General Design Principles for DuPont Engineering Polymers - Module

Figure 11.30 Designs of pressure cartridges Figure 11.31 Tightness test using vacuum
X X
A (poor) B (good)
However, it is not possible to draw any conclusions
about water or gas tightness from the mechanical
strength of the joint.
Pressure vessels and floats must therefore also be
tested in the appropriate medium. Containers which
will be under internal pressure are stressed to about
half the burst pressure, which should enable all weak
points to be detected. Floats and other tight containers
are inspected by dipping into hot water and looking
for bubbles at the joint.
It is, however, quicker and more reliable to test them
under vacuum and a simple apparatus like that sometimes
used for testing waterproof watches will often
be all that is necessary.
• Figure 11.31 illustrates the basic principle.
A cylindrical glass vessel a, big enough to hold the
part, is covered with a loose-fitting lid b and sealed
with a rubber ring. The test piece is kept under water
by the sieve c. Since the water level is almost up to
the top of the vessel, only a small volume of air
need be pumped out to produce an adequate
vacuum; in fact, only a single stroke of a small hand
pump will do. The rig should preferably be fitted
with an adjusting valve to limit the degree of
vacuum and prevent the formation of bubbles by
boiling.
Checking Weld Joints by Inspection of
Microtome Sections
Correct design and proper welding should render
microtome sections superfluous. The making of these
sections requires not only expensive equipment but
also a considerable amount of experience.
However, such sections can occasionally result in
the discovery of the causes of poor welds as, for
example, in Figure 11.32, which clearly shows how
the V-groove was forced open by the welding pressure
and the matching profile was not welded right down to
X
92
c
d
the bottom of the V. The resulting sharp-edged cavity
not only acted as a notch, but increased the risk of
leaking.
Testing of spin welded joints should only be carried
out at the beginning of a production run, and thereafter
on random samples, except when there is a risk that
some parameter in the injection molding or the
welding process may have changed. The percentage
of rejects should remain negligible if the correct
procedure is followed, and systematic testing of all
welded components will not be necessary.
Figure 11.32 Microtome of badly welded V-groove
b
a