Week 9 - Attachment Feature Creation - Challenge 2
Screw Boss & Dog House
Bosses are one of the most common features seen in plastic parts. However, similar to ribs if bosses get too thick relative to the nominal wall thickness, the sink can occur. A boss-rib combination can eliminate sink marks. By using ribs to connect the boss to a side wall, this method of part design will provide the strength necessary to support screws, inserts, etc. Also, adding small radii to break the sharp corners will also greatly reduce stress concentrations.
Design Rules for screw Boss Design
- Draft - 0.5 degrees on the walls of boss
- Thickness - Typically 40%* of the nominal wall at the base.
- Height to Diameter Ratio - 3:1
- Radii - 0.25mm maximum radii.
Dog House
The dog house is an engineering feature used in the plastic trim design. Dog houses are used as supporting features. Sometimes other engineering features like snaps; locators etc. are mounded on them to increase their
strength. Dog houses are subjected to draft analysis to prevent breakage of the component during ejection from mold cavity. Dog house and other engineering components are built on B-surface.
Rules For Designing Dog House
- Doghouse wall thickness at the intersection of the part wall ( C ) should be 40% or less than the main wall stock ( T ).
- The height of ( B ) should be between 3.0 – 6.0 mm
Tooling Axis
The Tooling Direction is basically the direction that the mold pulls apart. The mold is built in two separate pieces, basically. You have a Cavity side and you have the Core side. The Core pulls apart from the Cavity and that's the Dyeline or the Tooling Direction.
Side Core tooling Axis
When a product is designed in such a way that it cannot be cleared with the main tooling axis during the molding process, there we use the side core axis, another tooling axis perpendicular to the main tooling axis.
Design procedure of screw boss
- Extract the surface onto which screw boss is to be created and create a sketch for the same on it.
- Perform pad operation to convert into 3d with proper dimension.
- Perform drafts on the outer and inner surfaces of the boss.
- Create a sketch on the same extract for supporting ribs.
- Union trims with the boss to remove the inner material of the rib.
- Give draft to each face of ribs with main tooling axis as pulling direction and give proper fillets.
- To create other bosses, perform translation operations.
Design procedure of dog house
- Make a rectangle to enclose the screw boss in it but make sure it is open from one side.
- Sweep it with tooling axis as the pulling direction and give a minimum draft of 1.5deg inwards.
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Enclose it from the top by using the off-set of extract of the base of class-A surface. The offset distance determines the height of the dog house, so give it according to the design rules.
- The inward face of the dog house should be drafted too, so give them a draft of 0.5deg.
- Use the same extract of class-A, and offset it at the height of 3mm-6mm. 8. Extract the inside faces of the dog house and offset it outwards at 0.4't and trim them togather.
- Go to the part design and split it with the model
- After that we will fillet the entire model of the dog house and add it to the main body
- Later we mount the screw boss on the dog house by translating it.
Tree Structure
