Week 9 - Attachment Feature Creation - Challenge 1

                                                                COIN HOLDER WITH RIB DESIGN

Objective:

            The main objective is to create Ribs on Automotive Plastic Component - Coin Holder using industry principles and Design Rules. Concepts such as Rib Root Thickness, Draft Angles, Draft Analysis, Determining the Tooling Axis, Class A, Class B and Class C surfaces. The Component is Designed to manufacture maintaining a wall thickness of 2.5 mm using Surface Design and Solid Design techniques in Catia V5 application.

Introduction:

Design Rules for Plastic Components:

• Nominal wall thickness of a plastic component is 3 mm. For a strength component such as Instrument Panel, Bumper, Door Trims the values of thickness are larger than 4 mm. For example, Thickness of IP may lie between 5 - 6 mm, for Bumper it may be 6 – 8 mm. For Smaller components the thickness may be between 2 – 3 mm as in this case of coin holder.
• Deviation of +/- 30% from the wall thickness is allowed with gradual increase or decrease to avoid any plastic defects while manufacturing. For example: A component having wall thickness 3 mm should deviate within the range of 2.1 mm minimum and 3.9 mm maximum.
• Ideal draft angle should be 3⁰ for the overall component along for the faces lying in the tooling direction.

Ribs Design:

• Rib is a B-Side Engineering feature which provides a means to economical stiffness and strength in Plastic Molded parts without increase in overall wall thickness.
• Rib also facilitates locating and arresting components of an assembly.
• It provides alignment in Mating Parts.
• It acts as Stop and Guides for Mechanisms.

Guidelines for Rib Design:

• Proper Rib design is necessary to avoid plastic defects which involves the following critical issues that needs to be considered which are:
• Draft Angle
• Rib Thickness at base
• Thickness at Top of the Rib
• Rib Height
• Location of Ribs and Spacing between 2 Ribs
• Optional Radius or Fillet Radius
• Rib Root Diameter
• Quality of Ribs
• Manufacturability
• These guidelines are separate for Show and No – show surfaces.
• The Below table gives specific values for both cases.
• Consider ‘t’ as wall thickness of base for component.

Draft Angle:

• Draft is an angular tapering given at the design stage to every Plastic component that has to be manufactured.
• It is given in the direction of the Mold Movement.
• The draft allows the parts to release easily from the mold without creating friction between the surface of component and the core and cavity plates.
• When a Plastic component cools down inside the mold, the natural tendency of it is to shrink and attach to the core or cavity half.
• The movement of mold opening causes friction on the walls of the component as well as the core/cavity blocks.
• This reduces the life of the mold as well as the component itself.
• The component may also have rough surface finish as well as weak structure due to this method.
• This problem can be solved by giving draft to the component.
• Most applications require minimum draft angles of around 0.5⁰ to 1⁰, however 3⁰ draft angles are widely accepted for cost saving purpose.
• 7⁰ Draft angles are given to textured surfaces like Instrument Panels.
• Minimum possible draft angles are given to screw bosses, ribs and stiffeners due to their critical nature.
• Walls having 0⁰ or negative draft are difficult to manufacture thus, are required to redesign from the Class A stage.

Tooling Axis:

• Tooling Axis is the direction in which the mold opens and closes.
• Part Drafts are given along this axis.

Draft Analysis:

• Draft Analysis is a tool in Catia which identifies the zones which deviates from the specified values of angles along a defined draft direction using colour codes.
• It highlights the region of draft using colours.
• Draft Analysis is the first step performed after receiving the Class A of the component.
• Draft analysis should be passed in order to make the plastic component manufacturable.
• Acceptable draft angles are used as range for draft analysis.

Class A Surface:

• In automotive design, a class A surface is any of a set freeform surfaces of high efficiency and quality. It is the visible aesthetic surface of any automotive component.
• Class A surface refers to those surfaces which are visible and abide to the physical meaning, in a product.
• This classification is primarily used in the automotive and increasingly in consumer goods industry.
• It is a requirement where aesthetics has a significant contribution.
• For this reason, the exterior of automobiles is deemed Class A.

Class B Surface:

• Class B surfaces are engineering surfaces which are invisible to the eyes or often on the back side of the Class A surfaces.
• It is mainly the thickness of the component or Class A.
• These surfaces need to be designed as per the Class A surface i.e., the continuity, thickness of the component, functionality etc should be maintained.
• These surfaces include the main engineering features such as Screw bosses, Ribs, Stiffeners and fixations.
• B surfaces also do not contain any aesthetic styling content and textures since it is not visible to eyes.

Class C Surface:

• Class C surfaces are connectors between Class A and Class B surfaces.
• This surface connects both A and B surfaces along or angled to the Tooling Axis.
• It contains the draft as per the design.
• It is basically the thickened surface between both A and B surface.

Design Methodology:

            The design methodology of Coin Holder Design is explained through flow charts.

                                                   

Description:

Class A (Input):

• After receiving the Class-A for the Coin Holder check the connectivity of the surfaces on it.
• Use Join operation, boundary operation and disassemble.
• The check is over if any discontinuities do not arise.
• Delete the operations after check.

