Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 54

indeed, more than one third of all thermoplastic materials are processed with
injection molding (Fig. 3.6; Han, 2007). This is because of the suitability
of injection molding to mass production of parts with complex shapes and
precise dimensions. Injection molding machines are classifi ed using an
international convention, based on the T / P ratio, where T is the clamping
force (tonne), and P is defi ned as:

Vp
= 1000

Vp
m V xm
V xm Vp ax

P

where V max is the maximum shot size (cm 3 ) and p max is the maximum
injection pressure (bar). Injection molding can be considered to be the sum
of two distinct processes. The fi rst process involves solid transport, melt
generation, mixing, pressurization and fl ow, which are phenomena that
take place in the injection unit. The second process is the product shaping
occurring into the mold cavity.
in injection molding, the polymer melt is forced into a cavity in order to
reproduce its shape (Fig. 3.6). The injection molding cycle begins when the
mold closes, followed by the injection of the polymer melt into the mold cavity.
To perform this, the melt must be pressurized with a piston or a screw. Once
the cavity is fi lled, a holding pressure is required in order to compensate for
material shrinkage. When the part has reached a suffi ciently low temperature,
it is ejected. The design is similar to a single-screw plasticating extruder. The
screw can, however, slide back and forth in order to accumulate and inject
the melt. The presence of vents allows extraction of moisture and residual
gaseous monomer from the melt. At the end of the screw there is a non-
return valve which enables the screw to work like a plunger during injection
and to prevent the back fl ow of the melt which is injected in the mold through
a nozzle. The clamping unit has the task of opening and closing the mold,
avoiding the risk of fl ash during fi lling and holding.

￿ ￿ ￿ ￿ ￿ ￿

Mold

Hopper

Heating
zones

Runner

Cooling
zone

Heating
cylinder

Hydraulic
cylinder

Reciprocating
screw

Mold cavity

3.6 Screw-injection molding machine (from Han, 2007).