Page 197 - Injection Molding Advanced Troubleshooting Guide

20.3 Burns Troubleshooting 187

Sometimes buildup issues will come from a location of mismatch between the
nozzle and end cap or the nozzle and tip. It is good practice to have a free flow path
as the material moves through the front-end components of the machine. Any mis-
match creates a place where material can be trapped and degrade over time be-
cause it will have a long residence time.
Proper shutdown procedures are critical to reduce the chance for buildup issues.
When a machine is to be down for any amount of time it should be purged to elim-
inate the chances of degrading the material in the barrel. With some materials this
becomes more critical because the more temperature sensitive a material is the
faster it will degrade leading to potential buildup problems.

20.3.3.2„ Machine: Wrong Screw Design
Shear imparted on the pellets provides much of the energy to melt the pellets as
they travel the length of the screw. If a screw is designed without the correct
length/diameter (L/D) ratio or the wrong compression ratio it is possible to gener-
ate excess shear, which in turn can lead to degradation of the material, which will
create excess gas byproducts that must be vented from the mold. A typical gener-
al-purpose screw is shown in Figure 20.3.

Figure 20.3 Typical general-purpose screw

Keep in mind that if the mold has been running well in this machine and the screw
has not been changed the cause is probably not related to the screw. There can be
cases where degraded material builds up on the screw and leads to conditions
where flight depths of the screw are different due to the buildup on the root of the
screw. Also, in extreme cases a screw can have so much buildup of carbonized,
degraded material that the screw looks like it has a barrier flight from the buildup.
If this is causing problems it is critical to establish proper shutdown and purging
procedures.

20.3.3.3„ Machine: Overriding Barrel Heaters
As with excessive shear an area of overheating on the barrel can lead to problems
with degrading of material. If there is a zone on the barrel, end cap, or nozzle that
is running above the set points it could lead to a spot that is overheating and de-
grading the plastic.