Material Insights

TPU - The 3D Printing Polymer with a Twist

Flexible-TPU-Filament

 Thermoplastic Polyurethanes (TPUs) form a unique class of thermoplastic elastomers that have long been taking the manufacturing sector by storm. Structurally, these polymers contain both hard and soft segments, which give them a rubber-like elasticity along with plastic-like mechanical properties. It is this fateful combination of properties that has allowed TPUs to turn the world of additive manufacturing around.

However, it is not the easiest thing in the world to turn TPU into a 3D printing filament. After extensive experimentation, we successfully extruded a flexible, durable and easy-to-print TPU filament here at 3devo. In this blog, we are going to briefly discuss how we optimized the extrusion of TPU into 3D printing filament. Before we do that, let’s take a look at some relevant chemical and mechanical aspects of TPUs.

 

TPU Origins

Back in 1937, German chemist Dr. Otto Bayer and his co-workers discovered the reaction that resulted in the synthesis of TPU. Chemically, TPUs are linear copolymers composed of alternating hard and soft segments, which give the polymer rigidity as well as elastomeric properties. The synthesis of TPUs involves polyaddition reactions with diverse reactants.5 The soft segments are derived from a reaction between diisocyanate and polyester, polyether, or polycaprolactone resins. Meanwhile, the hard segments are obtained through a reaction between diisocyanate and a diol.

The thermoplastic elastomers thus formed are adorned with amazing qualities. They exhibit excellent flexibility, high resistance to abrasion, tear and chemicals, and good dielectric properties. TPUs also have great mechanical properties, including high tensile strength and impact strength.

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TPU in its granular form

Effect of TPU Chemistry on Physical and Mechanical Properties

Altering the reactants and their ratios results in different categories of TPUs, each with a unique set of properties. For example, polyester-based TPUs have greater shore hardness, tensile strength and tear resistance than polyether-based TPUs. At the same time, polyester-based TPUs also tend to have lower impact strength than polyether-based TPUs. Moreover, polyester-based TPUs are more prone to degradation in the presence of moisture and microbes than polyether-based TPUs.

Growing Industrial Applications of TPUs

Given its wide-ranging desirable properties, TPUs have found applications in a variety of industries. Including:

  • Shoes
  • Wire and cable
  • Automotive
  • Medicinal (implant and prosthetics)

More and more in industries are now realizing the special features that TPUs can add to their products. In fact, TPUs are often used as additives to enhance the mechanical properties of other materials. They have even found use as coating materials in the fabric industry.

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TPU is used in various sporting products, including Air Jordans!

TPUs Offering to 3D Printing 

So, what is so special about TPUs from a 3D printing perspective? Put simply, TPUs offer us a flexible yet strong 3D printing filament that most other 3D printing materials cannot (see the table below). For example, popular filament material polylactic acid (PLA) is highly crystalline, which makes it too brittle. Polyethylene terephthalate (PET), another commonly used filament material, has poor chemical resistance, which means chemicals like acetone can dissolve or weaken it. Acrylonitrile-butadiene-styrene (ABS) also has poor chemical resistance, and a high glass transition temperature that can cause warping in 3D-printed products.

Meanwhile, TPU has areas that are crystalline (which give it mechanical strength) and areas that are elastic (which impart bendability). Additionally, it has high chemical resistance and prints well at commonly used 3D printing temperatures.

Plastic Notable Characteristics Uses Mechanical Strength (on a scale of 1 to 3) Flexibility (on a scale of 1 to 3) Printing Temperature (°C) Bed Temperature (°C)
TPU Good tensile strength, high resistance to tear, abrasion and chemicals Medical implants, prosthetics, shoes, automobiles 1/3 3/3 200 – 230 60
ABS High impact resistance, high temperature resistance, leads to toxic emissions Decorative materials, air-conditioning equipment, wheel covers 2/3 2/3 230 – 250 100
PLA Strong but brittle, biodegradable Dental implants, resorbable sutures, non-moving machine parts 3/3 1/3 200 – 235 55
PET Lightweight, glass-like transparency Packaging (such as plastic drinking bottles), polyester fabrics 2/3 2/3 160 – 210 60

 

Filament Extrusion and 3D Printing TPU

Extruding TPU filaments is a tricky and optimization-intensive task. TPU’s dual set of properties – crystallinity combined with elasticity – makes it a bit challenging to arrive at the specific parametric settings that can yield a high-quality filament. This does not mean that it is impossible to work with TPU. It simply means that it takes more time to finetune the extrusion settings for TPU.

At 3devo, we were able to arrive at the optimum parameters to extrude a TPU filament using TPU Shore 75A pellets with our Precision 350 filament extruder. Note that commercially-available TPUs cover a vast range on the shore hardness scale, ranging from Shore A to Shore D. To get an idea of the hardness of the material we worked with, consider this: shoe soles typically have shore hardness values in the range 15A to 58A. Ultimately, with extensive experimentation, we successfully extruded a clean, egg-shell white spool of TPU filament. You can see this well-extruded spool in the photographs below.

TPU_filament

 

After extruding the TPU filament, the next step was to test its performance in its intended 3D printing application. We used the TPU filament to 3D-print a simple 3devo logo (as you can see in the photograph above). The filament printed perfectly, and the end product was tough yet flexible – exactly as we had hoped!

Note that studies have shown that printing temperature can influence the mechanical properties of the final 3D-printed TPU product. For example, changing the printing temperature from 200°C to 230°C for medical-grade TPU can increase the tensile strength of the final product from around 35 MPa to nearly 45 MPa. So, depending on your requirements of mechanical properties, you may want to play around with the TPU printing temperature.

Endless Possibilities with TPU

TPUs form a one-of-a-kind class of elastomeric materials that have the capability to meet the manufacturing demands of numerous industries. Their excellent flexibility and great mechanical properties have together catapulted their rise in the 3D printing industry too. Plus, you have so many options to choose from among TPUs – based on chemical properties, hardness, and other characteristics.

Of course, experimentation is necessary to determine which TPUs can become filament material and give excellent 3D-printed products. It is also important to know that working with TPU can be quite difficult and certainly involves a learning curve. Still, if you are in an industry where your 3D-printed product’s flexibility and mechanical strength are equally important, you can try out TPU!