Delicate die casting example of a transmission housing (small)

Forge & Foundry, Part 6 – Selecting a Shot Blasting Machine for Die Castings

Our Forge and Foundry Series continues with tips for selecting a shot blasting machine for die castings.

Considerations for machine selection include:

  • Are the work pieces sturdy enough to allow for somewhat more aggressive processing or must they be handled gently without any part-on-part contact?
  • Is batch processing possible or must it be continuous?
  • Which work piece handling system is best: rotary drum, troughed belt, wire mesh belt, or overhead monorail system?
  • Can the work pieces be handled by robot, etc.?

Rosler Metal Finishing builds shot blasting machines that are designed to expertly prepare the surface of delicate and sturdy die castings and everything in between. We can design a machine that is perfectly matched to your work piece and process.

How can loose metal flashes be discharged from the shot blast system?

Before being shot blasted, die castings are usually trimmed, i.e. sprues, runners, and gates are removed by trim press, saw, or robot. Even so, these work pieces can still carry material overflow in the form of flashes into the shot blast machine. There the flashes are either broken off by tumbling or shot off by the kinetic energy of the blast media. If not discharged regularly, they will accumulate in the machine posing the risk of clogging up screens and interrupting the media flow.

At a minimum, the media auger must be protected by a tiltable screen preventing the flashes from contaminating the media. Of course, this screen must be inspected and emptied regularly. Even better is replacing the auger with a vibratory screen conveyor that allows the automatic discharge of loose flashes and other contaminants.

Shot blast machines for die castings should also be equipped with a rotary screen drum at the elevator exit to discharge any loose particles from the media/dust mix before it passes through the media cleaning system. If not removed, these particles can clog up, even, damage the blast turbines.

Why is it so critical to clean the blast media, before it is returned to the turbines?

When blast cleaning die castings, the media gets contaminated with metal dust from the work pieces and media, pieces of broken-down media, and small particles of metal flash from the work pieces. These contaminants must be completely removed from the media, as they would negatively impact the shot blast results and cause premature wear of the blast machine, especially the turbines.

Therefore, the shot blast machines are equipped with an air wash separator that safely removes all of these contaminants from the media before it is recycled to the blast turbines.

Must blast machines for cleaning of die castings be protected against premature wear?

Generally, shot blasting of die castings is a lot less abrasive than, for example, the blasting of sand castings and forgings. When steel media is used, it usually has a smaller pellet size or, in the case of aluminum or zinc media, the material has a somewhat lower bulk density and is softer than media made from carbon or stainless steel. Therefore, it is also less abrasive.

Despite these somewhat less severe conditions shot blast machines for die castings still need some form of wear protection.

In some cases, for example with spinner hanger and wire mesh belt machines, the blast chamber is made from manganese steel. In addition, the areas of the blast chamber directly exposed to the blast stream are equipped with a replaceable wear lining.

Overall though, shot blast machines for die castings require less wear protection than machines for sand castings and forgings.

What design features must be considered in blast turbines used for cleaning of die castings?

The cleaning of die castings – whether flash removal, light deburring, masking of heat checking, or stripping of burnt-in die lubricants – requires somewhat less energy than the cleaning of sand castings and forgings, yet the required “fire power” is still substantial!

That is why turbines with curved throwing blades, such as Rosler’s Gamma 400 G blades, and their 25-percent higher throwing speed offer significant advantages. They allow the use of smaller-sized blast media, be it steel, stainless steel, aluminum, or zinc. This helps not only in reducing the overall wear rate of machine and turbines, but also creates a much finer satin-like finish and lowers cycle times with subsequent lower energy consumption.

The fact that both sides of the throwing blades can be utilized is an additional bonus that helps to maximize the overall cost efficiency of the blast cleaning operation.

How can dust explosions in dust collectors be prevented?

The blast cleaning of die castings allows the use of standard cartridge filters, but the dust from non-ferrous metals, especially aluminum and magnesium, is highly explosive. Therefore, special technical precautions must be taken to prevent explosions or, at a minimum, special explosion protection features must be installed to alleviate the effects of an explosion including burst plates, explosion relief chimneys, and relief valves.

Alternatively, wet dust collectors can be installed, which wet the dust particles with tiny water droplets and separate the air from the wetted dust particles in a diffusion chamber. The dust particles are discharged from the system in the form of sludge.

The Rosler Way

Understanding the finishing needs of die castings has been one of Rosler’s strengths for decades. Trust us to use our expertise to learn, develop, and deliver the exact finishing results you need while keeping your employees and environment as safe as possible. Contact us today to start the process.

The complete Forge & Foundry Series includes:

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