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Casting porosity


#1

Hi all, this is my first post so I hope I get it right.

I seem to have inherited a porosity problem lately and I cant
figure it out. Maybe someone could help me. In the last couple of
years, I have been getting “shrinkage” porosity in some of my
castings. The only thing I can attribute it to is the silicon
copper alloys that I have been using. It seems that when I cast
with “old” customers gold, I dont have any problems- but when I
use new or 50% new gold, I get porosity.

outlined below are the procedures that I follow :

  1. waxes are sprued at heavest areas. sprues are short and at
    least 3mm thick.

  2. Kerr Satin Cast 20 investment mixed and invested at 40/100
    ratio. I mix and vacuum the investment in a rubber bowl- pour
    into flasks, and vacuum again. Total work time is 8 minutes

  3. I dont dewax my flasks before burnout. I use a 12 hour
    burnout cycle starting at 300deg F ramped up to 1350deg F and
    held for 3 hours. Then I ramp down to 1000deg F and hold for at
    least 1 hour prior to casting. My oven is well ventillated and
    the temperatures are correct.

  4. I use at least a 50% mixture of new gold each time I cast. I
    alloy my own metal with either Stuller or United PMR non
    oxidizing alloys.

  5. I am using a Rey centrifugal caster (wound three turns) and
    an oxy-acetylene torch. I dont believe I am overheating the gold.

  6. after casting, I let the flasks cool down for at least 15
    minutes before removing the investment.

I have been using this procedure for about 15 years and until I
started using the non-oxidizing alloys, I didnt have too many
problems. Should I try increasing my flask temp another 100
degrees? This problem usually shows up on medium to heavy weight
high shine pieces. Should I start using vacuum to cast with
instead of the centrifuge? Would it help to coat the waxes with
a wetting agent prior to investing?

Also, when I carve a pattern from carving wax(Kerr or Matt)
sometimes I get bad spots in my castings. It almost appears to be
holes in the wax that the investment seeped into- although I
never see any holes in the wax before investing.

Someone told me that the non-oxidizing alloys love heat and that
I should raise my flask and melting temp more.

Any suggestions or ideas on this would be very helpful. Ken


#2

Hi,

I’m a dental tech with a ‘zillion’ castings under my belt. How
old is the investment? It ages. Do you use a reservoir on the
sprue? It will remain molten and ‘suck’ the porosity into the
reservoir on heavy castings. Why do you burn out so long? For
thirty years I have invested the flask, bench set for 1 hr. Put
it in a cold oven and run it up to 400 degrees for 1 hr., then
up to 900-1000 degrees (F) for 1 hr. and then cast it. I use
this technique in jewelry and dentistry. I’ve used Kerr
Luster-Cast, but other guys do the same thing with other
investments. Try it on a non-essential piece and E-mail me
back.

Good Luck,
Skip
Skip Meister
NRA Endowment and
Instructor
@Skip_Meister
04/28/9700:03:24


#3

Ken: I’m not a pro but have been casting about a year now or
more with sterling. I’ve been having major problems (well not
that bad really but just annoying as hell) with pits and porosity
and recenlty I’ve discovered vents. My pieces are fairly chunky.
I had to cast a size 13 ring for a commission and it scared me to
death. I have been corresponding with a friend online who’s
helped me with this and he told me to add vents to the pieces. In
studying some old books in particular one by Sharr & Choate I
found a fair amount of on venting. Anyway, I had
tried minor vents (just pieces of round wax wire added to the top
of the wax in various areas) in one cast and it didn’t really
help much. The size 13 ring however I ran a vent to the OUTSIDE
of the flask, venting into the air. This made a major difference
in the cast I got. I got some very minor subsurface pits in two
places and no porosity at all. If I do this next time I will
probably put two vents like this and probably get even better
results. It sounds like your problem is related to the silicon
alloy but maybe this might help you, and if there’s any beginning
casters out there this took me a long time to find.
It really puzzles me that this isn’t in any of the
new books I have on casting…Dave

Art Jewelry for Conscious People
http://www.opendoor.com/stephensdesign/crystalguy.html


#4

Did you try my recommendations? They come right out of Stuller’s
book… They state, the larger the ring, the higher the temp. Try
calling Stuller and ask them for help with their product.

