A Definitive Guide to DFM Success Injection Molding Issue III Dec 2014

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2 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS

Injection Molding Design Guidelines

Design Guidelines for Ribs

Issue III, Dec 2014


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Welcome once again to another issue of DFM Guidebook!

Whether youre counting down the days to Christmas or finishing your planning for 2015, we thought our DFM guidebook would provide some important tips to help you design effectively in the coming year.

This week we cover some important guidelines for Injection Molding.

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Injection Molding is one of the most commonly used technique for manufacturing plastic parts. A wide variety of products including automotive interiors, medical devices, packaging to the coolest toys that you gift your little ones on Christmas are manufactured using injection molding.

Ribs are longitudinal protrusions on a plastic part and are frequently used by product designers in injection molded plastic part designs. They are used to either increase strength of a part or as a decorative feature in the design. In this issue, read important design guidelines for Ribs such as Rib Height and Thickness, Minimum Radius at Base of Ribs, Draft Angle and Maximum Spacing between Two Parallel Ribs.

If you missed reading previous issues of DFM Guidebook, please visit our website, www.dfmpro.com

Happy Holidays!

Rahul Rajadhyaksha Senior Product Manager Geometric Limited


Contents

Recommended Rib Parameters .............................................................................. 6

Minimum Radius at the Base of Ribs ...................................................................... 7

Draft Angle for Ribs ................................................................................................. 8

Spacing Between Two Parallel Ribs ........................................................................ 9


Recommended Rib Parameters

Ribs are thin wall-like protrusions designed in a plastic part. Ribs are used by product designers

to mainly improve melt flow into a section, like corner or a large boss or to add strength to a

part.

The most economical way to increase stiffness of the part is by adding reinforcing ribs. Ribs should be placed on the molds opening direction. For parts under bending, ribs should be placed perpendicular to the bending moment to achieve optimum stiffness.

However, projections like ribs can cause cavity filling, venting, and ejection and strength problems. The problems are even severe when ribs are taller than recommended height.

Rib parameters should be considered in correct ratio to avoid defects such as short shots and rib strength. Thick and deep ribs can cause sink marks and filling problems. Deep ribs also lead to ejection problems. If ribs are too long or too wide, supporting ribs may be required.

As a general guideline, rib height should not be more than 2.5 to 3 times the nominal wall thickness. Similarly, rib thickness at the base should be 0.4 to 0.6 times the nominal wall thickness.

It is also advisable to use multiple smaller ribs than one large rib as it can lead to voids or sink marks.

Example

t = Nominal Wall Thickness

W = Width of the Rib

H = Height of the Rib

T = Thickness at base of the Rib


Minimum Radius at the Base of Ribs

A fillet of a certain minimum radius value should be provided at the base of a rib to reduce stress.

However, the radius should not be large enough that it results in thick sections. The radius

eliminates sharp corner and stress concentration. Flow and cooling are also improved.

Fillet radius at the base of ribs should be between 0.25 to 0.4 times the nominal wall thicknesses

of a part.

Example


Draft Angle for Ribs

Draft angles are important element of successful plastic part design. Plastic parts have higher

tendency to shrink on the core, causing contact pressure and friction between the part and core.

Providing a proper draft angle helps in easy removal of plastic part from the mold. It also helps in

improving cycle time and productivity.

A general guideline suggests that draft angle for rib should be between 1 to 1.5 degrees.

Minimum draft should be 0.5 degree on each side. Draft angles should be provided on interior or

exterior walls of the part along the pulling direction.

Example


Spacing Between Two Parallel Ribs

Mold wall thickness is impacted due to spacing between various features in a plastic part. If ribs

are provided too close to each other or to the walls of a part, thin areas are created which are

difficult to cool and affects part quality. Additionally, if mold wall is too thin, then it becomes

difficult to manufacture such molds and can also result in hot blade creation and differential

cooling eventually impacting mold longevity.

A general guideline suggests that spacing between ribs should be at least 2 times the nominal wall

thickness.

Example

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A Definitive Guide to DFM Success Injection Molding Issue III Dec 2014