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2 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS

Injection Molding Design Guidelines

Design Guidelines for Boss

Issue V, Feb 2015


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Welcome to another issue of the DFM Guidebook. We highly appreciate your feedback for our previous issues. Please continue sending us your comments, suggestions and ideas for subsequent issues.

This week we will provide a comprehensive summary of important design guidelines for Injection Molding, covering design guidelines for Boss.

Injection-molded parts should be designed such they slip easily out of their molds and can be produced economically at a better quality and shorter lead time.

Adhering to some basic Design for Manufacturing rules for injection molded parts can improve the moldability of the designs and the life of the mold, increase the operational life of the part, and reduce manufacturing cost.

The rules covered in this issue are Minimum Radius at a base of Boss, Spacing between Bosses, Radius at base of a hole in boss, Minimum draft for Boss OD, Minimum draft for Boss ID, Boss Height to OD ratio, Boss Height to ID Ratio, and Minimum Radius at Tip of Boss. Following these guidelines will improve the associated dimensional accuracy, surface finish and quality.

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

Rahul Rajadhyaksha Senior Product Manager Geometric Limited

http://www.dfmpro.com/


Contents

Minimum Radius at Base of Boss ............................................................................ 6

Spacing between Bosses ......................................................................................... 7

Radius at Base of Hole in Boss ................................................................................ 8

Minimum Draft for Boss OD ................................................................................... 9

Minimum Draft for Boss ID ................................................................................... 10

Boss Height to OD Ratio ........................................................................................ 11

Minimum Radius at Tip of Boss ............................................................................ 12

Chamfer at Top of Boss ......................................................................................... 13

Wall Thickness of Boss .......................................................................................... 14

Standalone Boss .................................................................................................... 15


Minimum Radius at Base of Boss

Bosses find use in many part designs as points for attachment and assembly. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware. Under service conditions, bosses are often subjected to loadings not encountered in other sections of a component. Provide a generous radius at the base of the boss for strength and ample draft for easy part removal from the mold.

A fillet of a certain minimum radius value should be provided at the base of boss to reduce stress. The intersection of the base of the boss with the nominal wall is typically stressed and stress concentration increases if no radii are provided. Also, the radius at the base of the boss should not exceed a maximum value to avoid thick sections. The radius at base of boss provides strength and ample draft for easy removal from the mold.

It is recommended that the radius at the base of boss should be 0.25 to 0.5 times the nominal wall thickness.

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Spacing between Bosses

When bosses are placed very close to each other, it results in creating thin areas which are hard

to cool and can affect the quality and productivity. Also, if the mold wall is too thin, it is very

difficult to manufacture and often results in a lower life for the mold, due to problems like hot

blade creation and differential cooling.

The general guide line suggests that spacing between bosses should be at least 2 times the

nominal wall thickness.


Radius at Base of Hole in Boss

Providing a radius on the core pin helps in avoiding a sharp corner. This not only helps molding

but also reduces stress concentration.

It is recommended that the radius at base of hole in boss should be 0.25 to 0.5 times the nominal

wall thickness.


Minimum Draft for Boss OD

An appropriate draft on the outer diameter of a boss helps easy ejection from the mold. Draft is

required on the walls of boss to permit easy withdrawal from the mold.

It is recommended that minimum draft on outer surface of the boss should be greater than or

equal to 0.5 degree.


Minimum Draft for Boss ID

Designs may require a minimum taper on the ID of a boss for proper engagement with a fastener.

Draft is required on the walls of boss to permit easy withdrawal from the mold.

It is recommended that minimum draft on the hole in boss should be greater than or equal to

0.25 degree.


Boss Height to OD Ratio

A tall boss with the included draft will generate a material mass and thick section at the base. In

addition, the core pin will be difficult to cool, can extend the cycle time and affect the cored hole

dimensionally.

General guidelines suggest that the height of boss should be less than 3 times of outer diameter.


Minimum Radius at Tip of Boss

Bosses are features added to the nominal wall thickness of the component and are usually used

to facilitate mechanical assembly. Under service conditions, bosses are often subjected to

loadings not encountered in other sections of a component.

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


Chamfer at Top of Boss

Boss should have chamfer on top. A chamfer at top of boss is good lead in for the fasteners.


Wall Thickness of Boss

Wall thicknesses for bosses should be less than 60 percent of the nominal wall to minimize sinking.

However, if the boss is not in a visible area, then the wall thickness can be increased to allow for

increased stresses imposed by self-tapping screws.

It is recommended that wall thickness of boss should be around 0.6 times of nominal wall

thickness depending on the material.

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Standalone Boss

Bosses and other thick sections should be cored. It is good practice to attach the boss to the

sidewall. In this case the material flow is uniform and provides additional load distribution for the

part.

For better rigidity and material flow, the general guideline suggests that boss should be

connected to nearest side wall.

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