy and fuse, while the unprinted powder remains loose and can subsequently be recycled and reused.

What can be made from PP using 3D printing?

In the packaging industry, PP is used for containers for alkalis, solvents, etc. thanks to its chemical resistance. In addition, it is also food- and skin-compatible and can thus be used in the B2C and end-consumer sectors. In the automotive industry, PP is used for dashboards, battery housings and other elements of car interiors. Electrical engineering also relies on the properties of polypropylene, for example in the manufacture of transformer housings or cable sheathing.

Somewhat more exotic uses of PP can be found in Australia and New Zealand, among other places. There, banknotes are made from polypropylene.

You can read about how other materials behave in 3D printing here:

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