Modelling of a Car Interior Door Trim model using CATIA

OBJECTIVE

The objective of this project is to create/design a CAD model of a side door trim component from the given Class A surface using CATIA.

INTRODUCTION

Importance of plastics in the automotive industry

Plastics play an important role in the automotive industry as the market in recent times demand lightweight, affordable, fuel-efficient, safe, comfortable and high material performance automobiles. Plastics have actually been used in cars since the early 1950s, but it’s the latest innovations that are really changing the industry for the better. Engineered polymers and plastics are continuing to replace aluminium and other metals in automobiles, with the average car interior now being made up of over 50% plastics. Over 1,000 different car parts are now made of plastics, and these plastics can be moulded together to replace the need for multiple parts, keeping manufacturing efficient and weight down.

Plastics also weigh significantly lower than their other counterpart materials(like sheet metal) with a weight reduction of up to 50% which can improve fuel efficiency by 25-30%. The plastic used in cars means that for every kilo lost, your car will emit 20 kilos less carbon dioxide over its operating life which contributes to a greener environment. Plastics also offer lightweight solutions which fulfil essential safety requirements. So it can be said that the plastic industry, in a way has revolutionized the automotive industry(and this is not just pertaining to cars).

One more reason for using plastics is manufacturability. It has low manufacturing costs along with easy manufacturability. Single mould components have allowed car manufacturers to decrease assembly time as well as to introduce design innovations at a decreased cost. The fact that plastic can be moulded more easily, allowing components to be tailored for more comfortable human-ergonomic features. It also allows for more streamlined and aerodynamic shapes to be made. As less material can be used compared to if steel was used for components, plastics ensure a longer and more reliable vehicle lifetime.

Now that we understood the importance of plastics in cars, we'll take a look at the major types of plastics used in a car. 

• Polypropylene

                            This is the most commonly used plastic in the automotive industry. Polypropylene is a saturated addition polymer produced from propylene. It is durable and unusually resistant to numerous chemical solvents, bases, and acids. It can also be easily formed into any shape. Given the properties of this plastic, it is used for car bumpers, gas cans, cable insulation and even the carpet fibres of your car's interior flooring. It's also a more economical alternative to expensive plastics of similar strength and durability, which helps drive down the cost of manufacturing.

• Polyvinyl Chloride

                            This is the second majorly used plastic in an automobile accounting for 16% of all plastic in the typical vehicle, PVC has excellent flexibility, is flame retardant, has good thermal stability, a high gloss and little to no lead content. Polyvinyl chloride works extremely well in a huge range of auto parts that can be through extrusion, injection molding, compression molding, and blow molding processes. Either stiff or flexible depending on the amount and type of plasticizers used, polyvinyl chloride is used to create instruments panels, electrical cable sheathing, and door parts.

• Polycarbonate

                             This plastic provides a distinctive combined rigidity, hardness and durability. It has superb weathering, impact, optical, electrical, and thermal qualities. Polycarbonate is also lightweight, so it reduces a car's overall weight, which in turn improves vehicle and fuel efficiency. It is used predominantly for car bumpers.

• Acrylonitrile Butadiene Styrene

                             Acrylonitrile Butadiene Styrene(ABS) is created by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The styrene provides the copolymer with a shiny, tough exterior with a sleek design. The rubbery butadiene supplies resilience down to very low temperatures. Steering wheel covers and dashboards are often made of ABS plastic. It's also well-suited to heavy-duty applications, so it can be used for automotive body parts, too. Plastics used in cars, like ABS, helps the body absorb and redistribute energy during an impact, keeping passengers safe.

These are the most commonly used plastics in an automobile. Now that the importance of plastics and their various types and applications are understood, we proceed to give a brief overview of the steps that we have followed while developing this Interior side door trim.

