Definition:  Nozzle; 1. the orifice fitment at the delivery end of an injection cylinder.

The issue of pressure drop through our nozzle is important to the overall success of our processing.  Should we have a process that is using a majority of the pressure on the press than anywhere we can save (gain) pressure is important.  What this really means is how much pressure can a machine generate, and how much are you using and where? If for example our machine generates 20,000 pounds of plastic pressure and our need is for 15,000 pounds of pressure at fill (to maintain fill time) that leaves us with 5,000 pounds available.  If the machine has a delta P of 3,000 pounds, than we could only afford to have a loss through the nozzle of 2,000 pounds.  This is where length of nozzle comes into play when in fact we match orifice and type of nozzle.  This is usually not much of an issue until the machines are starting to get maxed out.  The only caution here is what safety factor have you built in to the process.

Nozzle orifice size is the key, given that we can create shear in the nozzle prior to going into the feed system of our mold.  I have actually downsized nozzles to achieve greater shear on the materials so as to fill parts, and in fact had a client open his nozzles once so as to be slightly smaller than the "O" of the sprue bushing only to find out that the shear created in the nozzle was assisting him in filling the parts.  (Note that in most processing information it states that the nozzle orifice should be slightly less than the sprue "O" dimension.)   In fact in multicavity tools this is critical given that the volume of material flowing through our nozzle can be 10 to 20 times more than what flows through the gate orifice into our part. This is a point often forgotten when going from the prototype tool to the production tool and then finding an issue with splay and or degradation of the materials.

The orifice size is also important to the fact of when does it freeze off or does it freeze off?  If anyone has ever fought with a stringing of material from the sprue and nozzle this could be one of the issue's. It is important to understand that when the mold opens on a 2-plate cold runner mold system that the sprue must release from the nozzle so as to be ejected with the parts.  Though technically we are breaking this joint, with a puller in most cases, or in those difficult cases using sprue break, what is trying to be done is create that break joint.

The Nylon nozzle or reverse taper nozzle has a larger opening at the tip than back inside the nozzle itself. This allows for the break point to be a bit warmer, and smaller in size thus being able to break with nylon.   This also brings up the point that shear created by the nozzle opening is less than the shear created inside the nozzle due to this reverse taper.  The other issue that need be address here about nylon nozzle is it a 2 piece type, with body and tip or a 1 piece type as they are designed differently and though having the same orifice and length may have different pressure drops.  A good reference to look up is with any suppliers' of nozzles. An example would be IMS supply ordering page/ description: http://www.imscompany.com/pdf/202-211.pdf

In trying to summarize about nozzles it is important to know what we are using, from a design point of what is going on inside of it and how we are heating it. They are little things that can create many opportunities for the processors and others trying to trouble shoot issues with finished parts.

SL Silvey

360-882-3183

silveysplastics@hotmail.com