Page 36 - Plastics News January 2022

FEATURE

How Much of a Taper? plates that are heated with hot water or
As the molten plastic cools, it shrinks oil. There is also the common and more
onto the core pins. Effectively, the concerning scenario where the processor
pins are an immovable cooling fixture. intentionally uses a hotter temperature on
Therefore, core pins, like the cores the A-plate to prevent parts from sticking
themselves, typically require more draft in the cavities. The larger the mold, the
or taper than the cavity does. Materials more this can become an issue. In a case
with higher shrink rates will apply more like this, consideration should be given
holding force than materials with a low to interlocking the individual cavities and
FIG 2 Various core-pin configurations. shrink rate—and therefore require more cores, as opposed to interlocking just the
taper. The type of material can also A and B plates. The cavity or core should
it is not perpendicular to the parting dictate the amount of taper required, as be allowed to “float” a little when they
line. That is when either jig grinding, or well as the core pins’ surface finish. If in are individually interlocked, otherwise,
high-speed milling is required. Small core doubt, you should contact the material the interlocks can end up “fighting” each
pins have a high risk of bending. They supplier for guidance. Occasionally, you other. If the cavities or cores are cut in
should be piloted into the opposing mold may want to add negative taper so that a solid, as opposed to individual inserts,
component by whatever method you the core pin functions as a puller to pull there are very few options to counteract
prefer. (Also see last month’s article for a feature out of the cavity. thermal expansion issues between the
various methods of piloting.) mold halves. One option is to calculate
Add Clearance
Method L: This is an alternative to the amount of thermal expansion and
Method K. The pin enters the cavity When installing core pins in a core insert install a bushing. The bore of the bushing
only by the amount of the interlock. or B-plate, it is common to add a little can be offset from the centerline by the
As in Method J, a small flat is on the clearance between the pin and the hole. amount of thermal expansion.
face of the pin to prevent side flash. Obviously, the clearance needs to be Pin Removal Tips
The pin is spring-loaded to compensate less than the vent depth for the plastic
for any variation in height or injection material to be molded. The advantage Core pins are typically removed by
pressure. Since the pin diameter is small, a of this small amount of clearance is to tapping on the face of the pin with a
hardened spacer is used between the back allow the core pin to float slightly due piece of brass, aluminum, wood, or dead-
of the pin and the top of the stacked to inevitable misalignment. It also acts blow hammer. But core pins are often
Bellville springs. This allows the use as a stationary vent for trapped gasses too thin, small, or fragile to be removed
of springs with sufficient force. Align and can improve the strength of the by this method. Therefore, a suitable
Precisely. No matter which method you weld or meld line of the plastic as it method of pulling the core pin out
use to form a hole in a molded part, it flows around the pin. Note: If weld line from the back of the plate is required. A
is extremely important to have precise strength is a primary concern, especially tapped hole in the head of the pin works
alignment between the cavity plate and for load bearing items such as threaded well for pulling out core pins. Especially
the core plate. Conical interlocks may be inserts, it might be best to use a very if the mold had a water leak and the pin
acceptable when a pin butts off against short and pointed core pin. This will is rusted tight in its hole.
a flat surface. They are also acceptable form a locating position for a secondary Deflection
for pins that are interlocked, as long as drilling operation. Contrary to having a Mold designers should account for the
the angle on the pin’s interlock is greater slight clearance around core pins, if you length-to-diameter ratio, or L/D. This
than the angle on the conical interlock. need to maximize the amount of heat ratio refers to the unsupported length
For piloted core pins that enter a cavity, transferred from the exposed portion of of the exposed core pin divided by its
straight-sided interlocks are required the pin to the insert that retains it, then diameter, as shown in Fig. 2. The higher
with an engagement length longer than the pin should have a light press fit—not the L/D value, the more the core pin is
the engagement length of the core pin a slip or slight clearance fit. susceptible to deflecting.
in the cavity. Proper alignment is the key Consider Thermal Expansion
to a mold’s longevity. Accounting for Core-Pin L/D Ratio
A mold may be perfectly fitted on the
bench, but it will start to expand after a While researching this topic, I found
few minutes of exposure to hot plastic or several rules of thumbs from various

36 PLASTICS NEWS JANUARY 2022