Rapid, gentle blending maximizes strength of powder metal parts

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Courtesy of Munson Machinery Company, Inc.

Nestled in the heart of the powder metal capital of the world, Allegheny Blending Technologies (ABT) specializes in custom blending of ferrous and non-ferrous powder metals that are sold to molders of powder metal parts. These blends of iron powder and additives (copper, nickel, graphite, manganese sulfide and dry lubricants) play important roles in products and equipment. In the U.S. automotive industry, nearly every car has about 25 lb (11 kg) of powdered metal parts ranging from timing gears to anti-lock breaking mechanisms. Powdered metal parts are also found in the drive gears and motors of lawn mowers, snow throwers, and other gardening tools, as well as in home appliances such as washers and dryers. 

ABT has been blending custom powder metal batches from 500 lb (227 kg) to 5 tons since 1997. Four double cone blenders handle the 500 lb (227 kg), 1.25 ton, 2.5 ton and 5 ton batches. A 25 lb (12 kg) capacity double cone blender handles samples. 'With double cone blenders, we find it's best to match the load to blender capacity for optimal homogeneous mixing,' said Paul Reed, president of ABT. 'Under-loading a double cone blender reduces mixing action efficiency. The material tends to roll around in the barrel rather than blend thoroughly together.'

When customers requested blended batches of 22.5 ton truckload lot sizes, rather than purchase another double cone blender, Reed chose a Munson 700-250 HD rotary batch mixer for high bulk density materials. 'A double cone blender for this capacity would not meet our needs,' said Reed. 'Higher weight loads increase the friction the powder particles experience during blending, affecting the final properties of a mix. In addition, double cone blenders of that capacity are tall, requiring a ceiling height of nearly 25 ft (762 cm).'

The rotary batch blender is a physically-compact piece of equipment. 'Plus, I had heard that rotary blenders' mixing action is more gentle than that of double cone blenders and of shorter duration,' continued Reed. 'These features will reduce particle rounding caused by friction between the metal powders, maintaining our high-quality product.' 

In addition, the rotary batch system can be under-loaded down to 10% with no effect on batch uniformity or cycle times, handling batches from 22.5 tons down to 2.25 tons with equal efficiency.

Blending more than 22 tons of material quickly 
The blending process begins when iron powder arrives at ABT in loads consisting of 2.5 ton palletized cardboard boxes. Forklifts load the boxes into a powder dumper, which tilts and dumps the contents into a steel hopper positioned above a fine-mesh screener. 

Additives arrive in either small bulk containers or paper sacks and are poured into individual hoppers that rest on coarse mesh screens. 'We sift everything prior to blending,' said Reed. 'We ensure contaminants such as wood slivers from the pallets or bits of cardboard or plastic do not enter the mix, as foreign material could compromise the finished powdered metal part.' 

After screening, three floor scales weigh the powders before they are loaded into the blender. The iron is weighed on a 5 ton scale in 2.5 ton loads. The 500 lb (227 kg) scale weighs the nickel and copper additives. A 200 lb (91 kg) scale provides the greatest accuracy needed to weigh graphite and dry lubricants. 

The iron powder is gravity-fed one box-load at a time from the steel hopper into the blender. Before the last 2.5 ton load is added, the hopper containing the additives is raised into position by a forklift for gravity discharging into the blender.

Rapid, gentle blending preserves green strength
The rotary mixer consists of a horizontal, rotating drum with a stationary inlet at one end and a stationary outlet with a discharge gate at the other. A self-adjusting face seal at the inlet allows dust-free operation. Internal baffles (mixing flights) and lifters create a four-way mixing action that tumbles, turns, cuts and folds material throughout the filling, mixing and discharging phases, achieving 100% batch uniformity and preventing the separation of ingredients of varying particle sizes.

The mixer can achieve 100% batch uniformity in three minutes, but ABT mixes 22.5 ton loads for about 15 minutes depending on the properties required for specific powder metal mixes. 'The rotary batch mixer saves us time and ensures a quality product,' said Reed. 'A double cone blender for this size load would take more than 50 minutes to blend because the material tends to roll around rather than mix together. Plus, the long blend time builds more heat within the mix. When the mix gets hot, then you know you're rounding the particles, reducing the mixture's green strength.' 

A powdered metal part that has not undergone sintering is a green part. 'Green strength is something we measure for our customers,' continued Reed. 'We mold test bars of green metal that are about 1 1/2 in. (38 mm) long, 1/2 in. (13 mm) wide and 1/4 in. (6 mm) thick and break them in a tensile tester to assess a blend's strength. We have found that green parts from material blended in the Munson unit have a higher green strength than parts molded of material from double cone blenders.'

Homogeneous blends essential for consistent performance of molded parts
Uniformity of powdered metal blends is crucial to the performance of the finished part. 'Even though our customers' parts typically weigh only a few grams to 17 lb (7.7 kg), they need repeatability and consistency to meet tight dimensional tolerances,' adds Reed, 'so they see the benefits of our homogeneous product.' 

Sampling ensures homogeneity, but it is done primarily on new blends to obtain blending times. 'Once we've mastered a blend's mix and mixing time, there's no need to sample. But, if a customer has an order for a specific feature in a blend, such as apparent density, we will sample to ensure we meet that specification,' said Reed. 

Total evacuation simplifies cleaning
After loads are mixed and the plug gate is opened, the flights and baffles in the rotating drum elevate and discharge the entire contents of the batch, with no residual, into hoppers that are transferred to the packaging area. 'We found that if we let the blender run for a few minutes after full discharge, only minute amounts of dust remain,' said Reed. 'If we stop the blender too quickly, maybe 1/2 lb (0.23 kg) to a 1 lb (0.45 kg) of material remains. If the next load must be absolutely contamination free, we use a shop vacuum and remove any remaining dust, but even then, clean up is quick and easy.'

The blended material often goes back into the bulk packs that held the raw material. Some material goes into fiber drums or bulk bags that hold up to 2.5 tons. These packs have been cleaned to eliminate contamination, with new liners installed where needed.

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