Design and Development of Door Panel Trims - Automotive Interior Plastic Component Design

DESIGN AND DEVELOPMENT OF DOOR PANEL TRIMS – AUTOMOTIVE INTERIOR PLASTIC COMPONENT DESIGN

 

Objective:

Development of Door Trim Panel containing Arm rest, Lower Substrate, Map Pocket and Bottle Holder using Class A and Master Sections as Inputs. The Design and Development is to be done using CATIA V5 software with implementation of concepts and rules of Plastic Product Design.

The components in consideration are Automotive Plastic Component and has to be developed as per the DFMA (Design For Manufacturing and Assembly) rules used in an Automotive Industry. These rules basically define the capability of the designed product to be Manufactured by Injection Molding or similar Plastic Manufacturing techniques.

These rules include various factors such as Constant Part Thickness, Draft direction, Parting Line, Parting Surface, Draft Analysis, B- surface Creation, C- Surface Creation, Draft Analysis on the part, B- Side features such as Locators, Push Pin, Doh Houses, Flanges for fitment (assembly) of parts.

 

Introduction:

Automotive door is a vital component in a vehicle as it allows entry/exit access to the vehicle. These Doors consist of Sheet metal components which may not have practicality and Aesthetics. Thus, interior panels are fixed to these doors to give it an aesthetic fell as well as practical usage. It is used as an interface between the Sheet Metal surface, Power Window mechanism, other wirings and the Passenger Cabin

Moreover, door panels also give a stylish look to the vehicle, as they have the ability to integrate various interior parts, which are mounted on door trim. Such door panels differ from one manufacturer to another and from car model to model. Further, door panels are considered as one of the crucial components in vehicles as they provide reliable side protection, ergonomic comfort, and convenience coupled with attractive and soft-touch surfaces. They are usually made up of Plastics with sometimes wrapped with Leather or velvet cloth material for soft touch feel. Figure1 shows the Interior Door Panel of a Car.

 

 

                  Figure1: Interior Door Panel of a Car.

 

 

Door Trim Panel Overview:

Figure 2 shows a complete Door Panel Trims of a Modern-Day Car.

Figure2: Door Panel Trims - Source:Daco Engineering.

 

Some Terms Defined:

 

Draft:

• 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.
• This problem can be solved by giving draft to the component.
• Most applications require minimum draft angles of around 0.5º to 1º , however 3º draft angles are widely accepted for cost saving purpose.
• 7º 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.

 

                                       Figure3: Draft Angles

 

 Tooling Axis:

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

                            Figure4: Tooling Axis

 

 Side Core Axis:

• Some features or walls are difficult to clear by the Main tooling axis itself.
• These features may have negative draft angle or are a part of a sub feature which is an important part and cannot be ignored.
• These features need a different tooling direction for clearing the part in the mold.
• This tooling direction is called Side Core Axis which often tends to clear the sub features or walls at different angle with rwspect to the Main Axis.
• The following part requires 2 Side Core Axis due to clearance requirements as shown in Figure 4.

 

                      Figure5: Side Core Axes for Arm Rest

 

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 regions 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.
• Since it is the factor which decides perfect the ejection of part from the Mold.
• Acceptable draft angles are used as range for draft analysis.
• Acceptable ranges are decided before planning to manufacture the product and it may vary from product to product.

               Figure6: Draft Analysis on Class A of Arm Rest along Main Tooling Axis.

  

Class- A Surface:

• In automotive design, a class A surface is any of a set of 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, Snaps, 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 B surfaces.
• This surface connects both A and B surfaces along the Tooling Axis.
• It contains the draft as per the design.
• It is basically the thickened surface between both A and B surface.
• Below Figure 6 shows class A,B and C surfaces: Blue surface is Class A, Purple surface is Class B and Red surfaces are Class C surfaces.

 

                   Figure7: Depiction of Class A, Class B and Class C surfaces

 

Some Materials Used in Door Panels are:

• ABS
• Polypropylene
• Reinforced Elastomers
• TPE
• PC-ABS Blends

 

 

Design Rules for Plastic Components:

• Nominal wall thickness of a plastic component is 3mm. For a strength component such as Instrument Panel, Bumper , door trims the values of thickness are larger than 4mm.
• For example, Thickness of IP may lie between 5-6mm, for Bumper it may be 6-8mm.
• For smaller components the thickness may be between 2-3mm.
• 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 3mm should deviate within the range of 2.1 mm minimum and 3.9 mm maximum.
• Ideal draft angle should be 3º for the overall component along for the faces lying in the tooling direction.

 

B-side Features:

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
  • Quantity 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.

