Page 81 - Injection Molding Advanced Troubleshooting Guide

8.3 Velocity Linearity 65

To determine if the problems stem from the non-return valve or a worn spot in the
barrel try moving the shot size and transfer position back approximately 25 mm
and doing the 10-shot study again. If the fill only weight variation is better after
changing the shot size and transfer position there is probably a worn spot in the
barrel that is causing the problem rather than the non-return valve.
If a non-return valve appears to be an issue the end cap of the press should be
removed and the screw tip and non-return valve should be inspected for wear,
damage, or contamination. If an issue is detected the non-return valve should be
replaced.

„ „8.3„ Velocity Linearity

Processors often assume that the setpoint that they enter in a machine controller
is what the machine will provide. A classic area where this may not be the case is
velocity. To determine if the machine is providing the velocity that is requested
follow these steps:
1. Set the process to run a fill only shot, no second-stage pressure.
2. Adjust transfer so that the part is approximately 80% full.

3. Record shot size, decompression, and transfer position.
4. Set the fill velocity to the minimal practical value.
5. Record fill time.
6. Double the velocity setpoint.
7. Record fill time.
8. Repeat steps 5–6 until machine maximum is obtained.
9. Calculate the actual fill velocity for every step (Equation 8.4).
10. Calculate percent difference for each velocity (Equation 8.5).
11. Calculate the total range of fill difference by subtracting the minimum differ-
ence from the maximum difference. Maximum range should be below 10%.
Calculating actual fill velocity:

( shot size + decompression)− transfer position
fill time = actuall velocity (8.4)

set velocity − calculated velocity = % difference (8.5)
set velocity