Common Defects in Lost Jewelry Wax Casting / Rough skin (rough casting surface)

★This column is a commentary on lost wax casting with a block molding method
This column has been presented by Yoshida Cast

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Casting defects come in various forms, such as cracks in castings, surface roughness and porosity and so on.
Casting defects are classified into several types. And each of these has different causes.
In addition, since the casting process is diverse, the cause may not always be one, and most cases are complicated.

Typical failures are roughly classified into six categories.
Second, let's take a look at the causes and countermeasures for "rough skin".

 

 

What is 'Rough Skin'?

Rough casting surface is a phenomenon in which the entire surface of a cast product or some surfaces is not smooth compared to the surface soo degree that appears in buried materials.

Mainly due to the temperature of molten metal. Some of the castings are exposed to high temperatures, causing "rough" in parts of the product, or depressions caused by gas generation due to the mixing ratio between buried materials and water and breathability inhibition of molds.

 

Why rough skin takes place?  And what is countermeasure?

The causes of rough casting surfaces can be broadly classified into seven categories. Countermeasures for each cause are introduced.

CAUSE 1 / Molten metal temperature is too high

 

CAUSE 2 / Temperature of a casting mold (Flask temperature) is too high

If temperature of a casting mold is too high, temperature of the melting metal will not drop in a short time, and chemical heat reactions on inner casting mold surface may roughen the casting surface.
Appropriately lower flask temperature before casting.

CAUSE 3 / Generation of residue gas

Residual gas may roughen casting surface. This is a phenomenon seen in metals that solidify quickly in pressure casting.
By the way, generally speaking, in vacuum suction casting, it appears as "double skin" in casting of slow-solidifying metal, and in centrifugal casting, it appears as "gas porosities".
If a casting mold is not completely burnt out and some residue remains in a casting mold, the gas generated from residue during casting stage will cause micro-dents on all or part of the product.
It is necessary to change burn-out temperature gradient so that no residue remains.
Those gas sometimes may have already been contained in casting metal itself even virgin metal is cast.
Alternatively, this problem may be related to poor mold permeability (too low mixing ratio between investment powder and water).

CAUSE 4 / Moisture content of a casting mold

①　High water content

If investment powder mixed with high water ratio, density between powder molecule will decrease and gaps between particles will increase, resulting in rough cast surface.
Make sure that the investment powder is not mixed with too much water.

②　Little residual moisture in a casting mold

If casting molds are dried excessively, the casting surface becomes rough.
In case of gypsum system investment powder, if a longtime elapses from completion of investment works to start of burn-out process or if casting molds are left in an extremely dry environment, necessary moisture will be lost in the flask and casting surface will become rough.
when casting with casting molds that has been elapsed more than a day from investing, it is possible to soak it into water for about 10 minutes to rehydrate.
However, as a prior notice, casting surface in this case is inferior to that of casting molds made by proper methods.
On the other hand, in the case of silica system investment powder, it is impossible to add water afterwards.
Deliberately adding water will cause casting molds to crumble.
Also, overdried casting molds or/and short-time dehydrated ones with silica system investment may crack during burn-
out process.

 

CAUSE 5 / Flow Lines (Casting flow lines)

This is a phenomenon in which wavy wrinkles seen on the coast of ocean occur on surface of castings.
It is mainly due to insufficient casting temperature, and it can be said that it is on the verge of underfilling.
Increase casting temperature of casting molds or/and casting temperature during casting stage.
In addition, observe the flow line that appears on a casting. If you find texture of investment powder on the flow lines, the flow lines may not have been created by casting process but by wax injecting process. Wax flow lines on wax patterns are sometimes indistinguishable from casting flow lines.

 

CAUSE 6 / Lack of Permeability of casting molds

Gypsum system investment powder tend to lose permeability at low water mixing ratios.
In the case of investment materials that are solidified using a binder such as silica system investment powder, lower air permeability of casting molds causes this problem.
Check if dilution rate of the binder and water is too low or too concentrated.
Simultaneously, check if mixing ratio of aqueous binder solution and investment powder is too low.
Residual gas may cause micro-dents on castings.
In order to secure air permeability of casting flasks, adjust mixing ratio of the investment powder and binder.
In addition, the binder used with silica system investment powder changes to silica at high temperature.
If casting molds burnt at maximum temperature (900-950°C) for a long time, sintering of the binder will be excessive and sintering bonding of silica will proceed too much, impeding air permeability.

 

CAUSE 7 / Partly broken or/and local peel-off of inner surface of casting molds

For some reason, when inner wall of a casting mold is partially missing, it causes local surface roughness.

 

 

Summary of Rough skin

Countermeasures for underfilling are summarized blow.

There is not always one cause. Please review all the items again and see how they work.
When casting condition is desired to be changed, change it one by one. 
If you change two conditions at one time, you will not be able to clearly find out an exact and direct cause for the casting error you have. 

Author : M. Yoshida