Week 9 - Attachment Feature Creation - Challenge 2
ATTACHMENT FEATURE CREATION (SCREW BOSSES & DOG HOUSE) FOR A CENTRE CONSOLE COIN HOLDER USING CATIA v5
OBJECTIVE
To design screw bosses and dog house features for a previously designed model of a centre console coin holder according to proper guidelines required for plastic moulding
INTRODUCTION
Manufacturing process: Injection Molding
Brief about the process
It is a manufacturing process used for the mass production of plastic components. It works by injecting molten material (plastic) into a mold. Under the application of heat and pressure, the material solidifies into the required shape. Once the product cools, it is ejected from the mold with the help of ejector pins.
Terms in the injection molding process associated with this project
Draft
To facilitate easy removal of components from the mold, it is necessary to add a taper all along the surface with reference to the tooling direction. This taper is termed as a draft and its value ranges from 0.5 degrees to 3 degrees, with the latter being the most widely accepted norm.
The Draft analysis command in CATIA enables to detect if the drafted part can be removed in an easy manner. This analysis is performed based on colour ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.
Tooling direction
The direction along which the core pulls apart from the cavity is termed as the Dyeline or the Tooling direction.
Software : CATIA V5
CATIA (an acronym of Computer Aided Three-dimensional Interactive Application) is a multi platform software suite for CAD, CAM, CAE, PLM and 3D , developed by the French company Dassault Systèmes.
The version of CATIA used for this project is V5. For the target model, the features in the part and surface workbench were extensively used.
Target Model: Coin holder used in automobiles
A coin holder is a plastic component usually found in automobile interiors especially as a part of the door arm rest.
Methodology in CATIA associated with this project
Bisecting line method
Since draft is measured with reference to a tooling axis, the first step after importing the model into CATIA was the creation of a tooling axis. To incorporate for tool clearances due to the presence of tapered surfaces in the part, the bisecting line method was used to create the tooling axis.
After converting the surface into a 2D profile using the Intersection command from the surface workbench, two inclined lines were created using the endpoints of the 2D curves which represented the tapered faces. Using a bisecting line type, a third line was created along the centre point which signified the coincidence of the tapered faces. This line was the required tooling axis for this component.
NOTE: This project did not have features which needed a titled tooling axis and hence the tooling axis was created using a bottom face extract and creating a line through the centre point.
Draft Analysis
The draft analysis command in CATIA helps to detect whether the drafts provided in the model will facilitate easy removal of the component during manufacture.
This analysis is performed based on colour ranges identifying zones on the analyzed element where the deviation from the draft at any point, corresponds to specified values.
- When the draft angle value of the surface is equal or smaller than a graduated value, the color below the graduated value is applied
- When the draft angle value of the surface is greater than the greatest graduated value, the color above this graduated value is applied
In this project, the required draft for all surfaces were chosen as 3 degrees and hence the quick mode option in draft analysis was chosen which incorporated two different colours for two input values.
DETAILED PROCEDURE
- Create a sketch for the screw boss design
1. Using an extracted surface from the bottom of the coin holder part as the reference, a sketch was created with two concentric circles.
2. The inner circle was of 5.2mm dia while the outer circle had 8.2mm dia which gave around 3mm wall thickness for the component and hence was proper according to screw boss design guidelines
- Extrude the sketch along the tooling direction
1. Using the pad tool, the sketch was extruded to a height of 12mm, forming the screw boss (Height:Diameter= 12:5.2=3:1.3=close to 3:1 which is the accepted norm)
2. A draft of 0.5 degrees was applied to the outer surface of the screw boss, with the base of the coin holder part as the neutral plane and the main tooling axis as the pulling direction
- Add strengthening features to the screw boss
1. The screw boss is usually strengthened using ribs to support the height. These ribs were designed using symmetric lines across the screw boss.
2. These lines were extruded to a height less than the height of the screw boss (=10mm)
3. A proper draft (0.5 degrees) was applied on the ribs
4. The sharp edges were smoothed using a fillet of 0.25mm
5. The inner portion which intersected with the screw boss was trimmed using the union trim tool
- Create the design sketch for a dog house feature
1. Added another screw boss by translating the previous body to a new location on the same surface by choosing a different coordinate.
2. Using the centre of the translated screw boss as the reference, a rectangle sketch was created for the dog house
- Design the dog house
1. This sketch was then formed into a drafted surface with a height of 21mm (extra 1mm for trimming) using the sweep tool with a draft of 1.5 degrees
2. To cover the top of the swept surface, the base surface was offsetted upto a height of 20mm
3. The offsetted surface and the swept surface were trimmed together to form the surface design for the dog house
4. The surface was then thickened using the thick surface tool to create a solid part
5. Using proper surfaces, the dog house (solid part) was trimmed to form the final component
6. To create the intersection surface (C) with a wall thickness of 40% T, (in this case, =1mm) the offset and trim commands were used to create a surface which was then used to split the solid feature
- Add a side core axis
1. To clear the dog house feature a side core axis needed to be created in addition to the main tooling axis.
2. The same centre point used to create the main tooling axis was reused with the Z direction for line extension
- Adjust the positIon of the translated screw boss
1. The translated screw boss was adjusted so as to make the design intersect with the top of the dog house
- Perform draft analysis
1. Perform draft analysis to ensure that the part is manufacturable
2. Observe the results and make modifications if needed
DRAFT ANALYSIS: RESULTS
1. Draft- Main tooling axis
2. Draft- Side core tooling axis
3. Draft- Analysis under the running point
STANDARD VIEWS OF THE SOLID MODEL
TOP VIEW
SIDE VIEW
FRONT VIEW
ISOMETRIC VIEW
CONCLUSION
This project imparted a great deal of knowledge regarding the creation of screw bosses and dog house features in a plastic component by following proper DFM guidelines. It also clarified the need for attachment features in a plastic component and how it helps the designing process from an economical point of view.