Thermoformed ABS plastic Bodywork on RB13 University of New South Wales – Redback Racing Team

In a bid to minimise real world costs, lean manufacturing principles have become a critical part to the process of design for manufacture. Strong appreciation for such principles, alongside sustainable decision making allowed the UNSW Redback Racing Team push the design envelope of SAE vehicle bodywork in 2013.

RB13, the teams 2013 vehicle, utilised thermoformed ABS plastic in place of CFRP bodywork to reduce work time, skills requirement, safety risks, costs, and improve repeatability, part lifecycle while delivering a part of similar weight at cost of some stiffness.

Lean Manufacturing Principles

Lean design philosophy that aims to optimise or limit the total resource input, to achieve improved output quality, reduce waste and cost. In essence, achieve maximum gain with minimal input. In manufacturing, additional processes that do not add value to the final product are reduced or eliminated in the design phase to reduce manufacturing wastes that include transport, inventory, motion, waiting, overproduction, over processing and defects.

Lean manufacturing principles can be traced back to Just-In-Time (JIT) Manufacturing principles, and the Toyota Production System (TPS) from Toyota.  Such principles allowed for lowest costs per item in mass production systems by eliminating or reducing total wastes which can be applied to FSAE racing to increase time available for construction, lower skill requirements for manufacturing and assist teams with smaller monetary budgets.

Redback Racing Team Lineage

The Redback Racing Team current vehicle lineage is well regarded for its design simplicity, finesse, and light-weight performance. The team’s design and manufacture strategy has been to, in essence, build it simple, build it light, allowing the team to maximise in-house manufacture with low complexity tooling without sacrifice to vehicle performance. This has been achieved with a hybrid monocoque setup, a design legacy that balances vehicle stiffness, packaging and in-house manufacturability. Of recent, vehicle lifecycle and manufacturing costs in all areas have been optimised to excel in a unique design direction in the FSAE – Australasian Division Competition.

RB13 at FSAE-A 2013

Endurance FSAE-Australasia 2013

For 2013, the team continues its design strategy to produce RB13, a matured design of the current generation vehicle. RB13 features 13” rims on Avon wheels, aluminium control arms, hybrid-monocoque, Aprilia SXV550, locked rear differential, Wi-Fi telemetry, Braille Li-on battery in a package that weighs in 165kg with fluids. This makes RB13 the lightest car built by the team, and the lightest vehicle at the 2013 FSAE-A competition, 10kg ahead of the next lightest vehicle by Tokyo Denki University. This weight advantage improves agility, acceleration and fuel-economy, while also allowing the team to excel in the cost and manufacturing event.

The push for lifecycle consideration came about in 2012, where RB12 utilised Self-Reinforced Polymers (from Curv) to replace CFRP for abraision resistance and minor impact resistance to the front monocoque. Sustainable outcomes were achieved with lower emissions during production of material, and end-of-life recyclability. However, production gains and cost-reduction was also achieved, with less processes required to bond the sheets onto the vehicle. Toxic chemical treatment processes were also removed, thereby reducing skills training and safety risks.

Sustainable manufacturing and further cost reduction outcomes were furthered in 2013 with the introduction of ABS thermoformed plastic bodywork. Lean outcomes such as repeatability and reduced processing time were also achieved in the thermoforming process.

History of Bodywork

past nose types

past nose types

Redback Racing Team bodywork through its lineage of vehicles has been constructed as a three part design. Two side-pods and a single piece nose-cone that extends from the frontal tip of the vehicle to the front roll hoop.

The bodywork has always formed a non-structural aesthetic piece that protects the chassis and protruding powertrain components. In 2012, RB12 featured a two-piece nose-cone that allowed the front nose-tip to detach, improving access to the now detachable impact attenuator. This allowed for quick access to the pedals, and quick adjustment to the new adjustable pedal box.

Up to 2012, the bodywork has been primarily constructed from composite materials such as fibreglass and carbon fibre, similar to other FSAE teams. This method provides the team with the ability to produce the bodywork in house using the wet layup process, requiring only low-complexity tooling thereby reducing team capital costs.

While the wet-layup process is flexible and has low initial setup costs, it is at expense of many labourious hours of cutting individual pieces of fabric, and repetitive distribution of the resin matrix. It also requires a higher level of skills that need to be passed onto new members, and uses costly materials.

Thermoplastic Bodywork Development                                                        

At the start of the 2013 season, the team looked toward implementing lean manufacturing principles and improved life-cycle considerations in the team’s most expensive department, bodywork. RB12 bodywork would cost $1325.26 in the Cost and Manufacturing event, and nearly double in real-world costs. With tightening of budgets, the team sought to reduce costs both real-world and in the event.

During the early design stage a variety of changes and concepts were suggested including; changes to the shape of the bodywork, adjusting the manufacturing process, using alternative materials and even the possibility of running the vehicle without bodywork.  A cost and manufacturability review of the various concepts was then conducted. Ultimately, the vacuum forming process was selected as the team believed it was the most ideal for the purposes of a Formula SAE vehicle.

