Design of backdoor
AUTOMOTIVE SHEET METAL (BIW) DESIGNING OF BACK DOOR (TAILGATE) OF A SUV CAR
Aim:
To design a backdoor of a SUV Car with its components by following its design requirements and industrial standards of automotive in NX CAD Software.
Introduction:
A tailgate(backdoor) is a door at the back of a truck or car, that is hinged at the bottom so that it opens downwards.
Like hood and side doors, back door is also a swinging closure commodity. We first decide hinge axis and latch and striker point, we join the trajectory to decide the motion of this and find out the door design. Also, like side doors, the sealing flanges are important in back doors. We prepare master on 0Y top, bottom, hinge area top and bottom. We develop the sealing surface like that on corners. Since gas stay is used here, we use the gas stay top mounting and bottom mounting also to sealing flanges. With respect to the styling we position our master and then we develop sealing surface. Once that is done, we continue with the construction. But if that alone sufficient for a backdoor? No. This backdoor will also take responsibilities of rear visibility and the luggage area. More thinner our back door section, More better our luggage area is. For that we cannot make it straight with the styling surface/outer panel. Because we need strength to the panel in rear crash. We need some space to accommodate the latch and striker’s motor. We need some space to accommodate the wiper motor. So, we will have a deepness of inner panel from outer panel. How long the deepness go is the factor. And at that deepness on corners/edges we have sealings. The sealings characteristics is similar as on side door. This will be contact with the rear floor of the boot, roof on the top and with body on side. As a tailgate engineer, we also contact roof engineer, body side engineer and rear panel engineers. Because the sealing flange in the tailgate is the input for bodyside engineers. Sealing flanges on the top will be the input for roof engineers to develop rear roof rail in accordance with rear flange. There will be a rubber section between the roof rear flange and sealing flange of the tailgate.
As we have given with only the styling Class A Surface. We are going to design the tailgate from scratch with all design requirements and industrial standards.
Different approach on materials for tailgate:
We are going to prepare the backdoor with steel or aluminium.
Table. Difference between steel and aluminium
Why we have to choose different material is, in order to reduce the cost or reduce weight of the whole part.
Inner panel of the aluminium cannot be deep drawn but outer panel of the aluminium can be deep drawn, because of the structure. The purpose of using aluminium is, it is having less density, so the final weight of the product will be very small. So, we cast the part which means we vary the thickness on different regions so I avoid the reinforcements completely.
At the end, the inner panel assembly will be one part and weight will be drastically reduced. Designing methodology of aluminium part is quite different when compared to steel on approach how we design. Because we are varying thickness, it cannot be just one sheet metal panel and it cannot be like designing one sheet metal panel as its way different.
Now as we see more about material challenge activity like, how we can change material and reduce weight of the part?
And in steel parts there, only inner panel can be deep drawn, we can have only one thickness. Which means we will have to add reinforcements on local areas like hinge reinforcements, gas stay reinforcements, latch and striker reinforcement in the bottom, wiper motor reinforcement in centre. We add gas stay reinforcement because gas stay will have one mounting on tailgate and one mounting body side. As gas stay mounting embosses reducing the strength, we add gas stay reinforcement. As we all know the reinforcement purpose itself compensate the weight reduction to add strength locally. Hinge reinforcements are added to avoid the loss in strength of the inner panel due to addition of hinges.
In steel parts we have prepare reinforcements separately, we will be investing the tool for the reinforcements. We will be investing on individual parts. And with whole parts we increase the weight. Having done all this, the price will be less for steel and the price of the aluminium is more that is the cost the product. So, depending upon the projects budget we decide the part what material we use. Even though aluminium has many advantages over steel, the cost is more.
In aluminium we need not to give minimum 3 mm radius (3+ thickness) we can even have the deep drawing angle need not to be 7deg or minimum 5 deg, it can even be 3deg open. Because we are anyways going to cast the part. Our casting tool should just come out, so it should be positive draft it cannot be negative. But it need not to be that open as steel. So, these are all the advantages of aluminium over steel.
When we reduce the reduce the opening angle, we increase the area outside the part. So, we increase the luggage area. When it comes to back door, we increase the luggage area and in windshield glass region we increase the rear visibility. The point here is when we use aluminium, we need not give 7 deg open or 5 deg open, even minimum 3 deg open is sufficient enough for aluminium parts. So, in this situation we can get an increase in luggage area and increase the rear visibility criteria, but in steel we have a reduced luggage area and reduced rear visibility criteria. And these are all differences of aluminium over steel. Having seen all things aluminium is way costlier than the steel. So, in inner panel we have 7 parts. The cost of these parts is lower than on one aluminium inner panel part.
Assembly sequence:
In steel parts it will be the vice versa of this ratio.
Back Door Assembly:
Inner panel
Outer panel (upper and Lower)
Hinges and its reinforcement
Wiper motor reinforcement
Latch & Stricker and its reinforcement
Gas stay reinforcement
The outer panel and inner panel are hemmed only so we give reliefs on the corners.
