polycarbonate

Hello! could you help? I am injecting a plastic ball (2 halves) made out of polycarbonate,the mould temp is 105c,melt temp is 320c and the mould is a hot runner with temperature controllers set at as follows CAVITIES 1 AND 2 340C CHANNEL 340C AND NOZZLE 340C.The problem is that the ball when assembled by ultrasonic welding ,cracks at the injection point.Could this be because of stressin the plastics as when the parts are viewed with a polarizing filter there are lines coming out of the injection point while the remainder are concentric circles. Any tips on hoew to reduce the breakage?

 

[gambro]

Yes, your problem is most likely internal stresses due to the injection miolding process. To reduce stresses do the following (1) make sure the material is properly dried (2) try to increase cylinder and mold temperatures (3) try to reduce back pressure. If these measures do not lead to satisfactory results, anneal the parts in an dry air oven chamber, appx 130°C for 30-60 minutes. You will see that the internal stresses (birefringence markings = concentric circles under polarized light) will be reduced. If you find warpage in your parts, reduce annealing temperature and increase annealing time. For large-scale production, you can also use "mid-range" infrared lamps. The irradiation wavelength maximum of the lamps must be between 2 and 3.6 micrometers, since the optimum absorption wavelength of polycarbonate is 2.5 micrometers. The lamps should be placed about 100mm from the parts. The surface temperature of the parts should not exceed 130°C (warpage!). An annealing time of appx 50 sec is sufficient using IR-lamps.