Technical Report Draft 1
An Affordable Carbon-Fiber Bicycle For The Masses
Team name: Lapis Bikes
Lecturer: Professor Brad Blackstone
Module: MEC1281
Date:
Proposal
Contents
Executive Summary
Introduction
Background
Problem Statement
Purpose Statement
Proposed modification
Benefits
Evaluation
Limitations
Methodology
Concluding Statement
References
Appendix: A
Doi.
Executive Summary
Background
Today, the cost of a carbon fibre bicycle ranges from $2000 to upwards of $10,000. A majority of the costs incurred is due to the research and development required to develop these bicycles for the market.
According to Becker (2018), the cost of a mold of a carbon fibre bicycle “costs between $60,000 and $100,000 depending on a number of variables”. The high costs are due to a number of factors. Firstly, in some production methods, molds are subjected to high pressure and temperatures. As a result, these molds need to be made out of strong materials that can withstand such pressures and temperatures without deformation. In addition, costs can be driven up due to the complexity and design tolerance of the design. The more complex the part to be produced is, the more complex the mold will be. Tighter design tolerances for some of the parts of the design will also require more precise molds to be produced. Once produced, molds are impossible to modify, hence requiring another mold should there be any changes to the design. If multiple iterations are required the costs are further increased.
In standard carbon fiber production, the carbon fibre mold is required to undergo high temperatures in order to bind the resin and carbon together. This process requires a huge amount of heat, which is necessary to burn off the non-carbon molecules in the chemical, which requires a large amount of energy. Energy is expensive, and manufacturers must use a massive amount of energy to bring internal oven temperatures to the thousands of degrees necessary to force this chemical process. Additionally, non-carbon molecules are industrial pollutants and must be carefully—not to mention, expensively—disposed of in order to prevent pollution.
Another reason for the high cost is the manufacturing methods adopted by companies. These include unidirectional prepreg, resin transfer moulding and filament winding.
The most popular method would be unidirectional prepreg method, due to the higher specific properties and a more straightforward specific fibre angle lay-up. To make the carbon fibre weave, a lot of manpower is required, thus increasing costs.
Resin transfer molding is a closed-mould process for medium-volume manufacturing. Molds typically consist of matching metal tools into which a dry fibre preform is inserted. The mold is then closed and clamped shut before pumping resin into the tool cavity to thoroughly wet-out the fibres. It will then be heated to cure the resin, after which the part can be removed from the tool.
For filament winding, construction starts with dry fibre, where fibre tows pass through a resin bath and wound at various angles onto a speed-controlled rotating mandrel, controlled by a fibre feeding mechanism.
All these are complicated processes involved in manufacturing carbon fibre bikes, which results in high costs. The cost of carbon fibre is still expensive for mass manufacture. According to a report by Meredith, J. et al (2015), the cost of carbon fibre is USD 8.31/kg, as compared to steel and aluminium at USD 0.39/kg and USD 1.75/kg respectively. As such, the costs of carbon fibre bicycles will also be more expensive.
All these are complicated processes involved in manufacturing carbon fibre bikes, which results in high costs. The cost of carbon fibre is still expensive for mass manufacture. According to a report by Meredith, J. et al (2015), the cost of carbon fibre is USD 8.31/kg, as compared to steel and aluminium at USD 0.39/kg and USD 1.75/kg respectively. As such, the costs of carbon fibre bicycles will also be more expensive.
The ideal bike frame is a sustainable carbon fiber frame made through eco- friendly manufacturing process while maintaining the same performance.
Problem Statement
Current bike frames in the market are too expensive to manufacture due to tooling and R&D cost. Also, materials such as aluminium and carbon fiber are used excessively and wastefully due to current machining techniques
Purpose Statement
The purpose of this report is to introduce the concept of 3D printed carbon bikes and molds to independent bike companies that do not have the budget required to compete with other companies in terms of R&D cost. This way they are able to construct a bike without having to invest too much money into it
Problem Solution
A hybrid carbon fibre frame coupled with 3D printed sandwich structure created from a 3D printed mould.
