Documenting Design Part I: WheelAir

Documenting Design Part I: WheelAir

I met Corien Staels, founder of Staels Design, at her office in the one of the buildings at Glasgow University. Corien had spent the last year or so working towards solving a problem that was illustrated to her by her wheelchair-user tutor at University. This problem was one that never would have occurred to me as long as I had uninhibited use of my legs: many wheelchair users struggle with overheating as a result of sitting in their wheelchair for extended periods of time. 

USING 3D PRINTING FOR MAXILLOFACIAL MEDICAL TREATMENT

Recently we've been spending our time reaching out to the medical community in Scotland, trying to understand the needs of the different departments within the medical and dental spheres.

We were pleased to get a call back from Fraser Walker from the Maxillofacial department at the Southern General Hospital in Glasgow. He was really interested in speaking to us and kind enough to invite us in to see what they do. The second I put the phone down, I went straight to Google to ask "What does Maxillofacial mean?". Okay, so I'm not so clued up on the medical jargon... 

Put basically, the British Association of Oral and Maxillofacial Surgeons describe it as:

Oral & Maxillofacial (OMF) Surgeons specialise in the diagnosis and treatment of diseases affecting the mouth, jaws, face and neck.

Fraser Walker is the honorary secretary at the MaxFac laboratory at the Southern General, where they use 3D printing to improve their treatment of maxillofacial patients.

The Southern General Hospital

The Southern General Hospital

pon our arrival, Fraser was extremely friendly and keen to show us all around their lab, explaining to us in detail (without breaching confidentiality, of course) how they've used their Objet 30 3D Printer to produce 3D models for different purposes. The majority of the models they make are full scale, and mostly serve as a reference for planning surgical operations. 

He then introduced us to Micheal O'Neill, an award-winning maxillofacial prosthetist who gracefully granted us some of his valuable time to show us how their CAD workflow happens, from CT scan right through to 3D printed models and final surgical procedure plans. 

Using Materialise's very-high-end Mimics software, they take raw data from a CT scan, and filter it down to exactly what parts and what tissue type they need. This can then be converted into an interactive 3D model.

Materialise's Mimics Software Package - (SOURCE)

Once the 3D model is created, the prosthetist has a range of tools they can use to plan surgical procedures:

  • isolating bone area to be removed
  • design cutting guides
  • using removed bone to find suitable replacement bone to harvest from elsewhere
  • replacing old bone with harvested bone to analyse suitability
  • planning of welded titanium plate to hold the new bone in place

Over the course of such a process, typically more than one 3D printed model will be used to validate the surgical plan, and to communicate with the patient exactly what is going to happen.

Fraser went on to describe how invaluable 3D printing has become as part of their treatment, describing how surgeons have become so reliant on 3D printed reference models that they have almost forgotten that as little as 10 years ago, they had to perform facial re-constructive surgery without them.

As a 3D printing service, the ability to produce models that will potentially reduce surgery time, risk of infection and improve overall results is quite humbling. We will always enjoy making and perfecting aesthetic & functional models, but when you can help to make a difference to someone's quality of life in this way it really illustrates the life changing benefits of 3D printing technology.

PRINT ME A HORSE'S HEAD

It’s not every day that you get a request to 3D print a life-size replica of a horse’s head from a statue, but that’s what happened when Fokus Grupa – an artist collective based in Rijeka, Croatia, contacted us this summer.

Their idea was to create a new art piece based on a 3D scan of an equestrian sculpture which stands in the main square in Zagreb, the picturesque capital of Croatia. A 1:1 replica would require a print standing over a metre tall, a metre deep and 60 cm wide – quite an undertaking when working with printers whose maximum print size is 28.5 x 15.3 x 15.5 cm.

