MIM and AM: Solvent finishing of complex parts


As the pressure to produce smaller and more complex parts faster and more economically increases, manufacturing using MIM (metal injection molding) and AM (additive manufacturing) processes such as 3D printing is gaining widespread acceptance. Manufacturing options like MIM and AM have opened up design possibilities that are almost limitless, allowing designers to create parts that would otherwise be impossible to make using other manufacturing methods. For example, in the past, design compromises had to be made for parts that were too intricate or too small. But today with MIM and AM, there is now a way to make detailed, precise and individually customized parts easily and affordably.

MIM is already widely used in a variety of applications and AM is quickly being embraced by industries ranging from aerospace, to medical devices to consumer goods.

MIM (Metal Injection Molding)

Material advances in MIM technology allow manufacturers to produce large quantities of parts with intricate geometries, often eliminating machining completely. The quality and precision of parts made with MIM often result in parts requiring minimal post-processing to create a finished part with excellent dimensional repeatability. In addition, using MIM fabrication is often faster and more cost-effective than other traditional metal production methods like forging or casting.
The MIM process combines a fine metal powder with a binding agent to make a moldable admixture that is then formed or molded into the desired structure. Metals available include stainless steel, tool steel, and many other ferrous and nonferrous alloys. Common types of binding agents are typically paraffin wax, carnauba wax, and specialty polyethylene waxes.
The binding agents serve a critical purpose in the forming process, but in many instances must be at least partially removed before the part can be exposed to the high heat required for sintering. In many cases, the binders require a specialty solvent that is engineered to selectively remove some, but not all of the binders. The binder is removed to avoid contamination of the metal during sintering, but it is essential that some of the binder remains so that the part maintains dimensional accuracy during the sintering process.
Selection of a debinding method is a balance of removing the binder in the shortest amount of time and with the least amount of damage to the structure because as the binder is removed, the structure becomes fragile. Solvent extraction of the binder can be done in either the solvent vapor or liquid phase in a vapor degreaser. Both rely on the solvent flowing through the pores of the structure to dissolve the wax. This is where the physical properties of the solvent become important. The ideal solvent will be nonflammable, have a low surface tension to work its way into the part structure, and be 100 percent volatile so the solvent can be easily removed before sintering. Once the debinding solvent is fully removed from the part structure, the parts are then thermally sintered under high heat to bond the metal powder into its finished solid mass state. The remaining binder in the part is essentially burned off at sintering temperatures.
New solvent blends have been developed to speed the debinding process without the use of n-propyl bromide, methyl pyrrolidone, polyethylene glycol, heptane, or trichloroethane, which all carry health and or environmental baggage. The new debinding fluids boast low viscosity and surface tension ratings, are nonflammable, and are engineered for selectivity so the right amount of binder is removed without damage to the part structure. The debinding fluids are distilled and reused in the debinding process.

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