Page 393 - Injection Molding Advanced Troubleshooting Guide

40.3 Short Shot Troubleshooting 391

1. Nozzle type. A mixing nozzle creates much greater flow resistance than a stan-
dard free flow nozzle. Mixing nozzles have static mixing elements inside them
that are intended to improve the color mixing of the plastic. Mixing nozzles are
really a Band-Aid to compensate for poor mixing as the material travels through
the screw. As with many things there is a trade-off to the improved mixing that
the mixing nozzle provides: negative aspects include increased pressure drop
and potential material degradation.
2. Length of nozzle or nozzle tip. Establish the habit of using the nozzle and nozzle
tip of the shortest length possible. The normal need for increased nozzle length
is due to a hot runner with a recessed sprue. If the mold does not require an
extended nozzle then do not use it. Longer nozzles and nozzle tips can impact
pressure drop and will often have issues with proper heating. An example would
be using an 8-in nozzle with a 4-in heater band; the result will be impacted by
location of the band on the nozzle especially in relation to the thermocouple.

3. Nozzle tip style. The three most common are the general-purpose (GP), nylon,
and full taper tip. There are also mixing tips on the market. The different tip
styles will produce different pressure drops, different shear effects, and risks of
contamination. A general-purpose nozzle has dead spots where the nozzle tran-
sitions into the nozzle orifice and can lead to contamination.
4. Nozzle tip orifice. The orifice of the nozzle tip should match the orifice of the
sprue bushing on the mold. Normally a nozzle tip orifice that is 1/32 in smaller
than the sprue orifice gives the best results.
During process development document the nozzle and all specifics including
length, orifice, and style. All of these specifics should then be verified during every
following mold run, because they are some of the details of molding that can lead
to a great deal of problems. In molding it is important to “sweat the small stuff”
because really there is no small stuff when replicating a process.

40.3.3.3„ Machine: Worn Barrel
Sometimes what appears to be a defective non-return valve is actually a worn bar-
rel. To determine if the problem is the non-return valve or the barrel try moving
the shot size and transfer position back 1 in and running a shot. If the machine
holds a steady cushion at the larger setting it is an indication that the machine has
a worn spot in the barrel.
Use of glass-filled materials will lead to increased barrel wear. There are cases
where glass-filled material running in a low hardness barrel lead to a worn-out
barrel in less than six months. Barrel selection for glass-filled materials should
lean toward bimetallic abrasion-resistant materials up to and including bimetallic
carbide barrels and CPM-10 V high wear resistant barrels. For proper barrel selec-
tion determine what material will be run and choose the proper hardness.