Forge & Foundry, Part 7 – Selecting a Shot Blasting Machine for Forgings, Non-Sand Castings, and Powdered Metal Components
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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:
- Part 1 – Shot Blasting Systems.
- Part 2 – Efficient Recycling.
- Part 3 – Shot Blasting & Sand Castings.
- Part 4 – Selecting a Shot Blasting Machine for Sand Castings.
- Part 5 – Cleaning Features & Dust Precautions for Sand Castings.
- Part 6 – Selecting a Shot Blasting Machine for Die Castings.
- Part 7 – Selecting a Shot Blasting Machine for Forgings, Non-Sand Castings, and Powdered Metal Components.
- Part 8 – Frequently Used Shot Blasting Machines for Sand Castings.
- Part 9 – Frequently Used Shot Blasting Machines for Forgings, Non-Sand Castings, and Powdered Metal Components.
- Part 10 – Shot Blasting Machines Commonly Used for Die Castings.
- Part 11 – Top Mass Finishing machines for Cleaning Die Castings
- Part 12 – Media and Compound Selection, Effluent Handling Drive Success
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