This is pretty obvious, so forgive me if this sounds too
simplistic. Are your crucible and stirring rod clean? Is the
metal cleaned of all foreign matter? I steel brush all buttons,
and grind when necessary. Old silicon creates problems.

The acetylene concerns me, I cast with oxy-hydrogen when torch
melting, and get consistently better results.

I had the same problem and it vanished when I raised flask and
metal temps… I also switched to vacuum casting all large
castings, there is much less turbulence which helps on large
castings.

Have you ever tried waxing a ball next to (but not touching) the
ring ? it helps with shrinkage problems. One more thing, the
sprue can be too heavy… it should be about the same thickness
as the ring, just ever so slightly larger. Otherwise, it creates
a hot spot at the juncture that creates shrinkage porosity.
Aurthur Avalon (King Aurthurs Court Casters) use to use
rectangular sprues back in the 70s, they really work great, you
might try getting away from the round sprues.

The bad spots in the carving wax patterns are a mystery to me.
Maybe you could hold the flask at about 400 until the carving wax
is evacuated, and be sure not to put it in the oven before it has
had a chance to fully harden (at least one hour). You could be
experiencing steam erosion problems with the investment. If you
are building up the carving wax with a hot pen, you could be
putting air bubbles under the surface that suck investment, and
you might not see them because they don’t break the surface,
gotta be careful with those. You might try NOT vacuuming (the
second vacuum step) one flask that doesn’t require it (vibrate
it) and see if the problem persists, you could narrow it down
this way. One more thing, if you oven has lost its calibration,
you could be overheating the investment at the highest
temperature, you might try restricting the upper temperature to
1200 one time and see if that helps.

I get great castings using Stuller’s gold, even castings
weighing up to an ounce. My tree castings are usually porosity
free too (referring to gent’s rings).

One important tip, DO NOT MIX ALLOYS! (sorry about the yelling)
I have experienced many problems resulting from mixing alloys
from different manufacturers.

One side note, I usually don’t use Stuller’s alloys, I use the
mix from AAA Precious Metals (Portland OR). I think it’s the best
in the business, and their 18k yellow and super white alloys are
the best as well. (my personal opinion)

       Jeffrey Everett, jewelry craftsman

Handmade 18K, 22K, and platinum gemstone jewelry.
Diamond setting, rubber/metal molds, casting, lapidary
Die and mold engraving, plastic patterns for casting.
Jewelry design, cad/cam, milling, scroll, filigree, & more.
P O Box 2057 Fairfield IA 52556 515-469-6250


#5

Thanks for the advice Jefferey.

I have tried raising my flask temp an additional 100 deg. but
still had some problems. I searched through Stullers catalogs and
tips packages and it seems that they recommend a lower flask
temp when casting heavier pieces. Maybe I should raise the temp
another 100 deg?

My crucibles and stirring rods are clean. I coat them with borax
before I use them for the first time, and before every cast, I
remove any particles of old metal and then heat them up and sling
any excess boric acid into an empty flask.

When you refer to a “ball” next to the ring, are you refering to
a resevoir on the sprue? I just ordered in some plastic sprues
that have a resevoir on them from a dental supply outfit. I am
going to try them on Thursday, when I cast again.

I haven’t tried casting with hydrogen, although I use an aqua
torch for alot of soldering operations and must admit that I love
it. It burns much cleaner that acetylene or propane. Do I need
special regulators for hydrogen or can I use the same ones I
currently use?

