Customized ski boots for better comfort and performance
4 October 2018
The combination of additive manufacturing, 3D scanning and innovative customization software enables Tailored Fits AG to manufacture individual ski boots. An application which was illustrated in the Swiss AM Guide 18 and awarded at the AM Expo 18.
Anyone who has ever skied knows this problem: after a longer skiing trip, feet hurt – and finding comfortable ski boots can be a challenge. Ski boots are usually offered in different sizes but are not tailor made to the customer’s foot. A conventional ski boot consists of a hard outer shell and a soft inner shoe or ski boot liner.
Conventional solutions without additive manufacturing
In case of a non-optimal fit, the shoe will squeeze, blood will accumulate and as a result, the wearing comfort is lowered. Since there is no tight fit between foot and boot, the necessary support is reduced and thus the force transmission between foot and ski is lowered. In addition, buckles and straps are required to maintain a stable fit of the foot in the shoe, which leads to increased pressure points. The ski boot industry has tried to address this problem with foamed ski boots. Foaming enables the individual adaptation of an inner shoe to the geometry of a foot. Although this results in a better fit, foamed inner shoes have the disadvantage that after extensive use, the foam is compressed and wears out.
Solution of Tailored Fits AG
Tailored Fits AG, a young start-up from Horw near Lucerne in Switzerland, has focused on this problem and is now offering customized ski boots. It is the inner shoe or liner of a ski boot that is adapted to the foot shape of a customer by means of 3D scanning and fabricated with the help of additive manufacturing. The entire process utilizes a digital, end-to-end value chain that allows the creation of tailor-made ski boots. In addition to ski boots, Tailored Fits AG offers its customers other customized footwear such as insoles for running, hiking, cycling or football shoes. In the following, the entire process, which has been set up in cooperation with the Belgian company Materialise, is illustrated using the example of the customized ski boots.
Production of individualized ski boots
For the individualization of ski boots, the customer’s foot geometry must first be captured. In a sports store, the seller scans a customer’s foot with a 3D scanner. The 3D scanner is an easy-to-use, portable tablet with a structured light sensor attached to it. The contactless scanning process, which records the customer’s foot geometry with sufficient accuracy, takes only a few minutes. By pulling the toe tines, the customer puts the foot sole in a stretched position, just as it is the case in an active skiing movement. In addition to the scan data, the retailer transmits the customer’s weight, sole length and other preferences to Tailored Fits AG.
Once the scan of the foot geometry has been transferred to Tailored Fits AG, a quality control is carried out. Anatomical features are used to orientate the scan in space. A virtual bone skeleton is inserted into the scanned foot to morph the geometry and rotate it around the ankle in different directions. In this way, the alignment of the leg position and the angle of inclination can be adjusted and corrected in a targeted manner. This modified scan of the foot serves as an input for the design generation of the inner ski boot.
Design process of ski boots
The design creation process of the inner ski boot geometry is fully automated. For the customer’s individual scan and preferences, a software generates the corresponding inner ski boot, which possesses a varying hardness or softness at different areas. In addition to the use of a flexible material, this is achieved by the targeted implementation of distributed cavities and infill structures. Their stiffness or damping properties are determined by the specified customer weight. The closed cavities also serve as insulation for the inner shoe against the cold during skiing. The process for the automated creation of shoe insoles is similar. Depending on the chosen sport and customer preference, a different distribution of the cavities is generated for varying the insole softness.
Fused Deposition Modeling
As previously mentioned, the inner ski boot is additively manufactured with a flexible material, in this case Thermoplastic Polyurethane (TPU), using fused deposition modeling (FDM). Compared to powder- or liquid-based processes, this AM technology allows the creation of closed cavities, which serve as a thermal insulation in the ski boot. The employed German RepRap FDM printers are very cost efficient compared to laser sintering machines, which can also process TPU material – but not with enclosed cavities. For support structures, PLA material is used. The inner ski boots are manufactured at Tailored Fits AG and Materialise in Belgium. After additive manufacturing, the inner shoe is flocked.
Cost-effective production due to fewer individual parts
The final ski boot consists of an additively manufactured inner shoe and a conventionally manufactured hard shell, which is produced in various sizes by injection molding. Since the inner shoe can be adapted to a wide range of different foot geometries, only a reduced number of sizes is required for the outer shell. The final assembly of the ski boot is done by inserting the inner shoe into the shell. Since an optimized form fit is achieved through the custom-fit to the foot geometry, a single closure strap with a button snap fit is sufficient to close the shoe. Therefore, unlike conventional ski boots, several buckles are not necessary to firmly hold the foot in the ski boot. This simple design of the ski boot reduces the number of individual components from around 200 to less than 20. As a result, cost and lead-time of manufacturing, assembling and logistics are significantly lowered throughout the entire process chain. In this way, the price of individualized ski boots becomes competitive with comparable solutions.
Advantages for customers
After a quality check, the ski boot is sent to the retailer’s sports shop, where the customer receives his personalized ski boot. There are many advantages for the customer. The custom-fit shape enables a more precise and direct force transfer during skiing. Better blood circulation and reduced fatigue ensure increased comfort. In addition, insulation against cold is improved. The simplified fastening mechanism makes it easier to get into and out of the shoe, and pressure points are prevented compared to a shoe with several straps.
History of brainstorming and implementation
The individualization process described above is highly complex in terms of design, production and business model. An iterative and exploratory process was required and several problems had to be solved. At the very beginning, the first idea was to directly additively manufacture the outer shell of the ski boot and use a generic, standardized inner shell. The first prototypes were laser sintered from polyamide (PA). After a joint workshop with Materialise, the idea came up to additively manufacture and adapt the inner boot. A suitable, easy-to-use, cost-effective scanning technology had to be defined together with a suitable scanning positioning of the foot. In addition, an appropriate flexible material had to be chosen together with the right additive manufacturing technology and optimized process parameters. As soon as flexible TPU with FDM was printable, the question came up at which regions the shoe had to be hard or soft. Repeatedly different models were discussed with orthopedics as well as tested by the team on the ski slope regarding comfort and performance
After extensive testing, the decision was made to reduce complexity, focus first on the lower part of the inner shoe, and develop a shoe insole. The properties of the insoles were progressively improved and after some development time it was decided to offer them as a first product for different sports like running, hiking or cycling shoes. The automation of the design creation process posed a further challenge. Depending on the foot geometry and customer preferences, it was necessary to automate the generation of distributed hollow structures with variable stiffness. Besides creating a continuous, traceable process chain and robust production, there was also the challenge to implement it in an industrial and scalable supply chain as well as convincing first sports retailers. Based on the gained knowledge and technical expertise, it was possible to adapt the process chain of insoles in a similar form to ski boots.
From a holistic viewpoint, it becomes clear that an innovative approach and a restructuring of the process chain for conventional ski boots was necessary in order to implement the described process and rethink the underlying business model. In this context, doubts had to be overcome as to whether the quality of additively manufactured products could be competitive and whether customers would accept such an individualized product. Overall, there are many advantages for the customer and the manufacturer. However, also the sports retailer benefits. A sports shop is no longer dependent on buying and storing a larger number of standardized ski boots long before the start of the next winter season. This significantly reduces storage costs as well as the margin due to unsold goods. Since individual ski boots are custom made, payment is usually required in advance. In summary, the described process creates new opportunities for manufacturers, retailers and customers becoming part of a shared platform.