DESIGN OF FRONT DOOR TRIM USING CATIA

DESIGN OF FRONT DOOR TRIM:

AIM:

To create a door trim solid part along with B side features with CLASS A surface as input.

INPUTS:

We have skin data for the Lower substrate, Map pocket, Bottle Holder, Arm rest to create solid model. Along with skin data we also have the Master sections of how the parts were joined together which helps us to create its B side features.

Lower Substrate:

Image below shows the Class A surface of the Lower substrate. Tooling axis (Yellow dotted line) is created along Y direction so that it will be easy to create heat stake which makes assembly easier.

 

Arm Rest:

Image below shows the Class A skin data of the Door Arm rest model. Main tooling axis is crated along Z direction and Side core axis is created along Y direction.

Bottle Holder:

Bottle holder is one of the trim parts in the door. Image below shows the Class A surface of the bottle holder. Tooling axis is created along Y direction.

Map Pocket:

Image below shows the Class A surface of the Map pocket with its tooling direction along Y axis.

Draft Analysis of Class A Surface:

Before proceeding to solid body creation, we must ensure that the given skin is manufacturable. It is done with Draft Analysis.

Lower Substrate:

Draft analysis result of the Lower substrate shown below. It clearly shows that surface has draft greater that 3 degrees in all the areas. So, we can proceed to create solid model of the lower substrate of the model.

Map Pocket:

Door Arm Rest:

Draft Analysis of main tooling direction

Draft Analysis of side core axis

As we can see here in this model there is negative draft in some areas which makes it difficult to manufacture. So, we need to escalate this report to Class A surface designing team to alter the Class A surface with proper draft angle. But here in this case since we don’t have any Class A designers we are designing the solid part of Arm rest model with the Class A surface what we have.

Bottle Holder:

Like arm rest model this model is also having negative draft. But, as we did not have Class A surface designer we are designing the solid model of the Bottle holder with the skin what we have.

DESIGNING SOLID MODEL:

Class A surface is converted into solid model as shown below.

Lower Substrate:

Class B surface is created by offsetting the Class A surface to 2.5 mm as our part thickness is 2.5. But due to the fillets some of the patches were not offset and they can be manually created using Multi section surface or fill option. 

C surface is created by sweeping the outer edges of the Class A surface. In most of the areas 90 degrees of draft is maintained as the C surface is tooled with Class B surface.

CLASS A AND CLASS C JOINED:

COMPLETE SHELL:

Image below shows the complete shell of the lower substrate.

DOOR ARM REST:

B Surface:

In this arm rest model, there were 2 parts of the surface were offset separately. As we can see some of the patches were missed out in the surface which is offset due the fillets.

The patch is created by Fill command or the Multi section surface command. Guide curve for the outer edge is created by using the spline option. Then the two separated surface is extrapolated and joined together using the trim option.

C SURAFCE:

C Surface is created along the edges of Class A surface using sweep option. In some areas where the Class A surface is perpendicular to the tooling direction sweep along tooling direction with 3 degrees of draft is used and, in some areas, where the Class A surface is parallel to the Tooling direction sweep is created either at 90 degrees with tooling direction or 90 degrees with reference surface. In places where the 3 degrees drafted surface and the 90 degrees these two surfaces were joined using the blend option.

COMPLETE SHELL OF ARM REST MODEL:

The image below shows the complete shell and the Solid model can be created using Closed Surface command.

MAP POCKET:

CLASS B SURFACE:

As we can see in the image below some of the patches were deleted as the Class A surface has large curvature locally. When offsetting the patches will create complex geometry therefore, they were deleted. We can create them using “Fill” or “Multi-Section Surface” command.

Repaired Class B surface:

CLASS C SURFACE:

Class C surface is created by Sweep Command as we did in the Arm rest model.

JOINED SURFACES:

BOTTLE HOLDER:

B Surface:

B Surface of the Bottle holder model is created just by offsetting the Class A surface for 2.5mm. As this model did not have any large curvature areas it will be created fully without any distortion.

C SURFACE CREATION:

C Surface is created by sweeping the edges of Class A surface at 90 degrees with reference surface, where the reference surface was the Class A Surface.

JOINING THE SURFACES:

A, B, C surfaces were joined together to form complete shell. Then the shell is used as input to create solid using the “Close Surface” command.

Now, we have solid models of Door trims.

Now we must create a B side features with the Master section we have as a reference. Images below shows the Master Sections which shows us the details such as the geometry of the flanges and the Heat stakes.

Cut sections of the Master Sections 1 to 4 were shown below.

FLANGES CREATION:

Flanges were created by using the edges of the solid model as a reference. Class A surface of the flange is created by Extruding the edge of the Solid model and the direction of extrusion being the line in the master section. Further sections of the Class A surface were created either by using the “Sweep” option or “Extrude” option. Once the Class A surface is created the Class B surface is created by offsetting it directly and the C Surface is created by using Sweep option. Now all the surface were trimmed and joined together to form a Closed Surface.

