Page 354 - Injection Molding Advanced Troubleshooting Guide

350 36 Part Sticking on Ejector

account for sticking. Draft angles allow a molded part to “break away” as it is
ejected and allow clearance as the part moves away from the core details of the
mold. Review standard requirements for the material being molded and apply the
appropriate draft to the part.

36.3.2.4„ Mold: Buildup

During molding, gases from the material will be formed as the plastic is melted,
and these gases can result in a buildup or plate out on the mold surface that can
lead to sticking. The mold surface should always be kept clean and if buildup is an
ongoing problem, venting improvements should be made (see Chapter 7 on venting
and Chapter 19 on buildup).

36.3.2.5„ Mold: Slides or Lifters
If a lifter or slide does not travel enough the undercut detail may not exit the com-
ponent. All lifters and slides should be reviewed during mold design to determine
if adequate travel is achieved. During mold setup it is important to verify that the
proper amount of ejection stroke is being used to allow lifters to reach a full for-
ward position.

It is important for mold components to pull away from the part allowing clearance
between the component and the part. There are cases where even though the com-
ponent travels far enough the part will travel with the component, i. e. it sticks to a
lifter rather than the core steel. In these cases it may be necessary to add a rib or
other undercut to the core steel to prevent the part from moving with the lifter.
Whenever possible, lifters should be designed to travel in opposing directions,
which will help balance the tendency of the part to travel with lifters in one direc-
tion.

36.3.2.6„ Mold: Damage, Erosion, or Burrs
Any time the mold is damaged, sticking could be the result. The damage to the
mold may be a matter of a burr on the mold that is creating an undercut that traps
the part; any burrs should be removed to prevent sticking. Erosion can occur in
areas of the mold that have inadequate venting, where the trapped gases will start
to erode the steel over time, which can lead to a surface finish that is rough and
will trap the molded part.
Other items of damage may include hydraulic cylinders on either core pulls or hy-
draulic ejection. Verify that any cylinders used for part ejection are not leaking and
are connected properly.