Hot bending the polycarbonate-sheet for speaker backpack

Here we have placed our hotbending machine on the joint that is to be bent.

We used aluminum-foil and paper to isolate material that we didn’t wanted bent.

The corners seems to match!

Vilse removed some plastic left from the CNC.

We used our laser-cutted acrylic angeles to bend the polycarbonate-sheet in to the right angle.

We placed this böp to let the joint cool down

15 seconds left of our 4:25 timer, the heating of the polycarbonate is almost ready.

We used straps to hold the last bend

The last bend is a round corner, the radius is 200mm. We knew it would be very hard to bend it.
We heated like 100mm of the polycarbonate with both hot air gun and the heat bending machine. We heated the area for at least 20 minutes, it wouldn’t bend.

At last the polycarbonate was bending, but it turned out ugly as hell. A lot of bubbles and ugly artifacts, at least this side won’t be that visible.

Our heating machine turned out to be far more superior to the hot air gun.
Lesson learned, never make round curves in polycarbonate!

This is what the backpack will look like. The 12kg weight of the polycarbonate didn’t feel heavy at all. I think the weight will be ok.

To hold the corners of our speakerbox, we use a two-component poly-urethane glue (3M Scotch-Weld Urethane Adhesive DP-610). We got a test-sample from KA Olsson and it worked really great. It’s quite expensive, but we got it a little bit cheaper thanks to KA Olsson.
We used straps to tighten the polycarbonate around the corners. We also applied tape at the edges to stop the glue from seep out.

DIY Heat bending machine

As me and Vilse were picking up our CNC-milled polycarbonate sheet for our speaker backpack we noticed the heat bending machine they used at Plastmästarn:

We asked them how it was working, apparently it was a heating wire set for a fixed temperature. They didn’t know how it worked, just how to work it.

The profile on the bottom was set for 210° Celsius, and the one on top for 180°.

They placed a polycarbonate sheet around 5mm on the heat bending machine for about 4 minutes, after that it was ready for bending.

Here is two other commercialized heat bending machines:

It’s really hard to bend stright angles in polycarbonate with a heatgun.

Me and Vilse started to think of a way to make our own heat bending machine.
We bought some Nikrothal from Elfa

2.83 Ω/m

We angle-grinded a steel-profile to a U-shaped profile.

We cut holes in to the profile and placed isolated plates to hold the nikrotahl-wire from shorting.
The isolator plates were found in a old salvaged heatgun, you could find equivalent in a old toaster.

We placed the wire in a U-shape inside the profile. Half way in to the U-shape we attached a spring to make some tension on the wire. As we heat the wire the spring will retract around 20mm due to the expanding wire.

A total of 1,54 meters of wire give us 4,36Ohm. At 30V it will give us 220W of heat energy.

To make a even distribution of the heat we made two heating beams, on for the bottom and one on top.

We tested out the heating properties and it turned out pretty good. We got about 200° Celsius under the material.

It turned out great! We tested out different kind of thickness of polycarbonate, it was very bendable. It’s much easier to get a straight lines with this method compared to the heating-gun method.

One important factor to get the bending look clean is the timing. If the timing is too short it will be hard to bend and the plastic might break. If it’s to long bubbles will appear.

Material Heatingtime [minutes:seconds]
Polycarbonate 8mm 4:00
Polycarbonate 8mm with V-milled 90° angle 3:45
Polycarbonate 10mm 4:30
Polycarbonate 10mm with V-milled 90° angle 4:25
Acryllic plastc 3mm 1:30