A cylinder with two things taken out of it
The slurry volume is the flask volume up to the fill line, less whatever pokes into it. That is normally two things: the cone on the rubber sprue base, and the wax tree itself.
With the defaults on this page — a 2.5 by 4 in flask filled to a quarter inch below the rim, a 1.5 by 0.75 in base cone, and 18 g of wax at 0.95 specific gravity — the arithmetic runs like this:
| Step | Volume |
|---|---|
| Flask brim full, 2.5 in across by 4 in tall | 321.7 cm3 |
| Filled to 3.75 in instead | 301.6 cm3 |
| Less the base cone | -7.24 cm3 |
| Less 18 g of wax at 0.95 | -18.95 cm3 |
| Slurry in the flask | 275.4 cm3 |
The base cone and the wax between them are about 8.7 percent of the fill volume here, which is not nothing but is also not the thing that ruins a mix. The fill height is. Move the fill line half an inch and you have moved the volume by 40 cm3, which is more than twice what the wax displaced and five times the base cone.
Water to powder is a weight ratio
Investment is sold with a water-to-powder ratio printed on the bag, given as parts of water by weight per 100 parts of powder by weight. It is not a volume ratio and the two are quite far apart, because the powder is roughly two and a half times as dense as the water.
To turn a slurry volume into a powder weight you need to know what volume the mixed slurry occupies per 100 g of powder. That is the powder volume plus the water volume:
volume per 100 g powder = 100 / powder specific gravity + water parts
At a powder specific gravity of 2.6 and a 40:100 ratio, that is 38.46 cm3 of powder plus 40 cm3 of water, so 78.46 cm3 of slurry carries 100 g of powder and weighs 140 g. The slurry density falls out as 1.784 g per cm3, and the powder needed for any volume is that volume times 100 divided by 78.46.
For the 275.4 cm3 above: 351 g of powder and 140 g of water for one flask, before any allowance for what stays in the bowl.
Why the ratio moves everything
Thinning the mix does not just make it runnier; it changes how much powder a flask holds. Run the same 275.4 cm3 flask at three different ratios:
| Water:powder | Slurry volume per 100 g powder | Powder for the flask | Water | Slurry density |
|---|---|---|---|---|
| 36:100 | 74.46 cm3 | 370 g | 133 g | 1.827 g/cm3 |
| 40:100 | 78.46 cm3 | 351 g | 140 g | 1.784 g/cm3 |
| 44:100 | 82.46 cm3 | 334 g | 147 g | 1.746 g/cm3 |
Ten percent more water is about five percent less powder in the same flask, not ten — the powder volume does not change, only the water added to it. Whether that matters to the casting is a question for the investment maker and for whoever is running the burnout, and this page has nothing to say about it. What it does say is that guessing the water and weighing the powder, or the other way round, produces a mix that is neither of the numbers on the bag.
What stays in the bowl
The allowance field exists because a bench mixer never gives back everything you put in it. Some sits on the blade and the walls, some goes down the side of the flask, and on a multi-flask pour some is left when the last flask is full. Eight percent is a starting figure. The honest way to get your own is to weigh the bowl empty, mix a batch, pour it, and weigh the bowl again with what is left in it.
What this does not cover
Working time, vacuum cycles, bench set, burnout schedules and how long a mixed flask can sit are all properties of the specific investment and of the room, and they come from the data sheet rather than from arithmetic. So does whether a given ratio suits what you are casting. The volume and the split are the parts that are pure geometry, and those are what is here.
Dry investment powder is largely silica and it goes airborne readily while it is being weighed and mixed. Burnout kilns and the flasks that come out of them stay dangerous long after they stop looking it. Those are hazards worth naming and not things this page has a procedure for.
Questions people ask
How much investment does a 2.5 x 4 inch flask take?
Filled to a quarter inch below the rim, with a 1.5 x 0.75 in base cone and 18 g of wax on the tree, the slurry volume is 275 cm3. At a 40:100 water-to-powder ratio and a powder specific gravity of 2.6 that is 351 g of powder and 140 g of water. Change any of those inputs and the answer moves, which is why they are all fields.
Is the water to powder ratio by weight or by volume?
By weight, on every investment bag we are aware of, and the difference is large. At a 40:100 ratio the water is 40 g per 100 g of powder, but because the powder is roughly two and a half times as dense, that same water occupies more volume than the powder does. Measuring one by weight and the other by volume is how a mix drifts from the ratio it is supposed to be at.
How much slurry does the wax tree displace?
Its own volume, which is its weight divided by its specific gravity. Eighteen grams of wax at 0.95 is about 19 cm3, roughly 6 percent of a 2.5 x 4 in flask. It matters more on a heavily loaded tree in a small flask. The same volume becomes the cavity the metal fills after burnout, which is a separate calculation.
How many flasks does a 25 lb bag of investment fill?
With the defaults on this page, about 29 flasks of 2.5 x 4 in including an 8 percent bowl allowance. Larger flasks drop that quickly — a 3.5 x 5 in flask takes about two and a half times the powder of a 2.5 x 4. The page counts it for whatever flask size and ratio you enter.
What happens if I mix the investment thinner?
Less powder ends up in the same flask, and the slurry is less dense. Going from 40:100 to 44:100 in a 275 cm3 flask drops the powder from 351 g to 334 g and the density from 1.784 to 1.746 g per cm3. Whether that is acceptable is entirely a question for the investment maker and the data sheet, and not something this page has any view on.