LOWER SUBSTRATE FLANGES:

Flange of the Lower Substrate in Master section 2:

Flange of the Lower Substrate in Master section 3:

Flange of the Lower Substrate in Master section 4:

ARM REST FLANGES:

Image below shows the Flanges created for the Arm rest model. In this Flange 1 is shown in the Master Section 2. The second flange is created additionally to increase the number of mounting points as helps of rigid fixation of the arm rest with the lower substrate.

MAP POCKET FLANGES:

Flange 1 is shown in the Master Section 3 and the Flange 2 is shown in Master section 4.

BOTTLE HOLDER FLANGES:

These flanges were not shown in the Master section. But to enable the proper fixation of components these points were added taking the reference from the Master sections we have.

All the above flanges were made as a solid and attached to the base parts using the Boolean operation. Heat staking holes were made in the solid flanges using the “Pocket” command in the Part workbench.

LOWER SUBSTRATE SOLID MODEL WITH ITS FLANGES:

ARM REST WITH FLANGES:

MAP POCKET WITH FLANGES:

BOTTLE HOLDER WITH FLANGES:

HEAT STAKES AND RIBS DESIGN:

Ribs are the engineering features which are designed with the plastic parts on the B Surface of the part. Ribs are usually designed to improve the strength of the plastic part without increasing the thickness of the component. These ribs were also used to position the part on its parent during assembling.

Design considerations:

While designing any part we should consider the manufacturing process and we should try to avoid the defects which will arise in manufacturing by proper design of the product. The plastic parts were usually manufactured in injection moulding process. Some of the common injection moulding defects listed below.

1. Flow Lines
2. Sink Marks
3. Vacuum voids
4. Surface delamination
5. Weld Lines
6. Short Shots
7. Burn marks
8. Warping
9. Jetting
10. Flash

In the defects listed above the ribs were commonly prone to Sink marks and Short Shots. To avoid these defects in manufacturing process some of the design rules were listed below

Rib thickness:

Usually, thickness of the rib should be maximum 40% of the wall thickness. This is decided to avoid the sink mark on the visible surface. If the thickness of the rib at the base is high, it will increase the thickness at the junction therefore this will retain more heat than other surface. Due to this when it cools slowly it will try to shrink the material which is outside this region. Since the area at the fillet have high strength, it will shrink the flat surface causing the sink marks.

Rib Thickness at Top:

Thickness of the rib at the top must be maintained at least 0.75mm so that molten metal can flow easily. If the thickness is reduced further, it becomes difficult for the molten plastic to reach those corners of the ribs as they are almost perpendicular to the flow direction. They will leave unfilled mold region which leads to the defect called short shots.

Draft Angle:

Draft on the surfaces of the ribs is provided to ease of ejection of the part from the mold.

Usually, 3-degree draft is provided on the part in the engineering features like ribs, doghouse etc, were provided with minimum draft angle of 0.5 degrees.

Rib Height:

Rib height must be less than the 5 times that of wall thickness.

Rib base fillet radius:

Nominal base fillet radius is 0.125 times of wall thickness must be applied and maximum fillet radius is 0.25 is allowed.

Ribs spacing:

Ratio between the rib height to the spacing between the ribs must be maintained at 2 and this ratio is allowed maximum of 3.

Rib root thickness:

In general, the entire plastic component should not have the thickness deviation greater than 30%. As our part is 2.5mm thick and thickness must be maintained between 1.75mm and 3.25mm.

It is measured by creating the tritangent circle with base and the two fillets. Now the circle diameter should have the dimension between 1.75 to 3.25mm.

HEAT STAKES:

Heat stakes are the vertical cylindrical structures added to B surface. These are used to attach the child part with the parent part. This is joined to the parent surface by melting the heat stakes which protrudes beyond the attaching or parent part. This is also called as plastic welding. This kind of attachment provides advantage of avoiding any additional fastening methods like bolting, Push clips etc, which will add the weight of the component. But it has a disadvantage that once it is fixed it cannot be removed without damaging the component. Here since we will not have any need for removing or replacing these parts therefore, we are fixing it with heat stakes. The maintenance works can be done by removing the entire panel itself.

Design considerations:

Although they have different name based on the application, in general they are ribs. We will be following the design rules which are followed for the ribs to avoid the sink mark and short shots.

 

HEAT STAKE AND RIB SKETCH:

Centre point of the hole in the flanges is projected in the Lower Substrate and that point is taken as a centre for the sketches shown below so that Heat stake matches with the hole perfectly. Ribs attached to the heat stake will provide support to the Heat Stake and it also acts as a support where these flanges will be rest on the rib top after Heat Staking to avoid NVH problems.

Heat Stake sketch:

Ribs Sketch:

The Heat Stakes and ribs were created from the sketch using Pad command and draft angle of 0.5 degrees is provided to all the surfaces. The ribs were joined with the Heat stakes with the Union trim operation where the material inside the circle will be removed.

