Acrylic Bender

This weekend I decided to make an acrylic bender. I got the idea from a video on the internet.



Construction:

I bought a cheap hairdryer ($18) and removed the filament. I made the following measurements with a multimeter across the mains lead:
  
Fan

  0
 1 2
  0
 inf115
95.5
 Heater 1 inf 70.6 52.3
  2 inf 61.6 42.9

All in all, I was impressed by the design of the hairdryer. The heater seemed to run in series with the motor, there was what looked like a bi-metallic thermostat that would cut out if the outlet was blocked, and there was a non-resettable thermal fuse as a last resort. It seems like that it would be quite difficult to start a fire or do damage with this.

The construction consisted of having the heater elements inside U-channel aluminium. To either side I screwed a wooden board to make a flat work surface.

I cut a piece of aluminium U-channel to length, then used hot-glue and scraps of wood to seal the ends up. Mixing some plaster (50g plaster, 35g water) and pouring it into the trough made a nice heat resistant, electrically insulating base to run the wires on.

How much resistance to use?:

I did a brief survey of commercially available acrylic benders on the internet:
 21" unit
 200 W 375 W/m
 33" unit
 375 W 375 W/m
 12" unit
 125 W 410 W/m
 24" strip heater
(Seems to rely on contact though)
 55 W 90 W/m

My unit has approx 14" (35.5cm) of usable length, and I intended to use a 24V power supply.
  • For 375 W/m this requires I=5.55A and R=4.3ohms
  • For 200 W/m this requires I=2.95A and R=8.13ohms
  • For 100 W/m this requires I=1.47A and R=16.3ohms
I wasn't sure how much power to use. Odds were the contact heater would be too underpowered for my application, but the commercial ones would be overkill.

I decided to start with 16 Ohms and work from there.

Photo of the parts:



Positioned the filament inside the plaster trough, and secured the ends of the cables with wooden wedges:


Use:

I used a 24V lab supply for power. It's nice to have a current adjustable version, but for a project like this it's not essential. Using a 24V supply is really, really preferable to using mains on this one.

One more thing to consider is that when it heats up, the filament expands dramatically. Since in this case I have it in a conductive channel, that's a little alarming. Pay attention to the length of cable you need.

First I ran with about 16 ohms of resistance (equivalent to the 90W strip heater above). This heated up a bit, but was way too cold for bending acrylic. To give you an idea, I was able to hold my hand flat across the opening for more than 15 seconds.

I powered down and changed the length of the filament to be about 8 ohms. This worked much better.


The result:

After leaving the acrylic strip above on the heater for about two minutes, I was able to bend the right angle below fairly easily.

I was also able to bend a larger sheet (~20cm wide) in a right angle too. However when I tried to heat it up and bend it flat again I cracked it. Opps.


Stuff to do:

  • The filament keeps breaking on me. One thing to do is make the connections more sturdy.
  • Also, it takes a fair bit of time before the acrylic gets hot enough to bend. I may have to reduce the resistance a smidgen more.

It may be possible to solve both these problems by just using some straight nichrome wire. Unfortunately this means I send an innocent hair-dryer to its doom for no reason.

My previous method for bending acrylic was a heating gun, and would often leave bubbles and discolourations in the plastic. This works much better.
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