The development and progress to volume production of a completely new tailgate concept presented challenges to both the developer and the production team. Collaboration and visible, shared objectives enabled completion in a relatively short time.
From a presentation by Achim Dockweiler, Tailgate Concept Designer, BMW Group, Munich; Dipl.-Ing. Martin Imgrund, Magna Exteriors & Interiors (Bohemia), Liberec, Czech Republic
Fig 1: Lightweight tailgate of the BMW i3 (photos: BMW)
When BMW gave the go-ahead in 2008 to develop the i3, a completely new, electrically-powered vehicle with optional range extender, clearly specified basic requirements included sustainability, along the entire value chain. This is reflected in the use of sustainably produced and recycled materials; about 95% of the materials used in the BMW i3 can be reused. The electricity required in the Leipzig plant’s entire production process is generated by on-site wind power installations.
The use of innovative technologies and lightweight materials has a significant influence on the range of the vehicle and also helps to reduce energy requirements. Under the new “BMW i” brand, the styling had to satisfy particular appearance requirements and resulted in the requirement for a black surface. A new assembly sequence in the Leipzig plant led to a shorter production line, which enabled almost the whole tailgate to be fitted within one work cycle. Outer body parts are painted without using the energy-greedy cathodic dip process.
Selection of materials
Table 1 illustrates the parameters applied in the comparison and selection of materials. The decision to build a tailgate structure of plastic was followed by a process of competitive analysis.
Table 1: Selection of materials for the tailgate structure
The tailgate of the 1-series BMW consists of a plastic inner shell into which is glued a circumferential tube frame. The outer skin panel is then attached using clips and screws. The concept was already fully realised and comprehensively tested at an early stage of development, using prototype moulds.
The tailgate of a competitor from the premium segment consists of a plastic inner shell in GMT, which is bonded to a painted thermoplastic outer shell and rear window. There is a separation at the hinge, which meant that part of it was laminated into the inner shell. A tailgate from the high-volume segment has a plastic inner shell with a glued-in, circumferential tube frame. The painted thermoplastic outer skin is mounted using screws and clips.
The concept selected for the prototype is made of an ‘internal inner shell’ (PP GF40 LGF), glued with partial steel reinforcements in the lock, hinge and gas spring areas, along with an ‘external inner shell’ glued on top. The rear window was also glued in place. The painted components – the spoiler and lower outer skin panel (PP TV20) – were to be fitted on the assembly line.
Styling challenges
The taillights should, as design icons, be integrated with zero gap into the tailgate. The concept was modified to give a single-part rear trim panel. The requirement for a “smartphone look” was achieved by using a glass panel with black overprinting. Glass was selected after intensive comparison with a panel made of polycarbonate (table 2).
Table 2: Selection of material for rear trim panel; a minimum wall thickness of 3.0mm is required with polycarbonate; its panel would have to be in-mould laminated with at least 3mm of black ABS PC.
Reasons for choosing glass included cost, the “depth effect” and its contribution to the tailgate’s rigidity. “Manufacturability” was critical; the styling specified a very challenging shape. Colour-matching also had to be achieved between the green-glass rear window, and the rear panel’s clear glass, which improves taillight visibility.
Rigidity
Several simulation loops and topological optimisations were necessary before required rigidity was achieved. One of the basic assumptions in production was that the same adhesive should be used for all bonding work, covering glass, steel, sheet and cast aluminium, PP GF 40 and PP TV 20. Changes during development are shown in figs 2 and 3.
Fig 2: i3 before topological optimisation
Fig 3: i3 after
Assembly
When the body shell arrives on the line the tailgate seal is mounted by roll forming. The major challenge in assembly is fitting the complete tailgate against the pressure of the seal, without causing damage. This is difficult because the only outer skin reference part present at this stage is the roof; the rear side panel, bumper fascia and side windows are fitted later. Once the tailgate is in place, cable connections and the rear spoiler are installed.
Fig 4: Handling device for i3 tailgate assembly
The visible surface of the inner shell is made of long fibre reinforced PP and it was necessary to take steps to optimise the production process if acceptable surface quality was to be obtained. This required optimisation of the injection parameters (screw speed, back pressure, etc), gating geometry and melt and mould temperatures; adjustment of the hot-runner control system; and graining of the injection mould with BMW fine grain (fig 5).
Fig 5: Visible area of inner shell with PP LGF 40
Dimensional monitoring during production
An automatic laser measurement device has been used from outset on finished tailgates to conduct on-line measurement after removal from the bonding unit, and after one hour’s curing. It measures inside and outside edges to an accuracy of 0.1mm. The inner shell and glass sheets are checked with laser measuring heads.
Fig 6: On-line measurement equipment (1)
Fig 7: On-line measurement equipment (2)
Sealing quality is also checked on-line, with automatic testing conducted at underpressure. Results from all tests are assigned to the bar-code and documented. Parts are then conveyed to final assembly.
Fig 8: Seal quality measurement
Bonding and assembly
The following basic requirements specified for the bonding process included two-pack semistructural adhesive; surface activation by flame treatment; and an initial setup of two bonding units. A complete tailgate is bonded in one bonding unit. Adhesive is metered by a dynamic mixing head and applied by robot. Two bonding units are currently in service, with the option of more if required.
After application of adhesive and joining of the individual parts, the adhesive cures under infrared emitters. The next step is on-line dimensional monitoring and measurement of sealing quality. Assembly continues with the addition of the wiper motor, wipers, lock, emergency unlocking device, cable harness, interior trim and emblems. The fully assembled tailgate modules are then transported in special containers to BMW Leipzig.
Fig 9: Bonding and assembly line
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