Definition: Nozzle; 1. the orifice fitment at the delivery end of an injection cylinder.
The machine nozzle is what mates the barrel/ plasticizing unit of the injection molding machine to that of our mold. Its basic function is to convey plastic though it into the mold with minimal pressure and temperature lose, and not leak. Further the orifice of the nozzle should always be slightly less in diameter than the O dimension of our sprue bushing. The radius of the two should match so as to create the seal. The nozzle tip and sprue O at the junction or inside the nozzle (nylon tip) is where the sprue breaks or is suppose to break, if the material is not set enough at this point stringing occurs and if set up too good, the nozzle freezes off.
The nozzle itself screws into an adaptor which is bolted to the barrel and forms the end cap on the barrel of the plasticizing unit. The adaptor has a conical inside shape so that the screw tip conforms to but does not touch the side walls when fully forward. This is the area where the 2" (50mm) diameter is tapered down to the .250" diameter nozzle opening on the seat area. Note that this is speaking generally and each and every machine mfg is different. What is important is that the seal area between the nozzle and adaptor is doing just that and sealing. If in fact there is a step and or dead area we may have issues later in the manufacture of parts due to this dead area and the type of polymer we are processing. To take a term from extrusion we wish to have a streamline flow of material thus no hang-up spots.
Nozzles come in a variety of lengths, orifice diameters and types. The types and configuration of nozzle assemblies is numerous, there are 1 piece nozzles, but 1 piece nozzles of nylon type, taper and straight through. Than there are the 2 pieces or body and tip type which allow the very tip to be changed instead of the body and tip as 1 unit. There than are those nozzle which on the body of said nozzle are either externally heated or internally heated, with and without the thermocouple. The concern here is to note which one is being used and document that to the mold. What needs to be remembered is that the nozzle should be treated as part of the mold. This is not to say each mold has its own nozzle, (which would be nice) but that the nozzle configuration is married to the mold.
This configuration includes the following:
1- orifice diameter
2- Type; nylon, straight through, taper
3- Length
4- Heater band location, wattage, along with thermocouple location
5- End radius on nozzle.
Though the above is a short list, that which defines the nozzle assembly should be recorded for that mold and should be in the mold /project set up guide.
Much as we move a mold from machine to machine, expecting the same results the nozzle configuration / type for that mold must be used to get the same results. Too many times in production the mold is moved but no documentation is written for the nozzle and or type. The result being that the tool does not run at projected yield and process on that machine.
Two other points in this documentation are the temperature of the nozzle, and the pressure drop through our nozzle. In the temperature arena we only need to maintain our melt temperature, thus it appears easy to set our temperature. But at the same time we have contact to a metal mold which is now drawing heat away from the tip of our nozzle, which in the worst case can cause it to freeze off. This concern typically leads the processors to increase the nozzle temperature, insulate the nozzle from the mold, and or possibly to use the nozzle break function on the press.
The other issue in temperature is heater bands and location. If the body is internally heated than no changes, but if externally heated with bands there is location and wattage of band along with the location of our thermocouple.
An example would be a long nozzle body and the heater band is towards the rear, the front than has more of a chance to freeze off, due to various thermo activities going on. In most cases adjusting the nozzle band forward allows for heat to be placed at the tip to maintain the opening of the nozzle tip. But with certain materials it may be found that the material is not setting up and the band is moved back to take care of this issue.
End of part 1
Steven L Silvey
silveysplastics@hotmail.com
