How to use a hot wire (Kanthal) to cut a plastic bin containing a liquid

When I was a teenager, I attempted to build a hot-wire cutter for Plexiglas. (I was awfully tired of sawing it with a hand saw and had not yet discovered those great knives for cutting plex.) I learned that the reason you do not cut Plexiglas with a hot wire is that it welds itself back together in the trail of the hot wire, and the rewelded seam is almost as strong as untouched Plexiglas.

Good piece of information. I have an additional element in my setup; the liquid inside the bin. As the plastic is melted around the wire it could be that the liquid pours out and cools/lubricates the hot plastic surfaces, preventing them from welding together again. I'm certainly not sure about this and am just throwing ideas in the air... It might be that the liquid evaporates as the hot wire is very hot and the problem of re-molding is still present...

/Lars

I think the liquid in the bin will cool the wire and it will stop cutting until the liquid leaks out. I have experimented with hot wire to cut styrofoam and even the styrofoam will cool the wire. Thin sections will cut quickly and thick sections will cut very slowly and require more current.

Apparently I don't understand the configuration. But this sounds a lot like trying to solder a pipe that is half full of water; Which is impossible.

If the wire needs to melt plastic that is filled with liquid, at some point the wire must contact the liquid, which will cool the wire to the point where it will not melt the plastic and the cut will stop.

Evaporation produces cooling !!--hafta try it out

I think the current will be close to the same, whether the bin is filled or empty-- The voltage will be less, because the wire in the liquid will be cooled and it's resistance lower---the rest of the wire,however that in the air outside, and within the molten plastic will still get hot... However,The temperature gradient from air to plastic, to liquid, may be such that you can't melt the plastic before the wire in the air burns out.

This sounds like a possible scenario. If true then this will be a showstopper for this method.

Any other suggestions how to saw a plastic bin into 2-4 pieces with as few moving parts as possible (to decrease the spill of the liquid that pours from the bin to the mechanism&surroundings)?

A grid of wire saws maybe? Possibly the same way of setting up as a hot wire (connected to a spring at the one end and the other end connected to a oscillating force (e.g. rotating disc connected to a rod):

|-\\\\\-o-----w-i-r-e---s-a-w-----o-

I can see one disadvantage, the liquid may become contaminated by small plastic particles that need to be filtered out later. An advantage is that the temperature of the liquid is not that important for the process, it just makes cutting a bit slower.

/Lars

What does the number of moving parts have to do with spillage? If it had one thousand moving parts but cut the bin in one millisecond, would that spill more than a machine with one moving part that cut the bin in 10 seconds?

What is the liquid? Would contamination with a little water be a problem? If not, a waterjet cutting machine would be able to cut it very cleanly and quickly.

Here's a good link:

On 29 Jul 2004 10:01:23 -0700, snipped-for-privacy@tfif.fi (Lasse) vaguely proposed a theory ......and in reply I say!:

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I have to ask. Have you thought of simply emptying the bin?

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

Kanthal wire is good for pretty high temperatures. I don't think you will need anything that hot for most plastics. So I would think that Nichrome would be more than adequate. But you might tell us what kind of plastic you are wanting to cut. And also what the liquid is.

I don't think you will have much luck regardless of what plastic with the liquid cooling the wire. If I were going to try something, it would be taking an old circular saw blade ( non -carbide tipped teeth ) and grinding it to be a knife edge. Try using a sabre saw with a knife blade to see how easily the plastic cuts with a knife. Then if that works use a table saw for more power and speed. You will probably want to hang some plastic sheeting as I expect it will be messy.

Dan

The idea of having as few parts as possible is that I'm actually interested of the liquid. If the equipment can be placed inside a tube or similar with as few connections through the wall, less liquid is spilled. Otherwize I could just saw the bin with a chainsaw... The simpler the mechanism the easier it is to maintain and keep running for long periods.

Water is definitely a bad thing and needs to be boiled off the liquid before I can post-process it.

/Lars

The liquid is not always liquid, it is often in steady state and has to be warmed up before it can be poured out. The big amount of bins also implies to have some kind of equipment to open the bins fast. Also it would be great if the bins get a bit smaller to save some space.

A commercial heavy duty shredder is just too big & expensive (~60kEUR and above).

/Lars

Obvious method #1: Step 1, punch two holes in bin. Step 2, drain liquid to recover it. Step 3, cut/crush bin any way that suits. Since you have not explained any reason why you must cut these bins with fluid in them, it is the simplest solution. Cutting with fluid in place just complicates things, especially since you want to recover the liquid, as you've posted later on.

Obvious method #2: Place bin in containment/collection shield (giant funnel). Drop gillotine cutter on bin. If liquid splashes out, make containment sheild better/bigger until that does not happen.

Completely silly over the top method #3: place bins in press, crush, drop mess into centrifuge directly under press, spin, collect liquid.

On 30 Jul 2004 04:41:11 -0700, snipped-for-privacy@tfif.fi (Lasse) vaguely proposed a theory ......and in reply I say!:

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OK. This ius all very interesting. But I have had no solution to much simpler problems, with far less mysterious input!

My thougt here is that you need to describe _exactly_ what you are doing, and people (Probably not me. I am the bean-counting logician here, not the expert) will be able to give much better ideas.

Seriously. When this group decides it _will_ help with a problem (he says a bit wistfully) they really think outside the square, and have a vast anount of expertise. But they need the truth first.

***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.

How about a blast freezer to solidify the liquid, then have a gang of men dismantle the bins with axes?

I want an NC waterjet tomato cutter in my kitchen someday.

Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ ) Student member SAE for one year. I love: Dona, Jeff, Kim, Mom, Neelix, Tasha, and Teri, alphabetically. I drive: A double-step Thunderbolt with 657% range.