Problems with Concrete castings

[whalej]

Hi all, you were very helpful with my mold shell problem and I am now using burlap which helps lighten the weight of everything. I still have problems with the the finished casting having voids and bug holes. I bang on the outside of the mold but to no avail, I still end up with airbubbles. Does anyone have suggestions on what to use to vibrate the mold or concrete? Any help would be appreciated.

Regards to all

Joel Flora

[glass attack]

Depending on the size and shape of your castings, you may be able to solve your problems with a Concrete Vibrator. You should be able to rent one from a tool rental place, but they might only have ones that are too big for small castings when compared to building foundations. Northern Tool ( www.northerntool.com) sells a inexpensive/small sized one for just over $100, I have no idea how well that particular one is.

Instead of vibrating the exterior of the forms that you are casting into, the above device is inserted several times into the concete, just after it is poured. You will be amazed how fast the tools works. It's not a hard tool to master.

[warren01]

Something else that you can try is using an electric sander. One of those that use the small square sheets, some call them finishing sanders. I have used them before on some concrete work. Use without any sand paper. Take a tarp strap (bungee cord or whaterver you call them) and fit the sander with the rubber pad against your mold. Turn it on and let it vibrate away. You may have to move to other locations depending on the size of mold. If mold very irregular shape you can hold it place and move around.

warren

[Mordachai]

a bit late for a reply,

but better late than never right?

I worked for a few years at a Architectural Detail Shop. (I worked as one of 3 mold makers)
Learned a few simple tricks along the way, and will be glad to share.
The first 2 are about how you prep and pour.

We found this to be a large factor of the quality of your castings.
The first is what kind of release you are using? we were using an industrial lubricants( I think it was chevron- not too sure though) It was a mix of Hydraulic Fluid and machinists grease. The mix should be thin enough to not build up(and loose surface detail) and also thick enough that a thin coat stays on vertical surfaces.
The second thing is about how you are pouring the material in the mold, Start out in one corner and pour all the material in SLOWLY. as long as you pour slowly, and let the leading edge of the wet concrete roll over the surface. This will make a cohesion (sp?) effect with the grease and actually push the air out of the way.
If ya don't trap it in the first place then you're good.(also think about your mixing method, it might be whipping air into the mix!)

I know that depending on the mold shape, you cannot always do this though.

The other thing gets back to the virbation. The concrete vibrators are great , but expensive and hell on the wrists. What we would do( very low tech mind you- think stone age) is once the mold is filled, to simple push the material into the corners and recesses witha stick or curved metal rod depending on the shape. We never did this very long on any mold, just a few pushes here and a few there.

After the model material was pushed into the corners, recesses, etc; it sat on a vibrating table. I know it sounds more $$ than a concrete vibrator , but i's really easy to make. Just get an old motor- just about any size will do.( 110 makes it easy to power it with AC) on the end of this motor, weld or bolt a little arm on it, and put a weight on the arm(washers, fishing weights, whatever) . Securly bolt the motor to a donated table and you're about ready to go!

As the motor turns the counter weight will wobble like mad , and get the whole table moving. If it moves to much, take off some weight, and if ithardlt moves, ya need more weight.

Trust me it really works wonders.

Our set up had a bolt welded to the motor , so wee could add or remove washers depending on how much vibration we needed. Our table also had bumps and spikes welded to the top to keep the mold from moving around too much( It was always clamped or fixed to the table somehow) and it also had it's feet mounted on old springs to keep the table from walking all over the shop! ours were springboard springs, but I've seen small car springs used to do the same thing!

Hope this late and long answer gives you some help.

M

[notso]

lol Mordachai you little star, had been thinking about how i could make a table myself :cool:

[Mordachai]

lol Mordachai you little star, had been thinking about how i could make a table myself :cool:
He he

It's really easy

I am about to make a table too, but for a different reason now.
I have been starting a nusery business and screening the soil is a huge PITA.

I am going to make a vibrating table, with several shelves on it. The highest shelf will have 3/8" screen. The next lower shelf will have 1/4" screen. The one under that will have 1/8" screen, and the one under that will have some window screen on it.

By doing this, with that so very not fancy vibrating motor attached to the table directly, will allow me to put soil on the top of the thing, and then turn it on.

a few minutes later, I should have perfectly screened soil for use for different kinds of plants.
And I won't be sifting by hand.

Just build a variance of the vibration table above, to suit your needs for casting. I'm thinking a plywood top with formica or mable on top for easy scraping clean up.

You can get creative and use whatever materials you might have laying around, or you can go out and buy lumber/metal/ etc.

lots of luck out there

|M|

[Lilwolff]

He he

It's really easy

|M|

This has got to be one of the greatest tips I have found, thank you Mordachai!

[Landseer]

I worked for a few years at a Architectural Detail Shop. (I worked as one of 3 mold makers)

The first is what kind of release you are using? we were using an industrial lubricants( I think it was chevron- not too sure though) It was a mix of Hydraulic Fluid and machinists grease.

The second thing is about how you are pouring the material in the mold, Start out in one corner and pour all the material in SLOWLY. as long as you pour slowly,

Our set up had a bolt welded to the motor ,

I worked about 5 years for a place that produced commercial hydrocal casts. I never heard of using oil or grease as your boss did, it goes totally counter to what I know about concrete and plasters in that you don't want to coat the mold with something that will either degrade the mold OR come off on the casts- especially if they are to be painted, stained, sealed etc!

Really all you NEED is a spray bottle and mist plain water in a quart size with about a teaspoon of "jet dry" for automatic dishwashers in on the rubber mold surface. The water alone will eliminate most of the cohesion and air bubbles to begin with, the jet dry makes it even more effective and it's not soapy or greasy.

Next, unless you are using chemical super plasticizers your concrete should never POUR! if your concrete is thin enough to pour it has WAY too much water in it and will be crappy concrete. Concrete should be only moistened enough to be workable- not pourable like pancake batter- you should be able to pick up a heaping shovel full of it without its running off the sides.
When I cast my concrete that is close to about how I mix it, but the way I avoid bubbles is with the above spray and the way I pack the concrete into the mold- you don rubber gloves and press a handful or two into the face of the mold and work it in and around like clay, add some more, do the same, when the face is layered in you can then dump the rest of the concrete in and fill. I rarely get any bubbles- with a little care in pressing.

Here's a picture of a cast done this way in white concrete, no grease, oils, no vibrating tables;

http://i23.photobucket.com/albums/b375/Randall2/294-g-c.jpg


I have read that if you get too carried away with vibrating concrete it can and will cause the heavier particles to settle down- the result again is a crappy casting.