Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 7)

Stress concentration factors

Stress concentrations are areas that, by the nature of their

design, tend to concentrate or magnify the stress level within

a part. This increase in localized stress may allow the part to

fail prematurely by serving as a crack-initiation point. Design

features that can serve as stress concentrators are:

• Holes and slots

• Corners

• Ribs, gussets, and posts

• Sharp wall thickness transitions

• Surface roughness

• Bosses

• Notches or grooves

Inside corners are especially critical. The curve in Figure 1

shows how the stress concentration factor in an inside corner

will increase rapidly as the radius decreases. If the radius is

very small or if there is no radius, stress levels will be very high.

On the other hand, if the inside radius is too large, a thick

section will be formed, which can lead to high levels of shrinkage

and molded-in stress. The best radius value is a compromise

between these two behaviors. In general, a radius 1⁄

to 1⁄

the

wall thickness is suggested for most inside corners, with a

minimum radius of 0.4 mm (0.015 in.) in most cases.

Figure 1 Stress concentration factors

Coring thick wall sections

A part can rarely be designed with uniform wall thickness

because of such features as ribs and bosses. When wall

thickness is not uniform, it affects moldability, molded-in

stress, color uniformity, and structure.

One method of providing uniform wall thickness is to core

thick sections of a part. Often, a coring pattern can be chosen

that reduces the thick sections while incorporating structural

features such as ribs, gussets, and bosses into the part.

Rib and boss design

A rib can be thought of as a simple projection off the part wall.

Generally, ribs should be designed with a thickness of 1⁄

the

wall thickness to avoid a thick section at the base of the rib,

which can cause sink marks on the opposite side of the wall.

Designers typically try to limit rib height to 3 times the wall

thickness; if the height is much more, the rib tip may become

dangerously thin and the rib may be subject to buckling (see

Figure 2). However, for parts with fewer structural requirements,

rib height-to-thickness ratios as high as 18:1 have been used.

Figure 2 Typical rib design guidelines

The curve gives an indication of the proper radius

to be used for a given wall thickness.

Courtesy of S.P.I. Plastics Engineering Handbook

3.5

3.0

2.5

2.0

1.5

1.0

0.2

0.4

0.6 0.8

1.0

1.2

1.4

Stress concentration factor

R/T

Wall thickness

Force

Radius (R)

T/2 to avoid sinks;

/3 T on noncritical

appearance surfaces

U = 1° Typical,

more if textured

• Rib height-to-thickness ratio

should be 3:1 or less in most cases.

• Spacing between ribs should be

at least 2 times the wall thickness.

R = T/8, 0.4 mm

(0.015 in.) min.

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