survivability of a microwave oven for melting metal - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: April 13, 2018, 07:17:53 pm
content: If a microwave oven was lined with heat resistant brick and nicely made so that a pool of molten metal does not run out of it incase a crucible cracks, could it be used as a reliable shop tool?If these things have drastically lower operational times then it would not be worth my while.

author: ikrase
author_rank: Regular Contributor
timestamp: April 13, 2018, 07:29:14 pm
content: People definitely have done this with appropriate modification. I'm not sure how.There exist microwave kilns for pottery, too.

author: gregariz
author_rank: Frequent Contributor
timestamp: April 13, 2018, 07:31:07 pm
content: unlikely unless your metal was encased/mixed in something.. anything that gets hot in a microwave needs to be a "lossy material"placing metal into the oven will simply scatter the waves - with the potential that you blow the magnetron up due to a reflection back into the feed. Its the old rule... don't put metal into your microwave.

author: C
author_rank: Super Contributor
timestamp: April 13, 2018, 07:36:42 pm
content: Microwave ovenhttps://en.wikipedia.org/wiki/Microwave_ovenA microwave oven has a foundation of aCavity magnetronhttps://en.wikipedia.org/wiki/Cavity_magnetronThink of radio reflections an what damage to transmitter they can causeStanding wavehttps://en.wikipedia.org/wiki/Standing_waveSo the microwave lives or dies based on how well the design of microwave prevents reflections from getting back to magnetron. Reflections harms magnetron and to much burns it out.EditShould add that microwaves only effect surface of things that reflect waves so it would only heat the surface.Think of something frozen, takes longer to heat center do to limited penetration of microwaves.

author: CopperCone
author_rank: Super Contributor
timestamp: April 13, 2018, 07:41:49 pm
content: Yea I know that reflections will kill it, but im asking practically how the system will behave. I cant think of a good way to measure VSWR in a microwave oven.In general I see that people manage to do it by using a suceptant crucible, but I am wondering how good of a solution this is. I don't want to spent alot of time tricking out a microwave for high temperature saftey if it turns out that these youtube solutions are cooking the magnetron and its only gonna last for a few hours of operation. \I want to know specifics here. I cant exactly put a VNA* across the weird cheap waveguide the microwave is built with while I am experimenting. I don't know if I can maybe put a thermocouple into the magnetron (where?) to measure average temperature rise.*i meant some kinda probe/slotted line

author: C
author_rank: Super Contributor
timestamp: April 13, 2018, 07:49:37 pm
content: To prevent total reflections from getting to magnetron you would use a directional coupler.This is a reduction not prevention.With reflections harming tube and limited heating of medal think this is a fail.Better idea could be induction heating.

author: CopperCone
author_rank: Super Contributor
timestamp: April 13, 2018, 07:53:02 pm
content: I can't see myself getting a high power directional coupler to modify a shitty microwave oven.I am just wondering how much life expectancy is reduced on these tubes with the average microwave smelting setup.I am only asking because I have alot of firebricks from something else that I could with some minimum effort put in to a microwave.

author: C
author_rank: Super Contributor
timestamp: April 13, 2018, 08:12:42 pm
content: You are assuming that fire bricks will not reflect or absorb microwaves.Esther could be case.Life is how long it takes reflections to harm tube. An unknown based on specific microwave design. Best case is never but unlikely, guess is minutes.

author: Distelzombie
author_rank: Frequent Contributor
timestamp: April 13, 2018, 08:20:50 pm
content: I know you can melt glass in a microwave by heating it red-hot beforehand. Maybe you have to do the same with metal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2018, 08:22:17 pm
content: Low density porous firebrick has a low dielectric constant, and low loss tangent until it gets hot (which is the case with everything, by the way). Put a susceptor inside, that effectively absorbs ~100% of microwaves, and you're done. Put thing-to-be-heated inside there, and off you go.I've seen a mixture of magnetite and SiC used. Note that magnetite is a flux in the 1100C+ range, so don't try doing things hotter than that, especially if it's in contact with the firebrick, which will melt as a result. Putting that inside a less reactive refractory, like an alumina dish, might be a good idea.Tim

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2018, 08:33:22 pm
content: Quote from: Distelzombie on April 13, 2018, 08:20:50 pmI think glass becomes conductive, or at least resistive above a certain temperature. There are notes about the insulation properties of vitreous wirewound resistors when running at high temperatures.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2018, 08:35:41 pm
content: Quote from: Distelzombie on April 13, 2018, 08:20:50 pmThis is a cool trick, you only have to heat a small spot red hot with a propane torch, then start the microwave and the red spot spreads until the whole thing melts. Obviously don't do this in a microwave you care about.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2018, 08:50:32 pm
content: From the ever-quirky Bill Beaty:http://www.metacafe.com/watch/1004040/melt_a_frickn_beer_bottle/Tim

author: Marco
author_rank: Super Contributor
timestamp: April 13, 2018, 09:01:12 pm
content: Robert Murray-Smith used to have a video online how to make a microwave kiln using perlite+fireclay to shape it and making his own magnetite paint using sugar for a susceptor on the interior. He took it offline though, I remember the sound quality being atrocious so maybe that was why.

author: nctnico
author_rank: Super Contributor
timestamp: April 14, 2018, 12:20:35 am
content: Quote from: C on April 13, 2018, 07:49:37 pmI second that. Induction heating seems much more suitable to me.

author: rhb
author_rank: Super Contributor
timestamp: April 14, 2018, 01:21:07 am
content: Quote from: nctnico on April 14, 2018, 12:20:35 amAnother option would be rewinding the secondary for use as an electric arc furnace. However, the most practical thing is a simple resistance heated furnace if you want electric, or a gas fired furnace if you want to be economical. And for limited use, charcoal. Check the books by David Gingery.https://www.amazon.com/Charcoal-Foundry-Build-Metal-Working/dp/1878087002https://www.amazon.com/Building-Gas-Fired-Crucible-Furnace/dp/1878087444https://www.amazon.com/Bertha-Compact-Electric-Resistance-Furnace-ebook/dp/B019X2U05ASteve Chastain's designs are more professional, but also more complex to construct.http://stephenchastain.com/store/index.php?main_page=product_info&products_id=3

author: Audioguru
author_rank: Super Contributor
timestamp: April 14, 2018, 01:24:53 am
content: A microwave oven rotates the water molecules in food then they heat up. How much water is there in the metal you want to heat??Maybe the metal must be molten with a torch first?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2018, 01:48:29 am
content: Quote from: Audioguru on April 14, 2018, 01:24:53 amDielectric or magnetic loss is not unique to water. Molecular rotation is a commonly repeated lie, but it's meaningless in liquid form (broad spectrum hydrogen bonding dominates), and way, way off in gaseous form (the first absorption peaks are around 60GHz). FYITim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 14, 2018, 01:49:10 am
content: One of my neighbors made jewelry and melted glass in her microwave. Eventually killed the thing but it's doable.So find a craft jeweler and ask for a microwave kiln.BTW, you can also just buy a 120V kiln, small enough for countertop use.

author: floobydust
author_rank: Super Contributor
timestamp: April 14, 2018, 01:54:21 am
content: This guy falls for the "put it in a microwave oven to make it shiny" ... the mirror-polished aluminium ball out of aluminium foil trend from Japan...It's as I expect- metal does not absorb microwaves enough to do anything.A sphere does nothing, but crumpled aluminium sheet arcs badly. Nothing melts.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 14, 2018, 01:59:17 am
content: Quote from: T3sl4co1l on April 14, 2018, 01:48:29 amThey make industrial 900MHz microwave ovens. So it's not like 2.54GHz is somehow the "resonance" of water or anything like that.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 14, 2018, 03:26:26 am
content: There's something in the back of my mind that really old microwave ovens had to have something inside to absorb the energy or it would reflect back and damage the magnetron. Newer ones don't have that problem because they redesigned the cavity/waveguide to look like a stub (quarter wave?, half wave?) which naturally looks like a short so there's no reflection. I'm not an RF guy so I don't remember the details.Ed

author: gregariz
author_rank: Frequent Contributor
timestamp: April 14, 2018, 03:47:46 am
content: Quote from: edpalmer42 on April 14, 2018, 03:26:26 amMicrowave magnetrons still do fail due to poor loading but there are a bunch of tricks they've found to help out, including playing with the launch polarization so its more difficult for a reflection to couple back in (is non reciprocal under most circumstances). Nobody cares these days though since you can buy the whole oven for about $50. I was at a conference last year listening to a talk on an aspect of plasma physics whereby they used a bunch of them - it was cheaper for them to just go and buy a heap of ovens and trash them than to source the magnetrons individually.

author: amyk
author_rank: Super Contributor
timestamp: April 14, 2018, 03:48:31 am
content: If you're going to be using it for long periods of time, I'd be more concerned about the circuitry driving the magnetron; transformer-based ones are a bit more robust to overheating but still not designed for continuous use, and HF inverters are not good at all.

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2018, 03:51:30 am
content: Simple enough to do, just get a nice fire brick base to put on the base in place of the turntable, and use a graphite crucible to hold the metal you want to melt. you are going to be limited in the melting point to things that do not destroy the graphite, and melt the fire brick, but you will just have to remove the inner plastic coating on the door, but leave the mesh that completes the cavity, and slightly modify the air flow so the magnetron still has cooling air but it does not run through the cavity, simply by removing the plastic shroud by the interior lamp that directs the air from the magnetron cooling into the cavity, instead venting it into the case. The magnetron will survive longer than the thin steel of the microwave, though you probably will want to have another fan on the case to provide a lot more airflow across the transformer to keep it cool, just take care to not interfere with the magnetron cooling , so the fan should exhaust air from the case rather than blow it in so it augments the inside fan. Do not bridge out any of the door interlocks or the overtemperature thermal cutouts on the magnetron or the cavity, if they operate you need more airflow to keep them cool.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2018, 06:49:37 am
content: Quote from: Alex Eisenhut on April 14, 2018, 01:59:17 amHave also seen an, uh... it wasn't 13.56, but 27.12MHz I think? oven, that did pizzas of all things. AFAIK, inside was two plates, across which the voltage was applied. Tube oscillator no less.Tim

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2018, 10:46:03 am
content: Those are in a way more like inductive heaters than microwaves. They heat through dielectric losses in a conductor, much deeper penetration, but also much greater dependence on the content of the food. Also very high voltages involved.

author: kosine
author_rank: Regular Contributor
timestamp: April 14, 2018, 11:08:02 am
content: When high temperature superconductors appeared in the late 1980s I spent a day in the chemistry department at Sheffield university making some in a domestic microwave. (Was still at school but they let me have the day off.)We ground up the ingredients and microwaved them in a crucible inside a fume cabinet. The only modification was a tube through the side of the oven to let the dense brown fumes out. It arced and sparked like a firework show, but it did the job. I still have some of the pressed pellets.Not sure how their oven fared in the long run, but they had it down to a routine. There's probably a research paper out there somewhere with the details.More recently I've experiemented with plastic loaded with magnetite, which will melt without sparking or fuming in a standard microwave. You can try it with some of the blue metal detectable cable ties used in the food industry (which are widely available if you do a search). Not sure what you'd want to do with the resulting molten blob, but it's a fun experiement you can try at home.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 14, 2018, 11:30:09 am
content: Quote from: T3sl4co1l on April 14, 2018, 06:49:37 amMakes sense, they also use something like that to make plywood.

author: CopperCone
author_rank: Super Contributor
timestamp: April 14, 2018, 05:08:25 pm
content: Quote from: blueskull on April 14, 2018, 02:01:48 amI want to know more about this.This is what I thought. Your not dissipating 900W into the microwave oven walls, I opened it before after it ran with nothing in it and it was cool. This would imply all the energy is lost in the mangetron with 100% reflection? So does putting something reflective in there actually destroy them quicker?? Why?This would imply that the empty microwave is the worst case load, and that any thing in there that causes dissipation in the form of heat, sparks, etc... actually improves the VWSR and decreases magnetron heating.

author: floobydust
author_rank: Super Contributor
timestamp: April 14, 2018, 06:55:06 pm
content: MOT has magnetic shunts to limit the oven's transformer output, I thought to limit max. power.Faintly recall the chamber has mismatched SWR on purpose, so an empty oven does not destroy the magnetron.

