When Subtractive Applies To Additive
January 28, 2013 | Category: Post-Processes
While additive manufacturing provides improvements in realizing designer's functional intent, it still is limited within some additive processes due to necessary residual materials requiring removal. Any additive process using either polymers or metals that requires support structure must subsequently employ means to remove such unwanted support structure. For powder-based processes that support structure is the non-sintered or non-melted materials themselves. In these cases, non-processed materials must be removed post-processing. Figure 1 depicts the challenges found in porous metal (“titanium foam”).
A question then arises, how can we better remove residuals (e.g. powder) from our additive parts? While there are several ways to remove residuals from additive parts such as media extrusion, aqueous-based ultrasound and liquid carbon dioxide (CO2); in this article I wanted to investigate the liquid CO2 process. I am curious of its technical differentiators and best-use applications. Toward that end, I recently had the opportunity to speak with Cool Clean Technologies founder and CEO, Jon Wikstrom, to “talk tech” about this technology.
Wikipedia defines “supercritical” CO2(1) “Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. [This] supercritical fluid [expands] to fill its container like a gas [lowers viscosity which increases accessibility to features] but with a density like that of a liquid.” Whereas the “dense” state is just below critical and still maintains the lower viscosity and surfaces tension for improved residual removal capabilities but with an increased density to better remove oils, particles, etc. that you would not otherwise be able to efficiently remove with a supercritical fluid(2).
1 http://en.wikipedia.org/wiki/Supercritical_carbon_dioxide. Accessed 1/10/13. 2 Also, equipment to support supercritical fluids is significantly more expensive than equipment to support “dense” fluid states due to many pressure vessel codes which not only increase capital expenditure but also increase operating costs.
The centrifugal, liquid CO2 differentiators are most relevant when you consider cost of phase management (temperature and pressure manipulation to maneuver between liquid-gas-liquid states), renewable, green attributes (uses industrial byproduct CO2 that is recyclable in a closed-loop system with negligent waste and no industrial volatile organic compound or greenhouse gas emissions) and effective material removal (accommodates high geometric complexity and full feature penetration).
While water is also environmentally sound, readily available (though conservation is relevant; especially outside the US) and affordable (used in many cases for low feature complexity, gross cleaning) its relative surface tension and viscosity attributes are not sufficient enough for many applications. In particular, water's insufficiencies are noticed within complex geometries and small particulate sizes; both of which are characteristics of powder-based additive processes.
The fluid shear force produced by centrifugal action is a promoter that advances the material separation and removal effort augmenting the chemical, fluidic action provided by the liquid CO2 and ultrasonic excitations. Therefore, you reduce processing time, improve the efficiency and lower required pressures to remove unwanted materials.
For additive manufactured part cleaning, an immersion system is most effective as it allows for the combination of cleaning (using environmentally friendly solvents which without CO2 following cannot effectively be used independently due to vapor pressure and other constraints) with the particle removal, distillation and filtration processes, and drying (using liquid CO2). While immersed following in the cleaning event, an ultrasonic frequency is applied in order to excite particle removal which then flows to filtration. As the dense phase CO2 completes the cleaning / removal / drying process, distillation occurs by manipulating the temperature and pressure which forces the gaseous dense state of CO2 into a pure liquid state; recovering approximately 99% of original pure liquid CO2. For Cool Clean Technologies' approach, a patented compressor solution enables optimal control of the flow and pressure of the liquid CO2 thus eliminating the need for a high maintenance pump system. The same compressor is also used in the recovery and recycling of the remaining CO2 which enlists the onboard distillation system.
...is if you require high volume, gross cleaning of simple features use an aqueous-based solution; however, for low to medium volume, intricate cleaning of highly complex components (like many additive parts), then an immersion process that employs a dense phase CO2 for cleaning and removal may be a more effective and economical approach over other pure compound options and approaches. While I have focused on the immersion process due to its inherent best-use application for additive manufactured parts, there are also CO2 based composite spray solutions. These spray solutions are more focused on flatter, low-complexity parts found in the microelectronics and hard drive industry that support high volume, robotically controlled cleaning with all the benefits of the liquid CO2 as mentioned for the immersion solution. This technology is also being used in the medical and aerospace industries for flatter geometry parts and can be used in combination with CO2 based plasma for surface modification and activation.