Load Cases – General Analysis:
In backdoor, we will see some load cases. How will we check whether the part is right or not? When we sent to CAE team. They not only test production feasibility. They also test some load cases, so these are all some of the load cases. And this is common for all parts not only for backdoors, for side doors also there will be some similar tests. For hood they do some relevant tests.
Bend Test:
In bending test, we constraint all the four regions and we give force on centre. For every door there will be a nominal value at which it should come back to its original position. The upper two are hinge positions and bottom two are corner positions. And in the centre of the door we give weight.
Torsion test:
In torsion test, we constraint the hinge positions on upper and latch and striker position in bottom. And on the bottom corners we apply weight on reverse direction. We can assume how it behaves; the thing is we have to see how it comes to its original state.
Abuse Case:
In abuse case, we examine the causes of over slamming or hanging when the door is open. Because some people tent to lie on the door when the door is open.
Dent Test:
It is done to evaluate whether dent is coming back to its original position.
Assembly load Case:
Assembly load case is examined to check the backdoor during assembling the vehicle, how much force is given to the body and whether the door is withstanding or not?
Endurance test:
It is the workability tests. The backdoor will be opened and closed for a particular number of cycles. And we see how the door behaves at the end of the cycle. If it’s a side door the number of cycles will be more. If it is back door the cycle will be relatively less. Because we don’t open that many times. If we open side doors for 10000 times, we open backdoor only for 2000 times. So, this endurance test will happen with variation in cycles with respect to the commodity. For Hood, we approximately open only 100 times. We open hood only if there is a problem.
So, these are all the load cases which is tested and when tested we get a result of inner panel and how does it behave.
There is another important factor why we use Y emboss or V emboss. It’s from the predefined study done by Mr. Alex Bolgert.
And from the thesis done by Mr. Alex Bolgert from Germany, we can clearly see that on six major tests over tailgate gives us that on simulation results, the force normally tends do go over a V or Y direction and that is reason we use V or Y shape emboss in tailgate. While doing this we dissipate the energy outside the car and we create strength locally in that region. And the latch and striker reinforcement which will locally help.
All Optimized Structure comparison over Manufacturing effect cases.
And over manufacturing effect cases what we have seen before, it can see from the report with eight cases the forces acting on the y direction.
Deep drawing:
Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch.
It is thus a shape transformation process with material retention. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing the part through a series of dies. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property.
Hemming:
We already designed the inner panel and outer with the thickness of 0.75mm. After that when we join these panels we will produce the bends along the edge of the outer panel towards inner panel. These bending processes is called hemming. By doing this with provided nomenclature we avoid spot welds.
And when we do the hem, on the corner of the hood outer panel we have to give the relief.
There will be different rollers assembled on different corners. Each roller will roll over the edges of the sheet metal to close it with the inner panel. And these rollers will roll two or three times to enable perfect rolling provided with different rollers. To avoid intolerance issues, we do hem with different rollers and it will also save time.
On first roller pass, the minimum closing is happened.
On second roller pass, it completely closes the corners and
On third roller pass, it justifies the closing what it attends.
Normally hemming is done for assembling the outer panel with inner panel. And when you pass the rollers over the sheet metal there are some tendency that the sheet metal vibrates. Simulation team also studies that is these forces making the panel come back to its original position or it is changing the design of the part. That is what the spring back effect is.
Hemming region:
Like we do in hood, side doors and other commodities. We have this 0.2 mm gap in top and bottom and 2mm clearance on the side. Why do we have this, this is because we will have a sealant applied on the after the hemming which will whoosh out for which we will need a little space to whoosh out and get it stuck properly. On the windshield region we do brazing to join the inner panel and outer panel. And the joining of the inner panel and outer panel lower regions is also carried out by brazing.
We give some reliefs on corners. And wherever we have a step we ignore the hem. Ignoring in sense we give some small hemming flange region than usual.
Mastic Region:
This is region where we use mastics to join the outer panel upper and inner panel. We use mastics to reduce / avoid any rattle noise. As we can’t spot well on the outer panel as it reduces the aesthetics look of the tailgate. Anyway, the hem itself is sufficient enough. But additional strength is provided by this mastic.
Draft Analysis:
We do draft analysis our part before giving it to the drafting team, to check whether any negative draft is made. Because if manufacture a part with so many negative draft angles as it costs more and consumes time to produce the parts. So, we maintain the positive draft angle throughout designing as possible. We consider minimum draft angle for this part as 7 deg.
Latch and striker trajectory:
When we get a hinge axis, we prepare a circle keeping the hinge point and striker point (approximate point) to form the trajectory. When we have this circle, it should perpendicular to the striker. If it perpendicular our hinge axis is right. Or else we have to move our hinge axis accordingly to it to ensure the perpendicular striker point. We can also change the angle in striker region.
Final Model of Backdoor Assembly:
Inner panel:
Back Door Assembly
Isometric View
Front View
Side View
Conclusion:
An Automotive Sheetmetal (BIW) designing of a backdoor of a compact SUV car is successfully designed with its components with its design requirements and its industrial standards in NX CAD Software.