The 3D Printing Footwear Take-Over

3D printing

Traditionally, shoes are manufactured using a myriad of cutout patterns for the upper plus a sole comprised of two or three layers. Constructing an upper and fitting it to a last is largely a manual process, making it labor-intensive and time-consuming. Soles are produced with expensive molds for each size, driving up the cost of a shoe. 3D printing offers the opportunity to create shoes in a largely automated way, in a more cost-effective way for limited series of 100-400 pairs of each size, and custom-made for each wearer improving aesthetics and comfort. We are investigating the possibilities of constructing a modern shoe entirely through 3D printing techniques.

In constructing such shoes through 3D printing, several functional and ergonomic requirements have to be taken into account:

  • Airflow providing ventilation to the foot.
  • From the eyestay that holds the lace openings to the sole, there is a large tensile force occurring upon landing of the foot that will need to be absorbed by the structure of the shoe. We can take inspiration from many current shoes where the quarter panel is reinforced with heatbonded films, wires, leather, TPE, or thicker knit structures.
  • Many current shoes have a heel notch: a cutout at the top of the heel to keep the achilles heel from rubbing against the shoe. The upper part of the heel touching the achilles is often cushioned to provide better support. This is a critical area in all-knit uppers as they wrap more tightly around the foot and can cause heel chafing.
  • Multimaterial 3D printing allows for a mixture of materials in various parts of the shoe to give rise to a graded functionality to meet the desired mechanical properties in every part of the shoe. For example, harder materials could be used in the heel in a lattice structure to give rise to the same compressive properties while reducing weight.
  • Grassroots innovation. A very enthousiastic 3D printing community is emerging that is developing more and more skills and knowledge of 3D modeling and product development. Production is on-demand and does not require any stock keeping. This way, a product is always in development as customers are sharing experiences, and work on improvements which can give the product a distinct advantage. These improvements can include compartments for electronics like GPS tracker chips and batteries that are being upgraded over time.
  • Scan data of the customer’s foot will have to be translated into a set of parameters to drive the 3D model customization algorithm.

Construction: FDM

With rapid innovation in available materials, as well as much lower production costs compared to other 3D printing techniques and the capability of using multiple materials on some machines, Fused Deposition Modeling (FDM) is the most interesting technique for 3D printing running shoes. Materials are extruded from a spool or container and laid down in strands layer upon layer. This way, an entire running shoe including sole and upper can be built in one print run with hardly any manual labor needed.
A drawback of this method is that layer-to-layer adhesion tends to be weak, though in some materials it is strong enough to be able to produce functional shoes. These are elastic materials that increase linearly in hardness with the amount of infill printed.

TPE
The most interesting elastic filament for commercial purposes is Flexismart because of its price ($33/kg), color availability and good elastic and tensile properties. The surface is relatively hard so to improve wearing comfort on the inside you could use a flocking gun to apply nylon, rayon or suede fabric fibers.

Filaflex is a more proven filament for footwear applications with a higher price at $85/kg and availability in many striking colors. Its properties can be altered by varying the additives used. Future research is in full motion and includes scented polymers. It can be printed multiple times faster than other flexible filaments because of its low viscosity so up to two pairs a day can be printed with a 1.0 mm nozzle. It is stretchy, softer to the touch than Flexismart and comes in two diameter sizes. Also it has been tested to be food safe, resistant to oils and acetone, non-toxic and non-irritable to the skin. Footwear design school SLEM and several startups are currently printing with Filaflex.

Ninjaflex is a more elastic and less expensive TPU than Filaflex but also weaker – it will be stretched irreversibly at 65% elongation. It is also harder (85 shore A). It can be used to print shoes and insoles in dozens of interesting colors but is mechanically less suitable than Filaflex. Also it has no UV stabilizer so it will start to degrade after 4-6 months of outdoor use. Ninjaflex comes in spools of 1.1 or 5 pound spools at 1.75 or 3 mm at $76/kg.

