Page 473 - Injection Molding Advanced Troubleshooting Guide
P. 473
47.3 Weld Lines Troubleshooting 473
47.3.1 Weld Lines Troubleshooting Molding Process Issues
Some of the molding process conditions that impact weld lines are:
Fill velocity
Melt temperature
Second-stage pressure
Mold temperature
47.3.1.1 Molding Process: Fill Velocity
In general, if the mold is well vented faster fill velocities will lead to better weld
lines. A common issue is when the mold is not well vented and technicians will
slow down fill velocity, which will impact the weld line quality. If weld lines can be
“slammed” together they will typically be stronger and less visible.
Also with fast fill times the material will maintain a more consistent viscosity and
allow for improved ability to pack the weld line with second-stage pressure. Main-
taining consistent viscosity during first-stage fill helps minimize the pressure drop
across the cavity, which allows a more consistent cavity pressure during second
stage.
Verify that the fill velocity is optimized. Look for venting improvements that may
lead to improved fill rates. Lack of venting is not a good reason to reduce fill
velocity; instead make the tooling adjustments to allow for optimized processing.
47.3.1.2 Molding Process: Melt Temperature
The plastic melt temperature needs to be high enough to allow the weld to have
some bonding across the weld. If melt temperature is too low the weld line will be
weak because the molecules will not have a strong attraction.
Fountain flow is the description of how plastic flows from the inside out when fill-
ing through a mold wall section. The advantage of fountain flow is that hot material
is always coming to the front of the flow, which helps provide hot enough material
to a weld line. Results from flow analysis show that in some cases melt tempera-
ture rises during fill and in other cases it decreases during fill. It is critical to pro-
vide an adequate melt temperature to allow the weld line to “bond” together well.
Figure 47.3 is a magnified photo of a weld line that shows a valley where the flow
fronts welded together.
