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Powder Preparation for the Additive Manufacturing Industry

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Additive manufacturing (AM) is a rapidly growing industry that has the capability of being used for an infinite number of applications. Whether it be for automotive, aerospace, pharmaceuticals, or any other industry, the preparation of the powder for this market is extremely important.

There are a number of different types of 3D printers and various powders that can be used to make parts in these printers. For Electronic Beam Melting (EBM) the required powder specification is from 107 microns down to 44 microns. For the Selective Laser Melting (SLM) process, a fine powder is required as the powder must be sized to the range of 44 microns down to 15 microns.

The main reason that powders used for AM have to be in these specified size ranges is the effect they have within the bed of the 3D printer. The way that the material layers in the 3D printer bed is critical, and removing the sub 44 micron and sub 15 micron particles from the metal powder going into the 3D printer is as critical. Without removing these fines, the powder will not layer properly in the bed and can significantly affect the way the laser melts the powder. There are a couple ways to ensure that the material that is going into your powder is going to meet these tight specifications.

When preparing metal powders for the AM industry, the name of the game is maximizing yield without sacrificing efficiency. There are two main processes currently being used to effectively prepare powder for AM. The first way to size this powder is using an air classifier and the other is to use a screening or sieving system. Each method has benefits and drawbacks that dictate where each is best applied. As technologies have advanced, the ability to create more precise particle size distributions has been enhanced.

Air classifiers use air velocity and a classification wheel to separate a powder based on size. The feed rate, air flow, classifier wheel size opening, and wheel speed are all adjusted to determine where the separation occurs. The classifier allows for separations at small sizes, and at high-volume processing this is clearly a separation technology. There are drawbacks to this type of system for AM powders.

The selective laser melting method requires almost all -10 and most of the -15 micron particles be removed from the powder. To attain these targets with an air classifier, there will also be 15-25 micron particles that are removed in the fines in order to make sure all of the fines are classified out. This affects yield as well as the distribution of the powder that makes the bed material for processing.  Air classification also comes with a large capitalized installation and ongoing operational costs.

The other option to meet tight specifications is to use an advanced screening system. Until a few years ago, making a cut at 15 microns on a screening system would have been extremely difficult and time consuming. Recent advances in technology have made it possible to remove this fine product using a screener. The key is energy. Energy on the screen surface allows the fines to easily fall through the hole openings that now do not blind. The higher energy is just what the heavy and finer powder needs to screen at high volumes and excellent efficiencies. The ability to successfully and continuously separate this material at low micron sizes means that companies making powder for the AM industry can maximize yields by not losing as much good product as they would with an air classifier. This creates huge benefits for the AM manufacturers, as well as other industries.

However, as with classifiers, there are limiting factors to using a screening system. Currently, very few companies are able to produce screen cloth at less than 20 microns in stainless steel. The AM industry requires stainless steel contact surfaces and expensive 20 micron cloth is currently being used to remove the sub 10 micron particle fines. The performance benefits of these new systems outweigh the added costs, and as the market continues to grow, many screen cloth companies are using new techniques to develop stainless steel cloth in these lower micron ranges.

Bob Grotto is owner and president of Elcan Industries Inc., Tuckahoe, NY. He has over 25 years of screening experience and has been running screening equipment at Elcan’s toll manufacturing facility since 2000. Grotto is a former board member of PEMA.

Screening out though agglomerates

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Recently, a chemical company opening a new $60 million manufacturing plant discovered a major problem. The company built the plant to manufacture a spherical bulk solid product, but the manufacturing process also generated particulate fines that were forming agglomerates.

As the 1/8-inch product spheres dried, the fine powder would clump together and stuck to the product. These agglomerates needed to be removed before the product could be delivered to the client. Traditional screening methods weren’t working because the agglomerates were similar in size to the product and the screeners weren’t capable of breaking up the agglomerated material.

“We attempted to remove these particulates in the plant by both a gyrator screener and a standard circular vibrating screener,” says the plant’s senior manager. “However, these processes were not vigorous enough to either break up the large agglomerates or break the agglomerates off the product.”

Searching for a solution

With a production deadline approaching, the company needed a fast solution to remove the agglomerates. The company turned to Elcan Industries, a screening equipment supplier basic in Tuckahoe, NY.

“Elcan Routinely does a particle separation work for us,” says the plant’s senior manager. “We knew they had several types of equipment that we could explore to solve this problem.”

The supplier has a 20,000-square-foot testing facility with 10 tolling bays, where equipment can be set up to either perform a test or for full-scale processing. Elcan tested several different models of screeners with the product to try of screeners with the product to try to remove the agglomerates. Once option was a tumbler screener that used air wands underneath the screen to push the material up and break off the agglomerates. However, the agglomerates were too aged to be broken up by the action of forcing the particle up and into the roof of the screener.\

Next, the supplier tried a high-energy screener that imparted energy directly into the 1,500-micron screen cloth. The hope was that the energy in the screen would break up the agglomerates to a small enough size to pass through the screen cloth but this didn’t work either.

Breaking down the agglomerates

While talking about the problem in a conference room, the supplier’s engineers noticed that if they dropped a marker pen with enough force, the agglomerates would break, but the final product was strong enough to withstand the impact. The supplier went back to a curricular, high-energy screener but added several hundred 35-millimeter polyurethane balls on top of the mesh to impact the mixture of agglomerates and products.

The high-energy screen vibration shot the balls into the air, and when the balls came down, they broke the unwanted material off the product and crushed the agglomerates. This allowed the fine material to pass through the screen while leaving the finished product behind. However, the screeners circular shape encouraged everything to discharge from the top of the screen, including the balls.

To combat this, the supplier decided to use a rectangular, high-energy screener and placed partitions inside the divide the screen into three sections and prevent the balls from being discharged. A small gas below the partitions allows the product and agglomerate to pass through freely. The balls break around 70 percent of the agglomerates in the first section, finish up most of the remaining agglomerates in the second section, and polish off whatever remains in the third section.

A vacuum then sucks down the crushed material through the screen and collects it in a bathhouse. The product exits the screener and can then be shipped tp the client. The supplier set ip four large machine in two of the tolling bays to process the truckloads of product, allowing the the chemical company to meet its deadline. PBE