Recently, a bio-compostable elastic filament has been developed called Willowflex. It comes in two diameters with a cost of $110/kg and requires slow printing speeds. It is strong, has good adhesion between layers and has been proven for footwear applications by professional institutions.

The FlexMark series filaments by TreeD come in shore A 70, 81A, and 93A with a 750% elongation at break and 1.75mm diameter. That makes them softer and more elastic than Filaflex. Disadvantages are that they are supplied in black only and costs $266/kg.

Flexfill 92A by Fillamentum, at $77/kg, comes in 1.75 and 3mm filament and 7 colors. It is a bit less flexible than Filaflex but also elastic with 600% elongation at break property, and very good against abrasion. Unlike Filaflex, this material should be printed slower and with a hot bed.

An experimental material that can prove useful is Rubberlay Solay. This newly developed filament is flexible and elastic with a 90 shore A hardness and costs $160/kg. It has hardly been used but may prove a viable alternative to TPUs. It has a pleasant rough finish and can be dyed in any color.

Another alternative elastic filament is flexible Polyester (FPE) which at a hardness of 90 shore A is suited for shoe soles and cost-effective at $49/kg.

TPE Printers

Most interesting is to use the Lulzbot TAZ 4 printer with multiple printheads to combine softer filaments like Filaflex or Flexismart and harder filaments like Ninjaflex for parts needing more stability or reinforcement. This is a machine from a trusted brand at a price of $1050, including a heated bed, large build volume of 298 x 275 x 250 mm which allows for printing a pair of size 12 sneakers, and the capacity for a dual extruder. It uses 3mm filament and is suited for Ninjaflex and Filaflex.

For printing in a single material, we recommend the Witbox 2 printer, that has been proven to print well with Filaflex. It costs $1,500 and with a build volume of 297 x 210 x 200 mm it enables you to print up to two size 12 shoes simultaneously.

The Lewihe Sneaker printer with Filaflex is the choice for fast printing. Because of its print head adapted to flexible materials, it can print much faster than other FDM printers – speeds up to 140mm/s have been used successfully. Printing a shoe will take about 5 hours. With a build volume of 320 x 185 x 160mm it suits sneakers up to size 15. The machine is priced at $2,995 and uses 1.75mm filament. The XL version is twice the price and its larger build volume allows you to print five sneakers instead of two per cycle. However more production-efficient, due to possible printing errors and not having to replace spools, going for the smallest possible print bed will be more advantageous. You may opt to design a clean break between the sole and upper part, program the machine to pause the print at the parting line and refill the machine with a new filament to create a dual-material duotone shoe. Also you can make a monomaterial shoe much more interesting with hydrographic printing.

The Stacker S4 is the only FDM printer that can print with four materials so allows using a tough material for the sole, a soft flexible for the insole and upper, and a hard material for the stabilizers and eyelets, in a single print. With three or four materials used it can print one shoe per print run, when a single material is used it can use the four heads to print four shoes simultaneously and with two heads it can print one dual-material pair of shoes per cycle. The Stacker S4 works with 1.75mm filament and has a heated bed.

The Fusion3 F306 3D printer is priced at $4,000 and with a dual extruder it allows for the creation of three sneakers up to size 14 in one print run. It is compatible with 1.75mm filaments and has a build volume of 306 x 306 x 306 mm.

Silicone
Next to printing with TPE and TPU polymers, it has recently also become possible to use RTV silicone with paste extruders such as the Hyrel 16A. UV-cured silicones are suited because while they are a paste-like liquid, they can be cured within seconds with UV light. Silicone offers the advantages of a softer feel, lower cost, and provides more grip while being less durable and requiring lower print speeds than TPU. Another disadvantage is lower surface smoothness, because when printed the material is gooey. Recommended is to print largely flat structures without overhangs, and fold these into shape. The Hyrel printer works with multiple 60cc syringes that need to be refilled during printing for large objects. It can also work with various 1.75mm filaments. At a price of $11,000 it has a build volume that allows the printing of eight sneakers simultaneously.

