Page 30 - Plastics News December 2021

FEATURE

more difficult it or injection pressure. Springs also
will be to keep help to protect small, fragile core pins.
cool. I once Bellville springs work very well for this
saw a medical application. One spring washer or a stack
mold that had of washers may be required, depending
extremely small- on the size of the pin and the amount
diameter core of preload desired.
pins—so small I have used this method quite frequently
that even tiny on return pins and other mold
bubblers made components. It greatly reduces the
FIG 2 Copper-alloy core pins can provide of stainless- amount of shock inflicted on the pin and
faster cycle times. steel surgical tubing could not be inserted on the mating plate or mold component.
into them. The designer knew that the Since the pin may need to expand or
against the cavity or another pin, I prefer exposed length of the core pins would contract slightly, the bore hole it rides
the harder hardness. In cases where the get extremely hot, even with a water- in should be relieved. The amount of
core pin enters a hole in the cavity or wash around their shaft. Therefore, he bearing surface or land length in the core
other component, I prefer the softer core designed the mold with air jets in the insert should be at least twice the amount
pins, which will wear faster. It is cheaper cavity directly opposite each of the core that the pin protrudes from the core. It
to replace the core pin if it wears than pins. When the mold started to open, is also a good idea to grind (“dust”) a
it is to repair a worn hole in the cavity. the air blasted against the hot pins. It small amount of steel off the back of the

Copper-alloy pins (Fig. 2) can also be used was certainly an unconventional design, core pin to ensure it is perpendicular to
in any of the depicted methods. They but it worked well in solving the cooling its centerline. Now the spring will apply
can be very beneficial for maintaining problem. even pressure—preventing any forces
circularity of a hole, especially in thick- Method A that would try to push the pin slightly to
walled parts. They can also help reduce Method A in Fig.1 is the least expensive one side or the other.
the cycle time of the part. But you need Consider the hardness of the steel that
water running around or inside the way to form a through hole in an injection the head of a core pin sits on.
molded part. The face of the core pin
copper pin to keep it cool.
butts off against the cavity impression.
However, since copper-alloy pins are The downside of this method is that it Method C
much softer (90 to 98 Rockwell B) than can either make a slight impression in the Method C uses a core pin that pilots into
steel pins, their face can deform if there cavity as the plates start to “settle in” over a hole in the cavity, which is the same
is excessive force applied. At least one time, or it can generate side flash if the diameter as the pin. The pin is bullnosed
mold-component supplier offers core injection pressure opens the parting line in case of any misalignment. This design
pins made of 420 stainless steel with a a little. It is also important to consider the eliminates the need for any spring
through hardness of 50 to 52 Rc. These hardness of the steel that the head of the loading, but it is prone to deforming the
are also a very good choice for core pins pin sits on. Plates made of prehardened through hole and possibly the core pin
itself if there is significant misalignment.
in many applications—particularly those 4130 steel (28 to 34 Rc) are preferred As the through hole or the pin begins to
for the medical industry. But compared over the softer 1030 steel (80 Rb). wear, it can generate down-flash.If the
with copper pins and carbon-steel pins, Alternatively, a hardened retainer about hole in the part that the core pin forms is
stainless steel has considerably lower 1/8-in. thick can be inserted in the softer tall, the tip of the pin needs to be retained
thermal conductivity. While a stainless plate. The counterbore for the head of a by some method, such as entering a hole
pin is resistant to corrosion, it can also core pin should be 0.002-in. deeper than in the cavity. If you think about a pen
require an increase in the mold’s cycle the thickness of the head itself. barrel or a syringe, you can easily see that
time. Method B core shift is big concern. When a core pin
Regardless of the type of material a core Method B is the same as Method A, is supported on both ends, core shift is
pin is made of, the smaller the diameter except that the pin is spring loaded to less of an issue.
and the longer the exposed length, the overcome any variation in plate thickness

30 PLASTICS NEWS DECEMBER 2021