Page 69 - Injection Molding Advanced Troubleshooting Guide
P. 69
7 Venting
Proper vent design based on the material being molded and the process is critical.
Venting of a mold allows gas to escape from the cavity as the plastic fills the mold.
It is important to remember that the cavity is filled with air at the start of fill and
the plastic will usually have outgassing during the molding process, so if the mold
is not vented properly the gas will become trapped in the mold. Trapped gas will
lead to defects including burns, short shots, splay, and poor gloss replication.
In addition to basic venting to remove trapped gas from a mold cavity it is vital to
vent any weld line locations (see Figure 7.1).
Figure 7.1 Weld line locations are critical for venting
7.1 Venting Basics
Every vent has the three dimensions depth, width, and land. Figure 7.2 shows a
cross section of the vent depth and land. Also shown in Figure 7.2 is the relief
channel of a vent. Vents should always be considered in area rather than simply
depth. The width and depth provide the total area for the gas to evacuate.
