Peristaltic Pumps have been in the market for about 80 years and offer zero-contamination pumping. When the fluid flows in the tubing and the driving force is generated by the compression; neither the fluid nor the pump head or the pump mechanism gets contaminated and it is easy to keep them clean and sanitized. As the fluid does not leave the tubing these pumps are virtually immune to abrasive media and many chemicals. Due to the fact that the fluid does not come into contact with the pump itself, problems associated with instrument contamination are also eliminated. As a result, cleaning the pump internals is not necessary – this is particularly important when dealing with ‘sticky drugs’ that are difficult to remove.
The present invention provides a 3d printed peristaltic pump with an optimized, automated gear assembly process and a rugged design for precise and zero-contamination pumping applications using an FDM (Fused Deposition Modeling) type 3d printer. 3D printing enables easy scalability of the device and low wear & tear during its operation.
An Early 3D Printed Prototype of the Device ©ankitaryan
The gear assembly consists of a sun-gear in the center with three planetary-gears around it with a geared ring outside the planetary gears. The shaft of the motor passes through the center of the sun-gear and is held in place by a screw that passes through the motor-shaft-connection and then through the shaft. Thus, the rotation of shaft leads to the rotation of sun-gear which in turn rotates the planetary gears and the outer ring remains static, being held by the assembly holder. This leads to squishing of the tubing which is held between the planetary gears and the outer ring. The squishing of the flexible tube and the revolution of the planetary gears causes a contraction and relaxation pattern that moves like a wave in a cylindrical symmetry system. This whole process mimics the working of the human digestive system in which oesophagus (food pipe) contracts and relaxes in a similar fashion to push the fluid/solid matter to the stomach with the only difference being the pattern waves move in a straight line. The planetary gears have teeth on their top and bottom but are cylindrical in the centre which supports the squishing at a fixed contact angle and minimises the damage to the tubing. When a planetary cylinder is revolving and has just compressed the tubing near the inlet and as the tube regains its shape, a momentary vacuum is generated at the inlet and the fluid is sucked inside till another planetary cylinder reaches the inlet. This leads to the formation of packets of fluid between the cylinders whose volume can be calculated by simply multiplying the circular cross-section of the tube with the arc length between two cylinders. This allows in precise calculation of fluid flow rates and hence, is useful when high precision is required in fluid dispense volume or flow rate. Moreover, the generation of fluid packets helps in preventing backflow and supports the transport of high viscosity fluids.
The adaptive gear assembly holder holds the gear assembly and the motor in place. It ensures that the outer geared ring of the gear assembly remains static and only the planets revolve to maintain a constant flow. As mentioned earlier, the flow is dependent on cross-section area of the tubing and in order to decrease the flow rate or dispense volume, the size of the gear assembly also needs to be decreased in order to maintain the squishing angle and prevent backflow due to poor compression. But this would mean that the holder needs to be replaced with the gear assembly if the user wants a significantly smaller or larger flow rate which is done by adding two spring-attached flexible arc-strips which could support gear assembly of different sizes. Whenever a gear assembly is inserted between the two strips, the springs elongate due to an increase in the separation between the two strips and the spring force helps in holding the gear assembly in place and keeping it static. This is further ensured by the embossed pattern on inside of the strips which stop the outer geared ring of the gear assembly from rotating as the pattern fixes inside gear-assembly’s outer surface gear teeth.
Complete Assembly of the 3D Printed Peristaltic Pump rendered in Fusion360 ©ankitaryan
All the components of gear assembly i.e. the sun and the planetary gears are 3d printed in a single step procedure. The design ensures that the three planetary gears and the sun gear are well fitted in the encapsulating geared outer-ring. Hence, all the four individual components of the gear assembly need not be assembled later-on but are already fitted inside when they are being 3d-printed. A tolerance value of about 0.15 mm ensures smooth revolution of planetary gears but poses a challenge for 3d-printing.
The basic 3d printing process involves pushing the filament (extrusion) using the feeder (motor which pushes the filament forward) into the extruder (heating element) which melts the thermoplastic and pours it over the previous layer and on encountering it, the plastic solidifies and this process is facilitated by cooling fans mounted on the FDM 3d printer.
The optimization of 3d printing parameters was done. Printing speed was increased so that the extruder moves fast enough to make the poured plastic-overhang between two sides which is analogous to making a hanging bridge between two close mountains when there is a deep narrow valley between them. The flow rate of thermoplastic is also reduced to ensure that the gear edges are sharp and the adjacent gears don’t stick to each other during printing due to over-extrusion. Another parameter of importance here is retraction length and retraction speed. Retraction is the sucking of filament back into 3d printer while it isn’t extruding which is when it jumps from one point to another and here it happens because of gear-gear separation due to tolerance. The low retraction means there would be thread-like hanging structures in the gap which could jam the gear assembly during operation. So, to prevent this, retraction length was increased and the retraction speed was increased. Increasing retraction speed ensures that the filament is sucked fast enough to prevent its overflowing.
The gear assembly is made up of thermoplastic material which enables easy control/ manipulation of gear tolerance for low friction pumping. The invention provides the fabrication of very low volume dispensing pumps and the worn-out gear assembly can be changed easily by an untrained user.
Patent Filed: Kumar, A, Satapathi, S & Indian Institute of Technology Roorkee 2018, “3D Printed
Peristaltic Pump with Automated Gear Assembly Fabrication”, IN 201811031020. (Patent
publication can be accessed at https://ipindiaservices.gov.in/publicsearch).
Ankit Kumar 