Week 8 - Challenge 2 - Base Bracket Design

Design of a bracket

Since thermoplastics shrink as they cool, they grip to cores or male forms in the mold. This makes ejection difficult and may lead to scuff marks on the part. To facilitate ejection, part surfaces parallel to the draw direction of the tool require taper or draft. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. The amount of draft needed depends on several factors:

• Material shrinkage – Some materials shrink more than others. In general, the higher the shrinkage, the larger the draft angle needed to prevent ejection issues.
• Height and shape of wall or feature – A shallow single straight rib having not enough draft causes less problems than a high cylindrical wall.
• Surface texture – A polished surface requires a smaller draft angle than a surface that is textured.
• Aesthetic requirements – Scuff marks might not be as big of a problem on a technical part "hidden" in an assembly while, for the cover part of the assembly, they may be totally unacceptable.

An intersection is created across the mid-plane of the component. Two lines are created on the parallel surface which are parallel or almost parallel to the die and mold movement. A bisecting line is obtained from the two lines.This is the tooling axis of the component. Draft analysis is carried out w.r.t. this tooling axis to check the draft angle of the surface.The min. draft angle is 3 degrees.

Given input surface

• Using the given A surface, a class B surface is obtained which has all the engineering features.
• The class A surface is extracted and offsetted to a distance of 2.5mm which is the thickness of the surface.
• Then C surface is created using the boundary of the class A surface.
• The class A & C surface are joined together and then trimmed using the B surface to form a thickned component.
• This surface is then taken into the part design workbench to obtain a solid component.

Final solid component is created.