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Hints on investment casting: Troubleshooting


#1

Hints on investment casting: troubleshooting casting problems

Defect
cause
solution

Cracking
Quenching too soon
Wait 15-20 minutes

Cracking
Metal cast too soon
Increase casting temperature

Cracking
Flask too cold
Increase flask temperature

Cracking
Incorrect sprueing
Modify sprueing

Cracking
Contamination of gold or alloy
Refine metal

Cracking
Oxide build up in metal, over-use
Refine metal

Cracking
Hydrochloric acid di-vesting
Use alternative di-vestor

Incomplete fill
Metal too cold
Increase casting temperature

Incomplete fill
Flask too cold
Increase flask temperature

Incomplete fill
Insufficient vacuum
Check vacuum for leaks and seal

Incomplete fill
Wrong speed on centrifugal caster
Adjust speed

Incomplete fill
Insufficient burn out
Modify sprue

Incomplete fill
Incomplete burn out
Use proper burn out schedule

Inclusions in castings
Sharp corners or bends in sprueing
Round out sharp corners and bends

Inclusions in castings
Crucible old and deteriorating
Replace crucible

Inclusions in castings
Oxide build up in crucible
Clean or replace crucible

Inclusions in castings
Foreign particles or oxides in metal
Refine metal

Inclusions in castings
Investment erosion or breakdown
Follow investment manufacturer’s mixing instructions

Brittle prongs on castings
Improperly alloyed metal
Pre-allay gold and master alloy

Brittle prongs on castings
Flask temperature too cold
Increase flask temperature

Shrinkage prorsity
Incorrect sprueing
Sprue to heaviest area of casting

Shrinkage porosity
Inadequate sprueing
Use larger sprue or multiple sprues

Shrinkage porosity
Flask too hot
Use lower flask temperature

Shrinkage porosity
Castings too close to sprue button
Leave 1" space on tree above main sprue button

Gas porosity
Metal overheated
Reduce casting temperature

Gas porosity
Inadequate burn out
Increase top end burn out time

Gas porosity
Inadequate air supply during burn out
Assure oven has good air supply & exhaust

Gas porosity
Flask too hot
Reduce flask temperature

Gas porosity
Scrap reused too many times
Refine metal

Gas porosity
Too much oxygen on torch flame
Use reducing flame when melting

Gas porosity
Investment residue on remelted scrap
Remove investment residue before remelting scrap

Rough castings
Flasks not cured before burnout loading
Let flasks set 1-2 hrs. before burnout loading

Rough castings
Incorrect water-powder ratio in invest. mix
Follow investment manufacturer’s instructions

Rough castings
Flasks heated too rapidly
Follow investment manufacturer’s instructions

Bubbles/nodules on castings
Investment not mixed, vacuumed or vibrated sufficiently
Follow investment manufacturer’s instructions

Bubbles/nodules on castings
Vacuum pump not working properly

Bubbles/nodules on castings
Wax patterns not coated with wetting agent
Coat wax patterns with wetting agent

Andy " The Tool Guy" Kroungold
Tool Sales / Technical
Stuller Inc
Phone 800-877-7777 ext. 94194
Fax 337-262-7791


#2

Andy, Thanks for sharing that on troubleshooting
investment casting with us.

Oh, and I enjoy your articles in the Stuller Standard too!

Cheers from Don in SOFL.


#3

Andy:

Thank you for posting your chart.

Everyone:

Is there some way, without a metallurgical lab, of determining if
porosity is caused by ‘shrinkage porosity’ or ‘gas porosity’.

Thanks in advance for any help.

Lorne
Ladysmith, BC


#4

Is there some way, without a metallurgical lab, of determining if
porosity is caused by ‘shrinkage porosity’ or ‘gas porosity’.

Lorne,

Examine the shape and concentration of the pits. Gas porosity will
tend to leave larger, spherical pits. Dirt (investment, carbon from
tools, foreign body in the wax contaminants) will present as jagged,
irregular or included pits. Over fluxing will leave rounded
depressions with a halo of bright metal – a result of the flux
scavenging off the oxides-- and, often, glassy flux inclusions.

These pits are the result of melting, burnout or casting procedures.

“Shrink spot” or “hot tearing” are the result of sprue placement and
solidification (cooling) and occur just after casting. Hot tears
are actually torn/fractured areas usually occuring in zones where
thick meets thin. Shrink spot is most often appear as spongy areas
that just won’t take a high polish that, when louped, can be seen to
actually be areas filled with fine sprays of porosity, sometimes
irregular in cross section. These areas will often be concentrated
around sprue attachments or in heavy cross sections.

When larger, heavier patterns are cast-- a gent’s signet ring–
at too high a flask/ mold temprature the large mass of metal takes a
relatively long time to cool. The crystals grow larger resulting in
a coarse grain structure complete with voids and small tears-- shrink
spot.

The object of a good casting is to produce a dense, finely refined
crystal structure. Shrink spot is not this.

I’ve found that by considering all the factors-- the shape of the
evidence, where it’s located and the mechanics of the process that’s
giving me trouble-- I can often ferret out the culprit.

Summing up: in my experience, gas porosity is larger, round and
regular shaped pits; shrink spot pososity is fine and spongy in
appearance showing up, most often at final polish.

Hope this helps. Take care, Andy Cooperman


coopermanjewelry.com


#5
Is there some way, without a metallurgical lab, of determining if
porosity is caused by 'shrinkage porosity' or 'gas porosity'. 

The difference can be determined easily. Shrinkage porosity will
appear as small “tearing” in the metal, jagged with dendritic
shapes. Gas porosity will appear as clusters of small spherical
voids, basically gas bubbles frozen into the metal.

Shrinkage porosity is most often caused by improper and/or
insufficient spruing. Gas porosity can be avoided by using a
reducing flame, proper fluxing of the melt and not trying to cast at
the “lowest possible temperature”. Molten metal expels gasses during
solidification. Low flask temperatures can cause the gasses to be
trapped in your piece by the metal freezing too quickly. This is
especially true with sterling silver. Gold is generally more
forgiving of gas porosity, and more prone to shrinkage problems.

Mark Moretti
Alexandria, VA


#6

Andy,

Thank you so much for this You have no idea how many
books and technical docments I have read that have overwhelmed be
with extreme technical albeit interesting, but
difficult to translate into what you just supplied in your post.

I have had many different types of problems throughout the years
with porosity on my volume castings, and I now know simply why I
occasionally encounter the problems that I do. Some of the problems
I identified, but I just didn’t know how to describe it or what to
call it - this helps a lot. Thank you so much for your insight, I
will share this with my casting friends.

Sincerely, Sara
Sara D. Grinnell
Studio C Designs
C&L Gems LLC
6005 Wayzata Blvd. Suite 100
St. Louis Park, MN 55416
Ph: 952 797 7777
Fax: 952 797 9065