Page 433 - Injection Molding Advanced Troubleshooting Guide

43.3 Sprue Sticking Troubleshooting 433

ƒ Low nozzle temperature
ƒ Sprue break

43.3.1.1„ Molding Process: Second-Stage Pressure
If the second-stage pressure is high it can over-pack a sprue and cause it to stick.
Because sprues are usually the thickest portion of the part and runner system a
great deal of pressure can be packed into the sprue.
Verify that the second-stage pressure is set to the documented process. If high
pressure is suspected running a fill only shot will validate the influence of pressur-
izing the sprue. If a fill only shot does not stick in the sprue it can point to issues
with over-packing of the sprue that may be limiting the shrinkage that occurs in
the sprue.

Low second-stage pressure can result in sprue sticking if the thick section of the
sprue develops a void. Sprues with voids in the thick section will often tear as
the mold opens. If the sprue is pulling apart and a void is present try increasing
second-stage pressure to eliminate the void.

43.3.1.2„ Molding Process: Long Second-Stage Time
If second-stage pressure is applied after the gate has frozen the only thing that is
being pressurized is the runner and sprue. Excess second-stage time may lead to
over-packed sprues that will stick. In addition to the potential for sticking sprues
excess second-stage time leads to increased cycle times and heavier runners, both
of which are costing the molder money. Furthermore, the machine will be expend-
ing additional energy to provide pressure for the extra amount of time, which will
also increase cost.

Second-stage times should be evaluated versus the required time for the gate to
seal (see Chapter 3 on scientific molding).
If the process is to run with gate seal it is a good idea to have an accommodation for
variation that may be 5–10% added to the gate seal time to ensure gate seal occurs.
Some companies will select a given amount of time to add to gate seal time to
establish second-stage time. Choose a method and make sure that it is followed for
setting second-stage time.
Examine the second-stage time and verify that it is set correctly. Also verify gate
seal time and determine if the second-stage time is set appropriately. If there is
extra time on second stage it will be worth reducing the time to determine the im-
pact on the sticking problem. To keep the cycle time consistent the time removed
from second-stage injection can be added to cooling time. As always, part require-
ments must be understood, and reduction of the cycle time for some customers
may lead to revalidation of the molded parts.