TPU (Thermoplastic Polyurethane) is a flexible and elastic 3D printing filament. Its rubber-like elasticity, resilience and durability make it an excellent choice for application that require impact absorption and a soft-touch surface. While TPU’s distinctive mechanical properties are valuable in specific industrial applications, it encounters some issues during the extrusion process. The results of these issues are summarized in the following table.
Unlike a rigid filament like PLA, bending TPU requires less force. It can be also easily deformed during the filament feeding process. If TPU deforms during filament feeding, it will absorb the compressive feeding force and stop the filament from feeding into the hotend, and TPU is squeezed into a tight space and buckled during extrusion. The quicker a filament is fed, the easier it is for it to jam while extruding. This issue can be avoided by eliminating the free-space for TPU to bend and buckle, minimizing the friction when feeding, minimizing the feeding and melting distance, and maximizing feeding grip by modifying the drive wheels themselves. In summary, all these 4 points are intended to prevent TPU from buckling and ensure a smoother TPU feeding into the liquefier.
1. Eliminate Possible Free-space for TPU Buckling Problem
A 3D printer featuring an unoptimized feeding structure that leaves plenty of unsupported space or length for the TPU filament buckling during the loading and feeding process is shown in the figure below. The free-space is usually long that contains gaps between the drive wheels and the filament inlet hole of sleeve.
Structure of old type extruder structure.
3D printers with a wedge-shaped inlet hole installed closely to the gear and pulley can fix the issues resulting from the space between the drive gear and pulley. A wedge-shaped inlet hole can closely fit the opening of the drive gear and pulley, leaving little space for the filament to bend during feeding. When TPU filament passes through a wedge-shaped inlet hole it encounters resistance from hotend, and the compressive stress is hard to be absorbed by a filament. Instead, the stress will converted to friction and feed the filament along the tube. An example of an FFF 3D printer that replaced a flat inlet hole with a wedge-shaped inlet hole is the E2 3D printer from Raise3D.
Extruder structure of Raise3D E2 printer.
2. Minimizing Feeding Friction Problem
Most of the existing resistance during feeding comes from the friction between the filament and the filament tube’s inner wall. All-metal hotends generally have a feeding tube with an internal metal surface that is not polished. On the other hand, the metal has good thermal conductivity and rapidly absorb heat from hotend. High temperature tube causes thermal expansion of the TPU filament, which further increases feeding friction. Following image shows an old type all-metal hotend structure.
An old type of all-metal hotend structure
A PTFE tube can be embedded into the metal heat break throat for a lower friction coefficient and better heat resistance of the feeding tube. The lower friction coefficient decreases feeding friction while the heat resistance of the PTFE tube can prevent the thermal transition from the hotend to the TPU filament, thus preventing thermal expansion and friction of the TPU filament.
3. Minimizing the Feeding Distance between Gears and Nozzle Solution
Since feeding friction comes from contact between the filament and the feeding tube’s inner surface, a shorter feeding distance results in a smaller contact area, thus lowering the feeding friction. When other conditions remain safe, an extruder with a shorter feeding and melting distance between the drive wheel and hotend is more suitable for printing TPU.
4. Maximizing Feeding Grip Problem
Ensuring there is enough feeding force against the feeding resistance is essential for a smooth extrusion flow. Due to the elasticity and flexibility of TPU, the drive wheel can easily slip and lose its grip on surface of TPU filament when the on feeding filament encounters resistance. In this situation, TPU’s real flow rate is much lower than the feeding rate or it comes to a full stop.
One efficient way to maximize the grip or frictional force during TPU filament feeding is to replace the smooth pulley with drive wheel meanwhile upgrade all drive wheels to suitable types. For example, drive wheels replaced with two coarse gears can increase the grip on the TPU filament comparing to fine gears. For softer TPUs, more suitable drive wheel surface should be selected, e.g. Epoxy or rubber wheels.
Left: Old type extruder structure. Right: Extruder structure of Raise3D E2 printer.
An optimized FFF printer for TPU printing can increase the print speed and printing success of TPU filament. In addition, optimized slicing settings regarding TPU filament characteristics can further improve the printing process and result. Click here to view suggested slicing settings and some practice for Raise3D Premium TPU.