Mold Coating Application- How to

a skilled demonstration of the application of a refractory mold coating.

Category:

Science & Technology

Tags:

• afs
• asm
• brass
• bronze
• casting
• foundry
• metallurgy
• radiography
• realtime
• sfsa
• stainless
• steel

[bobpuhakka]

DanteRizo:

Rule#1 - Usually what appears to be "shrinkage" probably isn't...

It is impossible for me to speculate on the nature of your issues.  What I can say with certainty is that the vast majority of casting defects are caused by air/ surface films entrained during filling - and by the evolution of core gasses bubbling through the just-poured casting.....

[12thgenord]

part 2. I use 50% reclaimed furon sand and 50% new silica sand for molds and 100% fused silica sand for cores. I melt carbon steel using 40% prime (virgin) and 60% re-melt on induction furnaces. I test for core sand hardness, loi, permeability and everything seems normal. I am not allowed to lag patterns to add stock to the affected areas. any ideas? Thanks!

[bobpuhakka]

@12thgenord please describe, in detail, your fill system design from the pouring cup to the mold cavity. Also, if you have photos of the defects you can email them to me at bobpuhakka@gmail.com

[12thgenord]

Did you spray the first coat of alcohol wash followed by a second brushed coat?

by coating the surface so thick with wash do you ever get porosity due to lost permeability at the surface of the cavities? You are protecting against metal erosion caused by tubulance but that entire surface is no longer permeable. Are your vents and risers enough to desipate all the gases and prevent gas porosity?

[bobpuhakka]

@12thgenord

1. the mold/metal interface does not require permeability

2. venting is to allow air out of the mold cavity so that metal can take its place. That's it. Core venting to atmosphere insures that generated gases do not pass through the surrounding molten metal. That's it.

3. The coating application is NOT to protect against turbulent flow. If your fill process has turbulent flow it's because your fill system is terrible.

[DanteRizo]

Bob, part 2. it just hit me. (Iam not a foundryman, it's obviously apperant) some venta areused to vent the mold, the others used to vent the core.

Why would you say cause the holes which look like gas or shrink holes under the vents which come off the casting? slow pouring speed? gating design, pouring temperature etc? Thank you Bob.

[DanteRizo]

Bob, on mold venting on vertically poured steel casting impellers with thin vane apertures, I've seen the top part of the vertically poured impeller vented at the core print only straight up to atmosphere and others vented directly from the upper part of the casting straight up to atmosphere. Which venting system would be better? The ones vented from the casting cavity fill with metal part of the way, half way, and all the way to the top. gas/shrink holes on od appear on some vents . part 1

[12thgenord]

@bobpuhakka. You are more knowledgeable than the molders I know.

What may cause porosity which is not at the surface but subsurface and found only after rough machining. I've tried flow coating the mold cavities, adding extra vents to the cores and molds, curing the cores longer (fused silica sand) allowing the molds to ( furon sand, binder & acid) sit over night prior to pouring.

pouring at temps 100 F over liquidus, refining the meltal with argon gas, making sure that the surfaces are dry.

[12thgenord]

How long do you leave the fan on after you evaporated just the alcohol off the wash by lighting it.? How do you test it , to see that it is dry enough not to create gas porosity? Do you use an infrared thermometer to check the ambient temperature of the mold compared to the sprayed cavity?