Page 267 - Injection Molding Advanced Troubleshooting Guide

27.3 Excessive Cycle Time Troubleshooting 259

lead to reduced recovery rates. If rotate time is increasing over time it can indicate
a worn screw or barrel especially when running glass-filled materials.
For long cooling times driven by poor cooling of the mold see Section 27.3.2. Do not
use the molding machine and mold as a cooling fixture! Look for opportunities to
improve cooling to help minimize cycle time.

27.3.2„ Excessive Cycle Time Troubleshooting Mold Issues

Mold-related problems include:

ƒ Lack of cooling
ƒ Damaged or plugged cooling lines
ƒ Gate size
ƒ Runner and sprue size
ƒ Poor ejection
ƒ Thick sections
ƒ Thermal isolation of hot runner

27.3.2.1„ Mold: Lack of Cooling
If a mold cannot remove the added heat from injecting molten plastic the cycle
time will become a problem. All molds must be designed with optimized cooling
and details such as cores should be cooled whenever possible.
See Chapter 14 for more information on mold cooling.

27.3.2.2„ Mold: Damaged or Plugged Cooling Lines

If a cooling line in a mold becomes plugged water will be restricted and may not
achieve turbulent flow through the circuit. Any lines that have become plugged or
have restricted flow will lead to cooling problems. Also beware of blades that have
been inserted parallel to water flow rather than perpendicular to flow. Also watch
for cases where bubblers or blades have been inserted too far and are restricting
water flow through the mold.
Verify flow through all circuits. A thermal image of the part can be useful for
identifying areas with poor cooling. Verify that the mold is watered correctly; for
example, Figure 27.1 shows a mold that has bubblers watered with the water flow-
ing in the wrong direction through part of the bubblers leading to excessively high
core temperatures.