METHODOLOGY

• Class A surface extraction.
• Separate and sort the single Class A surface into respective components using Geometrical sets.
• Necessary Main/Side Tooling axis creation.
• Draft Analysis on Class A surface component-wise.
• Class B surface obtained after offset and extrapolation in the appropriate direction.
• Class C surface created using Tooling axis as reference draft direction.
• Class A, B and C trimmed together to form the finished surface of different components.
• A closed surface is converted into a solid followed by a draft analysis on the solid component.
• Various attachment features created on the Class B surface while following the design guidelines.

PROCEDURE 

This is the given model (Interior Side door trim). We have been provided with the class A surface and as we can observe, the surfaces do not have any connectivity between them(highlighted by the green boundaries on the surface). 

We can observe from the image shown above that we have separated the necessary/required surfaces and aggregated(joined) them into components and the surfaces in yellow are not our focus of interest. We have four components to work on and we focus one component at a time starting with draft analysis using a Tooling axis.

The tooling axis is the main consideration for plastic part and mold parts as it defines the mold part and the possibility of plastic part coming out from the mold part. In Molding machine arrangement is given to open the mold along a direction which is known as Tooling Direction/axis.

We've provided a minimum draft angle value of 3 degrees which should be cleared by class A in every direction and the green colour indicates that the angle is above 3 degrees in all directions. 

For surfaces that don't clear with a single tooling axis, we have introduced a secondary side tooling axis/Lifter axis with which we clear the issue.

The yellow coloured axis clears for that specific region and this region, instead of moving in the main tooling direction, is taken out with a lifter in the secondary axis direction. 

With this, we proceed to Class B surface creation. Class B referees to the non-aesthetic surface that is not visible to the naked eye when looked upon from  outside but rather connected to the other parts of a car using attachment features which lies on this very surface. 

For obtaining a class B surface, we simply offset the class A surface by required distance(thickness of the component) in the correct direction.

After this, we create a class C surface that forms the thickness/cross-section of the solid part. For class C surface, we use a combination of commands such as sweep, multisection surface, extract and extrapolate to obtain the surface and then join them together to form a full surface. 

The brown colour indicates class A, red indicates class C and the blue on the bottom indicates the class B surface. All three are trimmed to obtain the closed part(no solid; only surface).

To ensure that the surface is indeed closed, we use the boundary command to check for confirmation. 

We can observe an error message stating that the body is closed and hence no boundary can be formed which means our component is closed. Now, we use the close surface command to obtain a solid. After this, we perform a draft analysis on the whole solid and ensure that the component clears for the draft angle in all directions. 

The draft analysis clears in all regions except for the region cleared using the side tooling axis for which we perform another draft analysis to ensure that.

In this manner, we create the solids for all the components in our model.

After this, we proceed to attachment features.

Attachment features

Attachment features on the plastics have many uses like improved strength of the component, attach one component with another, locate a component for mold to move it(to pour the molten plastic), arrest motion of one component with respect to another and to attach the finished plastic to the metal BiW(Body In White).

The first set of features that we are going to create are heat stakes and Locator(4-way and 2-way locators). 

Heat stakes or the process of heat staking also known as thermoplastic staking is the process of joining two dissimilar materials together. In heat staking, we use local heating and cooling to raise the temperature of plastic components and allow plastic reforming to be carried out. This reforming process attaches one plastic component with another as the plastic cools and solidifies.

Note that we have also provided ribs at the base of heat stakes for additional strength. 

Locator is another attachment feature that is vital to any component obtained through molding process as it acts as a guide for the mold to move into position and there is no mismatch between the two(top and bottom) half of the mold. There are two classifications of locators, 2-way and 4-way. A simple method to remember the difference between them is to consider the 3 axes of translational motion, and the 6 corresponding directions or Degrees of Freedom. A 2-way locator restricts motion along 1 axis (or 2 degrees of Freedom), like a round pin in a slot. Likewise, a 4-way locator restricts 4 degrees of freedom along 2 axes. A round locating pin located concentrically in a hole is a common application of a 4-way locator and helps to further secure the piece. In order to sufficiently locate a part, a minimum of one of each type of locator is required to prevent excess motion of the workpiece, and more locating components may be used as needed.

