Page 25 - Plastics News May 2021

FeAtures

Key Decisions for an Optimal Ejection System

Paul Thal

It’s a journey from a small business in Amritsar to traversing unknown territory and making a huge success of it.

njection molders want plastic parts to eject easily, Ejector Location—Flatter Is Better
Iconsistently, quickly. Parts getting stuck causes damage The ideal surface for ejection is a flat one allowing the
and can break mold components, resulting in longer use of KO pins ground to the proper height. In the case
cycle times, manufacturing delays and increased costs. where a feature does not provide a flat surface, the
An experienced moldmaker knows how to manage the ejector component can conform to the part geometry.
complexities of your mold design with the necessary In some cases, severe contours could actually require
ejection strategy, improving your part quality and saving part-design alterations. Keep in mind that recessed flats
you time and money over the life of your mold. promote proper ejection because they provide a surface
For the optimal ejection system, your primary parallel to the parting line, making the plastic part easier
considerations should include ejection surface area, to knock out.
location and type. Let’s delve into these separately. Best Practice: Locate ejection points at the deepest areas
of the plastic part.
Ejection Area—Smaller Features, Bigger Prob-
lems Insufficient ejectors cause plastic part imperfections
Proper ejection is based on part size and geometry. and ejector breakage, so molders should err on the side
Applying an insufficient ejection surface area results in of more ejectors than might be required. However, that
component breakage, part deformities and slower cycles. principle must be weighed against the fact that too many
Instead, you want to steer towards larger ejection surface ejectors ultimately increase wear and cost of the mold. In
areas. addition, ejectors placed too closely together will result
in thin-wall conditions in the steel mold.
Best Practice: Use as much ejection surface area within
reason and avoid using KO’s under 3/64 in. diam.

Smaller ejection features on the plastic part require
smaller ejection components. These smaller components
can introduce both installation difficulties and wear and
breakage risks during use because of their fragility. These
factors ultimately increase costs for both the tool build
and long-term maintenance. Best practice tells us to avoid
using knockouts (KOs) under 3/64-in. diam. Thin walls
in plastic parts require custom-made thin-wall ejector
sleeves. Molders should know that these types of sleeves
are not only expensive to manufacture but they will also
not last as long as a standard sleeve. Primarily used to form undercuts, lifters can provide
ejection functionality in conjunction with other ejection
systems. An adept moldmaker optimizes the number of
ejectors while maintaining proper steel conditions. You
want your moldmaker to balance the layout of the ejector
locations to uniformly lift the part.
Ejection Type—Pros and Cons
In an ideal world, your moldmaker would use only KO pins
in your plastic injection mold. However, most injection
molded parts are just not that simple. Common ejector
options include pins, blades, sleeves, bars, stripper plates

25 May 2021 Plastics News