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300 31 Flow Lines
31.3 Flow Lines Troubleshooting
When troubleshooting flow lines, a short shot analysis can be a significant benefit.
To understand the cause of a flow line, the starting point must be determined.
There will be cases where a flow line appears to be located with no features around
it. Short shots can often show that the initial starting point of the defect was actu-
ally further upstream in the flow path.
Often people do not spend enough time looking at short shot progression on parts.
Even when conducting a shear viscosity study many people fail to look at the parts
but rather just document the process results. Examination of short shots and flow
progression will often highlight areas of concern in a mold.
Always eliminate the other possibilities for flow lines; it is frustrating to hear that
someone worked on a flow line for hours only to find there was a scratch on the
mold surface. Be observant of scratches, color streaks, and splay to ensure that the
correct problem is being addressed.
31.3.1 Flow Lines Troubleshooting Molding Process Issues
Process settings that may lead to flow line defects include:
Fill velocity
Fill only weight
Melt temperature
Mold temperature
31.3.1.1 Molding Process: Fill Velocity
Fill velocity can cause flow-related defects if the velocity is too low or too high.
Always verify that the actual fill time and fill only weight match the established
process.
A fill velocity that is too slow can lead to hesitation lines in the molded part that
indicate where the part struggled to fill. A classic scenario is where what is referred
to as record grooves are generated, where the melt takes on a track-like appear-
ance that looks similar to record grooves (see Figure 31.1). A slow fill velocity can
also produce meld lines in the flow front that become obvious from a visual stand-
point. Try filling the mold faster to see if the flow defects change. During process
development observe parts made during filling experiments to determine what fill
velocity produces the best results.
