Boss Design Guide Designing bosses must follow specific guidelines since these features are subjected to higher loads than other sections of a part. Critical Considerations: Wall Thickness and Height When adding bosses to your plastic part design, wall thickness and height are critical to success. Here are some guidelines to help achieve success with boss features. A boss is a common design element in plastic injection molding. It is a round protrusion added to plastic parts to facilitate certain functionality such as: Wall Thickness 1. If the nominal part thickness is less than 1/8", the boss wall thickness should be 60% of the nominal wall. 2. If the nominal part thickness is greater than 1/8" the boss wall thickness should be 40% of the nominal wall. • Mounting fixture • Location point • Reinforcement feature • Assembly 3. To minimize sinking, wall thickness for bosses should be less than 60% of the nominal wall. 4. If the boss is not in a visible area, wall thickness can be increased. Wall thickness for bosses should be less than 60% of the nominal wall to minimize sinking. 5. If the boss is not in a visible area, the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws. Did You Know? Adding draft angles to boss features that lie perpendicular to the parting line of your part will help to ease ejection from the mold. Height 1. Height of the boss should be no more than 2-1/2 times the diameter of the hole in the boss. 2. For screws in plastic, usually 2 to 2.5 times the nominal screw size is recommended for thread engagement. 3. Boss height should be less than 3x OD of the boss. A tall boss with the included draft will generate a material mass at the base. The core pin will be difficult to cool, extending cycle time and affecting the cored hole dimensionally. Contact Us Today at xcentricmold.com | 586-598-4636 | sales@xcentricmold.com Xcentric Mold & Engineering™, Reference: Bayer MaterialScience LLC, Engineering Polymers Part and Mold Design Thermoplastics A Design Guide, Copyright © 2000, Bayer Corporation. Boss Design Guide Additional design guidelines and consideration when adding bosses to plastic part designs. Boss Design Tips 1. Minimum Radius at Base of Boss - Radius at base of boss should be 0.25 - 0.5 times nominal wall thickness. 2. Spacing Between Bosses - Maintain minimum of 2 times nominal wall thickness between bosses. 3. Minimum Draft for Boss OD - maintain minimum draft of 0.5 degrees on outer surface of boss. 4. Minimum Draft for Boss ID - maintain minimum draft greater than or equal to 0.25 degrees. 5. Add Fillet at Tip of Boss - adding a fillet reduces stress on the boss. 6. Chamfer at Top of Boss - adding chamfer provides a good lead in for fasteners. Boss Placement (see illustration) Attach Bosses to sidewalls for better rigidity and material flow. Avoid thick sections or placing bosses directly in the corner as doing so can cause sink. For standalone bosses, add four orthogonal tapered gussets. What Are Sinkmarks Sink marks are caused mainly by thermal contraction as the polymer cools in thick regions of plastic parts. The material on the outside of the section will cool and solidify faster than the material near the core. These different rates of cooling cause the surface to be pulled inward, creating a depression, or low spot, in the surface. Sink marks are often found where ribs and bosses are thicker than an adjacent wall. Contact Us Today at xcentricmold.com | 586-598-4636 | sales@xcentricmold.com Xcentric Mold & Engineering™, Reference: Bayer MaterialScience LLC, Engineering Polymers Part and Mold Design Thermoplastics A Design Guide, Copyright © 2000, Bayer Corporation.
