September - stock.adobe.com
Weight loss analysis is most commonly used method to measure debinding success.
Weight loss analysis is most commonly used method to measure debinding success.
For many AM (additive manufacturing) companies, sustainability is impacting the way they do business. They must minimise negative environmental impact, conserve energy and protect natural resources all while safeguarding the well-being of their employees. One way companies are achieving sustainability is by changing the solvents used in debinding operations.
Debinding with vapour degreasing
In some shops, removal of binders is done with a solvent inside a vapour degreaser. Dunking parts into the liquid solvent or holding them inside the solvent vapours, dissolves the binders. The solvent has a low surface tension and low viscosity to penetrate the parts, ensuring thorough debinding. The solvent evaporates out of the parts almost completely before sintering, preventing damage by gasification of the trapped solvent.
Historically, n-propyl bromide (nPB), trichloroethylene (TCE), perchloroethylene (PERC), terpenes and hydrocarbons were the chosen solvents for AM debinding. However, those solvents have serious health, safety and environmental concerns, and environmental agencies are implementing stricter regulations to reduce the negative impact to the planet and to workers.
Finding a better alternative
Fortunately, there are next-generation debinding fluids that debind just as well, if not better than the legacy solvents. They are aggressive enough to selectively remove just the right amount of binder, yet gentle enough to prevent part deformation and damage to delicate, uncured substrates. In many instances, modern debinding fluids do not require purchasing new equipment. Often, after emptying and cleaning the existing vapour degreaser, the new debinding fluid is dropped in without any significant down-time or appreciable change to the cleaning process.
Modern debinding fluids are also safer. They are low-boiling, thermally stable, and nonflammable. The PEL (Permissible Exposure Limit), or OSHAdesignated time limit that workers should be exposed to a solvent, for new fluids is about 200- 250 ppm compared with TCE (100-ppm PEL) or nPB (0.1 ppm PEL). Plus, most new debinding fluids have a Global Warming Potential under 10 and a zero Ozone Depleting Potential, making them better for the environment.
What about performance?
Many new debinding fluids are lab-tested and analysed to ensure their debinding results are just as good as legacy solvents. However, some AM manufacturers are conducting performance tests of their own.
Density testing allows manufacturers to measure how much binder is removed from their parts. The feedstock supplier typically provides companies with the “minimum brown density” to measure the brown parts debinding success. This accounts for the maximum amount of primary binder allowed during secondary debinding and sintering without causing part deformation.
“Minimum brown density” must be reached before the green parts move into the furnace for final debinding and sintering. After primary debinding, the density of the parts should match the “minimum brown density” of the feedstock, which is equal to the density of the feedstock minus the primary binders.
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AM manufacturers also use weight loss analysis to verify the correct primary debinding is achieved. The percentage of binder removal is determined by comparing the part mass before and after debinding. Manufacturers start by recording the initial weight of a part and debinding the part using their chosen fluid, allowing the part to dry completely before recording the final weight. The amount of binder removed is then determined by calculating the percentage of weight loss.
% Binder Removed = (Initial Mass – Final Mass) / Initial Mass x 100%
Although weight monitoring is the most commonly used measurement to determine primary binder removal, it does not take into account unwanted loss of feedstock powder or secondary binder removal. Although a part shows a 4% decrease in weight, this measurement alone does not indicate whether that loss is due to binder removal or damage to the part surface. Density measuring using a pycnometer measures the volume of the brown part in order to calculate the actual density of the material and ultimately determines the overall debinding fluid success.
Conclusion
Many companies are looking for better debinding fluid alternatives that will be sustainable, easier to maintain and less hazardous for workers and the environment. For many companies looking to make the switch to a better debinding fluid, it is imperative that they maintain their debinding performance with minimal impact to throughput and productivity.
