Thermoforming with a Bending Bar
The part to be bent needs to have a 90 degree groove routed in the material, which will form the finished bend location. The groove should be cut to a depth that leaves about 0.100”-.050” of material (more thickness for a rounder corner, less thickness for a sharper corner).- Preheat the bending bar to a temperature of about 300 to 350 ºF when bending high density polyethylene. Adjust the temperature for other materials.
- Place the part on the bending bar and apply a weight of about 10 to 15 pounds per foot of the part being bent. For example, if bending a 24” part, a weight of 20 to 30 pounds needs to be applied to the part uniformly to hold the part down to the bending bar. This weight may need to be adjusted if the temperature of the bar is warmer or cooler, but the weight is needed to force the plastic in complete contact with the bar.
- The part needs to be in contact with the bar for 60 to 120 seconds depending on the temperature of the bar. The best way to tell if the part is ready to be removed from the bar is to look for a bead forming under the part where the plastic meets the bar. When a small bead forms the full length of the part in this location on both sides, then it is ready to be removed.
- Pull the part off the bar by grabbing one end and pulling up, peeling the part off the bar while holding the bar down securely. The plastic will tend to stick to the bar, a small amount of molten plastic left on the bending bar is common.
- After the part is removed from the bar, immediately bend the part by placing it in a fixture or jig which will hold the part in its final position. It is important to hold the part in the jig for a minute or two to allow the molten plastic to cool. Allow enough time to complete the cooling. Sometimes the part will relax slightly after removing it from the jig and it may be required to over bend the part to get the final bend correct. Some experimentation may be necessary.
Forming a radius can also be done by using a heat gun, using a back and forth motion across the length of the sheet both top and bottom until the material is soft enough to start bending. Keep the gun at a distance of at least 8” to 12”, do not get the heat too close to the material as blistering may occur. Be aware, the thicker the material the longer it will take to form a desired radius, and scoring of the thicker material may be necessary. Once the material has been bent to the desired angle, clamp into place for at least 24 hours.
