Page 35 - Plastics News January 2022

FEATURE

How To Form A Hole With A Core Pin: Part 2

Jim Fattori

Designing an injection mold to work well and last a long time with minimal repair costs and down-
time 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 cap-
tive molders

ast month I discussed several basic the hole in the molded part ends up being cavity side helps prevent the part from
Lmethods of forming a through too large for the established process sticking in the cavity. It doesn’t matter
hole in a molded part. This month I will parameters, you can grind the steel-safe if the section with the reduced diameter
go into more detail and provide more pin slightly smaller. If the hole in the is made with the pin in the cavity or the
variations that you may consider. You molded part ends up being too small, core. The part design will dictate which
might also consider combining various the pin can be replaced without having configuration is preferable.
features from several of these methods to increase the diameter of the through
to suit your particular application. hole. This method works well for both Method J: Method J is the same as Method
I, except that the faces of the core pins
Figure 1 depicts six of the more common blind and through holes in a part. The are interlocked. There are various designs
methods for creating a through hole in a core pin retainer holding the pin in place for interlocking two mating pins. Ideally,
molded part. All of them use a core pin. is an inexpensive and effective method you the one you use should allow any debris
While Fig. 1 shows core pins mounted might consider for non-laminated B-Series to fall freely out of the female portion of
in a B-Plate, most of these methods mold bases, stripper plates, and other the interlock every time the mold opens.
designs where there is no retainer plate.
Or you should provide space between
Method H: Here, two opposing core the male and female interlocks to retain
pins butt off against each other. Using some debris. Notice how the interlock
through-hardened core pins is essential does not extend all the way to the edge
for this type of application. Spring of the pin. There is a small ledge, or flat
loading one of the pins will greatly portion on the face of the pin with the
prolong the longevity of both pins. I male interlock. This is done to prevent
wonder how many of you realized that side flash. Interlock two mating pins
the drawing is incorrect. (Yes, it was a whenever the concentricity is critical,
test.) Since the core pin on the core side the height of the smaller-diameter pin
of the mold is spring loaded and the is extremely long, or there is a high
can be applied to core pins mounted in hole is counterbored, the spring-loaded probability of the flow of plastic material
the A-Plate, an ejector sleeve, or even pin will try to push the molded part off bending the pin.Interlocks should allow
a cam. Additionally, while the methods the core—possibly causing it to stick in debris to fall freely out of the female side.
discussed here apply to cylindrical core the cavity. When a hole in a part is fairly
pins, they can also be incorporated into long and the hole diameter fairly small, Method K: This depicts a small-diameter
a variety of shapes with the same intent it is common to use two opposing core core pin. The through hole for small pins
and purpose—for example, forming a pins to reduce the length. If the two pins should start from the parting line of
keyhole in a molded part. have the same diameter, one of them the mold because when you machine a
should be intentionally a little larger hole from the back of the cavity or core
Various methods to form a through hole
(G-L). than the desired hole diameter. In the insert, you run the risk of the drill or
event one of the pins deflects due to the end mill “drifting.” This is also why the
Method G: This depicts a core pin flow of molten material, a screw, pin, through hole for small core pins should
designed for a hole diameter that is or other item can pass through the hole be relieved. Reamers, laps and hones
extremely critical. The pin is slightly unobstructed. Method I: This is basically can help repair a hole that is curved,
larger in diameter, and then stepped to a mirror image of Method H. However, or shaped like a banana, but they can’t
the desired size for the hole in the part. If in this case the spring-loaded pin on the straighten out a hole if

34 PLASTICS NEWS JANUARY 2022 35 PLASTICS NEWS JANUARY 2022