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Poor rib design can actually make a part weakernot stronger.

How do you like your ribs?

We recommend well done

because failure to observe

basic design rules on thickness,

height and radii could lead to pre-

mature failure of an otherwise flaw-

lessly designed part. Poor rib design

could also create cosmetic prob-

lems in a part or slow molding time.

The primary purpose of ribs in

plastic design is to improve the

stiffness of the structure. Ribs do

this by increasing sectional prop-

erties, specifically the moment of

inertia. Because stiffness is a

function of moment of inertia and

Youngs modulus, you can also

improve stiffness by increasing

the modulus of the material. You

can do this either by using more

glass fiber or by using a material

with a higher modulus. However, there are often practi-cal and economic limitations to this approach.

When evaluating the use of ribs, first consider potential

causes of stress.

When we work with customers on parts that failed, we

need know if it was due to excessive load or excessive de-

flection, explains Sebastien Petillon, CAE engineer for

Solvay Advanced Polymers of Alpharetta, GA. These are

two different reasons for part failure and cannot be dealt

with in the same way.

Failure due to excessive load can be remedied by

adding a rib, which increases the moment of inertia and

improves stiffness. Failure when deflected, often seen on

mating parts of a snap-fit design,

requires a change in geometry to

decrease the moment of inertia an

reduce stress on the part.

Rules for Ribs Use

Once youve determined that a ri

is the appropriate solution, you

must observe a few basic rules in

their design.

Start with rib thickness, which

can affect part weight, cosmetics

warpage and moldability. Thick

ribs can cause internal voids as

well as sink marks on the part sur

face opposite from where they ar

attached. The amount of sink is

also affected by the type of mater

ial, processing conditions, surfac

texture and relative location to a

gate. Materials with high stiffnes

and low shrinkage rates, such as IXEF

polyarylamidecreate less sink. One rule of thumb, as shown in the di-

agram at the top of this page, is to limit rib thickness t

40% of the thickness of the wall to which it is attached

to minimize sink marks. Go up to 60 percent to maxi-

mize strength. Consult your materials supplier for a rec

ommendation specific to your application.

The guidelines apply to the thickness at the base of

the rib. The rib should be tapered as it rises from the

wall to create a draft angle for easier ejection (see dis-

cussion of draft angle in Part IV of this series at

http://rbi.ims.ca/4398-502).

Keep in mind the surface characteristics of the oppo

site wall when considering rib thickness. If the appearance is critical or glossy, play it safe with thinner ribs.

Another idea: You can disguise slight sink marks with

steps, a textured surface, or through the use of produc

markings on the opposite wall.

Rib thickness can also affect moldability of the part. It

is a common misunderstanding that ribs act as flow lead

ers and help balance flow in a complex part, comments

Kirit C. Desai, CAE manager at Solvay Advanced Poly-

A Design Recipe For Ribs

S P E C I A L A D V E R T I S I N G S E C T I O N

[www.designnews.com] 10.10.05 DESIGN NEW

DESIGNING WITH PLASTICS:A Practical Guide for Engineers

Where to Find the First Four Parts?This is Part Five of a Design News series on plastics designsponsored by Solvay Advanced Polymers of Alpharetta, GA.The previous installments, listed below, can be accessed athttp://rbi.ims.ca/4398-502.High-Temperature PlasticsCan They Really Take the Heat?

Why You Must Consider Continuous Heat and Atmosphere

How to Manage StressAvoid Molding Pitfalls

R

H

T

R

H 4T

W 0.4T in order to minimize sink marks

W 0.6T in order to maximize strength

W

a

T/4==

=

=

Keep the thickness of the rib less than the thickness of the ad-

joining wall, but by how much depends on several factors.

Recommended Rib Design

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mers. As stated, ribs should be thinner than

the intersecting wall. By definition, a flow

leader is a local increase in thickness to im-

prove flow in a required direction. There-

fore, most of the time, ribs with thickness

less than base wall thickness do not enhance

the flow, continues Desai. In the majority of

cases, it winds up acting as a stiffener rather

than the intended flow leader.

One thought: If this is a gas-assist appli-

cation, location of gas channels at the base

of thick ribs can avoid problems associated

with excessive shrinkage, such as sink

marks or warpage.

Additionally, very thin ribs-particularly

those located close to a gate-can create filling

problems. Melt flow entering a thin rib can

slow down and begin to freeze off while

thicker wall sections are still filling. Thick-

ness of the rib (or any type of plate) also af-

fects shrinkage, and as a result the tendency

to warp.

Consider Rib Height

Tall ribs are a good idea, but height should

generally be no more than four to five times

the thickness of the adjoining wall. Ribs

that are too tall can create mold filling and

venting problems. One option is to design

multiple, smaller ribs that can provide the

same level of stiffness.

Its always easier to add ribs to a design

than to remove them. Incorporate a mini-

mum number of ribs in your original design,

then add them as testing dictates.

Another design consideration is radii for

the internal corners of ribs. Corners with

small or no radii are a major cause of failure

under load because they concentrate

stress. Its important to calculate the stress

concentration created by an internal cor-

ner. Use handbooks on strengths of materi-

als to obtain formulae that estimate the

stress concentration factor for different

geometries.

Rule of thumb: Choose an internal corner

radius equal to or greater than one-half ofthe thickness of the part, or at least 0.6 mm.

This rule applies to all internal radii, includ-

ing those for ribs.

S P E C I A L A D V E R T I S I N G S E C T I O N

24 DESIGN NEWS 10.10.05 [www.designnews.com]

FORCE

FORCE

FORCE

I

A

=

=

0.01

0.5 in2

I

A

=

=

0.06

0.5 in2

I

A

=

=

0.07

0.5 in2

I

A

=

=

Moment of Inertia

Area

0.5"

1.0"

0.25"

1.0"

1.0"

0.125" 0.125"

1.0"

0.35"

1.0"

0.15"

When to Consider Ribs

These drawings are shown with sharp

corners and no draft for illustration pur-

poses. In actual design, follow guidelines

for proper radii and draft angle.

To access The Design Engineers Portal forHigh-Performance Plastics, go to

http://rbi.ims.ca/4398-502.

Design engineers can con-

sider two approaches

when designing a wall sec-

tion to provide a given

amount of stiffness. They

can make a thick wall sec-

tion or use ribs. In the crosssection shown in the top il-

lustration, the wall thick-

ness is 0.5 inches to pro-

vide the required moment

of inertia. That provides

the necessary stiffness, but

will adversely affect part

quality due to limitations in

the molding process. Cool-

ing times will be extended,

creating longer processing

times and poor injection

molding economics.

What do you do? Add a

rib.

A rib allows a design that is

six times stiffer with a wall

thickness of 0.35 inches.

There is no increase in the

amount or resin used: the

total area for both cross

sections is the same. If the

opposite wall is cosmetic,

you may need to reduce

wall thickness even furtherto avoid development of

sink marks opposite the

rib. The third illustration

shows how wall thickness

can be decreased to 0.25

inches with the addition of

another rib. The total area

remains the same, while

stiffness increases. The po-

tential for sink marks is re-

duced because the wall

and ribs are thinner. Take

care that rib height doesnot exceed four to five

times the wall thickness.