Mould and core washes

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Dec 7, 2020
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I'm wondering what people use as 'washes' to fill in the gaps in the sand of cores and moulds, in cases where a fine finish is needed, or to prevent sand 'burning on' to the surface of the casting. Any suggestions?
You can fill in the gaps using talc or graphite just rubbing it in or mix it with metho and brush it on.
I have a video showing how I use graphite. Have a look at 2:45.
100model (ironman) gets some really good surface finishes on his iron castings, and I think he has worked on mastering his green sand recipe for a long time.

I tried Petrobond (tm) first, which is an oil-based greensand.
That worked well with aluminum, but tended to dry out between uses, and required the addition of alcohol in a muller.

I tried Petrobond with gray iron, and that worked better than I expected, but the high temperature causes some mold failures and some significant sand inclusions.

I ran across some local art-iron folks, and they were using a fine commercial-grade foundry sand (OK85), with a resin binder.
I tried the resin-bound OK85, and got very good results.
The resin requires a commercial respirator, which is rather a pain to wear on a 110F summer day in the shop, and I was thinking I would try some greensand, since ironman had such good results with it.

I bought some brand"X" greensand, and poured a flywheel in iron (photos below).
That flywheel pour was the first and last time I tried greensand.
I see folks online in videos who get only slightly better surface finish than my greensand attempt, with a lot of burn-in, and lots of post-casting wire brushing/grinding/grunt work.

I have also tried OK85 bound with sodium silicate, and that worked pretty well with aluminum, although sodium silicate bound sand is really sticky stuff, and tries to stick to your pattern.
I have heard that sodium silicate bound OK85 works almost as well as resin bound OK85, but I had not tried that combination yet.

I noticed a can of mold coat at the art-iron foundry, and so I purchased some alcohol-based ceramic mold coat, to use with resin bound sand (I think it would work with sodium silicate bound sand).
I spray it on with a slurry sprayer, which is very similar to a sandblaster, and then burn it off.

I lightly flame the mold with a propane burner, before spraying on the mold coat, so I don't trap any residual moisture under the mold coat.

I generally use one layer of mold coat, but if you decide to spray on more than one coat, burn it off between each coat.

There is a potential that the ceramic mold coat could work with greensand, but I am not positive of that.

Below is the greensand casting I made, with the incredibly bad surface finish (I could win an award for "worst finish ever in the history of mankind").
I did see one individual try the lost foam method with iron, and his casting looked like something that came out of a cow who had bad gas, so he actually wins the award, but I got runner-up and honorable mention.

One has to try stuff to figure out what does and does not work.


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Here are some examples of ceramic coated resin-bound mold castings, using a product called "Velacoat" (tm).

You have to shake the container really well, since the ceramic sinks quickly to the bottom of the container.
I shake the sprayer too while I am using it.

These are gray iron castings straight out of the mold.
The only cleanup necessarily is using a paintbrush to finish wiping off a slight amount of sand.
These castings do not require any post-casting cleanup or wirebrushing.

This is the molding process for the bearing cap.
(Sidenote: Probably not safe to burn off the molds in the shop.)

Runner and gates were cut after the sand set.

This was a freebie casting for someone else, and so I got in a hurry, and just used painter's tape to fill in some low spots.
Normally I would fill and sand off the pattern smooth.

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Another example of a ceramic mold coat used on a resin-bound sand mold.
I get consistent results with this material.
It totally eliminates any post-casting cleanup work, other than a light brushing with a paintbrush.

You can see the mold making and coating process in this video.
This is exactly how I make molds, minus the automated sand mixer/dispenser.

See 4:38 for the ceramic mold coat application.
Looks like they just use a standard paint sprayer.

One secret (perhaps not so secret) is to leave the castings in the mold overnight, and let them cool as slowly as possible.
I think this helps a bit with surface finish, but mainly is to eliminate chills and hard spots caused by rapid cooling of the iron, which does not give the graphite time to disperse evenly throughout the casting.

The only cleanup necessarily is using a paintbrush to finish wiping off a slight amount of sand.
These castings do not require any post-casting cleanup or wirebrushing.
In post 4 you posted photos of straight edges with a large amount of flash around the parting line. That should not happen with bound sand because when I use it there is no flash around the parting line.
In post 4 you posted photos of straight edges with a large amount of flash around the parting line. That should not happen with bound sand because when I use it there is no flash around the parting line.

