Modern Machine Shop and MoldMaking Technology present ADDITIVE MANUFACTURING, a quarterly supplement reporting on the use of additive processes to manufacture functional parts. More at additivemanufacturinginsight.com.
Issue link: https://am.epubxp.com/i/104365
Advanced Cleaning for AM: When Subtractive Applies to Additive By Tim Shinbara, Technical Director, AMT—Te Association For Manufacturing Technology While additive manufacturing provides improvements in realizing a designer's functional intent, some additive processes are still limited due to the required removal of residual materials. Any additive process that uses either polymers or metals and requires a support structure must subsequently employ a means to remove that unwanted support structure. For powder-based processes, that support structure is the non-sintered or non-melted materials themselves, which must be removed post-processing. Tere are several ways to remove residuals from additive parts, including media extrusion, aqueousbased ultrasound and liquid carbon dioxide (CO2). I will discuss the liquid CO2 process here. Wikipedia defines "supercritical" CO2 in this way: "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. [Tis] supercritical fluid [expands] to fill its container like a gas but with a density like that of a liquid." Te "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. Te centrifugal, liquid CO2 differentiators are most relevant when you consider cost of phase management (temperature and pressure manipulation to maneuver between liquid and gas states); renewable, green attributes (uses industrial byproduct CO2 that is recyclable in a closed-loop system with negligible waste and no industrial volatile organic compound or greenhouse gas emissions); and effective material 16— AM Supplement removal (accommodates high geometric complexity and full-feature penetration). While water also is environmentally sound, readily available and affordable, its relative surface tension and viscosity attributes are not sufficient enough for many applications. Why centrifugal? Te 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. Terefore, you reduce processing time, improve the efficiency and lower required pressures to remove unwanted materials. For cleaning of additive manufactured parts, an immersion system is most effective, as it allows for the combination of cleaning using environmentally friendly solvents with the particle removal, distillation and filtration processes, and drying using liquid CO2. While immersed, an ultrasonic frequency is applied to excite particles, which then flow 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 percent of the original pure liquid CO2. Te bottom line is, if you require high-volume, gross cleaning of simple features, use an aqueousbased solution. However, for low-to-medium-volume, intricate cleaning of highly complex components (like many additive parts), 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. For more information about advanced cleaning technologies or additive manufacturing, contact Tim at tshinbara@AMTonline. org or 703-827-5243.