Casting a work surface?

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Forum topic by RobynHoodridge posted 08-18-2013 07:48 PM 1580 views 0 times favorited 17 replies Add to Favorites Watch
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127 posts in 2503 days

08-18-2013 07:48 PM


I’ve been staring at the sawdust pile, then looking over to the chipboard stock (particle board), then back to the sawdust. Back to the chipboard, back to the sawdust, etc..
And i’ve pondered, why don’t we use this prolific waste product of ours and make something useful from it?
In particular, i’ve thought to acquire some casting resin, mix in as much of the micro-fine ‘scrap’ pile as possible, and cast this slurry into a slab that can become a work surface of some sort or another.

I’m sure one of the reasons that thick, solid, plastic ‘table’ tops aren’t used ubiquitously is that it would be hella costly. Chipboard makers get around this by using only a very little bit of plastic (resin glue) to hold wood chips together, and include a lot of air spaces too. I don’t have a factory to process my sawdust in such a regulated way, but what if i could replace a lot of the volume of a worktop that i might cast in solid plastic myself with wood chips? How strong would my product be compared to what the same money (as the casting resin) would get me in a store-bought work surface? Or conversely, how much would a home-made equivalent to the store-bought work surface cost me to make?

My initial plan of action for this casting task would be:
Find a used / damaged / scrap / trashed sheet of (float) glass.
Build a wall (dam) upon the glass sheet to enclose the shape i want to cast my slurry into.
Coat the internal surfaces of this mould with a release agent.
Mix a batch and pour it into the mould.
Keep the dog away from it till it sets.

Has someone tried anything like this?
Am i missing something, or nuts?
Any advice or suggestions bettering my approach?

Cast upon glass, the work surface should be dead flat. It would be dimensionally stable; hard; hefty (good for not moving around); more robust than airy chipboard; and could be shaped using many a woodworking method. So i’m seeing only positives for my application.
Also: no delivery fees (well, i’d have to get the sheet of glass home i guess); no offcuts since it’s cast to size and shape; things like vices and T-tracks could be cast into the slab, or spaces left for them during the casting.

If this is possible I’m imagining it could be a very useful approach to very many folks. Every woodworker abandoning centuries of tradition of laminating fine wood into a workbench slab, and instead casting their own cheaper better alternative. We will call ourselves cast-aways. High on the fumes from our workbenches and ecstatic over the new tool we bought with the money we saved. Or is this all just blasphemy?

-- Never is longer than forever.

17 replies so far

View papadan's profile


3584 posts in 3541 days

#1 posted 08-18-2013 08:03 PM

” High on the fumes from our workbenches and ecstatic over the new tool we bought with the money we saved. ” Sorry, but I believe the fumes are already working on you. LOL Too expensive to consider feasible and would cost you the new tool instead of affording it. Just do as the rest of us and mulch the flower beds with your saw dust. If you must use it productively, mix it with melted wax and make fire starter bricks. ;-)

View Manitario's profile


2653 posts in 3056 days

#2 posted 08-18-2013 08:15 PM

it is a good idea and would certainly be unique; the problem is that the epoxy resin is $$$$ for the amount needed for a whole top. As well, I imagine that casting a whole top eg. 3” thick would present some serious drying difficulties for the epoxy.

-- Sometimes the creative process requires foul language. -- Charles Neil

View RobynHoodridge's profile


127 posts in 2503 days

#3 posted 08-18-2013 08:31 PM

Oh yes, epoxy is many monies. So what about other casting materials? Polyester (general purpose, orthophtalic) resin? Other ideas?
I only considered this expensive casting concept after seeing Mr. Nubs considering using this $230 tabletop in another forum thread. That’s 28” x 40” x 1-1/8”. How much resin could i get for that. Especially if i didn’t use expensive epoxy.
I like the fire starter bricks idea.

-- Never is longer than forever.

View RobynHoodridge's profile


127 posts in 2503 days

#4 posted 08-18-2013 08:47 PM

I really have been seriously considering cement. (maybe with something like HDF laminated atop)
It just has to be so damn deep (thick) so as not to break under deadblows etc.. And then it’s too heavy to be reasonable. But there still might be a way.

-- Never is longer than forever.