Now, in order to attach/position one component with respect to another using heat stakes/locators, we create flanges on the necessary surfaces.

The image shown above gives an example of how flanges look in our model. Now, our flanges should also clear on all sides for the draft analysis. So we provide suitable fillet and draft wherever required. We also create holes on the flange for the heat stake and locator to pass through. 

We also provide a draft for heat stakes and locator in the direction of the tooling axis of the component upon which it is attached(Lower substrate in this case).

For the next feature, we consider the doghouse for the push pin/retainer clip. This doghouse is basically a provision for our retainer clip to be mounted and the purpose of having a retainer clip between the trim component and the BIW sheet metal is explained below.

The retainer prevents door trim from being ejected in the event of a side-impact and can be disassembled for servicing. Apart from these purposes, it also allows vertical and lateral gap compensation. Once in place, the clip becomes invisible, delivering added value in terms of design and aesthetics. 

Now in order to design the doghouse, we have to follow a set of design guidelines listed below.

GENERAL GUIDELINES FOR DOG HOUSE –

• Doghouse wall thickness at the intersection of the part wall (C) should be 40% or less than the main wall stock (T). 
• Height of (B) should be between 3.0 – 6.0 mm.
• 'C' is where the doghouse contacts the backside of the surface.

Here, we are going to create the doghouse in a frustum(of a right cone) shape. The sketch for that is shown below.

Then we use the shaft option to create the frustum followed by shell command to remove the unwanted solid.

Now, we cut out the region of plastic which comes in contact with another surface. We also add horizontal ribs on the doghouse to provide strength.

This is done to avoid sink marks(plastic defect) forming between the plastics. After this, we create a sketch to obtain the Hole and slot provision for the retainer clip.

We pad the sketch to obtain a solid and then remove the lump from the doghouse and provide fillet and chamfer wherever necessary. We also add a stopper to prevent the retainer clip from slipping out of position.

We then create a power copy of this doghouse and then place it in the other necessary/suitable locations. Finally, we trim the doghouse with the base component(lower substrate).

The next feature that we have added is called a snap fit. Snap fits are attachments features that arrests the motion of one component with another similar to how a heat stake works except that there will not be any external work done here(thermal/heat applied externally to attach 2 components). We create suitable sketches for snaps and their slots followed by creating solids for them. We have attached different kinds of snap fits that we have created in this project.

The final attachment feature that we have created are called ribs. Ribs are a feature in plastic injection molded parts. They are thin extensions that run perpendicular from a wall or plane. They are commonly used to provide additional support and strength to a part. Thickness and location are essential to rib design. 

Design Guidelines for ribs

Proper rib design tackles 4 main parameters - Thickness, Location, Height, Manufacturability.

We have considered these guidelines while creating the ribs. We create a single rib which acts as a parent body with which we create multiple ribs using the power copy option. 

The power copy takes the plane and point as the reference to create the copies. In this manner, we create ribs throughout the model and trim them with the parent component. 

With this, we have modelled the given model completely and this is how it looks(shown in the image below).

CONCLUSION

To conclude, modelling of a huge plastic component such as interior door trim requires us to follow certain procedures and design guidelines to achieve our desired output - a CATPart which when given to a manufacturing team should be completely manufacturable without much complexity and less cost to the manufacturer. This is achieved by our model as we have all these into consideration while developing this model. We have designed the required class B, C surfaces along with attachment features like Ribs, doghouse, snap-fits and locators among others while ensuring manufacturability and following design guidelines.

Google drive link to the model

https://drive.google.com/file/d/1zEQAeR59Oxv0q4n_1x_72h2YEuNKnaDL/view?usp=sharing