Wishing to make two 30 degree bends in a small acrylic sheet, 2mm thick, I set about
attempting this as one would for a small metal sheet having been given to understand
that it was possible to bend the material cold, provided that a large internal radius
was adopted. I tried bending it around a 12mm dia. bar whilst holding bar and sheet
in the vice but the sheet snapped. Knowing that if heated it would bend with ease,
I began to consider this as a project for a small bender.
Hot wire heater
The secret lies in the way that the sheet is heated only along the bend line. The
one that I had made for making the shrouds whilst in industry used a single strand
of resistance wire. This, when having an electric current passed through it, warmed
up and heated the plastic sheet along the bend line. Only a few volts was required
so was electrically quite safe. The wire does get very hot, but not red hot, and
could cause minor injury. However, as will be seen it is not that easy to gain access
to it and having very little bulk cool's down rapidly when switched off.
From memory the wire used in the original was in the order of 1.2mm (about 18swg).
Unfortunately, the only resistance wire that I could obtain easily was 28swg (about
0.4mm dia). Whilst the same temperature could be attained using a lower current,
there would be less heat for a given length and at best would take a very long time
to heat up the plastic, maybe never getting it sufficiently hot. With this in mind
I considered winding the wire in the form of a spring as this would concentrate more
heat into a given length.
It was with this in mind that I considered using a length of spring that I already
have available in various gauges and diameters, albeit these were wound with spring
wire rather than resistance wire. The idea quickly developed into the thought that
perhaps a length of spring curtain wire would suffice, and already having some available
nothing would be lost by giving it a try.
Battery charger power source
With a car battery charger and a length of curtain wire about 250mm long, a very
rough and ready set up was established. The charger had outputs for 6 and 12 volt
batteries with additionally high and low charge rates. Its ammeter had a red line
at 4 amps and was calibrated to 5 amps. As the process of heating and bending the
plastic sheet should take no more than a minute or two I was not too worried if I
stretched it a little beyond five amps, provided I gave the charger time to cool