View Loren's profile


10477 posts in 3821 days

#5 posted 08-18-2013 08:52 PM

I think particleboard is formed under a lot of pressure
to compact it while the resin sets.

You can embed expanded steel lath in cement to give
it more strength. It is cheap.

View RobynHoodridge's profile


127 posts in 2503 days

#6 posted 08-19-2013 04:17 PM

re: high pressure moulding of particle-board. Why do they do that? To achieve flatness of both faces; and to increase contact that resin and particles are making in order to be able to use less resin. (probably also to speed setting, and penetration of the resin into the wood chips)
In the casting case there’s plenty of contact between resin. A resin continuum that’s interspersed with volume saving sawdust. I wouldn’t choose to compress this if i could.

Yes, re-enforced concrete is the way.
I want to follow through the thought experiment of casting using wood chips though.

A quick look at prices of gallon volume poly resin returns $30 to $60. Lets say $45.
The $230 tabletop from rockler has less than 5.5 gallons volume. And assuming that wood chips could replace half of the volume in a mix like i’m suggesting (no precident, just a guess) one would therefore need to purchase 3 gallons of resin to match that particular table’s volume. That’s 3 x $45 = $135
The resulting table top should also be a LOT stiffer (if you’re into that) than the “melamine-coated MDF with 3mm black edge banding” version. So is that not better for ~$100 less? Wait, there’s T-track in rocker’s version too. Let’s say $40 for the T-track equivalent. Though you could create a T-track recess in the casting process IF you wanted to fiddle with that. Still $60 less for something better. BUT that rockler table top is expensive to start with…

-- Never is longer than forever.

View jmartel's profile


8194 posts in 2323 days

#7 posted 08-19-2013 04:28 PM

You can make fire-bricks with it.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

View Loren's profile


10477 posts in 3821 days

#8 posted 08-19-2013 04:47 PM

Give it a go and let us know how it turns out.

I know I learned a lot from doing experimental projects
over the years. I spent a fair bit of money at it
too but the education was worth it I suppose
and sometimes the outcomes led to fruitful

View Danpaddles's profile


573 posts in 2485 days

#9 posted 08-19-2013 05:06 PM

If you really want to try that, consider laying some fiberglass into the sheet. Wood chips will add very little strength. Plain epoxy is not resilient. Overall, though, I’d skip it. Fumes and costs will be onerous.

-- Dan V. in Indy

View crank49's profile


4032 posts in 3144 days

#10 posted 08-19-2013 05:48 PM

If you could get hold of a pug mill you could mix as little as 4% resin with wood dust and make a compactable material. Unless you have a huge press to compress it with, there will still be voids in it however. I gues you could then pour a thin layer of pure resin on top of that to make a void free surface.

Or, for about 25% of the cost and none of the hassle, you could just build it out of three or four layers of plywood like I did mine. Works great, stays flat.

View RobynHoodridge's profile


127 posts in 2503 days

#11 posted 08-19-2013 06:53 PM

Loren, i’m a product designer, so i know all about prototyping things that are essentially concepts till that point. :) I’ll see if i can do a test.

The effort, costs, and fumes, may well not be worth it. Especially when there are other ways as crank says. That’s an important point i think. The best alternate might not be the ultimate alternate. I still think that a home cast table top could be ultimate, but maybe living with one a little thicker, or rougher, or whatever, cause it could be made with easy materials like plywood is actually best.

-- Never is longer than forever.

View LakeLover's profile


283 posts in 2113 days

#12 posted 08-20-2013 11:00 AM


Great concept. But you would have to do it in small pours. 3 inches of poly resin curing will start a nasty fire.

I used to work in the prosthetic industry. Saw a few guys add to much hardner and Poof, smoke then flames. I ran because I don’t think that smoke is very good for you.

View bondogaposis's profile (online now)


5049 posts in 2524 days

#13 posted 08-20-2013 01:03 PM

This is a harebrained idea, and I mean that in the best possible way. I have no idea whether this will work or not but LakeLover brings up a very valid point. Any of the catalyzed resins, like epoxy or polyester resin give off heat as they cure, too much resin in one pour and you will have a fire. Now for a few random thoughts.

