Page 311 - Injection Molding Advanced Troubleshooting Guide

31.3 Flow Lines Troubleshooting 305

31.3.3.1„ Machine: Velocity to Pressure Transfer
If the machine is not doing a good job when transferring from first-stage velocity
control to second-stage pressure control a witness line may appear on the part.
This witness line can be considered a flow line because it actually indicates a spot
where the plastic flow hesitated. If the machine pressure is dropping significantly
during transfer the melt can hesitate and impact both the cosmetics of the part
surface as well as the ability to effectively pack out the part.
An example of a poor velocity to pressure response is shown in Figure 31.3. Notice
in this example that the machine pressure drops to zero, the screw bounces back
severely, and the cavity pressure drops to zero. This example created a distinct
hesitation just prior to complete cavity fill, which resulted strange flow/hesitation
lines near the end of fill for this part.

Figure 31.3 Poor velocity to pressure response

Poor velocity to pressure response can be caused by several factors including:
1. Transferring too early
Normally the transfer from first stage to second stage occurs around 95–98%
full. Transferring too early will create a lag in how the machine can build pres-
sure. Frequently early transfer will show a dip in pressure followed by a pres-
sure overshoot as the machine tries to reach the set pressure.

2. Incorrect second-stage velocity or volume setting
If the volume setting for second-stage pressure is set too low the machine will
lag as it tries to build pressure. Adjusting the second-stage volume setting can