12-19V strip heater for bending plastics

[Vincenzo]

You'll have a devil of a job unwinding a used toaster element and getting the wire straight.   The wire will be very brittle due to repeated cycling to bright red heat and oxidization.   The hair drier element probably didn't get as hot, but will be wound in a small diameter coil so is likely to be equally hard to straighten.  You *may* be able to use it still coiled if you support it with a tensioned glassfiber cord through it. 

Buying some 'virgin' Nichrome or stainless steel wire may well be your best option as you can then choose its diameter for a resistance per unit length appropriate for your desired heater power and supply voltage.  You should be able to get a 'feel' for the max heater power required from the specifications of the commercial units you've been looking at.

A 10A rated DC-DC buck converter module would give you a lot more flexibility so you can adjust the voltage for the temperature you require.  Assume a 1800W hairdrier.  At 240V its heating element draws 7.5A.   You probably don't need to drive the element so hard so a 5A buck module may well be sufficient - you wont know till you extract the element, see how many parallel sections it has at full power and try powering a length of it to get a feel for the max current and voltage required.  You probably need no more than 1/3 the current required to reach dull red heat.  Don't expect a 'chinesium' buck module to deliver more than 1/2 of its nominal rating for long. 1/3 is a safer bet.

Great advice. Slowpoke is trying to jump from 1200 quid to zero. To avoid the cost of a converter, an atx power supply with the jungle of wires with each one of the output voltages tied together (or even cleaner, desoldered from the board and single thicker wires soldered instead), especially the 12V. Both the 5V and the 12V in all of them should have more than 10A. Trial and error with the length and current. Nichrome small reels are very budget friendly for some gauges that are commonly used in other applications. How about a timer (to turn it off after an experimentally measured adequate ON time) with a short sound beep for the operator to bend?

[Vincenzo]

Is high temperature (shiny red wire, pushing it to the limit) fast application better for acrylic or slow lower temp/amps?
or is there a sweet spot somewhere. I have never done heat bending before.

[beanflying]

In order to generate heat you need resistance in the wire and a current to pass. P = I² R

So the problem becomes with coated copper is it has a very very low resistance so to generate a set power you need a much larger current so it makes a very poor element. Nichrome is circa 100 times more resistive than Copper for example (varies with heat/purity and lots of other things). Physical melting of the wire when it is dissipating the power is also an issue. Several other options can be used including Kanthal, Stainless Steel (NiChrome alloy in most cases), Titanium and more.

So if you feed the 5A figure and the Temperatures needed (300C) and feed it into the Calculator from earlier for say a 500mm length of 20AWG (0,8mm) NiChrome  I was throwing around https://www.easycalculation.com/engineering/electrical/nichrome-wire-calculator.php you get some nice easy numbers of around 5V @ 5A or 25W. This is not a lot of power to heat in particular the thicker Acrylic so I am planning on two runs for about 50W over the 500mm and with a bit of a fudge I can fit three wires safely into the 12mm channel for 75W.

[SmallCog]

Is high temperature (shiny red wire, pushing it to the limit) fast application better for acrylic or slow lower temp/amps?
or is there a sweet spot somewhere. I have never done heat bending before.

Don’t want to be too hot, you need the top side to be pliable/bendy before the bottom side gets burnt

My commercially made bender takes about 100 seconds to get 3mm acrylic just right for bending

[Vincenzo]

Is high temperature (shiny red wire, pushing it to the limit) fast application better for acrylic or slow lower temp/amps?
or is there a sweet spot somewhere. I have never done heat bending before.

Don’t want to be too hot, you need the top side to be pliable/bendy before the bottom side gets burnt

My commercially made bender takes about 100 seconds to get 3mm acrylic just right for bending

Thank you, sir. Great numbers to experiment with to get to the "sweet point" with some voltage/current converter.

[Jwillis]

Acrylic will bend easily at 160C .

[beanflying]

Part of the non science bit is heating it through to that yield point on the far side as quick as you can without cooking it on the close side.

