Open sand mould or not

[stragenmitsuko]

I'm gonna try to cast a T slot plate for a cnc router I'm building .
I have a very good quality alloy of alum that I can melt , and I have more then enough of it .

The pattern is made . It's dimensions are 425*274*26 mm . (17*11*1 inches ) .
For reference , the plan is printed on A4 size paper

If I calculate the volume needed , it will be about what my crucible can hold .
A full crucible is 3 liter , the pattern is 2.7 liter . Even the 2.7 liter is a bit abovethe safety limit I would say . Probably will pour over when degassing
No material left for a traditional mould with raiser and feeders .
And I wouldn't know where to place them anyway or how many would be needed for such a large surface .

I can live with some shrinkage , as all surfaces will be machined , and I have
made the pattern a few mm thicker then really needed .

So what do the more more experienced casters think .
Can I get away with an open mould , or should I get a bigger crucible .

 

[stevehuckss396]

Having a very small amount of experience with open molds I would expect the top to suck in due to shrinkage. You might want to pour maybe 1-1/4 thick to allow for an inch after machining. I have seen some thick castings suck in more than that. Other than that i would expect a good result.

 

[mikelkie]

I would try open mold with pattern placed grooves down if possible, shrinkage will be limited by
the large area at the top now, can even pour more material that way.

 

[bazmak]

To my mind I cannot see any advantage in casting such an item.It will warp,twist and suck in etc.Why not buy a piece of plate.Less work and machining with better quality control on the alloy

 

[Wizard69]

Personally id want to see a covered mold and plenty of sprue. Even then i would imagine excessive shrinkage.

Now my experience consists of a few years in a die cast shop which is obviously a different technology. Given that i cant see anyway to prevent shrinkage without a high sprue and likely many risers.

Beyond that i have to agree with the suggestion to buy a plate, ideally avoiding aluminum.

The other option would be to have a foundry cast the plate in cast iron. Shrinkage will be a lot less and a good foundry will be able to pour a ton of metal.

Your third option is a bigger crucible and the ability handle it. Make the plate extra thick and make sure you have plenty of pressure in the sprue.

 

[stragenmitsuko]

Maybe I should elaborate .

I'm building a little cnc desktop router .
It 'll look a bit like the on in the picture .
The t-slot plate usually has an mdf spillboard on it , and that is where
all the cutting is done .
Only light cutting forces , almost like an engraver .

I do agree that cast is much better , fi for a milling attachement on a lathe , but for this particular project its not needed .

The reason I'm trying to cast it is because I don't have any material of the right size available . Sadly round here ( belgium ) we don't have companies like
mc- master to order blanks .
And offcourse it's great to diy and learn something in the process . :thumbup:
Altough I admit if I could find a piece of suitable plate I would probably buy it and save me the pattern making and moulding .

Thx for all the replies , I'm gonna give it a go with an open mould and I'll post the results here .

Pat

 

[Wizard69]

Hi Pat,

If you can't have some fun with a build the what is the point. The only problem i see for you is being able to pour enough aluminum to cover the shrinkage problem. I could see you needing twice as much pour capacity.

Pictures of this machine would be nice once you get it built.

 

[stragenmitsuko]

I've changed my mind .

I've got me a quite large fire extinguisher at the local metal recycling plant wich I gutted and I'll use that as a crucible .
It'll fit my furnace and has enough capacity to hold the amount of metal I need .

So today I spend a couple of hours making more greensand , and tomorrow I'll make a flask large enough to hold the plate . So it'll be a closed mould after all .
Altough I still might justtry it out with an open mould to see what the result would be .

I think I will put a large raiser in the middle of the plate , probably 50mm ( 2inch ) diameter and 4 or 5 inches high , and second smaller one at one end with two gates towards the corners , and the ingate also with two gates towards the corners at the oppside side .

And then I'll just have to give it a go wont I

 

[stragenmitsuko]

Here's how I made the greensand .

I went to a shop where they sell bricks and cement and stuff like that .
I got me a bag of what round here is called white sand .
It is used to fill the joints in outside stone pavements and it is a lot finer
then mansonry or concrete sand .
Fill in th egaps guy's , I don't know the exact terminology in english .

