Finishing Fluids for Additive Manufacturing (AM)

Online Editor

Elizabeth Norwood explains how a correctly selected debinding solution helps advance design and production

Initially, additive manufacturing (AM) was thought to be too slow or too expensive to use on a large manufacturing scale. Many engineers only considered it for parts prototyping, jig making or low production runs. But recent advancements in software and hardware technologies mean lower costs and faster production speeds, making AM an attractive alternative, even for complex or high-volume production runs.

AM makes design possibilities nearly limitless. Engineers can rapidly iterate designs, make changes to custom-made prototypes and test them without setting up extra equipment or expensive tools. It is a quick and practical way to make detailed, precise and often customised designs.  

Smoothing It Out

Plastic is the material of choice for AM with over 80% of AM parts produced using thermoplastic or thermoset polymers. AM polymer parts are made using any variety of printing methods including FFF, SLS, SLA, DLP or material jetting processes. Common to all these methods is that the parts are made in layers. They are constructed in progressive layers by depositing or extruding polymers, one level at a time, until the parts reach their final shape.

However, building the parts is just the beginning of the AM process. Using the right smoothing and cleaning fluid is key. Since AM parts are built progressively, layer-by-layer, the process sometimes leaves them with a stepped or terraced surface that requires smoothing to get a finished part.

Traditional methods of smoothing out the terraces include grinding, buffing or sandblasting. However, these methods are often manual, time-consuming and typically leave particles behind. Some engineers opt to use a speciality smoothing fluid inside a modern vapour degreaser to finish their plastic parts. Immersing the unfinished parts in a fast-evaporating smoothing fluid vapour inside the degreaser slightly melts the surface of the plastic parts, levelling out any irregularities and removing the terraces. It leaves a smooth finish without any leftover particles or damage to the finished parts. The quality of the finished product is comparable in quality to parts using more traditional injection moulding processes.

Ensure Materials Compatibility

To prevent parts damage, it is necessary to understand the composition of the polymer parts. For instance, acrylic, ABS, polycarbonate and highly basic fluids with a pH 10 or above need careful consideration due to the potential softening and swelling of the polymer materials. Finding the best smoothing fluid can often be a delicate balance between selecting one with a high enough solvency to effectively level out the parts, but not so strong that it damages them or compromises their structural integrity.

In addition to smoothing, the vapour degreaser fluid also removes any soils or particulates left behind during manufacturing. The fluid dissolves and rinses a variety of oils, greases and waxes. For stray particulates such as dust or shavings that are non-soluble they are removed through displacement. The cleaning fluid gets under the particulate, dissipates any static charge and lifts it off the surface. The key to effective displacement cleaning is to use a dense, heavy fluid that undercuts and lifts the particles of dust and dirt off the substrate. Today’s modern fluids are typically 20% heavier than water and 50% heavier than alcohol, making them the ideal choice for displacement cleaning of AM polymer parts.

Debinding Metal Parts

Although plastic is still widely used for AM, using metals, ceramics and other composite materials is rapidly gaining momentum. Metal 3D printing employs the same layered-build process as plastic AM, but uses fine metal powders and a binding agent, typically paraffin wax, carnauba wax, or specialty polyethylene waxes, to create green-state parts. After layering, the green parts are sintered in an oven to make fully dense metal parts.

The binding agents help form the green parts but must be partially removed before the parts are exposed to the high heat of sintering. Too much binder left behind in the parts could result in cracking, deformation of the backbone, or part expansion during the sintering stage. Too little binder could cause the parts to lose their dimensional stability and collapse upon themselves. In many cases, the binders are extracted using a speciality fluid inside a modern vapour degreaser. The fluid is engineered for selectivity, so the right amount of binder is removed without damage to the part structure. Some, but not all, of the binder is removed to avoid contamination of the metal during sintering, but not so much that it affects the parts dimensional accuracy during sintering. Due to their low viscosity and surface tension ratings modern metal debinding fluids penetrate complex part structures including inside blind holes and end holes to ensure thorough debinding.

Once the select amount of binder is removed, the parts are thermally sintered under high heat to bond the metal powder into its finished solid mass state. After sintering, the parts are post-processed using standard metal finishing techniques such as grinding, cutting or coating.

Finishing Fluids For A Cleaner Environment

Today’s finishing fluids meet strict global environmental regulations and can help manufacturers replace chlorinated solvents such as trichlorethylene (TCE) that if not used properly can contribute to groundwater and air quality problems. Most modern finishing fluids have a low global warming potential (GWP) under 10 and a zero ozone depleting potential (ODP). In addition, modern finishing fluids, when used in a vapour degreaser can also be distilled and reused in the smoothing or debinding processes, saving time and disposal costs.

Companies looking for help in determining the correct finishing fluid or method to use should consult with a critical cleaning partner that specialises in vapor degreaser smoothing and debinding. Some fluid manufacturers have field engineers that conduct on-site audits to evaluate methods. They can also perform comprehensive in-lab tests with sample parts to ensure smoothing or debinding success.

Elizabeth Norwood is a senior chemist for MicroCare