Page 26 - Plastics News May 2021

FeAtures

and lifters—and each option brings its own tradeoffs.
Different ejection options will leave different sizes and
shapes of marks on the part and require different levels
of maintenance.Ejector pins offer the simplest and most
cost-effective ejection system. As the default ejection
system, they leave solid circular ejection marks on the
part. The injection molding process subjects ejector pins
to a great deal of pressure during ejection, so larger pin
diameters present a safer option.
When ejector pins cannot fit within the allowable ejection
area, blade ejectors offer a good alternative. These flat,
rectangular pins can eject on thin part regions. Blades the preceding ejection systems, these tend to be larger
have more surface area than pins, so they can wear down and can utilize multiple pins beneath the bar. Because of
more quickly. Because of their high wear, blade ejectors their bigger size and high customization, ejector bars are
can be integrated into the mold as inserts for easier typically high-cost and high-maintenance as the mold gets
replacement. more wear. Stripper systems eject a part on its perimeter.
Maintenance involves ensuring that the stripper plate or
Best Practice: An injection mold designer should know block sits flush with the rest of the mold.
the pros and cons of each ejector type, and choose
accordingly. At the part level, the need for mold maintenance can be
evaluated by checking for flash on parts at the parting
Ejection sleeves distribute force evenly across small, line of the stripper plate or bar.
circular features. The mark left on the part at the end of
ejection mirrors the shape of the sleeve. Maintenance for Ejector blades have more surface area than pins and can
sleeves involves checking the fit of the outside of the core therefore wear down more quickly. Used primarily to form
pin to the inside of the sleeve. Checking this reduces the undercuts, lifters also provide ejection functionality in
risk of plastic flashing into the ejection system. conjunction with other ejection systems. Unlike vertical
ejection systems, lifters angle in the core plate. This
Ejector bars offer a good option for ejection on lengths allows the lifter to clear the undercut for ejection to
of flat surface. Ejection bars utilize multiple pins topped take place.
by a flat bar that evenly distributes their force. Unlike
The injection mold designer should
follow the preferred order of ejection
types. KO pins are reliable, inexpensive
to purchase, easy to install, easy to
maintain and easy to modify. Sleeves
increase the expense to purchase and
install. KO blades require extra cost
for the pocket and fitting. Strippers
offer the most expensive of the
ejection options since they are a
completely custom component driven
by the part geometry.
A robust ejection strategy will improve
the quality of the parts, optimize
the immediate and long-term costs
related to the mold, and increase the
time the mold spends making parts,
while the wrong ejection system can
decrease uptime and curtail the life
of your mold.

Plastics News May 2021 26