 Now the Heat Stake is created but the Lower Substrate and the Heat stake were not joined together, They were joined using the Union trim with the Lower Substrate which removes the excess material protruding beyond the lower substrate.

Similarly, all the Heat stakes were created and joined with the lower substrate using Union trim option. The image below shows Lower substrate with all the Heat stakes with ribs.

The image below shows the assembled model of the Door trims parts.

To fix this above shown assembly with the sheet metal of the door we at least need three fixation points. In general, the door trim panel is fixed with the door outer panel using push clips which are attached on top of the doghouse.

DOGHOUSE DESIGN:

Doghouse is a support feature for the other engineering features like locating pins, Screw bosses etc on the B side surface of the plastic component. Whenever these engineering features need to reach the distances of more than five times that of base thickness the feature will become weaker and tend to fail due to the bending moment, to avoid this failure they were mounted over the dog houses where about half of distance will be covered by the doghouse.

Base wall thickness:

Wall thickness if doghouse where it is attached to the part must have the thickness maximum of 40% of the wall thickness. The distance B should be maintained between 3 to 6mm.

 

Doghouse dimensions:

Doghouse height must be at least 6mm and its width must be minimum of 12 mm. Width to height ratio can be maximum of 3 and it is preferred to be maintained at 2.

Draft Angle:

Draft angle for the outside walls of the doghouse should be maintained at 1.5 degrees and the inner wall draft should be minimum of 0.5 degrees.

 

In this model as we need to reach height of 65mm from the base of the lower substrate to its outer flange. Height of the push clip is 28mm therefore doghouse height must be minimum 37mm in height. In this model we have Width to Height ratio of 0.8 mm

 

DOGHOUSE DESIGNING PROCESS:

First the doghouse outer shell is designed.

Ribs inside the doghouse is designed and joined with the doghouse using Union Trim Boolean operation.

To improve the stability of the doghouse ribs were places outside the doghouse.

Then the Doghouse is attached to the lower substrate using union trim Boolean operation which removes the faces of doghouse which protrudes beyond the lower substrate solid.

In similar manner, all the Dog houses were created and attached to the lower substrate.  Image below shows the lower substrate model with all the B side features.

PUSH CLIP DESIGN:

Push clip is a part which is used to assemble the plastic parts. It is similar to snap fits where the push clip attached to the child part is pressed against hole in the parent part. Push clip compresses and gets locked inside the hole in the parent part. Unlike heat stakes this method of fastening can be reversed by pulling the child part from the parent part.

Sketch for Push clip:

Push clip solid model:

Solid model of the push clip is created by revolving the sketch with the centre line as its axis. Revolving is done by using Shaft option.

Now the push clip is imported into the model and its degrees of freedom need to be arrested to keep the push clip in position.

First the Axis of the hole centre in the lower substrate is coincided with the axis of the Push clip which arrests the movements in the X and Z directions.

Face of the doghouse top and the push clip face is coincided to arrest the movement along Y Axis.

To arrest the rotation of the push clip YZ plane of the push clip and the YZ plane of the lower substrate is offset at distance.

Thus, the DOF’s of the push clips were arrested.

DRAFT ANALYSIS OF THE COMPONENTS:

Class A of Lower Substrate:

B and C Surfaces of Lower Substrate:

Result shows the faces were drafted properly and does not have any negative draft.

SIDE CORE AXIS FOR DOGHOUSE 1:

SIDE CORE AXIS FOR DOGHOUSE 2:

SIDE CORE AXIS FOR DOGHOUSE 3:

DOOR ARM REST DRAFT ANALYSIS:

As we saw already due to the improper Class A surface, we received it shows lot of negative drafts. Usually, these drafts were corrected by Class A surface designer but unfortunately, we do not have any Class A surface designers and for the purpose of project work we continued to work to create solid part using the Class A surface what we have.

Class A surface of Map Pocket:

Class B and Class C surface of Map pocket:

Image below shows all the faces were clearing the draft.

Class A of the Bottle Holder:

Similar to Arm rest model, Bottle holder also have improper draft in Class A surface itself. For the purpose of the project, we are proceeding to create the solid part from the class A surface what we have.

Class B and Class C surface of Bottle Holder:

All the faces of the flanges and the holes were drafted with B Surface. Image shows that the faces have proper draft.

 

CLASH CHECK:

Clash check is done in the assembly to check any conflicts between the parts s in the assembly. If the assembly is sent directly to the manufacturing without clash check the final assembly may not be proper due to the clashing of parts which incurs lot of loss. To avoid such kind of loss occurring we are checking for the clash between the parts in the modelling software and correcting it there itself.

In CATIA using “Clash Check” option in the Product workbench is used to check for the Interference between the parts.

Preview window highlights the conflicts. From the clash check dialog box it is clear that we do not have any clashes and only contacts were there which is fine in any assembly.

CONCLUSION:

Thus, the Lower half of door trim assembly is designed along with its B side features using the Class A surface and the Master sections provided.