A company to watch in the future is Wacker AG, who have set up the ACEO lab dedicated to developing 3D printers for silicone. In October 2016 their first model, the ACEO Imagine Serie K, is being demonstrated. Another interesting 3D printer for silicone is the SiO-Shaping 1601 developed by Sterne Elastomere. This machine creates detailed prints in silicone in the range of 30-60 Shore A that can be opaque, translucent or phosphorescent and will be released at the K2016 trade fair. With a print volume of 250 x 200 x 100 it can print a single shoe up to size US 11.5 in a single print run.

Resins
Resins such as created with SLA and Polyjet modeling produce very high surface quality but are less abrasion resistant, can be irritating to the skin and degrade over time with UV exposure so therefore are unsuited for outdoor use.

Construction: SLS

With Selective Laser Sintering, powder-based materials are fed into a building volume, and layer-by-layer these particles are fused with high-power lasers. Unlike with FDM, no support structures are needed, and the direction perpendicular to the layers is the strongest so it is best to print shoes upright. Because of superior part strength, an entire shoe could be created in one print run.

3D SLS Printers
The most interesting SLS printer currently available for shoes is the Prodways Promaker P1000. It sells for $115,000 and with a build volume of 300mm cubed and speed of 0.6L/hour it is good for printing 6 pairs in about 6 hours. A drawback is wearing comfort because the material is stiffer so you may have to incorporate a separate inner layer. You can also use rubber dipping to provide a softer outer layer.

The EOS Formiga P110 is priced at $170,000 and allows for the creation of 2 pairs per print run, or 4 pairs for small size shoes.

The 3D Systems SPro 60 is steeper in price at $300,000 and allows for the printing of 8 pairs per print run.

SLS Materials

Two flexible materials are currently available for SLS 3D Printing:

1. TPU. There are a few suppliers that sell TPU in powder form, such as Luvosint with a 92 shore A TPU, also used in the Under Armour Architech shoe. Prodways sells a 70 shore A TPU. Adsint 90A GB30 powder is another soft TPU. These materials have very good abrasion resistance and are very durable with 350-400% elongation before it breaks. These sintered materials are very tough and not very elastic, though lightweight open structures can be produced with tendons and walls of 0.7mm diameter/thickness that can be made elastic by implementing latticework patterns comprised of elements that behave like springs. SLS TPU comes in basic colors and can be further imbued with fabric dyes. The pricing is around $ 200/kg so it will be around $ 80/pair production costs.

2. Duraform Flex. This is a rubber-like material produced by 3D Systems with comparative tensile strength to TPU. It has the added advantage that with added agents it can be produced to a range of hardnesses between 45 and 80 shore A. It is good against abrasion but not as much as TPU, and has only 151% elongation at break.

Construction: Hybrid

The optimal material for a shoe upper is a fabric for its tight fit to the foot, wearing comfort and breathability. It would require an innovative construction to 3D print the shoe this way though there are currently some possibilities. The strategy will be to create a textile-like pattern as a flat surface that can then be formed into a sock-like upper and connected to the sole. This will require the flexibility of a textile structure, the best example being a four-way stretch knit. The advantage of creating an upper from a flat pattern is that each layer comprises the entire pattern, so there are no layer seams crossing the structure as you would have with printing the shoe in three dimensions. That makes the shoe much stronger.

3D Printed Knit
In 2014, a knitting machine was developed that takes digital files as input in the OpenKnit project. Essentially this is a 3D printer for knitted fabrics constrained to tubular knits with up to 3 yarns such as sweaters, hats, scarves and socks. The open-source initiative has had immense success, and allows people to build their own knitting machine with 3D printed parts and components within 5 hours assembly and within reasonable budget. The only human labor required is to attached weights to each print and finish the open edges manually after printing. The printer did not prove reliable enough for repeatedly producing wearable products but is currently continued as Kniterate.com which may result in a very suitable machine.

Continuous Fiber extrusion Disney has developed a 3D printer that extrudes thick fabric thread and with a felting needle creates connections between layers, creating a three-dimensional fabric structure. However this machine is not commercially available and the structures are too fragile for extended use.

Markforged printers extrude continuous fibers at the core of a polymer filament. It will be possible to print yarns together with flexible filaments, creating strong fabric structures, though at the current time these printers are geared towards printing stiff composites using carbon fiber, fiberglass and Kevlar.

Japanese researchers (Matsuzaki et al.) have proved that it is possible to co-extrude textile yarn with PLA on a regular FDM 3D printer with some modifications to the extruder. A jute yarn sheathed with PLA proved to be significantly stronger than unreinforced PLA. Moreover, this method allows for the creation of fabric webs using 3D printers that will have significant mechanical advantages over polymer structures. Elasticity and strength can be controlled by incorporating stretchable yarns into elastic filament for creating a running shoe upper. The printer can be paused at certain points for replacing yarn and /or filament and an automatic cutting system can be developed to cut off the yarn when the nozzle needs to move to a different area.

To directly secure the fabric upper to the 3D printed sole, I recommend using a heat press with heatactivated adhesive with an activation temperature of 100-130 degrees Celsius, such as Heatnbond. I also suggest reinforcing the sides of the shoe by running stronger bands of fabric over the foot from one side to the other, or creating an adjustable strap connected to the sole that can be tightened on top of the foot which will also be a distinctive design feature.

3D Design Apps

1. Rhinoceros + Grasshopper with Lunchbox and Weaverbird plug-ins. A low-priced ($995) professional package with a plugin to create models that change dynamically based on input parameters. Because of the visual approach to generative design it is relatively fast to learn. I recommend setting up a last model with curves, then using Grasshopper to draw geometry for the upper onto the last. For the sole, I recommend to produce this entirely in Grasshopper. The output from Grasshopper are meshes that can be directly exported to a 3D printable format.

2. Solidworks. The modeling approach is to set up a surface model and alter this driven by sketches. Models are parametric so sketches, dimensions and features can easily be changed and the entire model will be recomputed accordingly. The dimensions can be driven by an external design table such as an Excel sheet. The program is quite expensive at around $3,995. Models can be exported to almost any 3D format.

3. Geomagic Freeform. This application allows for a sculptural approach to shoe design which brings the advantage of an intuitive user interface. Reasonably priced at $1,000, it is used by major brands such as Converse, Reebok, Adidas and New Balance to design uppers as well as sole models for mold tooling. With the PHANToM haptic interface, modeling becomes even more intuitive as it gives you the experience of directly working on the surface. Using Geomagic Wrap, scan data of a foot can be imported so the model can be tailored accordingly.

4. Autodesk Within. This allows the creation of complex parametric structures unique to 3D printing. Unlike the easier visual flowchart-based interface of Grasshopper, this program requires deeper level programming.

5. iCad 3D Plus. Relatively easy to use software creates 3D models from 2D drawings by wrapping them around a virtual last. It is targeted to footwear designers so makes for an easy workflow. The designs as well as the last can continuously be altered and output to IGES format as well as 3D printable STL file. In case you are only modeling a sole there is the Sole3D application

 

Mesh Editing Tools

When outputting a 3D file from a 3D design program, it often contains holes and imperfections making it unsuitable to 3D print. This is why you need a mesh editing program.

1. Materialise Magics. Very versatile program to edit meshes and powerful auto-repair and manual repair functions. It also has cut, boolean, retriangulation, smoothing and hollowing functions.

2. Netfabb. Free and easy to use mesh checking and repair software. Especially the wall thickness checker is useful.

3. Meshlab. Free mesh checking and repair software with many advanced features and file conversion options.

4. Meshmixer. Easy to use mesh editing tool with the advanced remeshing functions as well as the possibility to sculpt.

Slicers

To get the mesh file to 3D print well, it has to be converted into gcode including settings for bed adhesion and support structure. Because the possibilities vary across different slicers, it is necessary to carefully select one.