Creation of Tooling Axis:

• Identify faces which can potentially create problems while manufacturing.
• Surfaces at angles along a desired Tooling Axis.
• Extract a plain surface and create a point on its center for Axis system creation.
• In this case Y axis can be possible Tooling Axis Direction.
• The highlighted coloured faces can be problem in terms of draft analysis.
• Intersect the YZ Plane with the Class A.
• Since Most of the Faces from the Intersect are either parallel or at 90⁰ or having comfortable draft angles, the Tooling Axis will be created in direction normal to the Base Surface along the origin of the axis system.
• Publish the Tooling Axis as the Class A for easy access using Toolsà Publications as per the below image.

 

Draft Analysis on Class A:

• Change View Mode Customization to Material mode.
• Go to Insert – Analysis – Feature Draft Analysis and move the compass to the Tooling Axis. Change the range to 3⁰. Apply the settings as shown below image.
• As shown the target area has 27⁰ draft angle on a face.
• The input Class A also has walls having negative angle to the tooling axis and some having 0⁰ too.
• This can be an issue while manufacturing the component as it would be very difficult to draw from the molds.
• So, this design needs to be reconsidered at the Class A Styling stage where improvising on the draft angles is required.
• Minimum Positive drafts can be considered while styling after which can be considered to be thickened and manufactured.

 

Creation of Class B Surface:

• Extract the relevant surfaces from the Class-A surface which excludes the Fillets and holes.
• Here the selected faces consist of Side walls, Base, Top walls, Initial trims, Outer Flange.
• Extract of these surfaces as shown in below image.
• Use Untrim Command to untrim the extracted surfaces and create a natural extend.
• Extrapolate the untrimmed surfaces so that each of them intersects clearly and pass way forward than the Class A will help in its trimming in further operations.
• Offset the extrapolated surfaces to the value of required Thickness of the part.
• In this case of Coin Holder, the thickness value is 2.5 mm.
• The offset faces are the part of B-Surface.
• Then, trim the extra surfaces to obtain similar extended surfaces as in class A.
• The surface obtained after trim is shown in below image.
• Apply fillets while maintaining the part thickness between the Class A and Class B surfaces.
• The value of Fillet is given by the equation (B surface fillet = A surface fillet + Thickness of the part).
• The sign + or – must be decided so as to maintain the part thickness which in this case is 2.5 mm.
• Filleted image as shown in below image.

 

Creation of Class C Surface:

• For creating Class C surface, multiple extract command is used to highlight the boundary of Class A surface.
• Applying sweep command to create Class C surface with respect to class A surface and Extracted boundary lines.
• Then joining Class C surface and Class A surface to create solid body.
• The Class C surface is shown in below image.

 

Creation of Closed Part Body:

• Join all the sweep surfaces along with the Class B surface.
• The Final trim should be one single surface entity without any gaps.
• Join the final trim surface with the Class A surface to obtain the final enclosed surface for the coin holder.
• Insert New Part Body, select the mechanical part design tab to create solid part body of Coin Holder.
• The closed solid part body of Coin Holder is shown below.

 

Creation of Ribs On B - Side:

• In generative shape design workbench, Extract the base from B surface and create a Point on origin to Axis System.
• Create a sketch on the B-side Surface to develop Ribs.
• By clicking on the Sketch tab, create a line and add dimensions to be iso-constrained and click Exit.
• Select Extrude command, the dialog box appears in that select Profile as Line.1, Direction as Tooling Axis and Limit 1 as 12.5 mm (5 * t = 12.5 mm, where, Thickness (t) = 2.5 mm) and Click Ok to validate.
• Also, extrude it 1 mm below the surface, so that it intersects the base.
• Switch to part design workbench and thicken the Rib to a Base thickness Value as per Design Rule.
• Thickness should be 0.5 times the base wall thickness (i.e., 0.5 x 2.5 = 1.25 mm).
• Thicken it 0.75 mm on each side.
• Assemble the Ribs to the main Coin Holder Part.
• Use Draft of 0.5⁰ on all the walls which are along the Tooling Direction.
• Also, give fillets of 0.25 mm to all the edges.
• Do similar operations for the 2^nd Rib by Keeping in mind the spacing between the ribs to follow the design rules which is height to spacing ratio to be 3:1 or 2:1.
• Take a Section of the Final Part with the XY plane to check the application of Design rules of the Ribs are satisfied.

Draft Analysis on Closed Part Body:

• Change View Mode Customization to Material mode.
• Click on Inert, select Analysis and select Feature draft analysis, then drag the compass to the Tooling Axis and lock the compass at specified direction.
• Select the component to check the part producibility or not.
• In this case, the final surface draft analysis does not pass because Class A surface contain parallel walls and walls without draft.
• So, this component will be costly to manufacture or may even fail causing loss in Time and Cost.

 

Standard Views (Isometric, Front, Side, and Top View):

 Conclusion:

        Successfully created Ribs on Automotive Plastic component - Coin Holder by applying industry principles, Design Rules and Draft Analysis using Catia V5 application.