                        Table1: Rib Design Guidelines

Rib Specification	No-Show Surfaces	Show Surfaces
Draft Angle	Min 0.5º per side, Recommended 1.5º per side	0.5º per side
Rib Thickness at Base (w)	0.5 * t	0.4 * t
Rib Thickness at top	Min 0.75mm	Min 0.75mm
Rib Height (y)	Max 5*t	Max 5*t
Spacing Between 2 Ribs (x)	2*t	2*t
Fillet Radius (r)	0.125*t	Max 0.25mm
Rib Root Dia	`30 % of t	`30 % of t
y/x ratio for multiple ribs	3:1 2:1 preferred	3:1 2:1 preferred

 

 

                                  Figure8: Rib Design critical Sections

 

Screw Boss Design:

• Screw Bosses are Class B side engineering features which find use in many part designs as point for attachment and assembly.
• Most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts or other types of fastening hardware.

Guidelines For Screw Boss Design:

• To reduce stress concentration and potential breakage the bosses must have blended radii rather than sharp edges at the base.
• Larger radius reduces stress concentration but also increases chance for sink or void.
• Specifying smaller screws or inserts often prevents overly thick bosses.
• Normally, the boss hole should extend the base wall level even if the full depth of is not needed for assembly.
• Shallow holes can lead to thick sections at wall resulting in sink marks or voids.
• Deep holes lead to thin walls which are difficult to fill during molding.
• Because of draft requirement the tall bossed (>5 x outer diameter of Boss) can create filling problem at their top or thick section at the base. 

• Proper Boss design is necessary to avoid plastic defects which involves the following critical sections and design rules that needs to be considered:
  • Draft angle
  • Thickness at base (w)
  • Height/Diameter Ratio (H/D)
  • Optional Radii for fillets (r)
  • Boss Gussets or Ribs for support
  • Root Diameter
  • Planning ahead for type of Ejection
• The figure 9 shows the critical sections.

 

                         

                                 Figure9: Screw Boss Design critical Sections

  

• Design Rules considering above critical sections are as follows:
  • Thickness (w) = 0.6 x t.
    • Should be measured at base.
    • Should be measured at the tangent of the fillet.
    • Larger base area could lead to higher heat retention.
    • Retained heat could lead to higher shrinkage.
  • Optional Radii (r) = 0.25mm Max.
    • Must not violate `40% thickness requirement for the base thickness.
  • Draft = 5º on both inner and outer walls.
    • Different rule used for Sleeve ejection that needs to be planned for
  • H/D ratio should be 3:1 or 2:1
  • Boss Gussets should meet Rib Design Rules
  • Planning for Ejection:
    • If height of Boss < 25mm then draft is not required when sleeve ejection is used.
    • If height of Boss > 25mm then draft should be done only on outside wall for sleeve ejection.
    • If Sleeve Ejection is not used then draft of 0.5º is required on all walls.
  • If Boss Wall thickness must exceed the recommended ratio, consider adding recess around the base of the boss to reduce the severity of sink.
  • Avoid bosses that merge into the side walls because they can form thick sections that lead to sink marks. Instead position the bosses away from the side walls, and if needed connect it with ribs as shown in Figure 10.

 

                                       

                                                             Figure10: Screw Bosses near walls.

 

Dog House Design:

• Dog House is a B surface engineering feature used to avoid sink marks primarily.
• Its secondary use is to place an offset B side feature such as boss or locators on it.
• The cross section for dog house near the B surface is less and then increases rapidly.
• It can be extended to have another feature on it.
• Also, if Bosses are perpendicular to the B surface Dog houses are used to place it.

Guidelines and Design rules for Dog House:

• Proper Dog House design is necessary to avoid plastic defects which involves the following critical sections and design rules that needs to be considered:
  • Draft angle
  • Doghouse Walls Section
  • Part Surface
  • Bottom of Doghouse
  • Wall Thickness
  • Height and Width

                       Figure11: Dog house wall section.

 

• B = 3 to 6mm, C <= 0.4 x T

 

                              Figure12: Dog house width.

 

• Dog House width (X) = 12mm Minimum
• Height (H) = 6mm Minimum
• X/H = 3:1 or 2:1
• Draft of 5º minimum to draw direction, if slide is considered then this can change to 3º minimum.

 

 

4 Way and 2 Way Locators:

• Locators are used to locate a job or a component to arrest its degrees of freedom using the 3-2-1 Principle which is arresting of 3 DOF's first, then 2 DOF's and last DOF in a sequence.
• A 4-way locator will look like a '+'  and a 2-way locator will look like a '-' .
• Both these locators use the same Rib Design Rules and principles.
• These Locators are placed on the B side of the component and must have same height  with respect to the B surface.

 

Heat Stake:

• In manufacturing products with Thermoplastic Components, it is often necessary to join a thermoplastic to a part having dissimilar material such as a metal or any other thermoplastic.
• Heat staking is an assembly method that uses the controlled melting and reforming of plastic stud or boss to capture or lock another plastic or metal component of assembly in place.
• The Plastic Stud protrudes through a hole in the component to be locked in place.
• Heated thermal tips are used to melt the top of the studs which fills up the volume of the cavity to produce a head locking the component in place.
• The progressive melting of plastic under continuous pressure forms the head.
• When staking, the right combination of heat and pressure is critical.

