Week 9 - Attachment Feature Creation - Challenge 2
Designing a dog house and screw boss for Coin holder body:
Dog house:
During the process of injection molding, sink marks offer occur at the junction of plastic body surface and the engineering features attached to it. This is due to more contact area between the features and body surface. Dog houses help avoid sink marks by reducing contact area. They make a feature stand at a distance from B-surface. They also enables us to create a feature perpendicular to main tooling axis.
The design guidelines for a dog house is shown below:
For coin holder,
T: Main body thickness=2.5mm
C: Location where dog house contacts rearside of surface.
The thickness of C = 0.4*T =0.4*2.5 =1mm
Height(B) can be in between 3mm to 6mm.
The draft should be of 1.5 degree minimum.
The width of dog house must be min. of 12mm.
Dog house creation:
With the extract of class-B as reference draw the sketch as shown below.
Each side is taken to be 24mm. Since dog house should have a draft of 1.5 degree use the sweep command to create a drafted surface for a length of 21mm.
Untrim the extracted part of B-surface. Offset it to 20mm. Trim with the extrude to get the basic surface. Convert it to solid object using Thick surface commad giving 2mm as thickness.
The cuts in lower portion are obtained by following steps.
Taking the height to be 4mm extract and offset the portion of A-surface to 6.5mm. Since C=1mm, offset the trim of B-surface to 1mm as shown.
Both of them are trimmed to get the following surface.
Add draft to faces of dog house as shown.
Now dog house is split with the surface as shown.
Proper fillets are added at all locations to form the final dog house.
Side core tooling axis:
Apart from the main tooling axis, a side core axis is created to allow the clearence of doghouse. This is created as shown below.
The faces of dog house are drafted with respect to side core tooling as shown.
Screw Bosses:
Screw bosses serve as points for attachment and assembly of components. To avoid defects they shoud not be merged into side walls. The design rules for screw boss are shown below:
The outer and inner diameters are 5.2mm and 8.2mm.
The height is taken to be 12mm.
Procedure for creating screw boss:
Create a sketch containing two circles shown below.
Pad both of them to form a hollow cylinder.
Draft of 0.5 degree and proper fillets are given on inner and outer surfaces.
The boss formed will be as shown below.
To support the boss four symmetruc ribs are created following rib design rules.
The assembly of Ribs and boss should be as shown below.
The Boss copy is translated using coordinates option in translate command. It is made to rest on dog house as shown below.
The dog house and two screw bosses are then assembled to the main coin holder body.
Draft analysis:
The draft analysis is done with following steps:
a) Select "Materials" option in View mode customization toolbar.
b) Select "Feature Draft analysis" under analysis option.
c) Select compass button in draft analysis box and orient the compass in tooling axis direction (Main tooling axis, Side core tooling axis).
d) Lock the compass and click anywhere on the surface to get complete draft analysis.
Draft analysis of Coin holder Class-A surface:
About Main tooling axis:
Draft analysis of Coin holder with Dog house and screw boss:
About main tooling axis:
About side core tooling axis:
