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chiloschista

Vacuum forming machine: need a bigger one!

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chiloschista

Hello,

finally I'm coming to an end with the REDacro. The plane is almost finished, covered and waiting for final painting. 

I still miss the canopy, or better said, I miss a way to build it.

I need a bigger vacuum forming machine than the one I have.

Problems are:

  • the sheet frame does no more go into my oven, so I need a heating solution. I found a heating wire, but am no sure how safe it is to work with it. This will be a 220V, open system. How to hold the wire? I could build some ceramic holders, with mould ceramic. How to connect it, but this is a minor problem. Can I connect it directly to main or better to preview some safety, considering electrical system has its own already? Main is limited to 2200W, which should handle the vacuum pump too.
  • and now we come to vacuum pump. A vacuum cleaner is no more enough, but I guess a solution like that one should work.

Experiences, ideas, suggestions?

An alternative could be build it with composite, but every builder knows the feeling: a challenge is a challenge!

Best regards,

Ric

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CTR

Hi Ric,

I strongly suggest that you keep the voltage low! touching 220v is likely to be lethal. On the other hand, 48v = safe.

Another option would be to hire a 110v transformer use for site work by builders etc. Not as safe as 48v but certainly more healthier than 220v :cry: 

Don't know your level of knowledge, so maybe the following things you already know, but if not, it gives a starting point for you.

The wire used in the kiln element can be bought in spools; it is nichrome or resistance wire. This is available in a wide range of sizes. Each diameter will have a different resistance per meter

Try www.wires.co.uk for bare nichrome wire. Good quick service when I have needed some.

https://wiretron.com/nichrome-resistance-wires/ has useful charts giving current/ temperatures and resistance values

It can be heated to red hot without problems; just select the necessary wire resistance to give the heat you need.

current x voltage = watts, so if you need 1kw at 48v you need about 21 amps supply. This means you need about 2.3 ohms total resistance of the wire. Power supplies can be found on auction sites etc. but new price for RSP-1000-48 is about £200!!! However, a simple 48v step-down transformer able to supply 20 amps or so will be a lot cheaper.

For the 110v solution, the total wire resistance needs to be about 5.3 ohms for 1kw of heat.

You need to work out how many parallel strands of wire you need to give the area of heating needed. i.e. what spacing between the wires to give a constant heat over the whole surface of the sheet. This gives the total wire length needed.

Send me a PM if you need more info. Good luck.

Kind regards

Neil P

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chiloschista
17 hours ago, CTR said:

Hi Ric,

I strongly suggest that you keep the voltage low! touching 220v is likely to be lethal. On the other hand, 48v = safe.

Another option would be to hire a 110v transformer use for site work by builders etc. Not as safe as 48v but certainly more healthier than 220v :cry: 

...

Hi Neil,

thank you very much for that detailed answer. I know barcs forum is full of knowledge.

Open 220V wires are worrying me a bit too, so 48V is a solution I will definitely take in account.

Another solution could be using IR heater tubes, so almost everything is out of reach, but would find a source where I can eventually buy spares in the future.

Or I could use a finished IR heater, but then I have to deal with modifying it and build a box around. But I don't feel comfortable with those two way.

The DIY solution would be the more clean, if well designed, but needs to start from scratch.  I'm searching the net for 48V power supply and other hardware suppliers now.

Thanks for the links, both interesting.

Best regards,

Ric

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Hi Ric,

Sorry to have given you a link to the UK site for Wire. Didn't spot your location until later.

Look for a simple transformer rather than a power supply, it will probably be cheaper. You don't need DC.

There are also 24v step-down transformers used for site lighting. If you can find a 1kVA one of these, it would be good.

Kind regards

Neil P

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chiloschista

Hi Neil,

no problem at all! I order from abroad on a regular base and from UK is very easy. The most detract VAT, so I don't pay it twice, and I get stuffs in less than a week. It even happens that postage is same or less than from Switzerland itself.

Good point about DC. Avoiding expensive toroid ones, I found that one, which is reasonably priced and seems to match. I still have to compute the needed power to cover about 40cm x 50cm, but I guess that 1000W could be enough. Room left for the vacuum pump. Heating surface could be a little smaller. I'm wondering how well and how quick does NiChrome wire get hot.

Best regards,

Ric

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Hi Ric,

Sorry for delay in reply.

Nichrome heats quite quickly as it has low thermal mass. Probably less than 10 seconds.  The PSU is low cost compared normal and looks OK for the job.

Maybe 20mm spacing between wires and allow about 30mm vertical space between the wires and the plastic. With the wires in the 50cm direction, you need 20 parallel wires, = 10 meters of wire in total.

Based on 48v at 20 amps, divide this into 5 separate wire lengths, each wire carrying 4 amps. Each wire will be 2000mm long, each wire needs a resistance of 12 ohms. This is 6 ohms per meter which is about 25AWG,  26SWG or 0.456mm diameter size wire.

This will heat to about 550 deg C. The wire will get a little longer as it heats. 

I would use a piece of wood about 700mm x 600mm, with nails placed in 2 rows parallel to the 600mm edges, about 25mm from the edge and spaced 20mm apart. Use 75mm nails, with about 50mm between the nail head and the wood surface. Use extension springs under the heads of the nails with the nichrome wire looped through the free ends of the springs. This will keep the wires tight and stop too much heat reaching the nails and setting light to the wood! each wire will zig-zag 4 times. Make sure the wires are kept separate. Join 1 end of each of the 5 wires together and attach to the 0v output of the PSU. Join the other ends together and connect to the 48v output. Don't connect via the nails or the springs will get heated too.

May need to put aluminium foil over the wood to reflect heat but make sure it doesn't touch the nails!

Good luck with the project.

Best regards

Neil P

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chiloschista

Hi Neil,

useful data again, thanks!

Unfortunately the transformer is actually out of stock, so the project is delayed.

In the meantime I eventually was able to build a canopy in composite, so I should be able to finish the Red for the beginning season, or at least I hope so.

Best regards,

Ric

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