Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 55

The mold cavity is the central point of every injection molding machine.
it distributes the melt into and throughout the cavities, shapes the part, cools
the melt, and ejects the final product. During mold filling, the melt flows from
a nozzle through a sprue (the overall entrance into the mold). It is distributed
by a runner system which brings the hot melt into the mold cavity. runners
are designed in order to produce small drops at low pressure, while ensuring
a low amount of material waste generation and slow cooling. There are two
types of runners:

∑ cold runners and

∑ hot runners.

Cold runners are ejected with the part and trimmed after removal. Hot runners
keep the polymer at melting temperature. Finally, the melt is distributed in
mold cavities through gates which control polymer flow and isolate mold
cavities from the rest of the system.
Total cycle time can be expressed as a sum of three terms: closing time,
cooling time and ejection time. Closing and ejection can take a fraction of a
second or a few seconds according to the size of the mold and the machine.
Cooling time depends on the maximum thickness of the part. The process
can be divided into five steps which can be represented, for example, by
plotting specific volume against temperature in a v - T diagram (Fig. 3.7;
Osswald and Hernández-Ortiz, 2006):

Polystyrene

1.05

1 bar

200

￿ ￿ ￿ ￿ ￿ ￿

1.00

600

1000

D V

1600

0.95

0.90

0 50 100 150 200

Temperature (°C)
3.7 A v - T diagram (from Osswald and Hernández-Ortiz, 2006).