Page 29 - Plastics News December 2021

FEATURE

How To Form A Hole With A Core Pin

Jim Fattori

Designing an injection mold to work well and last a long time with minimal repair costs and downtime is
not difficult if you pay attention to the details says Jim Fattori, a third-generation molder with more than
40 years of experience in engineering and project management for custom and captive molders.

here are many ways to form a hole where the through hole in the part was 58 to 62 Rc, but can also be found with
Tin an injection molded part. Some formed in a solid—meaning the core was a lower hardness of 50 to 55 Rc. You
are simple and inexpensive. Others are a machined with a protruding shut-off and might consider using one of these
little more involved but have some added not with a replaceable pin. That type of through-hardened ejector pins for a core
benefits. This month I will review various design should only be used for prototype pin application because the tolerance on
methods I have used over the years and molds for several reasons—the biggest the diameter of an ejector pin is about
explain the pros and cons of each. one being the difficulty and cost to repair. -0.0003 to -0.0005 in. In the event of
future wear or corrosion, the ejector pin
Figure 1 depicts six common methods Core Pins can then be replaced with a core pin,
for creating a through hole in a molded Core pins used to make a hole in a which has a tolerance of +0.0003 to
part. All of them use a core pin. While part should be through hardened— +0.0008 in.
Fig. 1 shows core pins mounted in a not nitrided. Most ejector pins and
B-Plate, most of these methods can sprue-puller pins are nitrided and have All core pins are not created equal.Be
be applied to core pins mounted in an exterior case hardness of 65 to 74 very careful when selecting the type of
the A-Plate, an ejector sleeve, or even Rockwell C (Rc), with a core hardness pin to use. I found one supplier that
a cam. Additionally, while the methods varying from 40 to 55 Rc, depending on says its core pins are through-hard but
discussed here depict cylindrical core the supplier and the type of pin. The specify a surface hardness of 62 to 65
pins, they can also be incorporated into case hardening is typically only 0.001 in. Rc, and a core hardness of 50 to 52 Rc.
a variety of shapes with the same intent to 0.007 in. deep and is extremely brittle. Additionally, since we are only human,
and purpose—for example, forming a This can cause the edge on the face of you will find discrepancies between what
keyhole in a molded part. is specified in a supplier’s catalog, its
website, and the actual product itself. It is
always a good idea to check the diameter
of any pin to four decimal places, and
occasionally check the Rockwell hardness
on the diameter and in the center of
the face. If you buy molds offshore,
you might be very concerned about the
measurements you get. You might also be
surprised with the measurements you get
from a domestic supplier that purchases
its pins offshore.
Core pins are typically made of H-13
or M-2 tool steel. The H-13 pins have
a standard hardness of 30 to 35 Rc
throughout. Higher hardness H-13 core
FIG 1 Various methods to form a the pin to chip during production. pins are also commercially available with
through hole. a 50 to 55 Rc hardness, and M-2 pins with
However, some ejector pins are available a 60 to 63 Rc hardness throughout. In
On occasion, you may see a mold in a through-hardened condition, typically cases where the core pin butts off

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