This sand is wet , so it was put on stainless trays in the sun an left to dry .
In wintertime I put the trays on the wood stove , in summertime the sun does it in a couple of hours .
Once dry , the sand can be poured like water .

Next I went to a garden centre and got me a box of bentonite clay .
They sell it in a closed plastic bucket . It is powdered , as dry as can be , and it's as fine as flour .

The dried sand and the powder clay were mixed in a 100 - 12 ratio by weight .
That is 100 kg or lbs or grams or whatever of dry whitesand to wich was added
12 kg or lbs or grams or whatever bentonite clay .
I used my wife's kitchen scale , don't tell her

The sand/clay mixture was then put in an emty paint can , and shaken in a paint shaker for 3 minutes . The paint shaker was thrown away at a company where I do the maintenenace . Fixed it , and I've used it ever since .
All of the sand was mixed with clay like this .
When it comes out of the shaker it feels kinda warm .

Finally I added 7% weight of water for each 112% of mixture .
So I weight 2240 gramms of sand/clay mixture and added 140 gramms of water .
That's half a paint can , and it was shaken again for 3minutes .

The result is a very homogenous mixture wich is now left for a couple of day's to allow the clay to fully absorb the water .

 

[stragenmitsuko]

Made a little progress today .

The pattern was made of mdf and glued together .
First a layer of latex or emulsion paint was used as a primer .
Then after a couple of day's it was sanded with a 400 grit paper .
Two layers of high gloss paint were then sprayed ( can ) on .
The high gloss is supposed to make the removal of the pattern easier , and stop the mdf from absorbing moist .

 

[stragenmitsuko]

Then I also neededd a flask big enough to hold this pattern .

Easy enough , just screwed some wood together .

The flask has not to much depth , that is because my amount of moulding sand is not unlimited .

I decided to add a bottom board wich wil be screwed to the flask after the ramming .
As the depth is pretty small compared to the with and lenght , I can imagine the sand
not holding when moving the mold .
The bottom board ought to add some stability .

When the mold is ready , both halfs wil be screwed together to avoid lifting of the upper half .

 

[stragenmitsuko]

I'm going to use a fire extinguisher as a crucible .
Got one at the metal recycling plant for free , as long as I didn't take an empty one .
They don't like handling containers under pressure , they're happy to get rid of them.

You could offcourse arm it and emty it , but that's such a mess .
I opened it , and poured the powder in a trash bag .
I've been told it makes a good parting agent but I'll stick to talcum powder .

So here's how I "defused " and unloaded the fire extinguisher .
This is the way I do it , use at your own risk .
This is for a powder extinguisher not for a CO2 .


First press the valve at the nozzle , just to make sure the tank is not under pressure .
Nothing should happen when you do so .

Then unscrew the top cap . Do not press the plunger or remove the safety .
The cap should almost unscrew manually or with very little force .
Remove and discard .

Now lift out the inner CO2 tank with its cup and in most cases a piece of tubing wich is there to direct the pressure to the bottom of the tank .

Inside the cup there's a nut wich secures the cup to the cilinder .
Remove the nut using two wrenches .
Never do this by holding the cilinder .
The nut to the cilinder joint must not be slackend in any way , no torque should be applied to the cilinder .

With the cup and tubing removed , now it's time to emty the cilinder .
Hold it secure , I do this in my 3 jaw chuck . I guess a v block or a vice with v jaws can also be used . Make sure it cannot move or rotate .
Now slacken the nut a tiny little bit until the gas starts to blow . It takes less than 1/4 turn . Wait , and allow it to empty .
As the carbon dioxide inside is liquid , it can take quite some time .
In the nut there's a little hole ( arrow ) , this is where the gas will come out , so make sure you don't get it in your face .
If it's empty remove the nut and check with your thumb that there's no residual pressure .
Now you can safely remove the cilinder from the chuck .

Finally a nice little cilinder remains wich can be used for something else .
It's designed to hold 30bar (400psi ) .
Inside the nut you can see the rupture disk , wich is punctured when the extinguisher is armed by pushing the button on the top cap .

