Using a microwave oven to harden a sodium silicate core.

[100model]

I made a video to show an alternate way of hardening a sodium silicate core using a microwave oven instead of passing carbon dioxide through the core. If you use this this method 1-2 minutes seems enough to harden a small core, if a larger core or mold is going to be hardened you may have to experiment on how long it takes to harden it.

 

[wce4]

I made a video to show an alternate way of hardening a sodium silicate core using a microwave oven instead of passing carbon dioxide through the core. If you use this this method 1-2 minutes seems enough to harden a small core, if a larger core or mold is going to be hardened you may have to experiment on how long it takes to harden it.

I just finished watching your video.

 

[lee webster]

I was having a lot of problems the last time I tried casting a part. There seemed to be an air pocket of some sort bursting the mould. I couldn't figure it out and haven't cast anything since then. I was sure it was the sodium silicate/sand core. My CO2 gas ran out soon after the failures, and I wondered if maybe part of the core was uncured. Your video said that SS has water in it. If the core wasn't fully cured, water could have caused the gas bubble.

I would like to try the microwave cure, but all my core moulds are made from either 3D printed PLA or resin. I don't think they could take the heat.
Thanks for a very informative video.

Lee

 

[GreenTwin]

Cores are suppose to have a hole through the center, with either end of the core vented up and out the top of the cope.
I don't always vent the core.

You can also bake the core, to drive out any residual moisture; perhaps 200 F (I have not tried this).

And you can also lightly flame a core with a propane burner, but wear safety glasses unless some trapped moisture decides to explode the core.

It is important to use the right amount of sodium silicate ratio to sand.
I forget the exact amount recommended by the manufacturer, but I think it is in the 3-5% range.

Some folks use too high a percentage of SS, and end up with a rock-hard core that can be very difficult to remove.
Water helps break down a SS core that has the correct mix ratio.

I have not tried the microwave thing.
I have heard of others baking SS cores, but as lee mentions, the corebox will limit what can be put into an over or microwave.

And they make a liquid catalyst which will harden sodium silicate cores without CO2.

.

 

[olyn]

I was having a lot of problems the last time I tried casting a part. There seemed to be an air pocket of some sort bursting the mould. I couldn't figure it out and haven't cast anything since then. I was sure it was the sodium silicate/sand core. My CO2 gas ran out soon after the failures, and I wondered if maybe part of the core was uncured. Your video said that SS has water in it. If the core wasn't fully cured, water could have caused the gas bubble.

I would like to try the microwave cure, but all my core moulds are made from either 3D printed PLA or resin. I don't think they could take the heat.
Thanks for a very informative video.

Lee

You may be able to use the resin printed mould. I had to replace some petg parts on my FDM printer because the etg would soften with the heat. I printed the part on my resin printer an it has been working fine for two years now.

I was having a lot of problems the last time I tried casting a part. There seemed to be an air pocket of some sort bursting the mould. I couldn't figure it out and haven't cast anything since then. I was sure it was the sodium silicate/sand core. My CO2 gas ran out soon after the failures, and I wondered if maybe part of the core was uncured. Your video said that SS has water in it. If the core wasn't fully cured, water could have caused the gas bubble.

I would like to try the microwave cure, but all my core moulds are made from either 3D printed PLA or resin. I don't think they could take the heat.
Thanks for a very informative video.

Lee

 

[lee webster]

Thanks olyn, but my resin prints are very prone to warping and distortion. I wouldn't use them for something as important as a mould. It might be the resin (water washable) or it could be me. Maybe I cure them for too long (5 mins for a print half the size of a computer mouse) or for not long enough. I am still working on trying to get stable prints.

 

[100model]

There seemed to be an air pocket of some sort bursting the mould

As green twin said way to much SS is used, for a core 3% is more than enough. What is a common problem for beginners the SS is not mixed properly and there are lumps of unmixed SS when it contacts molten metal so lots of gases are produced which causes blowholes. Were you using green sand for the mold? If you were using that it can cause blowholes if there is too much water in the sand. Plaster core molds are better than plastic which can soften if too much heat is used.

 

[NapierDeltic]

Take a look at this video:

I consider it very interesting; though I do not necessarily recommend it as there are relatively cheap/ proven comercial devices for melting aluminum.
So what-s the point?
At 23.20min. you have the solution of freezing the core to keep its shape, taking it out of PLA mould and letting it harden naturally. Eventually you can harden finally the core in MW oven but very slowly and reaching a final temperature high enough to cure sodium silicate: 250-300-350 Celsius???. This is still a question for me as it seems the drying of ss occurs at room temperature, by loosing water. In this state it binds sand but remains soluble. The other is a chemo-physical transformation taking place at mentioned temperature (temperature is given from my memory) and following is conversion in an insoluble binder.
Heating has to be extremely slow, otherwise ss forms on the surface a sealing layer, so water from inside, converted to vapour, not having an escape, generates bubbles.
What I propose is a slight extension to ss cores of STF's presented method so It is not tested, but suggested.
STF has hundreds of hours of tinkering in MW/ss/sand (whatever nature)/molding, so his methods are proven and optimised- at least from technical performance point of view. I'm not speaking here about health safety and my suggestion does not relate to potential hazardous part of video content.

