Subject: Investment casting problem

From: Ferdinand del Carmen <[email-masked]>
Date: Tue, 23 Jul 1996 14:41:28 GMT

Hi,

	I am having problems with investment casting of gold.  The problems 
are the surface defects. If you have any information which can help 
me please let me know.

Here is the process (skipping) the wax pattern : 

1. The mold with the wax pattern is placed in the oven (1200 F) to 
burn out the wax.  The mold stays in the oven over night at 1200 F.

2.  The next day, the mold is removed and placed in a vaccum 
chamber, where vaccum is applied.  At the same time gold nuggets are 
introduce into the crusible, located on top of this machine, where 
they are melted and introduced (by gravity) into the mold.  The 
process is a close system.

3.  The mold is removed, and water is sprayed on the mold to 
solidify the gold.  After that the mold is broken and the gold rings 
are removed.

Problems encountered :  Lots of surface defects.  some are round, 
which suggested present of gas.  And some are of various shapes, 
which may be caused by the investment material.

Questions :  1. Where did the gas come from ?  The mold is under 
vaccum.  Could it be the inclusion in the nuggets? To buy nuggets 
without inclusion is too costly.  2.  Could the mold be braking 
apart due to the molten metal. If yes, any suggests of different 
investment material I should try.  The stuff I use is R2R (trade 
name).

If you have encountered these problems, or have ideas what I should 
do, please let me know.  I appreciate any help you have to offer.

Thank You,

Ferdinand

---

From: [email-masked] (Philip Duclos)
Date: 24 Jul 1996 15:22:52 GMT

First of all, you don't describe your investment method. Do you degas
the investment mixture prior to investing? Do you use vacuum or vibration
to do this? Either works but the techniques are different. Do you "paint"
on a thin coat of investment onto critically small parts and highly
detailed areas? Perhaps you should. Are you using the correct investment
for the detail required and the material being cast? Do you have the mixture
ratios correct? Is the investment old or in some way less than optimal?

Gravity casting works but a little help from centrifugal force or a vacuum
works better.

What is the flask temperature when you cast?
Do you do a stepped burnout?
Do you properly "dry" the flask before burnout?
Is the flask sized properly for the items being cast?

It is critical to allow the flask to cool a certain amount after casting.
The metal should solidify slowly to avoid problems. Typically the flask
is allowed to cool until it is below ~900F (no longer "red") before being
plunged into a water bath, The plunging primarily breaks up the investment
and allows easy removal of the cast pieces. The metal MUST have solidified
properly BEFORE this step.

Round balls of metal attached to cast pieces are usually caused by air
bubbles in the investment mixture. Proper degassing and investing will 
prevent these. There are also "wetting" agents which may be used to better 
adhere the investment to the wax model.

Sharp knife like attachments are usually caused by cracks in the investment.
Proper mixture, degas, investment and burnout are the keys here.

Phil Duclos