Taking MIM Tooling To the Next Level Originally published in The American Mold Builder Magazine, February 2014 1
Metal injection molding (MIM) merges two established technologies, plastic injection molding and powdered metal. It combines the strength and durability of metal with the design flexibility of the injection molding process. The MIM process is generally limited to parts less than 400g in weight, and is best suited to designs involving complex geometry and annual volumes of 10,000 pieces or more. The advantage of MIM is the design freedom to consolidate two or more parts into a single piece, which not only cuts costs and eliminates the need to work with several different suppliers but in most cases also produces a stronger finished component. Powder Metal Screw Machine Stamping One MIM component Stock Component 4- Piece Assembly The MIM Process Compounding/Feedstock 4-Piece Assembly Feedstock is comprised of very fine powders (particle size usually <15 microns) as compared with the >40 micron diameters typical to conventional powder metal. In fact, the powders used for MIM feedstock are sometimes referred to as metal dust. Both pre- alloyed metal powders and combinations of elemental metal powders are suitable for MIM processing, and the varieties of metal available for MIM are constantly expanding. NiFe, 316SS, 420SS, and 17-4SS, titanium, and copper are just a few of the materials suitable for MIM. To form the feedstock, the metal powders are mixed with a plastic and wax binder. The ratio of binder- to- metal is approximately 60% metal, 40% binder. To ensure tight tolerances, the powder- to- binder ratio must be closely controlled; feedstock consistency results in predictable uniform, isotropic shrinkage. Thus, the binder system must be stable to be repeatable. Unlike conventional powder metallurgy, which can achieve only 80-90% of theoretical density, MIM results in 95-100% of the theoretical density. This is how we achieve close tolerances and realize the economic benefits of producing small, very complex parts over high production runs. 2
Molding The feedstock is processed through a twin- screw extruder, pelletized and loaded into standard plastic molding machines or Dynacast s proprietary multi- slide machines. Once molded, the component is referred to as a green part. Its geometry is identical to that of the finished piece, but to allow for shrinkage during the sintering phase, it s roughly 20% larger in size than the finished component will be. Debinding Binder removal or debinding involves a controlled process to remove most of the binders. The process uses heat and/or chemicals to remove the binders and prepare the part for the final step, sintering. Once this phase is complete, the component is referred to as brown. Sintering The components are placed into continuous or batch vacuum furnaces. Batch furnaces are most commonly used as the parts are processed in a vacuum under partial pressure using an inert gas such as argon, which results in greater chemistry control. The part is subjected to temperatures near the melting point of the material, and the entire sintering process takes 15-20 hours. At the beginning of the sintering phase, the brown part is held together by just a small amount of the binder, and is thus fragile. Sintering eliminates the remaining binder, then densifies and gives the part its final geometry. The shrinkage is approximately 20%. During debind parts shrink ~2% During sintering parts shrink 16-22% depending on material and binder loading Molded Green Part Debound Brown Part Sintered Finished Part 3
Tooling Designers are constantly pushing the envelope with smaller and smaller components performing multiple functions. The complexity of these designs require precision and detail not seen in the past. Mold makers are challenged in producing molds with multiple slides with complex shut offs. Unlike plastic injection molding where the parting line on the final part may be visible and soft, the final MIM part is solid metal and parting lines, ejector pins, and gates witness lines can affect the function of the part. Just as in plastic the material flows very easily and is subject to flashing. Because of the paraffin in the binder the viscosity is extremely low compared to standard plastics. Venting, slides, parting lines and ejector pins need to be built tighter with vents as small as.0005 in. MIM has been around for over 30 years. In that time little has changed when it comes to tooling and molding. Dynacast s unique approach to MIM results in more consistent parts. This consistency partly derives from the excellent alignment offered by multi- slide tools. The registration of their die faces measures 0.0015 inches TIR versus as much as 0.005 inches with a typical MIM tool. Tooling engineers utilize the latest CAD/CAM software and mold flow simulation to help in the design of the part for manufacturing (DFM). Using the multi- slide tooling technology we can minimize ejector pins on the part and in most cases eliminate the ejector pins entirely. Engineers also insure datum points are optimized in the mold for better repeatability. The molds are built using 5- axis CNC milling machine for cutting cavity and/or electrodes, 4- axis CNC, surface grinders, wire and sinker EDM machines. This reduction in alignment errors, or die mismatch, dramatically reduces an important source of dimensional variation that can carry through from the green part to the finished component. Multi- slide tooling also helps keep part- to- part variation to a minimum compared to conventional multi- cavity injection molds. The reason why comes down to tooling size. Whether in single or multi- cavity configurations, multi- slide tools are more compact than comparable MIM tools. These smaller tools are less prone to parting line variation and its negative effect on the finished part s dimensional tolerances. Tooling maintenance is paramount in keeping the tool running efficiently. Another issue is that as the parting line wears the mold is closing on not just plastic but 60% metal. So tool wear and damage can result far sooner then in plastic molding. Constant cleaning of the mold is required while molding and most tools are pulled every week for cleaning and maintenance. In a recent report by Hartmut Walcher and Marko Maetzing in Powder Injection Molding/September 2013, 70% of defects in the final MIM component can be traced back to the tool itself and 15% in the molding process. This means approximately 85% of all defects in MIM components are a result of the tooling and molding process. Multi- slide reduces and in some cases eliminates the variations of standard injection tooling and molding. Originally designed for die- casting these machines are modified for metal injection molding. Multi- slide tooling is far less complicated than standard injection molds and allows for small (<18 grams) complex components to be manufactured with greater precision. The multi- slide tool is made up of the die block, sliders, crosshead and cover plate. Each die block has either a cavity and/or cores on its face, which together form the complete cavity and runner profile into which the MIM material is injected. These die blocks are mounted onto sliders, which fit precisely into a crosshead, ensuring repeatable opening and closing operations. A cover plate, bolted onto the top of the tool, holds all 4
these components together. Each slide is managed by a PC controller, and moves independently of the other, both during the closing and opening sequences. This provides tremendous flexibility, which ensures part integrity and prevents damage to the tool. Ejection of the parts is achieved with an air- blast, which blows the shot clear of the cavity and into a padded collection mechanism, or the parts can be removed with a robotic arm. Slide Cross Head Die Block Cores The converted die cast machines themselves cycle at speeds of up to 20 cycles per minute (1,200 shots per hour, over 6 million per year). This can eliminate the need for multi cavity tooling, reducing tooling costs, long runner systems, wasted material and cavity- to- cavity variations. This is achieved by using pneumatics, rather than slower hydraulics, to operate the different parts of the machine. Mechanical toggle mechanisms and hydraulic thrusters supplement the weaker locking force available with pneumatics, ensuring that the multi- slide tool is held together securely during the injection process. Dynacast has also developed fully hydraulic multi- slide machines, allowing us to further match the process to the part. With very little actual movement the tooling is more precise. In a standard injection mold there are slides moving in and out with the use of slide blocks and guide pins, plus the mold is opening and closing along ties bars. With all of this movement it can create wear and small mismatches causing parting line defects in the part itself. Water lines are extremely close to the cavities keeping the mold at a constant temperature eliminating inconsistent mold fills and molded in stresses. In conclusion, metal injection molding continues to evolve. Complex designs, exotic materials and multi- slide tooling are changing MIM technology. Parts considered impossible just two years ago are now manufactured in high volumes. Mold makers are evolving with newer technology to keep up with this demand. For more information on Dynacast and MIM, please visit dynacastmim.com. 5