Gypsum dehydration

How about using MGPo4----10 minutes to 1 hour to full strength.

OR-- Hydrolytic cements--expel the mix water upon reaction and full set in about 4 hrs.

Could you please explain this with more details...

thanks

Hi if you combine a electrical conductive addative to the slurry could you zap it with hi voltage perhaps in a very short burst to create a crystalin structure? I know of a water filter manufacture that utilized a pressureized mold system. You can mix in large batches and cycle time was approximetly 30 seconds. John Minnesota.

Why does the water cause a problem - does it expand and crack or what. When Gypsum cures it combines with the water (+/-10 water to 1 gypsum - CaSO₄ ½H₂O → CaSO₄ 10H₂O).

Use of gypsum for this purpose is new to me most moulds for metal casting I have been involved sands bound with resins or other binders.

How about the sand/sodium silicate mix cured with CO₂ - a common casting binder?

You must dehidrate the molds, because you have free water and cristalization water

When you pour alum,inum at 750 ºC the mold explode because this water turn to

steam quickly.

I'm not a Gypsum exppert but I had a similar problem with moleecular seive and water removal and I put the item in a microwave oven. The microwaves heated the waater in the powder, in thihs case, raised the temperature and helped evaporate and drive off the water.

As guest says there are 2 types af water here - water of crystalisation (combined) and free (absorbed) If moten metal is poured in, combined water could be released so drying acheives nothing.

Plaster is dehydrated by heating.

I know nothing about gypsum, except that wallboard for the inside of houses is made out of it.

However, it seems to me that the gypsum needs to be aerated, that is, air needs to be injected into the slurry and mixed in, so that the gypsum is not exactly solid.

Today, when parking at a multi-level garage, of concrete construction, formed with sheets of plywood, I first noticed the imprint of the woodgrain on the concrete. But then I noticed that there were many, many tiny holes in the concrete. I assumed these were air bubbles, which had been vibrated to the surface of the plywood, but had not been driven out of the mixture completely, before it dried.

That's where I got the idea.

Of course, I suppose you'd still have to find a way to finish the inside lining of the gypsum, so that it was completely smooth, with no air bubbles actually on the surface that the molten aluminun must cool against. But I'm sure you can work that out.

I just imagine that the air bubbles in the rest of the gypsum mixture will give it that quality to be broken apart with much greater ease, turned back into a powder and then shot through some kind of air dryer, to get it back to dry again.

Good luck!

P.S. Please let me know if my idea has any merit. tomfranpat@yahoo.com

Tom

The air bubbles just provide an insulation factor they serve no other purpose.

Gypsum is CALCIUM SULPHATE - plaster of paris stars out as the hemi-hydrate - nothing exotic or mysterious. One molecule can take on 10 molecules of water to form gypsum plaster.

I am not extremely familar with gypsum molds but know that plaster to water ratios, mix time, shear imparted, etc..are important factors and need to be controled.

Is this the only mold making process you would consider. Assuming you are using this process for environmetal reasons and surface finish. There are a number of other alternative molding methods that may be suitable depending on the casting requirements. If you are interested you can contact me a jjmuniza@ashland.com

Joe Muniza

Global Marketing Manger - binder systems

Hi,

gypsum is routinely used in gold casting, so why not aluminum.

There are minimum 8 different types depending on production (temperature and time?) and many more depending on preparation of the gypsum - water mixture.

How much water to add?

Howfast to add?

How to stir?

How long?

How fast?

At which temperature?

All these makes a lot of difference.

I would try (I have no experience with casting and limited experience with house applications):

Make the shell of gypsum as thin as suitable and beneath a mixture of dry sand and scarce gypsum - to be as porous as possible.

Think about vacuum evaporation with prior heating of molds.

Below 18 mbarwater evaporates at 20°C but takes the heat of evaporation with the vapor (624Kcal/kg) off the surrounding gypsum, so preheating is necessary in asmuch as to put in the heat to evaporate all the water that shall evaporate.

Don't put excess water into the gypsum, maybe it can be formed plastically with just the right amount of water. This would avoid any subsequent drying operation.

Have success, report about it please and try to register!

RHABE

Guys

Let me explain with more details.

I want a short time to dehidratate the molds after pouring. Free water and chemical

water ( cristalization ) in time to provide me a production in line, and not must to waitint for the dehidration process that takes me 6 -8 hours.

I would like to have a process that gives me a mold dehidrated, supose 40 minuties

up to 1 hour.

Please see post #1.

MGPo4 Magnesium phosphate would give demold in minutes or could be hours if desired.

Hydrolytic cements include Sodium Silicate and Potassium silicate. They can be made to demold in a short time as well. They have no retained water (hydrolytic reaction)

Both can take the heat and are used for refractory work

Use fine sand with sodium silicate and inject CO₂ using a thin probe.

Other wise you will need a heated binder - sand coated hot with resin and cured in a hot box - used for cores mostly - tricky and expensive.

Are you by chance using plaster as for slip or cold moulding - these are usually pure plaster and may be used again. For hot metal casting a diiferent mould (sacrificial) is usually employed. This generally only has enough strength to allow the metal to be poured and sometimes is supported in a box.

Try mixing the plaster with sand - experiment will give you the best for the mould strength use you will only need enough to sustain the strength until the hot metal is poured. Cut the amount of plaster to the least required to give you the strength you need and keep the bulk of the mould to a minimum - this will require much less water and set faster.

Perhaps better to make a thin shell and support this in loose sand for the final casting.

The thinner and weaker the mould the easier it is to get rid of it after casting.