I will contact AAA Precious Metals and order some alloys from
them. Hopefully these tips will help. I appreciate all the good
advice.Maybe I should stick to fabrication…Thats one thing I do
pretty well. :slight_smile:

                                                                 Ken

#6
Did you try my recommendations? They come right out of Stuller's
book... They state, the larger the ring, the higher the temp. Try
calling Stuller and ask them for help with their product. >>

Jeff

The person to contact at Stuller about casting questions is the
casting manager Dennis Busby. He is a friend of mine tell him
that Thomas Ray sent you. If it is to be known he will have the
answer. RED


#7

Ken,

Wrote this a few minutes ago and my system hung and I lost it,
so I hope I cover what I did before…

I do not think the oxyacetylene is a problem as Cynthia has been
using it for YEARS. If it takes more than a couple of minutes to
get the metal melted, get a bigger tip (rosebud). Also,
preheating the crucible and metal in the burnout kiln helps speed
up the melt, saves gas and heat saturated the crucible.
Cleanliness as your are doing it sounds good.

As to the reservoirs, if you are using a rather large
feeder/sprue and coming off of it with a smaller sprue to the
piece, the large feeder can act as the reservoir, but if not,
just add a wax ball about the volume of the piece just down the
sprue from the piece.

As you stated you are spin casting. Are you aware that there
are specific ways to orientate the wax to facilitate the metal
filling it? It is too complicated to discuss here, but if you
would like more info on it, Cynthia will be here Thursday and she
would be happy to explain it to you. She did a lot of work on
this in school.

Lastly, I would suggest that you give R&R (800-800-7496) and
your alloy supplier calls and see what they have to say. They
might want a picture of the spruing and a section of the porous
casting to REALLY be able to help you. Is the porosity from
metal loss (sucking) or loose investment getting incorporated
into the metal? Have you cut some porous castings to see if it
all the way through the metal or just on the surface?

Anyway, a few thoughts and a few ideas and an offer for further
info on directional casting. Have you ever tried vacuum casting?
It is wonderful!

John and Cynthia

John Dach and Cynthia Thomas
Maiden Metals
a div. of True wealth is what you are,
MidLife Crisis Enterprises not what you have.
PO BX 44
Philo, CA 95466
707-895-2635(phone)
707-895-9332 (FAX) new
@John_Cynthia_MidLife


#8

< I seem to have inherited a porosity problem lately and I cant
< figure it out. Maybe someone could help me. >>

Dear Ken

It sounds as if you are quite up to date about the mechanics
involved in casting. I have a few suggestions that might make a
difference. Although you can never tell. Anyway from your letter
my biggest concern is the use of acetlyenr as a fuel. This gas is
filthy and introduces a lot of nasty elements into the molten
metal. I would suggest propayn or if you need a hotter flame
hydrogen.

< I have been using this procedure for about 15 years and until I
< started using the non-oxidizing alloys, I didnt have too many
< problems.

If your problems started when you started using the non
oxidizing alloys then do not use them any more. If your castings
worked better another way go back to what worked for you. Thats
why your customers gold still seems to work better.

< It almost appears to be holes in the wax that the investment

seeped into

Sometimes wax will have hidden air bubbles and they can be hard
to find. Hold the wax up to a strong light and you will see these
flaws and be able to fix them.

< Then I ramp down to 1000deg F and hold for at
< least 1 hour prior to casting.

You might try casting at a little lower temp.I go at around 900
and it has always worked well for me. There are a lot of
technical reasons for this but as of late I have been accused of
lecturing and God forbid I try to explain something:-).

< waxes are sprued at heavest areas. sprues are short and at
< least 3mm thick. >

Not all waxes will work well with the same sprue. As a general
ruleit sounds like you are doing it right but you might want to
increase the sprue length and make it about 60 to 70% of the
thickest part of the ring. This can help prevent the button from
drawing metal back away from the casting as it cools. Also if
your doing vacuum casting you can keep the button hot and allow
the casting a resivoir of metal to draw from as it cools. I am
sure others here will have excellent suggestions Good luck RED


#9

One of you said you were using Hydrogen\Oxyegen for casting.
Marc Robinson from P.M. refining recommended this in an article
in AJM awhile back. Is this like surface of the sun heat? I am
thinking of switching to this but I don’t know anyone who uses
it. Please tell me if any of you use it and what you think.

Mark Parkinson

P.S. He also said you shouldn’t use one of those glug-glug type vacume
pumps, that your fingers should break if you put your hand over the hole of
your vacume caster when on.


#10

Something just occurred to me. You previously mentioned you use 3
turns on your centrifugal machine. The only time I use 3 turns is
when I’m casting a large heavy flask (3x5 inches at least).

Thanks Jefferey, That may be my problem. Come to think of it, I
usually never had problems when I would cast with vacuum(years
ago). The only reason I switched is for a better density. I may
try going back to vacuum and see if my problems dissapear.

Thanks to all for the great suggestions. I should get this
problem straightened out. :slight_smile:

                                                                  Ken

#11

I do not think the oxyacetylene is a problem as Cynthia has been
using it for YEARS. If it takes more than a couple of minutes
to get the metal melted, get a bigger tip (rosebud). >>

I am using the largest tip available (rosebud) on my melting torch.

As you stated you are spin casting. Are you aware that there are
specific ways to orientate the wax to facilitate the metal
filling it?

I always try to position the pattern so the metal will flow as
easily as possible. ie. avoiding backflow of metal.

Lastly, I would suggest that you give R&R (800-800-7496) and your
alloy supplier calls and see what they have to say.

I did give United PMR a call and Mark suggested that I raise my
flask temp another 100 deg. He said Silicon Copper - Gold alloys
require at least a 100 deg. higher flask temp an 100 deg above
liquidus on the metal. Also he said the long burnout is good to
insure that no Sulfur gasses remain in the crucible during
burnout. I have learned a great deal since joining this group.
Thanks to all for the help…I have alot of new things to try.
:slight_smile: Ken


#12

Something just occurred to me. You previously mentioned you use 3
turns on your centrifugal machine. The only time I use 3 turns is
when I’m casting a large heavy flask (3x5 inches at least).

I have a Neycraft spincaster I bought used with no instructions
and people told me three turns to use it. I have problems with
minor pitting and venting has helped alot but I wonder here too
if maybe two turns or less would have any effect? I use mostly 2
1/2 inch x 2 1/2 inch flasks. Dave

Art Jewelry for Conscious People
http://www.opendoor.com/stephensdesign/crystalguy.html


#13
Thanks Jefferey, That may be my problem. Come to think of it, I
usually never had problems when I would cast with vacuum(years
ago). The only reason I switched is for a better density. I may
try going back to vacuum and see if my problems dissapear. 

You actually get better density with vacuum. I didn’t believe
this until I saw some photos (500x) which showed the dreater
density of the vacuum cst piece. I have noticed some difference
in surface texture between the two processes, but only when
using bright cast golds. The silicon alloy has an almost fabric
like texture when vacuum cast, but usually not when
centrifugally cast.

Jeffrey Everett


#14
One of you said you were using Hydrogen\Oxyegen for casting.
Marc Robinson from P.M. refining recommended this in an article
in AJM awhile back. Is this like surface of the sun heat? I am
thinking of switching to this but I don't know anyone who uses
it. Please tell me if any of you use it and what you think.

I have been hydrogen/oxygen for at least 10 years and highly
recommend it. It can get pretty hot depending how much oxygen
you add, but typically you won’t add that much unless casting
platinum, in which case you’ll want to create a severe oxidizing
flame (won’t hurt the plat).

P.S. He also said you shouldn’t use one of those glug-glug type
vacume pumps, that your fingers should break if you put your hand
over the hole of your vacume caster when on.

Yeah, that’s great if you have $1600 or so to spend on a 10-20
cfm pump. He’s into getting the vacuum up very quickly. I seem
to do just fine with my little 3 cfm pump. One time I chained
together 2 - 3cfm pumps in series, that worked great. Marc does
BIG flasks, for production, it’s his specialty. If you’re like
me and usually do small flasks, I think you can get by with your
glug-glug. All the 10 cfm pump does is evacuate faster, and
maybe produce a slightly higher ultimate vacuum. It’s not like
air pressure where you can keep building it up, it is a lack
of air pressure. In vacuum casting what you really have is a
system for evacuating the existing atmosphere and produced
gasses (from the molten gold reacting with the investment) from
the cavity in the flask, creating the possibility for unimpeded
(by gas pockets) gravity casting, assisted by 14 lbs. sq/in (sea
level) atmospheric pressure. A bigger vacuum pump won’t produce
more of, or a stronger vacuum, it will just evacuate the
atmosphere more quickly.

