A slightly different hinge design is shown in Figure 3. Although not as efficient as the
basic design shown in Figure 1 and with a more limited angular movement, this design
will allow the hinge to be molded at an angle to the tool parting plane.
To make a tool steel-safe, it is advisable to start with a thin hinge (0.008 to 0.010 in.
(0.20 to 0.25mm) and adjust upwards if necessary, to a maximum of 0.0 15 in.
(0.38mm) Although thicker hinge sections may be employed where the required angular
movement is small, these cannot be considered as true living hinges because they
result in a lower degree of polymer orientation which may result in lower flex life.
Hinge sections below 0.008 in. (0.20mm) will cause excessive pressure drop, which
may, in turn, create excessive local shear, excessive shear heat buildup and underpacked
parts or short shots.
A well designed hinge will maximize molecular orientation — and orientation gives
strength. In the same context, sharp corners must be avoided because they will act as
stress risers (see Figures 1 and 2). Gates need to be positioned to ensure that the melt
will flow perpendicularly to the hinge. Gate positioning will be discussed further in
subsection 2.4. Hinges longer than 6 inches should be designed in 2 or more sections
with small gaps or breaks between sections, this improves hinge life and reduces tool
flexing.
For better hinge life, parts need to be flexed a few times immediately after molding (cold
drawing) while they are still warm. This will allow for the web to elongate well beyond its
yield point without breakage.