Page 166 - Injection Molding Advanced Troubleshooting Guide

17.3 Brown Streak Troubleshooting 155

17.3.1.3„ Molding Process: Residence Time
When plastic is allowed to sit for excess time at melt temperature the risk of ther-
mal degradation is elevated. The length of time that the plastic spends in the barrel
of the molding machine is referred to as residence time. The material being molded
will determine the maximum allowable residence time, and some materials such
as polyethylene and polypropylene are much less sensitive to degradation from
long residence time especially when compared to something like a PVC or acetal.
Figure 17.3 shows purged material that has been degraded because of excess resi-
dence time; if this material had been injected into the mold the part would have
had brown streaks and splay.

Figure 17.3 Degraded material in purge

The best method for determining the true residence time within the melt delivery
system is the use of tracer pellets of another color added at the feed throat. After
adding the tracer pellets, continue to mold parts and time how long it takes before
the new color shows up in the molded parts.

There are less accurate ways to estimate residence time such as calculating the
shot weight versus the barrel shot capacity. When doing this it is important to re-
member that barrel shot sizes on molding machine are based on polystyrene,
3
which has a density of 1.04 g/cm , but when using something other than poly-
styrene it is important to account for the different density. It is also important to
understand that solid density is different from melt density, making the calcula-
tion less accurate. Also, the plastic that is retained in the flights of the screw must
be accounted for and this is usually done using a multiplier (normally 40%).