Page 297 - Injection Molding Advanced Troubleshooting Guide

290 30 Flash

Support must be designed in before the mold is built and worked into a mold
around ejector pins and lifters, so the overall mold design must be considered to
achieve optimum results.
Bear in mind that even if a mold has adequate support the support pillars can hob
into the mold clamp plate or retainer plates over time. This hobbing of the support
plate steel will lead to a lack of support that will cause flash. The mold must be
disassembled to check for hobbing of the support, so it is an easily overlooked root
cause.

30.3.2.5„ Mold: Erosion

Erosion of steel on a mold often occurs in poorly vented areas. If gas is trapped in a
location it will eventually start to cause erosion of the steel, which will lead to
flash. If steel erosion is detected the location in the mold should be better vented to
allow gases to better escape the mold.

30.3.2.6„ Mold: Slide Deflection
Any slides that form action outside of the line of draw on a mold must hold forward
with enough force to overcome the plastic cavity pressure. If the force on the slide
is not adequate the slide will be blown backwards resulting in flash around the
detail that the slide is forming.
Hydraulic actuated core pull cylinders must have a large enough cylinder com-
bined with a high enough core pressure to prevent movement of the slide. The
calculation for determining cylinder size is simple but relies on an estimate of
cavity pressure. To determine the required cylinder size the estimated cavity pres-
sure acting on the area of the core must be compared to the machine core pressure
applied to the square area of the cylinder (see the calculation in Figure 30.7).
Hydraulic cores can also be supported with a heel block locking angle that will
lock the core in place with steel. In this method the hydraulic core is set before
mold close and then the heel block locks the final preload onto the core.

Slides that are driven with horn pins will need a heel block to provide adequate
support. The heel block is what will actually preload the slide and resist the force
from the cavity pressure. Proper engagement of the heel block is critical to avoid-
ing flash, do not rely on the horn pins to provide the lock for the slide.