Additive manufacturing of copper heat pipes
In this section, we give you an insight at the technology of Pulsating Heat Pipe, the challenges to overcome with traditional manufacturing and perspective of optimisation.
Introduction to PHP and additive manufacturing
A heat pipe is a passive heat transfer device. Heat pipes transfer heat using phase transition of a working fluid such as water. The heat source creates a hot zone, from which heat is dissipated when the fluid evaporates while entering this hot zone. The fluid then cools down and condensates in the condenser. There are several types of heat pipes, you can learn more about their applications on Calyos’ website. Calyos has chosen Beamler as his partner to 3D print a pure copper Pulsating Heat Pipe (PHP).
PHP consists of a closed, small diameter channel with multiple bends. The image below is a schematic of a PHP .
Schematic of a PHP 
PHP are qualified as pulsating because of the fluid movement inside its channel. In the evaporator section, evaporating fluid causes the pressure to increase driving the gas fluid in the condenser. Therefore, heat dissipation in the condenser reduces the pressure and the fluid returns to the evaporator, creating an oscillating movement in the channel. The high heat transfer coefficients of boiling and condensation makes PHP very efficient thermal conductors.
The hollow channel featuring multiple bends and a capillary structure is a perfect fit for additive manufacturing processes such as Laser Powder Bed Fusion (L-PBF). There are several processes developed for 3D printing of metallic parts, each with specific requirements and capabilities. Feel free to explore our capability page to find the most suitable additive manufacturing process for your part.
Challenges to overcome with traditional production process
Traditional production processes of heat pipes are effective for mass production. The manufacturing includes crucial steps such as filling the tube with a copper powder followed by sintering. This step gives the tube the inside capillary structure. Be quiet!, an electronic products manufacturer, made a video explaining the manufacturing process of their heat pipes used in CPU coolers.
Those traditional processes are perfectly suited for mass production but unadapted for small series or prototyping. On the other hand, additive manufacturing addresses this issue by cutting costs and production time. It also enlarges the design space and allows the production of parts featuring complex shapes and internal channels such as PHP. Moreover, since numerical simulations of PHP are challenging, an iterative design approach can be very effective and perfectly adapted using additive manufacturing.
Optimizing heat pipes
Complex physical phenomenon takes place in PHP. To design the most efficient device possible, numerical simulations are used. These simulations give important results such as the inner diameter of the channel. A first generative CAD model is built and corrected by the engineering team.
Several studies focused on optimizing the internal structure of PHP to enhance thermal conductivity. By adding interconnecting channels, one can increase the thermal conductivity of the PHP. However, using traditional manufacturing techniques, the design space is limited by the capacities of the machines. Additive manufacturing is then an excellent way to produce those complex hollow designs.
In depth presentation of the 3D printed copper heat pipe
In this section, we give an in depth presentation of the printed part, the printing technology used as well as future challenges to overcome.
Presentation of the PHP
Most PHP used for cooling electronic devices are designed with the condenser and evaporator at both ends of the device. Calyos designed a PHP with the heat source located in the center of the PHP, thus distributing the temperature equally in a limited space as shown by the figure below.
Calyos PHP with a central heat source – Calyos
Designed for electronic devices with a high heat flux, up to 50 W/cm2, and charged with methanol as working fluid, the PHP was experimentally tested at the University of Palma. A publication has been written with the results of the experimental campaign and submitted to peers. It is expected to be published later this year.
Technology used and material
The device was 3D printed with pure copper using Laser Powder Bed Fusion (L-PBF). Pure copper has one the highest thermal conductivity of all metals, about 0.4 kW/(m.K) . However, much higher thermal conductivity can be achieved with PHP up to 12 kW/(m.K) thanks to their working principles.
The 3D printed pure copper heat pipe is a square prism of about 100 mm lengths for sides and 11 mm height. The images below show the blueprints and CAD model of the PHP.
Blueprints and CAD model of the 3D printed PHP – Calyos and Beamler
Internal dimensions of the channel are computed using internal software. The groove visible on the surface of the printed part are used for thermocouples. The total weight of the part is about 0.690 g.
To prevent leakage of the working fluid and a loss of thermal conductivity, the part must be fully dense and the printed pure copper part is sintered to achieve this density. Sintering is the process of heating up a printed part to compact it. For most materials, sintering results in the shrinking of the part and inner dimensions must thus stay over a lower limit to avoid the closure of internal features.
The image below is a picture of the final pure copper 3D printed PHP.
3D printed pure copper PHP – Calyos and Beamler
Calyos has given excellent feedback regarding the printing quality of the part. Their quality controls qualified the 3D printed part for further experimental testing by the University of Palma.
Challenges to overcome with 3D printed copper heat pipes
Many challenges had to be overcome with the design and production of this part. Featuring a small hollow internal channel, this 3D printed heat pipe tends toward the limits of the current technology of additive manufacturing. Reducing wall thickness for example could allow to reduce overall weight and volume of a part. Those are of utmost importance in industries like aerospace.
Another challenge raised by Calyos regards parts featuring internal hollow structures. When printing using powder bed fusion, some unfused powder remains stuck in the hollow features. The CAD must include holes of minimum diameters to allow the cleaning of the remaining powder. Working closely from the design phase could allow us to use existing holes already included in the design such as the ones used to fill in the working fluid in a PHP.
Choose pure copper now and start your project
This collaboration between Calyos and Beamler paves the way for additive manufacturing of pure copper parts for thermal application. It demonstrates the capacity of both companies to provide strong scientific competence and high quality parts.
Pure copper additive manufacturing has proved its worth as a cutting edge technology. If you are looking for a material with excellent thermal properties, pure copper is a serious candidate. Contact Beamler and request a quote to start your project now !
Contact Beamler now to receive your Pure Copper quote
 Study : A review on pulsating heat pipes: From solar to cryogenic applications :
 Thermal conductivity of metals : https://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html
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