[beanflying]

Flushed out my current stash of heavier Resistance wires. 18AWG (0.9 measured so more like 19AWG) I will use Nichrome as it is what I have most of and TempCo doesn't matter at all. The Kanthal is the same diameter but harder to come by so I will save it.

First very crude test on 500mm of it 5V got to 4.4A on my bench supply. With a simple bead contact thermocouple I only hit 130C so more volts and amps need to get to the Acrylic yield point.

Breaking out some more muscle and heavier leads on my messy bench for more testing. Current on the Manson reads high but I will make a better rig and re run it with a calibrated shunt and properly terminated wires. The Croc clips I have with 4mm ends are getting warm at the 8A so not a good indicator.

Voltage is correct as checked on a DMM.
5V 130C
7v 180C
8V @ circa 7A (fudged for the reading) 230C

Looks like for the OP 12V on a 650mm'ish run will pull somewhere in the 250C+ range at 8A'ish so you should be fine (pending some more testing) with the PC supply and 18AWG Nichrome.

Playing with my newish Thermal Camera too the results might be a little questionable due to the surface emissivity of the wire. Still learning 

Seems like the calculator earlier is fine for electrical Calculations but off for Thermals too 

[Vincenzo]

https://www.ebay.com/itm/202945759465?hash=item2f408288e9%3Ag%3AYVgAAOSwfD1eexYC&LH_BIN=1
This is an amazing cheap little board that gives you constant temperature better than sensing indirectly through current. It comes with default hysteresis of 2 degrees C, but you can lower that. I use them in all cheap fan-less space heaters after replacing the little relay that they come with with a little bigger one and adding a tiny $1 12v  500mA  SMPS power sypply from ebay.

I once made these instructions graph for using their three button setup

[beanflying]

Temperature range isn't up to the 2-300C unfortunately. 300C is sort of in the area of K type thermocouples only and would be pushing RTD's and a lot of other sensors to get one that would cope.

I do have a pair of these fitted to some Fridges on my Mobile Coffee making bench as the built in controls were a bit rough on the fridges to keep the Health mob happy and stop the milk from freezing so it is a good board 

[Vincenzo]

are you talking about the little cylindrical "sensor" that comes with those, that it has an upper temperature limit it can withstand before it is damaged that is less than what is needed for this application?

[beanflying]

I have never looked at the sensor for ratings as I am running at 3-4C. Also I have never tried to program them to the top limit and I suspect you will find the board will likely be firmware limited to that 110C?

[Vincenzo]

where does the 110 number come from?
and I still don't get where the limitation is. I only imagined that the little sensor and it's wire will not do 300 degrees, but to the microcontroller, they are all numbers.
I see those who convert a toaster oven use a little heavier duty sensor that is sometimes threaded

[beanflying]

Read the evilbay add you posted  I would think this to be correct and mine are packed away in a container so I cant check. If you have one of your try winding it above 110C.

[Vincenzo]

"evilbay"
quite interesting
and kind of accurate

I just saw the range they posted on evilbay, sorry about asking.

But still, don't see why the sensor can be replaced with a more robust industrial kind of thing with same characteristics

[Marco]

What you like or don't about Crocodile clips and screws they work and have being for decades on model Aircraft Foam bows as a simply low tech adjustable resistor.

With undoubtedly some pretty bad burns along the way ... it doesn't matter whether you know those clips could be hot as hell, use it enough and it's going to slip your mind eventually and you're going to adjust them at the wrong time.

[beanflying]

  Yep Beers and having mates 'help' you make foam wing cores, the odd ouchy has happened over the years. These days I am spoiled for choice with adjustable power supplies so it is a non issue, it is just a quick and dirty option for those without like the OP.

Started my build I guess too. I had some 6mm Acrylic on the Laser so I cut the Angle bracket out and a test pattern in card to make a Template stack to route most of the hinge rebate to aid my average woodworking skills.