Week 9 - Attachment Feature Creation - Challenge 2

Crearte Screw Bosses and Dog House on the Coin Holder

Introduction

Screw Boss

Boss features are commonly found in injection moulding designs. They are used to aid in the assembly of moulded parts by providing a channel for a screw. Designing plastic screw bosses is fairly straightforward but there are a few considerations to ensure a strong connection between parts and mitigate cosmetic defects.

Screw bosses can be reinforced with ribs or gussets for added strength and durability. 

Design rules of Screw Boss

• Draft -5° on walls of the boss
• Thickness - generally, 40% of nominal wall at base
• Height to Diameter ratio - maximum of 3:1, preferred 2:1
• Radii – maximum of 0.25mm
• Boss gussets must meet Rib guidelines

Dog House

Dog house is an engineering feature used in plastic trim design. These are used as supporting feature and other engineering features such as bosses, locators etc are mounted on them to increase their strength. Dog houses require a slider for moulding and are subjected to draft analysis along the slider axis to prevent breakage during ejection from mould cavity.

Design rules for Dog house

• Draft -5 degrees on walls of boss
• Thickness - Typically 40%* of the nominal wall at the base
• Wall thickness at intersection of the part wall – 40% or less than the main stock
• Radii - maximum of 0.25mm

Process

1. Creating Screw Boss

• An axis is made at the centre point of the extracted bottom surface of the coin holder
• We draw a sketch for the screw boss on the XZ axis of the axis system
• Considering an M6 screw we dimension the sketch with the thickness of the boss being 0.6 times the thickness of the component i.e. 8.2mm

          Thickness of boss, W= 0.6*2.5 =1.5mm

          Screw circle dia = 5.2mm

          Boss circle dia = 5.2 + 1.5*2 = 8.2mm

• The sketch is Pad for 12mm of height
• Draft of 0.5° along the main tooling axis is applied on the inside and outside faces
• 4 Ribs are created around the boss to strength it and add support to the boss
• The ribs are pad for a height of 10mm and a thickness of 0.4 times the component thickness i.e.1mm
• The faces of the ribs are drafted for 0.5° along the main tooling axis
• Fillet of 0.25mm is added to all the faces of the boss and ribs

Axis system on extraced surface

Sketch for boss

Pad for 12mm

Draft of 0.5degress

.

Sketch for the ribs

Ribs are pad for aheight of 10mm and thickness of 1mm

Draft for the ribs

Fillet for all the faces

2. Creation of Dog house

• We create a sketch for the dog house in the XZ axis of the axis system
• The sketch is swept along the tooling axis with a draft of 0.5°, and a length of 20mm
• Using Multi-Section Surface, we create the roof of the dog house and join it with the swept surface
• The dog house is then thickened by 1mm
• A side core axis is created for the slider mould of the dog house, by creating a line from the midpoint of the bottom surface of the coin holder in the Z axis
• The inner surfaces are drafted by 0.5° along the side core axis
• Fillets are added to the inner surface
• The bottom extracted surface is then offset above by a distance of 4mm
• The inner surface of dog house is extracted and offset by a distance 1mm which is 0.4 times the component thickness
• These surfaces are trimmed with each other and the dog house is split using this surface
• Fillet is then applied on the outer edges
• The inner split face is then drafted for 5° along the side core axis
• Fillets are applied on the inner split face
• The Screw boss created earlier is copied and translated to position it above the dog house and assembled with it

Sketch for dog house

Swept along the tooling axis

Creating the dog house roof

Thicken

Side core axis

Draft along side core axis

Offset surfaces

Split

Slpit face draft

3. Draft Analysis

Draft analysis along main tooling axis

Draft analysis along side core axis

Report

• Faces in green have a draft angle of 3 degrees
• Those in blue have a draft angle lesser than 3 and greater than 0 degrees
• The faces in red either have negative draft angle

Views of the model