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Retouch


#1

Sunil:

I am not a commercial casting expert, only know what the caster
I work with uses and what I see in the catalogs. As far as the
rubber, I can’t advise you. I believe you had a post from
someone who advocated a silicone based rubber as providing
smoother castings. I hope they will provide the on
this. RTV silicone (room temperature vulcanizing) is rather
expensive, about $22/lb, whereas regular mold rubber is about
$6/lb. I don’t know whether this is what the message referred to
or not. There may be a vulcanizing rubber with some silicone in
it.

As far as sprue cutting, there are various sprue cutters for by
hand use, some with long handles and much leverage. There are
also power sprue cutters with similar jaws run by a compressor.
They will cut quite fast, as no muscular effort is required.
Commercial casters do use them. All these will leave something
(a “button”) to clean up. Cutting off sprues with a jeweler’s
saw will leave the least button, probably, and a cutoff disk on a
flex shaft will do about as well, but the operator has to watch
himself or he can cut into the ring shank or whatever. I use a
stiff abrasive cutoff disk at the bench, but these are brittle
and do break. There are metal, diamond impregnated cutoff disks
available, but they cost $20 to $45US. I would imagine they
last a long time.

If you use sprue cutters, the button can be ground off on a
small, fine grinding wheel on the buffer. If you cut well, the
button should be small, maybe 1—2 mm high or less. Once the
button is ground off, the finish should be fine enough to shine
up if you are using a vibrotumbler to sand and polish. I’m sure
you could get your mold so good that you needed hardly any
finishing, but the caster I use figures he is going to
vibrotumble thru three steps anyway (18 hrs total) so the cleanup
is somewhat limited. Small defects will come out in the first
cut with 240 grit cones (ten hours). If your castings are well
prepped and the molds are very good, the first step could be
shortened or maybe even eliminated, and gold loss would be less,
as well as loss of fine detail. I think what you need to do is
do some experimenting to see how well all of the steps can be
performed in your factory. You may find that the labor is
cheaper than the faster but less precise equipment. Maybe you
can get some workers who can use the cutoff disk or jeweler’s
saw well and save you the gold that you would have to reclaim
from the buffer filters if you ground off the sprues. I don’t
know any way to find all this out other than to set up your
production and see which way works best. Remember that if your
buffers have good dust collectors you should reclaim much of the
gold ground off. I hope this is helpful. Looking at a catalog
of the tools available (Like the Rio catalog) might help you
figure out what you want to do.