The vacuum forming process involves the heating of a flat sheet of thermoplastic sheet until it is made pliable, and can be draped over the mould. The mould is them pushed up into the pliable sheet, beginning the shape forming process. The edges of the sheet are allowed to seal against the mould, and vacuum pressure is immediately applied from under the mould, forcing the plastic onto the mould improving dimensional accuracy. Pressure is then released, forcing the part to pop off the mould. This method has little complexity, produces parts in less than a minute and requires very little capital and is achievable in-house. Redback Racing team was however fortunate to be supported by local Autofrost group to utilise thermoforming machinery.

The use of crystalline thermoplastics, such as Curv polypropylene, was considered as it would give good mechanical properties. However, the use of amorphous structured thermoplastics was selected to give improved pliability and the ability to draw over long surfaces at glass transition temperature. Acrylonitrile Butadiene Styrene (ABS) was selected due to its availability, good impact strength, formability, availability and low cost. In terms of weight, thermoplastics are surprisingly on par with carbon fibre. ABS is itself actually less dense that carbon (1.06 g/cc vs. 1.7 g/cc), in turn will allow for use of thicker material to compensate for stiffness.

Material Table of Properties

CFRP (Commercial) ABS Plastic Curv SRP
Elastic Modulus 70GPa 2.2GPa 4.2GPa
Tensile Strength 235MPa 45MPa 120MPa
Density 1.7 g/cc 1.04g/cc 0.9g/cc
MDF mould

MDF mould

after machining and bogging of surface

after machining and bogging of surface

The moulds used for thermoforming process were prepared in a similar manner to the wet lay-up process with some minor adjustments. The moulds were constructed from medium density fibreboard (MDF) and milled in a 3-axis CNC machine. MDF was chosen as it is readily available at relatively low cost and may be easily fabricated. Alternative materials such as plaster, clay and polystyrene foam were also considered however there were concerns about the moulds lifespan and dimensional stability at temperature.

primer added

primer added

To facilitate the vacuum forming process considerations to the shape of the moulds were required. The mould sits on a flat sheet of plywood to allow the plastic sheet to seal from the vacuum pressure applied from under the mould. Vent holes were added to features to allow suction pressure into detailed areas. Flat vertical surfaces were avoided by using a 6° draft angle, to prevent the plastic sheet from drawing excessively until it became too thin.

thermoforming maschine bench

thermoforming maschine bench

A 3mm ABS sheet was used for all parts, drawing out in areas down to 1.4mm in thickness, similar to that of 2-ply carbon. Curvaceous surfaces were utilised in areas where stiffness is required to resist traffic cones in attempt to maintain bodywork integrity.

Performance and Results

The thermoplastic bodywork performed well at competition and was a major factor toward achieving 1st place in the cost and manufacturability event. A total 83% reduction in cost reduction was achieve, with total parts and assembly costing $160.79 making a total saving of $1164.47.

Component Weight Comparison

 

RB12 RB13
Nose-Cone 1.1kg 0.9kg
Side-Pod 0.8kg 1.1kg

They bodywork weight targets were kept in check, giving parts of similar weight to RB12. Of note, the RB12 side-pods required additional supports, lending to its slight weight advantage.

The second key area of performance was to ensure the stiffness and strength of the bodywork was sufficient to deflect 0.7kg traffic cones at speeds of approximately 70km/h. With limited time to obtain testing results, a direct qualitative comparison is made against RB12 bodywork, which withstood the load conditions on track.

prepared for heating

prepared for heating

The nose-cone was found to give the same characteristics between RB12 and RB13 bodywork on surfaces likely to be struck by traffic cones. The slab sided faces, where no curves were present, allowed the RB13 thermoformed bodywork to flex significantly. This surface however would be supported by the mounts that would hold the nose-cone in position, giving a similar performance outcome to RB12 bodywork.

The top and bottom faces of the side-pods were found to be the weak points in the side-pods, allowing significant flexure at the rear where it is self-supported against its own weight. Components housed in the side-pods would then be used to help support the bodywork under its own weight.
The outer faces that come into contact with traffic cones, saw least draw under manufacture, allowing most of the 3mm thickness to remain. This allowed for greater effectiveness to resisting shock and deflection against traffic cones.

Reinforcements to the mounting points were done to ensure the bearing strength of the material around the zeus clip would be sufficient in event of traffic cone impact.

Summary

Redback Racing - RB13

Redback Racing – RB13

The team benefitted greatly from the reduced manufacturing time and ability to produce more than one set of bodywork. However, the ability to make lifecycle considerations, and significantly reduce the cost of the components allowed the team to be competitive in a unique design field in the 2013 FSAE-A competition.

Some compromise on components stiffness has been made, however, with good shock resistance and ability to shape bodywork into curvaceous surfaces, sufficient stiffness was achieved without significant penalty on weight with use the of thermoformed ABS plastic. This ultimately allowed the team to produce a low-cost vehicle that weighs in at 165kg for greater agility and performance.

Authors

Geoffrey Hung
RB12 Chassis Department Head
2013 Business and Sponsorship Head
Responsible for chassis design for RB12

Jason Wong
RB13 Bodywork Department Head
Responsible for bodywork design for RB13

 

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