Pill containers. Plastic furniture. Containers for cleaning fluids. Rubbermaid storage bins.
Since its origins in 1951, polypropylene (PPE) has become one of the most common thermoplastics, accounting for over 22% of global plastic production (73.7 million tonnes of 330 million tonnes) in 2016. PPE is similar to polyethylene in many ways, but is stiffer and has a higher melting point. While a large proportion goes into fibres and storage containers, it has found increasing use in automotive applications where its low density and high strength has made it very useful in the ongoing drive to reduce vehicle weight. It has also found growing use in medical applications and is used to augment asphalt for road construction. Polypropylene is everywhere.
Why is it only now gaining traction in the 3D printing industry?
Despite its many virtues, there are some challenges when working with polypropylene. The good news is that solutions have been discovered and those challenges can now be mitigated. Read on!
The Strengths of Polypropylene
- Lightweight and Tensile strength
PPE has a high tensile strength (4,800 psi) and yet is the lightest of commodity plastics, boasting a density of between 0.895 and 0.92 g/cm³ (compared to 1.04 g/cm³ for polyethylene and 1.15 g/cm³ for nylon)
- Elasticity and Fatigue Resistance
PPE is considered to be a ‘tough’ material in that it can plastically de-form without breaking. It will retain its shape following a fair amount of torsion, bending and/or flexing. This makes it an ideal material when making living hinges like those on shampoo or detergent lids.
- Chemical Resistance
PPE is resistant to diluted bases, acids and organic solvents. This makes it a good choice for gas cans, cleaning agent containers, first-aid products, and more.
- Water resistance
PPE is hydrophobic. This means it does not absorb water, and when removed from an immersive state, dries quickly. This property is essential for total immersion applications in medical and industrial applications.
PPE has a very high resistance to electricity and consequently is very useful for electronic components.
The USFDA has rated PPE food-safe. It is BPA free. Biological factors, such as bacteria and fungi, will not cause it to mold or deteriorate in quality.
- Transparency/Optical Clarity
Although PPE can be made transparent, it is normally produced to be naturally opaque in color. PPE can be used for applications where some transfer of light is important or of aesthetic value.
3D Printing Polypropylene: the Challenges
Like other thermoplastics, polypropylene has some drawbacks. It is UV-sensitive, has a high coefficient of thermolinear expansion, and is susceptible to chlorinated solvents.
When 3D printing, PPE has historically presented two sticky (or non-sticky?) challenges that have made it a bit daunting to 3D print:
- This material really doesn’t like to stick to anything other than itself
- Its semi-crystalline structure and high rate of thermolinear expansion make it prone to warp
Thankfully, these concerns can be mitigated successfully.
And the Solutions!
- Temperature Control
Do your best to control the temperature throughout the printing process. Stay within 5 degrees of the recommended extrusion temperature. If possible, print in a heated build chamber. If you do not have ready access to one, enclose the build environment to prevent drafts (and thereby minimize temperature fluctuations).
- Increase Bed Adhesion
Use brims and drafts in your design. These increase the surface area of the first layer of your print. This will reduce concentrated points of warping stresses and increase the quality of your print. After printing, they can be snipped or peeled off.
Alternatively, adhesives like Nano Polymer Adhesive while designed for high performance filaments have shown a dramatic, universal applicability.
- Select the Right Filament
Polymers like PPE can be formulated with different additives to enhance certain characteristics. For example, 3Dtech introduced Hyperlite PPE. This product features an ultra low density filament that is more resistant to warping. Some filaments are produced of blended or filled PPE product that features a particular mechanical characteristic, such as tensile strength.