Part Design

Selecting the Right Radius for Rib Root

When generating an internal radius between a rib and the common wall consider the mold machining method before choosing the radius profile, especially at rib-rib-wall intersections.  Choosing a common, simple radius will allow for quicker and more accurate radius cutting in the mold.  Generating a complex profile or intersection may require a cutter change increasing lead time and cost.

Scott Skurdalsvold
President
SAS Plastic Design
Mansfield, TX

Rib Design for Injection Molded Parts

A rib is a projection off the nominal wall thickness of the part. The base of the rib should not be more than half of the nominal wall thickness from where it projects from the nominal wall. Ribs that are more than half the nominal wall thickness will result in a sink mark on the opposite side of the part. This additional wall thickness can create areas of high stress concentration and internal voids at the base of the rib, thereby weakening the rib at that junction. The maximum height of the rib should not exceed three times the nominal wall thickness. Ribs that go beyond the nominal height of three wall thickness will not increase the strength of the rib.

Larry Schneider
President
Schneider Plastics, Inc.
Wadsworth, IL

Plastic Snap Fit Design Calculator Tools

The attached link to the Integral Fastening Program, developed by Rensselaer Polytechnic Institute, contains several snap fit calculators and a material property table:

http://www.rpi.edu/dept/mane/deptweb/student/snapfit/index2.htm

Robert V. Petrach, Jr.
Senior Product Engineer Delphi Thermal and Interiors
Troy, MI

Living hinge in plastics

A "living hinge" is a very thin portion of plastic that bridges two heavy walls and provides ability to flex repeatedly the part without the use of a classic mechanical hinge. When properly designed, living hinge made from polypropylene or polyethylene, can sustain a minimum of one million to a maximum of ten million flexing cycles without failure. To reach such long life cycles it is extremely important that the gate vestige be at least ½ - ¾ in (minimum 12 - 18 mm) away from the hinge area. Gates placed within hinge proximity create turbulence in the melt flow, hesitation, etc. It is best polymer flows perpendicular, across the hinge length. Both PP and PE, being semi-crystalline polymers, will align their molecular chains with the direction of flow.

Paul A. Tres
Sr. Technical Consultant
ETS, Inc.
Bloomfield Hills, MI

Radii on Injection Molded Parts are not just for Cosmetics

The primary functions of radii are the distribution of stresses, to facilitate the flow of material since plastic materials do not like to flow around or into sharp corners, to help with the ejection of the part from the mold, and to help maintain uniform wall thickness. The general rule of thumb for radii on plastic parts is 50% of the nominal wall thickness for the inside radii and 1 ½ the wall thickness for the outside radii. If, for some reason, you cannot put a radii of 50% of the nominal wall thickness on the inside corner it is recommended that you use a minimum of .015. In that same situation, you should use the same .015 plus the wall thickness for the outside radii.

Remember, any amount of radius is better than a sharp corner since you will probably not be able to fill a sharp corner in the molding process.

Larry Schneider
President
Schneider Plastics, Inc.
Wadsworth, IL