Page 167 - Injection Molding Advanced Troubleshooting Guide
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156 17 Brown Streaks
17.3.1.4 Molding Process: Back Pressure
If processing with excessive back pressure it is possible to degrade a material,
which can in turn lead to brown streaks. Typical back pressures of 1000–2000 psi
of plastic pressure are normally used. Evaluation of higher back pressures may be
required; sometimes high back pressures are used to improve mixing of color con-
centrates but this may lead to degradation of the material.
17.3.1.5 Molding Process: Recovery Speed
Running high recovery speeds can generate excess shear, which in turn can over-
heat the plastic. The ideal recovery speed should be set so that the screw recovers
2–3 seconds prior to mold open. If the screw is recovering very quickly the excess
shear and residence time at melt temperature may lead to brown streaks. Always
keep in mind that shear is the key driver of the melting of the plastic and excess
shear can overheat the plastic.
17.3.2 Brown Streak Troubleshooting Mold Issues
The main mold-related causes of brown streaks are:
Thermocouple location
Manifold leaks
Lubricants
Bleeding of mold saver
Hot runner insulator gap
Sprue bushing seat
17.3.2.1 Mold: Thermocouples
The thermocouples in a hot runner are the key components to maintaining proper
temperatures. If the hot runner manifold does not have enough thermocouples it
will be impossible to have uniform heat distribution in a hot runner manifold. On
long hot drops it is also key to have good thermocouple distribution. If only one
thermocouple is covering an excess length of manifold or drop there will likely be
areas that will run much hotter than the set point, which can lead to overheating of
material and brown streaking. It is also important to ensure that thermocouples
are properly seated in the thermocouple well to get an accurate temperature
reading.