Figure 1. Rendering of proposed bike frame
3D printed sandwich structure
A sandwich composite structure is composed of a core material, that is sandwiched between two pieces of composite fiber layers. This structure is used widely in aerospace, naval and automotive applications due to their high stiffness/weight and strength to weight ratio. According to Li,. et al(2017), Conventional honeycomb cores are mainly used in applications due to their superior properties over its foam core. They stated that they were able to show the correlation flexural stiffness and strength as the relative density of the core material increases. Through this insight, we can safely say that we are able to tune the stiffness and the strength of our frame through the use of a sandwich structure. By implementing 3D printing technology, we can optimise the topology of the core with relation to the forces that the component will experience. WIth this, we can keep the carbon fiber layup as simple as possible. We can further optimize the core, by using different types of thermoplastics. Different thermoplastics have varying properties that we can take advantage of. For example, Polyethylene terephthalate(PETG) has excellent impact resistance or Nylon that is durable and wear-resistant. Using a blend of thermoplastics, we can alter the characteristics of the bike frame to however we want it to function.
Figure 2. Cross section of a 3D printed core material that goes into the bottom bracket
3D printed mould
A 3D printed mould will reduce the tooling cost, as we will be moving away from aluminium as the raw material and instead replace it with a thermoplastic. According to ClintonAluminium(2017), they stated that 7075 Aluminium, Aircraft grade, is used extensively for prototyping tooling moulds. According to MidWestSupply.com, 7075 Aluminium costs $20,984 USD/m3. In contrast, the thermoplastic material only cost (insert price here). Using 3D printed technology, further optimization can be made to reduce the amount of material being used. By only reinforcing the parts of the mould that will undergo stress, further reduction of the material used in the mould can be done.
Figure 3. Female 3D printed mould
Manufacturing Process
The manufacturing process that we are proposing will use elements of the above mentioned processes. By combining the use of the 3D printed moulds and cores, we are able to manufacture a bike that has the stiffness properties of a full carbon fiber bike, without the high cost of tooling and R&D. The proposed process is that the designer of the bike has to take into account the forces that the bike frame will undergo. From here, using Finite Element Analysis, we are able to simulate forces that the bike will undergo. Using this data, the designers are able to design a 3D printed core with the required density that is needed. Penultimately, the designer just has to add layers of carbon fiber weave. Lastly, all of this materials will then be consolidated using the 3D printed mould, that will provide the compression force that the carbon fiber needs.
Benefits
Sustainability
One benefit of this method of manufacturing process is the reusability of the moulds. As moulds are made from thermoplastics, used moulds can be re-melted back and reused to make new moulds. This method reduces the amount of wastage as the 3D printed parts can be reused to create new iterations of designs. According to Tian (2017), “continuous carbon fiber and PLA matrix was recycled in the form of PLA impregnated carbon fiber filament from 3D printed composite components and reused as the raw material for further 3D printing process.” Even after reusing the thermoplastics, materials such as continuous fiber reinforced thermoplastic composites (CFRTC) showed no compromise during the recycling process. Instead, the material gave an increase in 25% higher bending strength than its original form.
Reduced Cost
As stated above, 3D printed moulds can reduce the tooling cost. This manufacturing process allows lesser wastage compared to conventional processes such as Computer Numerical Control(CNC) machining. For example, products made from CNC came from a block of aluminium. The block of aluminium is then machined down to its specified dimensions to create the product. Excess materials may be recycled but it requires high amounts of energy to recycle.
(- r&d cost)
https://www.emerald.com/insight/content/doi/10.1108/RPJ-07-2013-0067/full/html
Limitations and Evaluation
There are two categories of cyclists. The first being a serious athlete, whereby every milligram shaved off his bicycle contributes greatly to his performance. Next would be a cyclist who enjoys the sport but does not take part in competitive racing. A hybrid carbon fibre frame at half the price of the commercial carbon fibre bicycle at the slight expense of weight would be sure to appeal to the second group of cyclists.
