The single most important
factor to consider when designing with ceramic materials is that
they behave under stress as a brittle material. As such, a localized
defect, flaw or other "stress riser" can be an initiation point
for crack growth, fracture and potential catastrophic failure. Stress
riser points can result from a design that incorporates sharp internal
corners to undercuts in the material. When the part is mechanically
stressed either by the application of a load or due to thermal loads,
these points can end up in tension and may crack. To take full advantage
of the properties of the ceramic materials the design should minimize
tensile stresses in the part.
Avoid knife edge features.
Because of the brittle nature of the ceramic, these types of features
will be prone to chipping. Provide a 0.030" to 0.050" thick tip
if knife edges are necessary.
Provide a 0.020" minimum
radius on inside corners, whenever possible. The sharper this corner,
the greater the possibility it can become a stress riser. Also,
the sharper the corner the more expensive the part.
Provide a radius or chamfer
on all edges to prevent chipping. Whenever possible specify a simple
edge break with a minimum radius of 0.015" that can be applied by
a simple hand operation. Tight tolerances on radii or chamfers require
precise machine made features that are more costly.
Use standard fractional
values for radii and angles to avoid special tooling. Try to use
30 or 45 degrees for angles and chamfers.
From Bralla's "Design
for Manufacturability Handbook," McGraw-Hill, New York,
Since parts may sag or
be distorted if not properly supported during firing, it is preferable
to avoid large overhanging or unsupported sections. Otherwise, supporting
fixture costs may be excessive.
of sections of non-uniform thickness during drying and firing causes
stress, distortion, and cracking.
Gently curved surfaces
without abrupt break lines or angularity are normally preferred
with most ceramic-forming processes.
Molding of screw threads
in ceramic parts is not very feasible.
Ribs and fins should
be well rounded, wide, and well spaced and have normal draft.
Secondary grinding operations
can be very expensive on ceramic parts. Limit high tolerance regions.