General Design Principles for DuPont Engineering Polymers - Module

Rib Design
As discussed earlier, the use of ribs to improve rigidity
and reduce weight is acceptable only when such
product improvement is essential. This restriction of
course is due to the possible surface and warpage
problems that can be caused by ribbing. Once the need
for ribs has been established, they should not be used
arbitrarily, but rather to provide a specified improvement
in rigidity, weight reduction or both. Two types
of ribbing are considered here: cross-ribbing and
unidirectional. In both cases, graphical plots are
presented to simplify problem solution.
Guidelines for selecting rib proportions are shown in
Figure 4.07. Height of the rib should be determined
by structural design, and in some cases could be
limited by molding conditions. Fillet radius at base of
rib should be 1 ⁄ 2 the rib thickness.
Figure 4.07
Cross-Ribbing
Most housings—tape cassettes, pressure containers,
meter shrouds, and just plain boxes—have one
functional requirement in common: the need for
rigidity when a load is applied. Since rigidity is
directly proportional to the moment of inertia of the
housing cross section, it is physically simple (though
sometimes mathematically complex) to replace a
constant wall section part with a ribbed structure with
the same rigidity but less weight. To simplify such
analysis, the curve in Figure 4.08 has been developed
to help determine the feasibility of using a ribbed
structure in a product. The curve describes the dimensional
relationship between simple flat plates and
cross-ribbed plates (Figure 4.09) having identical
values of moment of inertia. The base of the graph
shows values from 0 to 0.2 for the product of the
non-ribbed wall thickness (t A) and the number of ribs
per inch (N) divided by the width of the plate (W).
The W value was taken as unity in the development of
the curve; thus it is always one (1).
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Figure 4.08 Conserving material by ribbed wall design
1.0
0.9
0.8
0.7
tB
tA
0.6
T
tA
0.5
0.4
0.3
0 0.05
0.10
0.15
2.20
0.20
2.15
2.10 2.05
2.00 1.95
0.99 = VB
VA 0.98
1.40
0.97
0.96
0.95
1.45
0.90
0.80
1.50
1.55
1.60
1.65
0.70
1.70
1.75
0.60
1.80
1.85
1.90
0.50
tA x N
W
Figure 4.09 Conserving material by ribbed wall design
tA
t B
W = 1
W = 1
It should be noted that the rib thickness was equated to
that of the adjoining wall (tB). However, if thinner ribs
are desired, their number and dimensions can be easily
obtained. The left hand ordinate shows values from
0.3 to 1.0 for the ratio of the ribbed wall thickness (tB) to the non-ribbed wall thickness (tA). The right hand
ordinate shows the values from 1.0 to 2.2 for the ratio
of the overall thickness of the ribbed part (T) to the
non-ribbed wall thickness (tA). Ratios of the volume of the ribbed plate (VB) to the
volume of the corresponding flat plate (VA) are shown
along the curve at spacings suitable for interpolation.
For any one combination of the variables T, tB and N,
these volume ratios will specify the minimum volume
of material necessary to provide a structure equivalent
to the original unribbed design, as shown in the
following examples.
1.00
T