In the past, the Metal Additive Manufacturing (MAM) process was mostly used to make simple items like support jigs and other tooling fixtures. But as MAM methods become easier and more affordable, many more machine shops are finding other uses.
For example, MAM is ideal for small-volume jobs where a full-scale production run is too costly or time-consuming, especially if the job requires expensive tooling or long wait-times for molds or dies. It also works well when manufacturing highly complex or intricate products that are too detailed for traditional subtractive tooling methods like milling, cutting or turning. Many shops are also finding MAM useful for rapid in-house prototyping. It allows them to iterate designs quickly during the development phase and since confidential designs are never publicly shared, helps protect their confidentiality, patents, trademarks and intellectual property.
Many MAM methods
There are several types of MAM methods used today. However, many machine shops choose binder jetting because it is less expensive, faster, and easier-to-use than other additive manufacturing processes. Binder jetting builds parts by progressively depositing alternating layers of fine metal powder feedstock and binders onto a powder bed in thin horizontal layers, until the parts reach their final shape.
A variety of metal powders are used in MAM – including stainless steels, low-alloy steels, carbon steels, Ni-alloys, tool steels and tungsten and titanium alloys – along with paraffin wax, carnauba wax, or specialty polyethylene wax binders.
The binders help form the metal powder into a specific shape. However, the binders are temporary. Most of the binders are removed – that is, the parts are “debound” – allowing for faster sintering times. Only a small quantity of binder stays inside the parts to keep them from deforming and cracking during final sintering. Successful part debinding is a delicate balance of selectively removing just the right quantity of binders as quickly as possible without damaging the fragile parts.
Then, after debinding, the parts are thermally sintered at near melting temperature, burning off any remaining binders and bonding the metal powder to its solid mass state, typically 96-99.8% density. After that, the parts are post-processed using standard finishing techniques like anodizing, plating, painting, or welding.
One-step debinding and cleaning
In some machine shops, parts debinding happens inside a vapor degreaser using a special, low-temperature, nonflammable debinding fluid. Submerging parts into the fluid or holding it inside the fluid vapors dissolves the binders and transports them away from the parts. The debinding fluid penetrates the parts, dissolves the wax binders and creates porosity inside the green parts, which allows the debinding fluid to evaporate quickly before sintering.
After a typical cycle of about 6-20 minutes, the green parts come out of the vapor degreaser debound, powder-free, dry, and cool enough to immediately transfer to the sintering oven. Extra cleaning or rinsing steps or a long cool-down time are not needed. Vapor degreaser debinding is an easily programmable process and allows for excellent repeatability and debinding consistency every time. It is quick, efficient and results in faster production runs, reducing manufacturing costs, overall.
Choosing the right MAM debinding fluid
Parts debinding is a straightforward process. However, it is important to get the fluid details right for the best possible outcome. The physical properties of the debinding fluid are important. Here are the four most chief factors to consider.
Materials compatibility: The debinding fluid should be safe to use with both the metal powders and the binders, to safeguard the green parts. The debinding fluid should be aggressive enough to selectively remove the right volume of binders yet still maintain the parts’ integrity.
Also, it helps if the debinding fluid has a low-boiling point. Low boiling debinding fluids will melt the wax binders but do not damage the non-soluble fragile parts. Plus, it allows the vapor degreaser to run more efficiently. A lower boiling point reduces the vapor degreaser start-up time, energy consumption and cost, and requires less heat input to operate.
Low surface tension and viscosity: Low surface tension and low viscosity debinding fluids work their way into, and more importantly out of, blind holes, end holes and tight, internal geometries for more complete debinding. This prevents parts from expanding and cracking during sintering should any of the gasified binders become trapped inside.
Safety: Modern debinding fluids do not use nPB (n-propyl bromide), methyl pyrrolidone, polyethylene glycol, heptane, or trichloroethene, all of which carry serious health and or environmental concerns.
A Personal Exposure Limit (PEL) is the toxicity rating of a debinding fluid measured in parts-per-million (ppm). High numbers, those approaching 1,000 ppm (the highest rating possible), are a safer option. Lower numbers specify a greater risk. The PEL (Permissible Exposure Limit), or OSHA-designated time limit that workers are exposed to a modern debinding fluid, is about 200-250 ppm. This is significantly better than nPB with its 0.1 ppm PEL.
In addition, nonflammable debinding fluids are safer overall and do not require specialty fire or explosion-proof equipment. Nonflammability is important to shop environmental health and safety managers because it helps protect workers from burn accidents in the event of a sudden arc flash.
Sustainability: Debinding fluids must meet strict global regulatory requirements and environmental sustainability standards. Most have a very low Global Warming Potential (GWP), under 10, to help reduce greenhouse gas effects. They also have zero Ozone Depleting Potential ODP) and low VOC formula content. This allows them to meet strict regional air quality regulations. Many are not considered to be hazardous air pollutants (HAP) and do not require National Emission Standards for Hazardous Air Pollutants (NESHAP) permits.
Also, sustainable debinding fluids adhere to global environmental directives including the European F-Gas and REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation. In addition, when used in a vapor degreaser, modern debinding fluids are distilled, recycled and reused, limiting environmental waste.
Work with an expert
Debinding fluid is an important tool for successful MAM processes. Fortunately, there are a number of good modern debinding fluid choices that not only debind effectively but are also safer for people and gentler on the planet. To learn more about fluid debinding using vapor degreasing, partner with a fluid supplier that specializes in vapor degreaser debinding. They can help conduct on-site audits, perform in-lab tests with sample green parts, and make environmentally sound recommendations on which fluids will work best.
Elizabeth Norwood is a senior chemist at MicroCare LLC, a provider of precision cleaning solutions, with more than 25 years industry experience. Norwood earned a B.S. in Chemistry from the University of St. Joseph, and currently holds one patent issued and two patents pending for her work.