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3-D printing at SATRA

Highlighting the benefits to members of SATRA’s 3-D printing capabilities.

by David Smith

The November 2012 issue of SATRA Bulletin included the article ‘3-D printing in footwear’, which described the main methods of three-dimensional printing currently in use, as well as potential applications for this technology within the industry.

SATRA currently uses the FDM system – fused deposition modelling – the technique of which is described in the above article. This system ‘prints’ in a durable acrylonitrile butadiene styrene (ABS) plastic material that can be sanded, drilled, tapped or sawn to suit. While the material itself is not flexible, it can be used with careful design to build models of surprising complexity and detail. These can include design concept models for new footwear, supports and clamps for bespoke test rigs and anatomically correct foot forms which can be used to make mouldings for the production of test feet.

The printer takes rolls of ABS filament, which it heats to melting point before extruding into shapes made up of multiple layers a mere 0.25mm thick. In addition to depositing layers of this build material, the printer also deposits layers of a ‘support material’ – a scaffolding material that is used to protect and encase delicate structures during printing, and which can be used to fill small voids in a structure which will later be removed. For example, a working chain can be printed by temporarily filling each link with support material which is later removed to allow the chain to flex as normal.

An associated wash station removes the support material from the finished model by subjecting it to controlled agitation in a mild alkaline solution, after which it appears exactly as it did when it was first envisioned on the designer’s screen.


Figure 1: Custom-made components (printed in white ABS) installed in SATRA test equipment


Child-sized foot forms printed on the 3-D printer from data collected during SATRA’s foot scanning project

SATRA’s 3-D printer has already been used in a variety of applications. Contract testing work in the research department often requires the building of one-off or prototype testing machines for the purposes of assessing some novel product. The framework for these machines is often extruded profile aluminium bar, and the working parts (such as pneumatic cylinders and push-fit couplings) are also off-the-shelf solutions. Sample holders or unusual fittings, however, must be custom-made specific for each machine (figure 1). With the bespoke testing carried out in the research department, some means of securing irregularly shaped samples is often required, and a custom-made holder is often the only option.

Previously, this was only possible by relatively costly engineering, or by creating a rough piece by filing a block of wood or plastic to the appropriate shape. The use of the 3-D printer allows complex support jigs and sample holders to be made in only a few hours. These are durable enough to be machined and then incorporated into the test equipment, and resilient enough to be used for the lifetime of the machine.

SATRA’s recent work with digital footscanning has helped us to build up a large database of foot size data, from extensive studies in China, the UK and the US. Processing this data has allowed us to accurately model foot shapes in 3-D, and to create ‘average’ foot models that are representative of a particular size or age range. Some of these models were then output to the 3-D printer to create a solid footform based on our anthropometric data. This can now be used to form moulds for the production of heated or similarly instrumented feet (for SATRA’s Endofoot machine). This level of accuracy and biofidelity gives us the ability to produce test equipment unique to SATRA, based on our own specifications and data.

One of SATRA’s customers manufactures insocks for footwear worn for outdoor pursuits. These insocks feature electronic circuits, including a lithium polymer battery. SATRA has been working with this customer for some time to refine the design of these insocks, and the latest revision incorporates a user-replaceable battery pack. In order to assist with the development of this new product, SATRA printed a number of 3-D dummy battery packs that are a perfect fit for the receptacle in the insock.

Once in place, the insocks were assessed for durability using the STM 528 Pedatron machine, assessed for comfort by means of in-shoe pressure mapping and tested for the ease of replacing and removing the batteries by hand. This was invaluable, as it allowed the electronics in the insocks to be assessed while the batteries themselves independently went through design iterations and were refined by the manufacturer. Feedback from the designers was received during SATRA’s testing process, after which changes were incorporated into freshly printed 3-D battery packs made to the new design, and these were then re-tested. Once materials and design were finalised for the battery packs, the factory fitted them to the insocks, confident that they would fit neatly into a framework of electronics with proven durability.

3-D printing technology
Three-dimensional printing technology falls into four main categories:
  • SLS (selective laser sintering) – a bed of powder is solidified into shapes where a laser strikes the surface
  • SLA (stereolithography) – a pool of reactive liquid solidifies where a laser strikes the surface
  • 3-D inkjet printing – a bed of powder is bound together to form shapes by a binding agent which is ‘printed’ from a travelling inkjet head which also adds colours to the model
  • FDM (fused deposition modelling) – ABS (acrylonitrile butadiene styrene) filament is heated to melting point and then extruded through a nozzle, to be deposited in thin layers that make up the shape being formed.

In addition, these dummy batteries printed in 3-D have been sent to the manufacturer to use in their assembly process. This is to ensure that sufficient clearance is left for the real thing and to allow for experimentation with different designs.

SATRA is frequently asked to assist customers with the design and manufacture of innovative footwear, and to help accelerate the process of bringing novel footwear to market. One area of particular interest is the goal of making high-heeled shoes more comfortable. In line with this, SATRA’s research department has worked with designers several times over the last few years to create 3-D printed concept components.

SATRA has found its 3-D printer so useful that it intends to invest in a second unit in the near future. To complement our existing capabilities, there are two main options – a stereolithography unit that prints in flexible rubber-type material, or another FDM unit with two heads that prints in two colours of ABS or in two materials of different hardness.

The acquisition of this latter technology would enable us to create pliable sole units in realistic hardness, or more detailed and realistic footwear models that would be more akin to the appearance of finished footwear. It would also allow for the creation of jigs and clamps for bespoke test equipment that have a greater degree of flexibility and adjustment.

3-D printing is a powerful tool that is being utilised to good effect at SATRA, and our present and future capabilities are another reason for choosing us as your premier research partner.

How can we help?

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Publishing Data

This article was originally published on page 44 of the March 2014 issue of SATRA Bulletin.

Other articles from this issue »