I think what happened there is that when resin bound sand sets, it feels and looks hard and rigid, but it is actually still curing.
You can stand a set mold on edge, or place it on a surface that is not completely flat, and during this not-fully-set time, it will actually move and warp just a tiny bit.

The warp is enough to cause the flash though.

I did not realize this was happening for a while, and then figured out that the mold has to be placed on a completely flat surface for at least 20 minutes after set time.

Another thing that happened on that mold was I was using the Adolf's aligment buttons (you can see the recesses in the sand), and so when the mold warped sighlty, then instead of getting perfect alignment from the buttons, they actually kept the mold from closing completely.

As I mentioned, you have to try many things to find what works.

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I use snap-flasks, which means that the flask is removed from the mold halves before the metal is poured.

That is why you never see burn marks on my flasks.

The two bound molds are adhered together with foundry cement, and weighted on top too, just in case the cement fails.

Normal flask have pins on either side which accomplish the correct alignment of the cope and drag when the mold halves are combined prior to a pour. I actually use a 1/4" rod on either side of my flasks, which go through holes drilled through the wood, which eliminates all flask harware.

When the mold halves are removed from their respective flasks (when using snap flasks), then you have to reassemble the cope and drag accurately, and precisely.

The Adolf insert provides the male/female impressions in the sand which will align the mold halves with they are assembled outside the flask.

I tried using the inserts by including them on matchboards of sorts.
I had trouble getting the inserts on the boards to match exactly.

I ended up using one insert only when making the drag, and then rammed the cope into those impressions.
This worked ok.

Finally I just started drilling a 1/4" hole through the cope and drag mold on either side, and putting rods in until the cement that holds the cope to the drag set. I don't use the Adolf inserts anymore.

Below is how I pour a mold, but with weights on top.
You can see that the pour below was at a good pour temperature for iron, since the iron came up through the vents.
And yes, you do need vents in the cope when using bound sand, since it is not very porous once it sets.

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You can stand a set mold on edge, or place it on a surface that is not completely flat, and during this not-fully-set time, it will actually move and warp just a tiny bit.
The exact same thing happened to me using SS sand. In the winter time the CO2 gas was very cold and would not cure the SS sand completely so I would put the mold on a uneven surface and it would leak when poured. To fix this I used to make the molds a few days before and sit them on a even surface when the sun was out to cure them completely. It took me a long time to realize that SS cores and small molds could be put in a microwave oven and cure them without using CO2. I also found out there are liquid hardeners for SS but they have a short shelf life.
I have some liquid catalyst for use with curing sodium silicate, but have not used it yet.
The catalyst comes in various set speeds, and you can actually mix the catalyst for a custom set speed.

I bought one catalyst that has a 40 minute set time, because it was the only one I could find, and then found one with a faster set time.
I have not tried either.

The microwaving of sodium silicate cores sounds promising.

Sodium silicate molds and cores are a lot more environmentally friendly, and don't require a chemical respirator.
Still need a good dust respirator for handling sand in my opinion.

This is one of the first castings I ever made, and I was just itching to try and pour something for the first time.

This was an attempt to make some flywheel spokes.

A rather crude pattern, and an open faced pour, but I was able to melt and pour aluminum, and it was a thrill.

The sand is Petrobond, and I quit using it since it dries out between pours and has to be re-mulled with alcohol added.

I decided to spray a mold coat onto Petrobond, which was a terrible mistake, since the alcohol flashed and bubbled badly on the bottom of the casting.

The only way to learn is to try and fail; then figure out what you did wrong and try again.

I did not know at first that hydraulic pressure can force the flask halves apart, if you don't weight the top.

For a long time, I had a problem with every casting I attempted.
I eventually did everything wrong that could conceivably be done wrong, and then figured out how to avoid those problems.

This was the first attempt to cast the base for the green twin.
Lots and lots of blunders on this one.
1. Used sand from the hardware store that was very coarse, to make a core.
2. Did not use weights on top of the flask, and so the flasks separated, which gave the runnout.
3. Horrible surface finish on the interior due to the coarse sand.

When I started casting metal, I had my doubts about ever getting all the problems worked out.
It took a lot of failed castings to get the techiques and materials all worked out.

In the early days, I was using pin hardware mounted on the side of the flask halves, and I used the same diameter pin on both sides.
I got in a hurry trying to do a demonstration pour, and was explaining as I was working and got distracted, and rotated the copy 180 degrees on the flask.

The result was mismatched mold halves.

Every time I poured metal, I discovered a new way to ruin a casting.
This went on for a long time.