Week 9 - Attachment Feature Creation - Challenge 2

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.

Draft analysis

An intersection of the part is taken along its midplane and 2 lines are created on its faces which are parallel to the direction of die movement. A bisecting line is obtained from the 2 lines which forms the tooling axis.

Creation of screw boss

• Screw bosses are used in part design as points of attachments and assembly.
• Most of the screw bosses are usually cylindrical which are used to accomodate screws, inserts or any other type of fasteners.

Design and considerations for screw boss

• Draft angle - 0.5 degrees
• Thickness - 0.6*T
• Fillet radius - 0.25mm
• Outside diameter - 2 to 2.4 times the diameter of the screw or any fastener.
• Height to diameter ratio - 3:1

Screw boss is created on the coin holder and draft analysis is carried out on the same.The screw boss created is found to be clearing the tooling axis and the design is acceptable.

Creation of dog house

A dog house is a feature which helps to avoid sink marks and also to accomodate screw bosses in certain cases.For example if a screw boss is perpendicular to the tooling axis it cannot be directly attached to the component, so it is supported with the help of a dog house.

Design criteria for a dog house

• Thickness - >/= to 40% of the main wall thickness.
• 'C' is where the doghouse contacts the backside of the surface.
• Height - 3 to 6mm

• Width - 12mm
• Width to height ratio - 3:1 & 2:1
• Draft angle - 1.5degree min. draft angle

A dog house is created on the coin holder and a side core tooling axis is created for the dog house.

Draft analysis is crried out w.r.t. side core axis and the component is found to be clearing the tooling axis.

Draft analysis is carried out on the final component.