PM Process

Blending

Most powder metal alloys are “alloyed” in the sintering operation and are more an elemental mix in the powdered state. Highly specialized blenders are utilized to create and homogenize that mix. Blends typically consist of tightly held ratios of Iron, Copper, Nickel, Molybdenum, Manganese, Carbon, etc., with a minute amount of lubricant so the part can be ejected from the compaction tooling. The blends used at Sterling Sintered are usually made in 5,000 to 20,000 pound batches. The blender in the animation is a Dual Cone Blender. Several other types exist. Blending can often happen such that thousands to millions of parts can be made from a single batch that takes only an hour or so to process. Visit our quality link to see the process controls utilized at Sterling Sintered during blending.

Compaction

Compaction of PM parts happen under very high pressures. Compaction pressures of 80,000 psi are not uncommon. In the animation you can see that powder flows downward into the mold die from the feed shoe using gravity. The feed shoe then retracts out of the way and a compaction punch enters the die, sealing it off from above. Next, that top punch in conjunction with punch(s) from below compact the powder in the die increasing the powder density as they do. The powder particles are compressed into each-other with so much pressure that the irregularities of their shapes intertwine and the particles interlock. The compaction ratio is typically 2.3 to 1. It takes about 2.3 inches of powder to make a 1 inch part. After the part is compacted the top punch retracts and in the animation the bottom punch pushes the part out of the die. In some press designs the die travels downward and strips itself off of the part. In either case the part has to travel out of the die in an upward direction. The lubricant in the blend is there to reduce friction during this travel and thereby reduce die wear and scoring. Many times, the part shape dictates the use of multiple punches and motions during compaction. Sterling Sintered has many compacting presses with multiple motions ranging from 4 to 110 Ton capacity. See our case study page (link) for examples of part designs. Compaction of PM parts typically happens at 500 to 3,000 parts per hour. Visit our quality link to see the process controls utilized at Sterling Sintered during compaction.

Sintering

Sintering of PM parts happens at elevated temperatures. In the old blacksmith days a Blacksmith would heat metal components to white hot temperatures and pound them together with mighty whacks using their hammers. With PM, the mighty whack comes first. That’s the Compaction. The white hot part comes afterward. That’s Sintering. Once the powder particles are intimately interlocked in compaction the next step is heat them and vaporize the lubricant out of the part. This vapor gets burned up (oxidized) and exits the furnace as mostly CO2 and H2O. The next step is heat them to a hot enough temperature that the metals weld together and the alloying ingredients diffuse and steel is created. A reducing atmosphere consisting of Hydrogen and Nitrogen is used to protect the metals at these temperatures. The atmosphere also serves to strip Oxygen off of the particles which would otherwise impede their fusion to adjacent particles. Next the parts are cooled down to room temperature at a cooling rate which creates the desired crystalline structure. Sintering of PM parts typically can happen at thousands to tens of thousands of parts per hour.

Specialized Sintering can occur at Sterling Sintered to accomplish specific effects. For example, copper infiltration can increase a steel part’s strength significantly. Varying the protective atmosphere can also affect certain mechanical properties. Sterling Sintered can vary that atmosphere all the way from 100% Hydrogen to 100% Nitrogen to swing those properties. Most of our competition is locked in at one ratio. We can also vary the cooling rates such that some steel materials can land in the Rockwell C scale without post-process Heat Treatment! Visit our quality link to see the process controls utilized at Sterling Sintered during sintering.

Coining

Most PM parts are nearly finished at this point but some require sizing or coining operations to improve dimensional or structural results. Coining and Sizing operations are used to reduce dimensional variation and/or improve part properties. Brass and Bronze parts typically change a decent amount dimensionally in the sintering operation. Any time there is change there is variation in the amount of change. Steels and Stainless Steels don’t usually change as much but sizing can decrease dimensional variation and coining can increase density and thereby strength. Some materials will possibly need to be re-sintered after coining to re-fuse the particle boundaries. Coining does not usually happen at as fast a rate as previous operations so it can impact part cost. Let the Sterling Sintered engineering department help you balance product requirements and cost. Less than 10% of the parts made at SST require coining to meet customer requirements. View our quality link to see the process controls utilized at Sterling Sintered during coining.

Other Secondaries

Nearly any secondary operation that can be performed to a wrought part can be performed on a mechanical component made by PM technology. Click here for more details on secondary operations available at Sterling Sintered Technology.

If you have any questions on the PM process, give us a call. We are available to discuss your PM needs M-F, 9-5 EST, at 860-379-2753.