1. Kisslicer 1.5 beta has the best support structures for shoe design. It allows you to peel the supports off as if peeling an orange.

2. Craftware slicer allows for custom support structures.

3. Cura 2.3. Free, fast and reliable slicing software with many customizable settings as well as custom Gcode plugins for altering printing paths, different settings at different print heights, the possibility to write your own dedicated plugins etc. Especially the zigzag support structure is recommended because it peels off easily.

4. Simplify3D. Popular comprehensive slicer package priced at $150.

Online services

1. 3D Hubs – a hub will charge around $150-200 for printing a pair of shoes in flexible filament. Some hubs print in SLS for example My3DPart – it will be several times more expensive but the shoe can be designed 25-50% lighter.

2. iMakr. They have a large variety of 3D printers available and also have printers for rent.

3. i.Materialise, SLS printing in 92 Shore A TPU. This material feels quite rough to the touch so will need some post-production work to improve wearing comfort. Cost of a pair of shoes will be around $1,300.

4. Shapeways, SLS in elasto plastic which is comparable to i.Materialize’s TPU though a bit stiffer. It can be colored with the use of fabric dyes. Cost of a pair of shoes will be around $1,150.

References

  1. 3Dhubs 3D printing community. http://www.3dhubs.com
  2. Benedict (2016) http://www.3ders.org/articles/20161025-sterne-elastomere-launches-sioshaping-1601-silicone-3d-printer-targets-medical-device-sector.html. Last accessed October 26th, 2016.
  3. Designboom (2015). Adidas and Parley launch 3D printed shoe made from collected ocean plastic waste. http://www.designboom.com/design/adidas-parley-3d-printed-ocean-plasticshoe-12-10-2015/ Last accessed October 26th, 2016.
  4. Engadget (2016). Reebok Liquid Speed shoes use 3D drawing for a better fit. October 26th, 2016.  https://www.engadget.com/2016/10/21/reebok-liquid-speed-3d-drawing/
  5. Feetz. http://feetz.com/collections/feetz-firsts-collection
  6. Kooser, A. (2014) Awww. Look what Disney Research 3D-printed. https://www.cnet.com/news/disney-research-3d-prints-cuddly-teddy-bears/. Last accessed October 26th, 2016.
  7. Krassenstein, B. (2015). SOLS Unveils ADAPTIV, 3D Printed Robotic Adaptable High-tops https://3dprint.com/44772/sols-adaptiv-3d-print/. Last accessed October 26th, 2016.
  8. Matsuzaki, R. et al. (2016). Three-dimensional printing of continuous-fiber composites by innozzle impregnation. Scientific Reports 6:23058.
    With all the structure and support knitted in, the Nike Flyknit Racer’s upper and tongue weigh just 34 grams (1.2 ounces). The whole shoe weighs a mere 160g (5.6 ounces) for a size 9, which is 19% lighter than the Nike Zoom Streak 3, a shoe worn by first, second and third place athletes in the men’s marathon at the 2011 World Championships (http://www.dezeen.com/2012/02/22/flyknit-running-footwear-by-nike/) Last accessed September 15th, 2016
  9. Scott, C. (2016) SLEM and BioInspiration Develop Compostable Shoes 3D Printed with WillowFlex. https://3dprint.com/150715/bioinspiration-willowflex-slem/ Last accessed October 26th, 2016.
  10. Yarwindran, M. et al. (2016) THERMOPLASTIC ELASTOMER INFILL PATTERN IMPACT ON MECHANICAL PROPERTIES 3D PRINTED CUSTOMIZED ORTHOTIC INSOLE. ARPN Journal of Engineering and Applied Sciences Vol. 11, No. 10. Asian Research Publishing Network (ARPN).
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