 

[stragenmitsuko]

A couple of pictures of the fire extinguisher crucible , and the lifters I made to handle it .

Second picture shows it in the furnace

 

[stragenmitsuko]

So this afternoon I started moulding , closed mould remember , and that didn't work out at all .

As soon as I lifted the cope , the sand simply collapsed and fell to the floor .
I even moulded some chicken wire in it to support the sand , but no avail .
I guess the surface to heigt ratio needs to be a lot bigger .

So , forget about the cope , but as my drag was still ok I decided to try that open mould after all .

Wich again went horribly wrong .
I calculated the volume of the pattern , then weighed some ingots to rougly determine
how much I needed to melt .... and forgot alum weighs almost twice as much as water .
So I melted nearly half of what I really needed .
Sigh

 

[stragenmitsuko]

Back to the molding table .

A second attempt made , the rigt amount of material melted and this went very well .

I poured a little hotter then I would usually do ( I need a pyrometer if I'm gonna continue doing this ) and filled that mould till it almost overflow .
It didn't flow over , but the metal is certainly higher then the sand .

There's some shrinkage , but it's limited to a few mm (1/8" maybe ) .
I most certainly can live with that :thumbup:

I was afraid of cracks or large shrink holes , but no it didn't .

 

[stragenmitsuko]

A couple of pictures of my furnace .

Put it on a lawmnmower chassis . Makes it easy to move it around .

For now propane as fuel , but I have the parts to build a waste oil burner .
Most likely when the propane tank is emty I'll build it .

The little blower is 95% chocked , otherwise it'll simply blow out the flame .
Someday a small 35watt 3 phase motor with a vfd will be installed .

 

[stragenmitsuko]

removed , sorry double post ,

 

[stragenmitsuko]

Finally , these are my "ingots" .

Buckets from a centrifuge that is used in a medical lab .

The company that sells these machines demands that the buckets be renewed every year , for warranty and safety reasons .
Must be a very good sales departement they have , I would never accept that if I were the purchase manager .
You don't want to know homuch they charge for one bucket .

But anyway , I have abt 50 of these buckes every year I have to discard .
They're exactly one kg ( 2.2Lbs ) , and it is a good quality of alum .
No cleaning , no sawing , just put them in a crucible and fire up the burner

 

[stragenmitsuko]

I put the casting on a scale . It weighs 9.65kg ( 20.5lbs ) .
I had melted 10 "ingots " , and there was some material left from the previous pour wich went wrong .
So that sounds about right .
A little dross , and some material left in the crucible .

Then I took some measurements . The pattern was 25mm ( 1") thick .
The casting measures 33.5mm( 1" 5/16 ) on the edges , and it's abt 6mm ( 1/4) thinner in the middle . Amazing that the liquid metal would be 8mm ( 5/16) above the sand and didn't pour over .

Anyway , at first sight I think after machining the excess material on the edges and truing up the surfaces I would have a plate more or less thesame dimension as my patttern .
Sweet

 

[stragenmitsuko]

Casting was cleaned up today .

First I clamped it as good as I could to the table , and gave the t-slot side a light skim pass to remove the high spots and to create a flat surface .

Then I removed it , cleaned and degreased the mill table and put double stick tape
on it .
The skimmed casting was also degreased and put on the double stick tape .

Then I could start machining the other side , the side that was open when it was poured .

The final picture shows the finished bottom side .
The lines from the facemill are optical , an effect of the flash from the camera .
I can see them , but can't feel them .
A little sand paper ought to take care of that , or maybe a pass with a flycutter .

All in all I had to remove 5.5 mm , les then 1/4" to clean up the entire surface .
As the setup was with double stick tape , I took only light cuts . 11 passes of 0.5 mm .
The face mill is capable of much more , but I didn't trust the double stick tape enough to
take heavier cuts .

The first passes only cleaned up the edges .
It was fun to watch that shiny edge grow bigger as the depth of cut increased .

The material machines beautifully . Gives a shiny finish , even with my roughing facemill .