MIM, or Metal Injection Molding, is a metalworking process where finely-powdered metal is mixed with binder materials to create a 'feedstock' that is then formed and hardened using injection molding. The MIM process allows high volume, intricate parts to be shaped in a single step.
Materials advances in MIM technology allow manufacturers to produce large quantities of parts with complex geometries, often eliminating labor-intensive machining completely. The quality and precision of parts made with MIM often results in parts requiring minimal subsequent machining to create a finished part with excellent dimensional repeatability.
The MIM process is capable of creating complex and precise structures by blending metal powder with various binding agents. Metals available include stainless steel, tool steel, and many other ferrous and nonferrous alloys. The actual process combines a polymer or wax known as a binder with a fine metal powder to form a moldable admixture that then is formed or molded into the desired structure.
Common types of binding agents are paraffin wax, carnauba wax, and specialty polyethylene waxes. The binding agents serve a critical purpose in the forming process but are ultimately sacrificial and must be removed once the structure is formed. Selection of a debinding method is a balance of removing the wax 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.
Extraction of the binder can be done in either the solvent vapor or liquid phase in a vapor degreaser. Both rely on the debinding fluid flowing through the pores of the structure to dissolve the wax. This is where the physical properties of the debinding fluid become important. The ideal debinder will be nonflammable, have a low surface tension and low viscosity to clean internal geometries better. It should also be 100 percent volatile so the debinding fluid can be easily removed before sintering. Once the debinding fluid 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 parts is essentially burned off at sintering temperatures.
Debinding fluid blends have been developed to speed the debinding process without the use of n-propyl bromide, methyl pyrrolidone, polyethylene glycol, heptane, or trichlorethylene, which all carry health and or environmental baggage. Two recommended debinding fluids include Tergo™ MCF Metal Cleaning Fluid and Opteon SF79 Specialty Fluid. These debinding fluids boast low viscosity and surface tension ratings, are nonflammable, and are engineered for selectivity so just the right amount of binder is removed without damage to the parts structure. Their lower boiling point still melts wax and additives but runs more efficiently in the vapor degreaser. It also evaporates from inside the pores of the parts faster, saving energy costs and speeding drying times. The debinding fluids are also distilled and reused many times inside the vapor degreaser before replacement is needed. They also do not require acid acceptance testing, maintenance or additional stabilizers.