Some other things I would like to point out about how particle board is made. It is not made from sawdust, it is made from planer shavings. It is laid down in a mat w/ the finest shavings on the outer surfaces. The shavings are tumbled with glue being sprayed into the mix until it is well distributed. To make 3/4” particle board a mat of shavings about 6” thick is laid down on a platen. The platens that I’ve seen are 8’ x 16’, enough to make 4 sheets at once. Than another platen comes down from above and compresses the mat into 3/4” and a great deal of heat is also applied. The glue used is heat setting and once enough heat is applied the the platens open and out comes a 8’ x 16’ sheet of 3/4” particle board, which is quickly zipped into 4 4’ x 8’ sheets. My point is that this is an industrial process that would be really difficult to mimic in a home shop setting.

Boat builders use what they call “fairing compound” which is simply a mixture of wood flour and epoxy resin. Wood flour is very fine sawdust. They mix it to the consistency of peanut butter and then spread into corners to make a fillet over which fiberglass cloth and resin are applied. Fiberglass doesn’t like sharp corners. What you are proposing is to make a top out of fairing compound. It does sand quite well and will yield a very smooth surface in the small quantities that I’ve worked with. The biggest problem that I see is overcoming the heat problem. Too big a pour and your work goes up in smoke. Too little and you have a lot of layers that may not adhere well to each other. I would really try this is on a small scale first before gear up for a big workbench top to see if you like how it turns out and how thick a pour you can make without generating too much heat.

-- Bondo Gaposis

View RobynHoodridge's profile


127 posts in 2503 days

#14 posted 08-20-2013 05:29 PM

LakeLover: wow, i’ve had mixed batches cure too fast, and heard of some extreme reactions, but flames and fumes is the worst case scenario. They must have got it seriously wrong. And you’re right to avoid that smoke, it’s very bad stuff.
I’m sure the way one would need to address potential overheating depends on what’s being made. A 3 inch pour (say for a work bench) sure would need to be done in stages. But if you were making a thinner table top (say the equivalent of that rocker one at 1-1/8”) then i’d say it’s possible in one go. No harm in working in stages though since each is poured before the previous is fully set and they actually fuse / merge.

Thanks BondoGaposis. Interesting stuff. I’d guess you already know that if talk powder is used in place of the fine sawdust in “fairing compound” then you’ve got bondo (body filler).
As i mentioned, The impossibility of replicating chipboard production is why i’m talking about pouring a slab of resin that just happens to have some space in it taken up by wood chips.

Casting resin certainly is a delicate thing. As i understand it, it’s not that more resin causes more heat. The heat developed, and the rate of heat production is determined by the catalyst. What happens when you have a thick pour though is that the heat moving from where it’s produced just moves to another place where there’s already hot resin rather than into the air say. So overheating is a result of there being nowhere for the heat to go in a thick pour. And because the rate of this chemical setting reaction producing the heat is itself determined by temperature, it turbo boosts itself.
Catalyst ratio determines the reaction’s rate – at a certain temperature. And temperature determines reaction rate – at a certain catalyst ratio. So the trick with thick castings is to use very little catalyst (“too” little) and if possible actually regulate the temperature of the pour yourself. That way you’re controlling the reaction rate, and can keep it slow enough to avouid that turbo effect. But without tonnes of experience and the means to implement fine control (eg: water baths) i tend to mitigate the overheating effect by staggering the bursts by pouring thinner layers in succession. Just as suggested.
So thick casting is tricky, but there are ways of dealing with it.

-- Never is longer than forever.

View Bluepine38's profile


3379 posts in 3258 days

#15 posted 08-27-2013 03:00 PM

Sounds like a good experiment, if you are going to make a table top, you could just use plywood for the
base form, and not worry about reusing it. Sounds like you know enough to keep from causing any serious
damage to yourself or others. Which brings me back to concrete, in Europe, where they require concrete
floors in some commercial remodeling of some old buildings, they have used small styrofoam type balls in the
concrete mixture, it does not affect the strength of floor too much, and makes a much lighter floor that the
older buildings can support. But I think that would still be too heavy for a workbench, although they are
using them for countertops. A local credit union even mixed in glass, then ground and polished the top to
make a very nice enviromentally friendly countertop. Please keep us posted on you progress, or lack thereof.
We all may just learn something.

-- As ever, Gus-the 79 yr young apprentice carpenter

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