In short, the Locators locate the component and lock 11 degrees of freedom whereas the Heat Stake is a Fixation which locks the Final DOF.

 

Flowchart for Development of Door Panel Trims:

 

 

Inputs:

Figure 13 and 14 shows the inputs of Door Trims including the Class A surfaces and the Master Sections for further development.

Denotations used in the following Figures:

1. Lower Substrate.
2. Arm Rest.
3. Map Pocket 1.
4. Map Pocket 2 with Bottle Holder.
5. Master Sections of Parts.

 

                               Figure13: Class-A input for Door Panel Trims.

 

 

                              Figure14: Master Section for Development of Parts.

 

 

Further Development of Individual Parts:

 

1.) Lower Substrate:

• Lower Substrate is the encapsulating aesthetic Part used in the door panel.
• It may house various features such as Wirings, Speakers and other sheet metal components behind it.
• Figure 15 shows the Lower Substrate and its working and development is explained below.

 

Figure15: Lower Substrate.

 

Class A Surface with Mater Section (Input):

 

Creation of Tooling Axis along Y direction:

 

Draft Analysis of Class A along Tooling Axis:

 

Creation of Flanges on Class A as per Master Sections:

 

Creation of Class B Surface:

 

Creation of Class C Surface:

 

Creation of Closed Surface (A-B-C Surfaces):

 

Creation of B-Side engineering features (Dog House, Push Clips and Heat stakes with ribs support):

 

Lower Substrate Final Part:

A Side (Front):

B Side (Rear):

 

 

2.) Arm Rest:

• Arm Rest as the name says it is the part where the driver or the Passenger can rest their hand on. 
• It may come with a soft touch surface such as leather, cloth or any soft material as a wrap around.
• It may also house various electricals such as the Power Window switches, Mirror adjustment switches etc.
• It may come with a small storage for coins or sunglasses.
• Figure 16 shows the Arm rest which is developed. 

Figure16: Door Arm Rest.

 

Class A Surface with Mater Section (Input):

 

Creation of Tooling Axis along Y direction:

 

Draft Analysis of Class A along Tooling Axis:

• Some walls are not clearing using the main tooling axis, thus a Side Tooling Axis is Required

 

 Creation of Class B Surface:

 

Creation of Class C Surface:

 

Creation of Closed Surface (A-B-C Surfaces):

 

Creation of B-Side engineering features (Flanges and Heat Stake Insert):

 

 

Arm Rest Final Part:

 

 

3.) Map Pocket 1:

• Map Pockets are genrally used as storage space on the door.
• In this case The Map Pocket is divided into 2 sections the upper and Lower
• The Upper Section acts as an enclosure from the inner parts and for aesthetics only.
• Figure 17 shows the Upper Map Pocket and its development is shown further.

Figure17: Map Pocket 1.

 

Class A Surface with Mater Section (Input):

 

 

Creation of Tooling Axis along Y direction:

 

 

Draft Analysis of Class A along Tooling Axis:

 

 

 Creation of Class B Surface:

 

 

Creation of Class C Surface:

 

 

Creation of Closed Surface (A-B-C Surfaces):

 

 

Creation of B-Side engineering features (Fitment Flanges, Dog House and Push Clip):

 

 

Map Pocket 1 Final Part:

A Side (Front):

 

 

B Side (Rear):

 

4.) Map Pocket 2 with Bottle Holder:

 

• Map Pockets are genrally used as storage space such as a bottle Holder, Umbrella Holder etc. on the door.
• The Lower Section of Map Pocket development is shown in further steps.
• Figure 18 shows the Lower Map Pocket with Bottle Holder and its development is shown further.

Figure18: Map Pocket with Bottle Holder.

 

Class A Surface (Input):

 

Creation of Tooling Axis along Y direction:

 

 

Draft Analysis of Class A along Tooling Axis:

 

 

Creation of Side Tooling Axis for clearing the walls:

 

Draft Analysis of Class A along Side Tooling Axis:

 

Cretion of Class B Surface:

 

Creation of Class C Surface:

 

Creation of Closed Surface (A-B-C Surfaces):

 

Creation of B-Side engineering features (Dog House and Locators):

 

Lower Substrate Final Part:

A Side (Front):

 

B Side (Rear):

 

Final Assembled view of all Parts as Driver Side RH Door Panels Trim:

 

 

Developed Assembly:

 

Assembly with Upper Substrate and Door Release Handle trims:

 

Conclusion:

The Design and Development of Interior Automotive Plastic Components Assembly – Door Panel Trims from Class A surface and Master Section as input is completed using CATIA V5 software with implementation of concepts and rules of Plastic Product Design.