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.

The complete Forge & Foundry Series includes:

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

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