Page 313 - Injection Molding Advanced Troubleshooting Guide

31.3 Flow Lines Troubleshooting 307

31.3.4.2„ Material: Viscosity
All materials will have a natural variation in viscosity lot to lot. This variation can
be compounded when running wide-specification materials and with the addition
of regrind. If a defect shows up immediately after changing to a new lot of material,
try switching lots again to see if the defect improves. Keep in mind that the change
in material may also be linked to moisture content and not a true material problem.
If running with regrind try running with 100% virgin material. Regrind can in
cases raise or lower the viscosity of the material and may create flow-related de-
fects. Viscosity can increase with regrind use because some of the lubrication
package may have been degraded during the original molding process. Viscosity
may drop because the material experienced a level of breakdown during pro-
cessing, resulting in a smaller average molecular weight of the molecules. Do not
assume that regrind will always have the same impact with all materials and pro-
cesses.

31.3.4.3„ Material: Filler Content
In general terms, the higher the level of fillers added to the plastic the more likely
flow defects will be seen. During the design and specification stage of the project,
evaluate if a 15% glass-filled material will work instead of a 30% glass-filled mate-
rial. Higher filler contents will create a higher-viscosity material resulting in more
challenges filling a given mold. Also when using filled materials the fillers may
come to the surface of the part and highlight flow issues. Metallic flakes are notori-
ous for showing distinct flow lines in complex geometries (see Figure 31.4).

Verify that the correct filler content is being used for a given material. If using a
colorant additive with filler content ensure that the correct letdown ratio is being
used. When using a volumetric additive feeder, the letdown ratio is based on a for-
mula for a given auger size, so make sure that the correct auger size is being used
for the job that is running.