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TPU (Thermoplastic Polyurethane) is a variety of TPE (Thermoplastic Elastomer), though it is more rigid while retaining most of the traditional flexibility and elasticity of TPE. As such, TPU lends itself to a broader range of uses.
TPU can be 3D printed, machined, and injection molded, but it does present some challenges in doing so. In this article, we’ll discuss what TPU is, its applications, the challenges of 3D printing TPU, and possible alternatives.
What is TPU?
TPU is a thermoplastic that provides an excellent combination of both plastic and rubber properties. Its chemical formation consists of both hard and soft bond segments, which enables it to have such a wide spectrum of shore values. The ratio of hard to soft segments is what primarily determines the hardness of the material, and by controlling this ratio, you can make TPU like hard plastic, soft rubber, or anywhere in between.
Along with the broad spectrum of hardness, TPU has many benefits that make it desirable in a number of industries. These characteristics include:
- High elasticity & flexibility
- High durability
- Abrasion, impact, & wear resistance
- Chemical resistance (oil, grease, many solvents)
- Works in low temperatures
- Vibration dampening
- Some varieties of TPU can stretch multiple times their length without deformaiton
TPU is produced in two main types: polyester-based TPU and polyether-based TPU. Both types are elastic and flexible but vary on other details, perhaps making one better than the other depending on the part’s intended use. For example, polyether-based TPU is resistant to hydrolysis, which makes it ideal for humid or even submerged applications, but polyester-based TPU will deteriorate in wet conditions. To learn more about these categories of TPU, you can check out this comparison of the two types.
Given these versatile characteristics, TPU can be used in a wide variety of applications. The primary industries that utilize TPU are automotive, electrical, medical, and consumer goods.
Automotive & Electrical
With its high resistance to oil and grease, TPU excels in the automotive industry where many flexible parts are needed in lubricated, and possibly messy, conditions. These can be parts, such as tires, hoses, seals, gaskets, tubing, belts, automotive instrument panels, and more, where the fit must be exact to prevent leaks and other malfunctions. Additionally, TPU is used for the jacketing of cables and wires in both the automotive and electrical industries due to its resistance to low temperatures in addition to its natural flexibility.
TPU is valuable in the medical industry because its flexibility gives doctors and patients access to easier and more comfortable care. Many different patient treatments demand flexibility to be possible and comfortable. This includes medical devices such as shoe orthotics, dental materials, instruments and tubing for administering anesthesia and other fluids, and much more.
TPU’s range of shore values also lends itself to the industry of consumer goods where many different ranges of hardness and flexibility are needed. It is commonly used in athletics for sporting goods and the soles of shoes. Much like in the medical industry, the TPU used in shoes here is for both comfort and performance. Furthermore, TPU is a common material used for cell phone cases and laptop keyboard protectors where flexibility to absorb shock and not deform is crucial.
Manufacturing with TPU
As its widespread use would indicate, TPU is readily machined and injection molded and can produce durable, functional parts.
Machining TPU may seem like it would be difficult due to its flexible and elastic characteristics. However, as long as its mechanical properties are taken into consideration, there is little difference in how it is machined compared to other materials.
TPU parts may also be made via injection molding. The main considerations when producing with this method are pressure and mold and nozzle temperature. If these factors are not controlled well, shrink or warp may occur.
While both methods of traditional manufacturing are suitable for simple parts, they cannot compete with the ability of 3D printers to produce complex geometry. In particular, if your part has fine features or complex or hidden geometry, 3D printing TPU would be the more efficient choice.
3D Printing with TPU
With filament extrusion of TPU, the flexible filament is heated and extruded onto a heated bed in cross-sectional layers. It is important to note that TPU typically does not warp when produced by extrusion, making the part more exact.
However, the process comes with many challenges that could deform the print beyond repair. While it is possible to print TPU using a desktop printer, flexible filament is difficult to work with as it can easily disrupt layer thickness and jam or clog the printhead. This is especially true when using a Bowden extruder because it is easy for the flexible TPU filament to bend inside the feed tube. Industrial FDM printers often have a direct drive extruder system that will prevent this from happening.
TPU 3D printed parts also take much longer than traditional non-flexible materials because the print speed must be reduced. At normal speed, the filament might bind or wrap itself around the machine. Desktop printers also produce much lower quality parts than industrial printers, so the combination of this fact and the above challenges can turn your print into a hot mess.
Additionally, depending on make and model of the extrusion printer, the shore hardness of the TPU filament can be limited. Some suppliers may only offer select shore values of TPU filament, making the production of your part even more difficult. The same problem can occur in industrial FDM printers. Stratasys, the leading manufacturer of high-end industrial FDM printers, only offers one shore hardness of TPU filament and the resulting part may still not meet your quality standards.
To achieve greatest success printing with flexible filaments such as TPU, it is often recommended to disable the printer’s retraction feature and use a layer of different material on the build platform to which the filament can attach more easily for better first layer adhesion. For TPU, these may include PVA glue or blue painter’s tape. The chamber must not be too hot, however, or these aids may stick too well to your part and cause an issue in post-processing.
As seen above, it can be difficult to work with TPU and produce the quality of part you desire. This is made even more difficult if the shore value you need is not available. If this is the case, settling for a different shore value could force a redesign or may not be an option. Fortunately, there are alternative methods to 3D printing flexible parts.
PolyJet 3D printing involves the deposition of photopolymers on a build platform that are then cured by a UV light. A primary advantage of using PolyJet is that it can print two materials at once, one being a hard plastic and the other being a soft rubber. Additionally, the printer can produce any blend of these two materials to create the same shore value spectrum that TPU has. As such, you can adjust the ratio of rigid and flexible materials to create parts with a variety of mechanical properties.
There are some drawbacks to using PolyJet, however. Because the materials offered by PolyJet are not true thermoplastics, they are not as durable. They are also not resistant to various chemicals and will degrade if left in water. As such, PolyJet is best used for applications such as cosmetic prototypes and fit testing rather than functional parts. For an in-depth explanation of PolyJet, check out our complete design guide.
3D Printing at 3 Space
Here at 3 Space, we offer PolyJet and other 3D printing services. If you’re uncertain whether PolyJet or TPU is the best choice for your part, our engineers can help assess which material may be best for your part and its intended use. Contact us for more information.