Page 478 - Injection Molding Advanced Troubleshooting Guide
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478 47 Weld Lines
priate for every mold that runs in the machine. Sometimes smaller molds will run
in a machine due to machine capacity, shot size, ejector stroke, or mold open stroke.
When running a small mold in a machine evaluate if the tonnage should be re-
duced. If the process is established with less than maximum clamp tonnage make
sure that the machine is running with the correct clamp force.
To check for over-tonnage on a machine reduce the clamp tonnage setting or back
off the die height setting on a toggle. If the weld lines seem to improve this is a
good indicator that more venting will help the situation. If the reduced clamp
tonnage provides an acceptable part either the process can continue to run with
reduced tonnage or ways to improve the venting on the mold have to be sought.
47.3.3.2 Machine: Velocity to Pressure Transition
Velocity to pressure transfer is what occurs when the machine changes control
from first-stage velocity control to second-stage pressure control. If this transfer
does not occur in a crisp and quick manner the delay can cause a stall in the flow
fronts. When flowing plastic stalls, the resulting weld lines will be weaker than
normal. This factor is often overlooked but can be critical for ensuring weld lines
are of the highest quality level.
Velocity to pressure transition is discussed in detail in the Chapter 8 on machine
performance.
47.3.3.3 Machine: Machine Performance
If the machine is not achieving the desired set points weld lines will become a
more challenging problem to correct.
See Chapter 8 on machine performance.
47.3.4 Weld Lines Troubleshooting Material Issues
Some potential material concerns are:
Filler type and content
Viscosity
47.3.4.1 Material: Filler Type and Content
Fillers in the plastic material will not travel across the weld line interface. Because
of this materials with fillers will usually have more visible weld lines. The plastic
at the weld line will usually not have as much of the filler present, which in turn
will lead to a variation in appearance between the surrounding plastic and the
weld line (see Figure 47.5). Notice in Figure 47.5 how the fiber orientation changes
at the weld line as well as the decreased level of fibers on the surface.
