Forge & Foundry, Part 7 – Selecting a Shot Blasting Machine for Forgings, Non-Sand Castings, and Powdered Metal Components

This installment of Rosler Metal Finishing’s Forge and Foundry Series continues with shot blasting machine selection considerations for forgings, non-sand castings, and powdered metal components.

While none of these work pieces contain sand, their surfaces may show oxidization or – in the case of ferrous metals – heavy scale/rust caused by iron oxide.

All forms of oxidization must be removed to ensure that subsequent manufacturing operations such as machining, coating, and painting are economical and efficient. Poorly cleaned work pieces may cause additional processing, premature wear on milling tools and drill bits, excessive pollution within coolant systems, and inefficient adhesion of coatings and paint.

Traces of oxidation may also impact the work piece’s functionality.

Like with any work pieces, before choosing a machine, the following questions must be asked:

  • Are the parts sturdy, allowing 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, overhead monorail system, heavy-duty crane, or trolley on rails for extremely heavy work pieces weighing several tons?
  • Can the work pieces be handled by a robot or is a custom-engineered shot blast system the best solution?

Therefore, it is extremely important to work with an experienced supplier such as Rosler to select a machine that is perfectly matched to your work piece characteristics.

How can coarse contaminants like broken off sprews, gates, and runners be discharged?

Despite not containing any sand, there are certain technical precautions that must be taken with these materials.

For example, investment casting assemblies (“trees”) require sprews, gates, and runners, which may occasionally be carried into the shot blast machine. These must be removed before they can cause any damage. That is why shot blast machines for investment castings may have to be equipped with robust vibratory screen conveyors in place of the standard augers. They allow the discharge of sprews, etc., and the safe transfer of the mix of media and dust (scale, rust) to the elevator.

Machines should also be equipped with a rotary screen drum at the elevator exit to discharge any metal particles from the media/dust mix before it passes through the media cleaning system. If not removed, these particles can seriously damage the blast turbines.

Why is scale/rust removal from the blast media important?

During the shot blast process, the media gets contaminated with oxide dust from the scale/rust blasted off the work pieces. This oxide dust is extremely abrasive and must be completely removed from the blast media to prevent premature wear of the blast machine, especially the turbines.

Therefore, shot blast machines having to deal with this type of dust are equipped with an air wash separator – preferably a dual stage system with two cascades – that safely removes the oxide dust as well as the broken down blast media from the media mix before it is re-used in the blast turbines.

Must blast machines for cleaning be protected against premature wear?

Even though forgings, etc., do not contain any sand, the high media impact velocity along with the abrasion of the media/oxide dust mix can quickly induce wear of the blast chamber, augers, elevator, air wash separator, and turbines. The wear exposure is more extreme when granular blast media or large steel shot is utilized.

For this reason, special precautions must be taken to prevent premature wear. For example, the blast chamber should be made from 3/8” (10 mm) thick manganese steel and the area directly exposed to the blast stream should be lined with replaceable wear liners made from manganese steel or chilled iron castings with a thickness of up to 5/8” (16 mm).

What design features must be considered in blast turbines?

Curved throwing blade

While sand is not an issue, the scale, rust, and surface oxidation these work pieces are susceptible to may require high powered turbines for removal. Turbines with curved throwing blades have proven to be exceptionally effective since, compared to straight-bladed turbines, the curvature of the blades generates up to 25 percent higher throwing speeds.

Increased throwing speeds help reduce cycle times and energy consumption while contributing to an overall higher cleaning efficiency. Also, the fact that both blade sides can be used, practically doubles the wear life of the throwing blades.

To minimize costly downtimes, the wear protection along with impeller, control cage, and throwing blades must be made from wear resistant materials such as heat-treated cast steel or forged tool steel.

How can dust explosions in dust collectors be prevented?

Unlike sand casted forgings, dust created in the finishing of non-sand castings and powdered metal components can be handled with standard pleated cartridge filters. If the work pieces have a somewhat higher temperature, special heat resistant filter material must be utilized.

In case of non-ferrous casting and forging materials, such as aluminum and magnesium, etc., the dust collectors require special explosion protection features including burst plates, explosion relief chimneys, and explosion relief valves to withstand a dust explosion.

Alternatively, wet dust collectors can be installed.

Rosler wet dust collector

The Rosler Way

Understanding the materials you use and the work pieces you create is the Rosler Way. Our experts learn about your processes, develop solutions, and deliver the exact finishing results you need while keeping your employees and environment as safe as possible. Contact us today to learn how we can help improve your finishing processes.

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