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A Basic Plastic Injection Molding ProcessBarrel Temperature Control, Nozzles & Nozzle Tips
Custom Injection Molding - Barrel Temperature Control Each zone of barrel heat is individually controlled and range from a couple of zones to 6 or more depending on machine and barrel size. They are controlled through the use of thermocouples which are attached in a central location of each heating zone and provide feed back to a controller which turns electricity on and off to the heater bands, maintaining a constant stable temperature for your process. Thermocouples come in two types on most injection molding machines, types J & K. It is always important to make sure you have the correct type of thermocouple when replacing these or you can have process issues due to inaccurate temperature readings. Same thing goes for heater band replacement. If you replace a 400 watt band with one half the size or double the size, it can dramatically affect your processing. You also need to pay attention to the voltage of the heater bands you are replacing as they typically will be one of three voltages which are 120, 240, or 480 volt. These bands are not simply interchanged. I have seen 120v heater bands hooked up to 240 volt circuits, and turn cherry red before they failed, which can be very dangerous. The opposite can also be the case where a 220 volt band is used in place of a 120 volt band and though these will not turn cherry red, you might never get enough heat to move plastic through it.
Nozzles and Nozzle Tips The nozzle and nozzle tip are the exit point for the plastic resin from the screw and barrel on it’s way into the mold. Nozzles come in a multitude of lengths and designs and you will need to know which is right for you and your process. The most commonly used are the two piece nozzle which incorporates a separate body and tip. The reason for this is that you have more versatility for use between the most common sprue bushing radii, which are ½” and ¾”, and also different material types which may require different tip styles such as a continuous taper or reverse taper for nylons resins and other high crystalline materials. It also allows you to easily change the tip opening size which can vary from mold to mold. You always want to use a tip orifice size that is equal to or slightly less than the sprue bushing orifice in the mold to aide in leak prevention between these two areas. If you use a nozzle tip that has a larger orifice size than your sprue bushing, or is damaged, be ready to replace some heater bands as at some point you will likely leak plastic between the tip seat and the sprue bushing sea. This will then leak back over and cover your nozzle and eventually the barrel if it’s not caught in time, which causes unnecessary down time and cost to replace destroyed heater bands and thermocouples.. It is very easy to change tips during production with this type of nozzle either due to a radius change or if the tip has been damaged in any way. Nozzles and nozzle tips come in a variety of flow through styles as well, such as general purpose, continuous taper, and a few others.
Written by: WM8C, July 28th, 2006. Not for use without written permission
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