Jeffrey Everett


#15

A bigger vacuum pump won’t produce more of, or a stronger
vacuum, it will just evacuate the atmosphere more quickly.

Jeff

You are so right. My unit has a 6 CFM pump. It is a little
slower than the big units but I installed an old 20 cubic foot
oxygen tank in line with the casting chamber. Now I have a
reserve of vacuum to draw upon and do not have to wait for the
pump to build up to a full vacuum. I just turn it on several
minits before I cast or invest and it is ready to pull an
instant vacuum. RED


#16
I have a Neycraft spincaster I bought used with no instructions
and people told me three turns to use it. I have problems with
minor pitting and venting has helped alot but I wonder here too
if maybe two turns or less would have any effect? I use mostly 2
1/2 inch x 2 1/2 inch flasks. Dave

HI Dave

The Ney Spingcaster is does take a little more juice to get it
going, but two turns should be sufficient. I had one of those
when they first came out, but I forget… signs of age I
guess… :slight_smile:

Have you tried putting blind vents with resevoirs at the extra
heavy sections? Have you identified the type of pitting
(shrinkage/hot-tear, gas, contaminant)?

What metals are you getting the minor pitting in. What temps are
you using? How are you melting? (so many variables…)

I’ll bet we can completely eliminate the pitting with a little
discussion.

Jeffrey Everett


#17

P.S. He also said you shouldn’t use one of those glug-glug type
vacuum pumps, that your fingers should break if you put your hand
over the hole of your vacuum caster when on.

Yeah, that's great if you have $1600 or so to spend on a 10-20
cfm pump. 

I got a 17 cfm vacuum pump (a glug-glug type), used for $200! I
think it is an Ingersol-Rand and they have rebuild kits available
for them. You are right about them costing around $1600 new. This
thing is fairly large and heavy with a big flywheel and uses a 3
horsepower motor ($100 Harbor Freight). Works great! I just make
sure there are no small children around when I turn it on,
wouldn’t want them plugging up the vacuum lines (grin). What I
like about a high cfm pump is speed. I plumbed in a 1" line to
the chamber, this thing really pulls. I can flip the switch and
pour. I don’t recommend vacuuming body parts.

I used to use a centrifugal caster and then built this vacuum
setup. I will never go back. Vacuum casting saves metal too, I
don’t have it slung all over the tub, the room and me (could get
expensive with gold). Since most of the casting I do is
production work with 4x6" flasks I use a propane powered melting
furnace. This system works well for me as I am not tied up with a
torch in my hands the whole time.

Building and using a vacuum setup has dispelled alot of the
myths about how centrifugal casting is better than vacuum. For
one thing how can metal become denser by being whirled around in
a circle. What ever gets rid of porosity makes metal denser and I
have had better luck with the vacuum setup.

Kenneth Gastineau
@Kenneth_Gastineau1


#18
You are so right. My unit has a 6 CFM pump. It is a little
slower than the big units but I installed an old 20 cubic foot
oxygen tank in line with the casting chamber.

I often considered doing this myself. How’d you go about the
nuts and bolts of it Bruce?

JE


#19

“Hi” Kenneth

I read with interest your comments on the vacuum casting unit
that you have built and also the propane fired melting furnace.

I am in the process of getting things together to build a vacuum
unit and would be greatfull of any advice that you can give me
eg.do you use a parallel flask or one with a flange,do you use a
plastic netting on the inside of your flask for better vacuum of
the flask. I have seen a set up when pouring the investment where
a jig held a number of drinking straws around the inside to do
the same thing.

As for the furnace I have a burner from a kiln for pottery but
have yet to see or design a muffle furnace to melt the silver. I
have an electric furnace but would rather go with the gas. What
means are you using to gauge the temp of the metal.

My thanks Kenneth for any help that you can give as well as
comments from other members that have been down the same path.

Best withes to all.

Major Boyce @pyramid