Page 412 - Injection Molding Advanced Troubleshooting Guide

41.3 Sink Troubleshooting 411

Part of the process setup must be verifying nozzle style and tip type/size. If the
nozzle or tip is wrong it needs to be replaced before further troubleshooting efforts.
The pressure drop of a mixing nozzle will be very different to that of a general-
purpose nozzle. Do not assume that the nozzle and tip are correct when trouble-
shooting.
A simple trick that can help ensure that the correct tip is used is to tack-weld a
7/8-in nut on the side of the mold and then thread the required tip onto the nut.
Every time a mold is set in a machine the tip is swapped. Anyone can walk up to
the mold and with a quick look determine if the nozzle tip has been removed from
the mold and presumably installed on the press.

41.3.3.5„ Machine: Gas Assist Not Working
The full details of gas assist injection molding are beyond the scope of this book;
however, this technique must be considered relative to sink defects. If the part is
molded with gas assist and is displaying sink problems start troubleshooting the
gas assist side of the process first. Some factors to check include:
ƒ Fill only weight
ƒ Gas inject pressure
ƒ Gas pin function
ƒ Overflow function
ƒ Nitrogen level supplied
ƒ Gas booster working
ƒ Gas timing

41.3.4„ Sink Troubleshooting Material Issues

Some potential material-related causes of sink include:
ƒ Viscosity shift
ƒ Nucleating agent
ƒ Blowing agent

41.3.4.1„ Material: Viscosity Shift
If the viscosity of the material varies enough it will impact the pressure drop
across the cavity. Viscosity can vary due to variation in the incoming material or
due to variation in moisture content (on a hygroscopic material).
A significant change in viscosity normally will also result in a change in transfer
pressure. If the transfer pressure is running high compared to normal, the viscosity
of the material may have increased. Verify the material incoming certification and
check moisture content.