 

[lee webster]

100model.
I am fairly certain that by the time I had the failures I was using 5% ss with fine dry sand, hardened with a 5 second burst of CO2. The cores were about the size of half a golf ball, if that. It was also after the failures that I ran out of CO2. This is why I believe that there was uncured ss in the middle of a skin cured core, if you get my meaning. My casting sand was green sand with an unknown amount of water, but not wet by any means. I relied on "old foundryman" amounts of ramming to get good moulds.
I tried a few cores for another part using fine sand and two part epoxy resin. I thought that the curing time of 24 hours was too much of a limiting factor. I should have carried on using the resin, but it could be difficult to remove.

 

[100model]

as it seems the drying of ss occurs at room temperature, by loosing water.

This is what I have found but it takes a long time. A microwave oven speeds up the drying process. The end temperature after two minutes the core is too hot to touch but the SS it not boiling so it is way below the boiling point of water.

 

[100model]

This is why I believe that there was uncured ss in the middle of a skin cured core, if you get my meaning.

I used SS for 10 years and know all about the dead spots. In cold weather SS will not cure properly using cold CO2. Foundries that made a lot of SS cores would heat up their CO2 to prevent under cured cores.

I tried a few cores for another part using fine sand and two part epoxy resin. I thought that the curing time of 24 hours was too much of a limiting factor. I should have carried on using the resin, but it could be difficult to remove.

I have used epoxy cores for the last 25 years and found that 24 hours was not a limiting factor as cores can be made a long time beforehand as they do not deteriorate. You can cure epoxy cores in a microwave oven but a plaster mold has to be used as the water in plaster is heated up the epoxy cures very quickly. you could use a fan heater that is used to heat up rooms in your house. I have found that 45-60 minutes of hot air is all that needed for a usable core for casting. If you use this method don't let the plastic core mold get too hot. It does not need to get too hot to cure epoxy. Removing epoxy cores can be difficult but cores made from epoxy are very strong and will not break.

 

[L98fiero]

This is what I have found but it takes a long time. A microwave oven speeds up the drying process. The end temperature after two minutes the core is too hot to touch but the SS it not boiling so it is way below the boiling point of water.

Having very limited experience with casting, all I can add is this study I found. https://scholarsmine.mst.edu/cgi/vi...sodium,decomposition of the alkaline silicate.

 

[GreenTwin]

Some backyard folks have mentioned adding sugar to their sodium silicate cores to help them breakdown after casting is complete.

I can say for sure that if you live where I do (the Mid-South), the very last thing you want in your shop is anything with sugar in it, since you will quickly become infested with ants and all sorts of other insects, and most likely mice and/or rats.

If you use the right amount of sodium silicate in your cores, they will break down easily after casting is complete, especially if you add water, and thus there is no need to add sugar.
One of my mottos: Don't fix things that are not broken.

.

 

[awake]

I have a .pdf copy of A Textbook of Elementary Foundry Practice, copyright 1910. It is fascinating to see what they talk about for cores - see the couple of pages I've attached. No mention of sodium silicate or epoxy; instead flour, rosin, linseed oil, glue, and molasses, along with some other odds and ends. Further along, it seems as though linseed oil and molasses were the preferred binders.

I don't recall where I picked up this .pdf copy of the book, but it must be out there on the internet somewhere ...

 

[GreenTwin]

I think linseed oil was widely used, with some additives depending on exactly what characteristics you want the core to have.

I have never used it, but I think the linseed oil cores had to be baked.

.

 

[awake]

Right, the book talks about baking most if not all of these binders.

 

[Jasonb]

And molasses is sugar based so maybe those suggesting adding sugar are not so wrong.

I suppose as sugar is organic it will burn from the heat of the metal and loose it's binding properties hence the suggestion to use it to make cores easier to remove.

 

[awake]

And molasses is sugar based so maybe those suggesting adding sugar are not so wrong.

I suppose as sugar is organic it will burn from the heat of the metal and loose it's binding properties hence the suggestion to use it to make cores easier to remove.

Interesting observation! The molasses cores were to be baked in order to harden them - perhaps through caramelization (sp?) of the sugar? A sodium silicate core hardened via CO2 or catalyst wouldn't be baked to prepare the core, so the sugar would not caramelize at that point, but as you say, presumably would under the heat of the metal.

One thought is that, as hard and sticky and nasty as burnt sugar is, it still will dissolve readily in water, so that may be a factor, at least in the molasses cores - not sure if it would play a role in the addition to ss.

 

[Jasonb]

I suppose it would depend on how you added the sugar. If just granular then it would not bind but if there is water in the sodium silicate that would dissolve the sugar into a sticky binder. Or added as sugar /water mix. Even if then just left to dry naturally it would bind the sand to some extent.

If it comes into contact with molten metal it will completely burn not just get caramelised and the bonding properties lost. Which is what the article suggest happens with all those organic binders.

Mention here about hardening SS by removing water which heating in an oven will do if you don't have CO2

https://www.sciencedirect.com/science/article/abs/pii/B9780750642842500156

 

[awake]

I suppose it would depend on how you added the sugar. If just granular then it would not bind but if there is water in the sodium silicate that would dissolve the sugar into a sticky binder. Or added as sugar /water mix. Even if then just left to dry naturally it would bind the sand to some extent.

If it comes into contact with molten metal it will completely burn not just get caramelised and the bonding properties lost. Which is what the article suggest happens with all those organic binders.

Mention here about hardening SS by removing water which heating in an oven will do if you don't have CO2

https://www.sciencedirect.com/science/article/abs/pii/B9780750642842500156

Again, excellent observations, and I find myself persuaded by your reasoning. Not that that is anything to be too excited about, given my level of inexperience and ignorance!