author: C
author_rank: Super Contributor
timestamp: April 14, 2018, 08:22:53 pm
content: Quote from: blueskull on April 14, 2018, 02:01:48 amThink this could be called a rash statement.A great microwave designer will know that this is a possibility and plan for this in the design.First some simple facts.The magnetron connects to chamber via a wave guide.Simple thing is length of wave guide can change SWR the magnetron sees.Next you have the chamber.The waves keep bouncing off the walls of chamber until they contain no power.The shape of the chamber will create hot spots and cold spots. Places where a lot of reflections travel through that space or few. In addition you have if the waves add or cancel each other.The designer tries to cure this problem with a wave stirrer. Most cases this looks like a medal fan to change wave path from wave guide.Better microwaves also add a turntable to rotate object.Anything in chamber can and do change the waves.There is not just one source of reflections.The end of waveguide can be a source.Any wave that bounces back to waveguide.There are many paths with different time delays.Do the many reflections add or subtract?Something that does not reflect microwaves acts to shorten the normal path and could make wave weaker.Something that reflects microwave does change the path, This could be good or bad. Do they add or subtract.When the user adds something that reflects microwaves is it something the designer planed for or is it an unknown or unplanned area.You are dealing with very short distance of what adds or subtracts.You also have skin effect.You have things defuse or concentrate.Normal use adds things that absorb the waves, makes then weaker and thus less effect on swr.Need to think of what was left out or less efficient to get lower cost.microwave oven for melting metal is in the luck and unknown working for or against it working.Unknowns are huge.

author: Distelzombie
author_rank: Frequent Contributor
timestamp: April 15, 2018, 09:36:15 am
content: QuoteI don't think you have to be "great" to build a functioning microwave that you're going to sell.

author: james_s
author_rank: Super Contributor
timestamp: April 15, 2018, 08:24:11 pm
content: When I was a kid I remember everyone said not to run a microwave oven empty because it could damage it. I don't know how likely damage was, or whether modern ovens are less prone to damage or just much cheaper so nobody worries about it. I've certainly done plenty of abusing old microwaves and so far I've never managed to kill one.

author: Marco
author_rank: Super Contributor
timestamp: April 15, 2018, 08:25:59 pm
content: They sell microwave kilns for glass making and people use them (and you can make one yourself).Dunno if there is enough power for metal, maybe enough for casting small things in silver like rings.

author: Distelzombie
author_rank: Frequent Contributor
timestamp: April 16, 2018, 04:33:38 am
content: Quote from: floobydust on April 14, 2018, 01:54:21 amThat guy seems to like Alzheimers disease. With all this fine aluminium dust he and his son is inhaling, they're sure to get some kind of problems later on. Don't do that.

author: rfeecs
author_rank: Frequent Contributor
timestamp: April 16, 2018, 04:23:47 pm
content: I noticed this at the grocery store yesterday:I got something in one of these containers and ohmed it out. It's a dead short, so it does seem to be an aluminum pan. I'm surprised they actually recommend putting it in a microwave.

author: CopperCone
author_rank: Super Contributor
timestamp: April 16, 2018, 05:37:21 pm
content: The plot thickens

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2018, 07:27:09 pm
content: Just means it only heats from the top.Tim

author: metrologist
author_rank: Super Contributor
timestamp: April 16, 2018, 08:53:22 pm
content: Quote from: T3sl4co1l on April 14, 2018, 06:49:37 amThat is right between channel 13&14, good buddy...

author: james_s
author_rank: Super Contributor
timestamp: April 16, 2018, 09:06:14 pm
content: IIRC the shape is important regarding any sort of metal in the microwave. If you have sharp corners or adjacent conductive parts that can develop a potential across them you're much more likely to get arcs and heating of the metal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2018, 10:36:35 pm
content: Quote from: metrologist on April 16, 2018, 08:53:22 pm'Fraid industry has a bigger blaster than you do. Sorry, keep on truckin'...(https://en.wikipedia.org/wiki/ISM_band#Frequency_allocationin case anyone was wondering)Tim

author: metrologist
author_rank: Super Contributor
timestamp: April 16, 2018, 11:40:38 pm
content: Oh yeah...cue up "the bowl" and enjoy the 20kW hammers coming down on you. One of those stations caused my SWR warning indicator to come on from a couple miles away, and I was just listening. That could be something to consider... I already had to deal with the PAV PAWS restrictions back when I was more into radio.