Despite the reduction in cost, the hybrid carbon fibre frame coupled with 3D printed sandwich structure will experience an increase in weight due to the addition of the core material. This unfortunately will bring down the performance of the bicycle.
However, weight is just one factor taken into consideration. The choice when buying a new bicycle involves other factors such as the feel, stability, comfort, geometry of bike, sizing, aesthetics, functions and presence of mounting holes in the frame. In addition, we could focus more on gravel bicycle, where weight is not such an important factor.
When it comes to a new product, everyone will have their doubts if the product is truly credible. This is especially the case for our product because our core material is plastic. When it comes to plastic, the word strong and sturdy does not come to mind. It will instill doubt on whether the frame is truly as sturdy as stated.
However, a series of tests on our hybrid carbon fibre frame such as the rockwell hardness test, lateral load fatigue test, falling mass fork impact test etc, with comparison video to the other more commercialized frame will allow customers to have greater faith in our product.
Methodology
Secondary research sources were used as a reference to obtain relevant information for the completion of the report. In addition, prototypes were also developed as a proof of concept. FEA (finite element analysis) using ABACUS was also done to simulate the forces acting on the components.
Secondary Research
A thorough research on the materials used and the printing method was conducted in order to determine the best combination for the production of the bicycle structure and mold. The team used manufacturer websites and secondary sources to back up our findings. We used official product websites to get the pricing of the bicycles to set it as a benchmark. Secondary sources were then used to explain the high costs of traditional carbon fibre bikes, and to explain the advantages and disadvantages of using 3D printing technique.
Concluding Statement
This manufacturing process is still in its infancy stage, and requires more time and research to be a fully viable solution. In its current stage, we are able to 3D print the core and the mould. More simulations and optimisations needs to be done, in order to find a perfect ratio of plastic core to carbon fiber. However, when it is fully realised, we will arrive at a bike frame that is sustainable for the environment, and a cheaper alternative to full carbon fiber bikes.
References
Becker, K. (2018, May 18). Carbon Fiber Bike Frames May Become A Whole Lot Cheaper. Retrieved March 2, 2020, from https://www.digitaltrends.com/outdoors/arevo-3d-printed-bike-frame/
Carruthers, J. (2018, April 25). What is Resin Transfer Moulding (RTM)? Retrieved March 2, 2020, from https://coventivecomposites.com/explainers/resin-transfer-moulding-rtm/
Filament Winding. (2019, January 24). Retrieved March 2, 2020, from https://netcomposites.com/guide/manufacturing/filament-winding/
How carbon fibre bicycle frames are made. (2018, January 4). Retrieved March 2, 2020, from https://cyclingtips.com/2018/01/how-carbon-fibre-bicycle-frames-are-made/
Li, T., & Wang, L. (2017). Bending behavior of sandwich composite structures with tunable 3D-printed core materials. Composite Structures, 175, 46–57. doi: 10.1016/j.compstruct.2017.05.001
Meredith, J., Bilson, E., Powe, R., Collings, E., & Kirwan, K. (2015). A performance versus cost analysis of prepreg carbon fibre epoxy energy absorption structures. Composite Structures, 124, 206–213. doi: 10.1016/j.compstruct.2015.01.022
Renner, L. (2018, December 10). The Rise of Carbon Fiber. Retrieved March 2, 2020, from https://blog.propelx.com/what-is-carbon-fiber/
Swaby, R. (2013, June 17). Why Is Carbon Fiber So Expensive? Retrieved March 2, 2020, from https://gizmodo.com/why-is-carbon-fiber-so-expensive-5843276
The Best Aluminum Alloys For Molds. (2017, May 22). Retrieved from https://www.clintonaluminum.com/the-best-aluminum-alloys-for-molds/#:~:text=6061
Why Are Injection Molds So Expensive? - Reading Plastic. (2019, March 29). Retrieved March 2, 2020, from http://readingplastic.com/why-are-injection-molds-so-expensive/
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