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Injection molding materials drying tips

As far as drying goes you could dry the material for 12 hours but you need to make sure that the hose that goes from your dryer to your hopper is not too long. I have seen cases where the mold will run fine for a while and once you get into a cycle the material has a chance to sit in the hose and gather moisture. If the problem is a burning issue you may want to make sure that your nozzle orifice on the injection molding machine matches the orifice on the hot runner. If your injection molding machine orifice is larger than the one on the hot runner you are causing shear heat when the material passes over the edge of the hot runner inlet. Other factors that the hot runner may be causing is degrading of the material in the system.

You need to make sure that you follow the proper start up and shut down procedures recommended by your supplier. You should also make sure that the hot runner supplier sized the flow channels correctly and you are not holding too much material in the system allowing it to either degrade the material or the colorant. This could also be happening if the system is allowed to sit and the material is cooking in the system. If your melt channels are too small you may be creating too much shear heat. You should be able to determine this by the injection pressures that are required to fill the parts.

I have conveyed PC long distances with stainless piping and morris couplings. If the dryer feeding system does not have sufficient cleanout time or there are leaks in other systems running off the line, then the residence time can cause moisture absorption. This was particularly difficult thing to diagnose but sometimes I would see higher incidence during spring or fall months where the temperature would drop in the early evening spiking the relative humidity. Do not be fooled thinking it is closed loop system and vendors claiming purging air to prevent this issue. Air isolation and cut off at the hopper are critical to make sure there is no material trapped in the lines to absorb moisture.

Also, leaking cutoff or flow dampers in the hopper when feeding can cause the material to starve the screw. As the vacuum during feeding pulls air from the feed throat hindering the pellets to fall be gravity. This forces air down the barrel and traps oxygen/moisture which appears to be drying or wet material. Sometimes this can be noticed as the plasticizing time increases.

Also, you can run hopper feed zone chilling water. To avoid bridging in the hopper throat when we process PC at higher temperatures, I will run the water temp at the hopper feed to 60-90C. If the water is to cold, you can be drawing moisture or condensation depending the ambient humidity and temperature. I have also had instance where there has been a seal failure and even a cracked hopper feed section that was allowing moisture to slowly seep into the feed section. If the temperature is too high, then you can experience pellets sticking together and bridging in the feed zone or pre-melting early, this can cause excessive back-pressure and push air down the screw compression section with no ability to vent.

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