Page 278 - Injection Molding Advanced Troubleshooting Guide

270 28 High Fill Pressure

pressures. Also if a mold requires a 3/8-in nozzle orifice to match the sprue open-
ing that is the tip that must be used, or otherwise again extra pressure drop is
added to the system, which will impact how the material fills and packs.
Some molding shops will require a tip change for every mold change to ensure that
this step is not neglected. One trick is to tack-weld a 7/8-in nut onto the mold to act
as a tip retainer; this will provide a visual check to see if the tip has been removed
from the side of the mold and hopefully installed on the machine.
Investigate methods for identifying tips when installed on the molding machine.
This type of approach will make it easier to determine if a nylon tip or a full taper
tip is on the press after the stamped markings have been “carboned up”. Also if the
tip is marked 360 degrees around it will always be easier to see directly when the
purge guard is opened.

Another factor to keep in mind is that nylon tips are much more restrictive than a
full taper nozzle with the same orifice. A nylon-style tip has a long taper from the
orifice to the break point, which leads to a much smaller minimum diameter that
the plastic must flow through. If a nylon tip is installed instead of the required full
taper or general-purpose tip the increased restriction will lead to higher fill pres-
sure as well as higher shear rates through the restriction. Figure 28.1 shows how
the nozzle orifice tapers to a restriction depending on the tip style.

Figure 28.1 Cross section of general-purpose, nylon, and full taper nozzle tips (left to right)

28.3.3.2„ Machine: Nozzle Style
In plants that use mixing nozzles there is opportunity to leave a mixer on the press
when changing to a job that does not require it. The increased pressure drop
through a mixing nozzle can be very large; for example, see Figure 28.2 for actual
data comparing a general-purpose nozzle, a mixing nozzle, and a mixing tip. The