Ban Josip Jelačić Statue: Source

Ban Josip Jelačić Statue: Source

The first challenge for us was the file editing required – the 3D scan of the horse which was provided was of fairly low resolution, which, when scaled to the correct size, leaves holes and gaps between surfaces that need to be repaired. This somewhat tedious task can be made easier with automatic repair tools found in 3D modelling software but there are always issues leftover which require manual editing. With the model fully repaired, the next stage was the gruesome task of ‘slicing’ the horses head into separate blocks which our printers could handle. In order to ensure each individual part would be printable we made each block at least 20mm smaller in each axis – allowing room for the base layer (or raft) which the printers use instead of printing directly onto the build plate.

Horse Ear Repair

Horse Ear Repair

Print Showing Raft

Print Showing Raft

At this stage we had a large patchwork 3D model of a horse’s head to play with. To reduce print time, weight, and ultimately cost, the inner section of the scan was removed to leave a thickened ‘shell’ of the original scan.

Patchwork Horse

Patchwork Horse

Patchwork Horse (Hollow)

Patchwork Horse (Hollow)

With 124 blocks of all different shapes and sizes needing to be printed it was time to go Excel-geek. Each individual block was saved as an .stl file and loaded into our printer software to give us a time and material usage estimation we could use to plan the printing process. Prints ranged from half hour – 6 gram jobs, to 15 hour – 366 gram ones. They were all recorded in our spreadsheet and formatted to show the varying print times. This allowed us to estimate both the amount of filament we would need to order, and a schedule for printing which would make the best use of machine time. So with images of the famous Godfather scene galloping (sorry) through our heads it was time to start printing.

Time Sheet

Time Sheet

arge blocks were printed individually, small ones grouped together to maximise efficiency until we were left with a pile of horse parts waiting to be assembled.

Collection of Blocks

Collection of Blocks

To ensure that all of the blocks had been printed and that they fit together correctly we assembled them using tape. Some of the larger blocks had warped during printing - creating gaps when assembled with neighbouring pieces. This process of 'pre-assembly' allowed us to identify where these gaps appeared, we subsequently split some of the larger blocks into multiple parts to alleviate this problem.  

 

Pre-assembly

Pre-assembly

With the new blocks printed and pre-assembled we were ready to start fixing them permanently using epoxy resin. Anyone who has ever used epoxy resin before will know that it is a messy and smelly business. Due to the working time of the epoxy each part had to be held in place by hand (curved geometry makes the use of clamps impossible) until the resin had hardened. 

Fixing

Fixing

Bit by bit, the horse's head was taking shape and the unidentifiable blocks (except for our cunning orange marker numbering system) were beginning to make up unmistakeable equestrian features - a mane, nostrils and eventually...ears! 

Building Blocks

Building Blocks

Taking Shape

Taking Shape

Inner Section

Inner Section

To enable us to work on the bottom of the head, we kept the final three layers separate allowing us to flip the whole piece upside-down. The next stage was to fill the gaps between the assembled pieces of our lobotomised horse's head. For this we used a general purpose filler which was quick-drying and easily sand-able.

DSC_0019_medium.png
Filling Gaps

Filling Gaps

ith both sections filled and sanded it was time for the final join and the completion of the horse's head.

Sanding

Sanding

The Mane Attraction

The Mane Attraction

Decimation Detail

Decimation Detail

Totaling 14 Kg of plastic (equivalent to 3.5 miles of ABS filament) and half a Kilogram of epoxy resin, the horse's head was finally ready to be picked up by Iva and Elvis (Fokus Grupa) and transported painstakingly to the exhibition at the Transmission gallery in Glasgow. This project's model was the largest we have undertaken and posed many new challenges. Having said this, it was one that we really enjoyed, and in the end, the result was quite spectacular. After months of e-mail communication and the odd video call it was great to finally meet Iva and Elvis who were kind enough to invite us to the opening of the 'People Love Monuments' exhibition and explain the story behind the series of works presented. 

Print me a horse's head?

Neigh bother.

Provoking Thought

Provoking Thought

Transmission 

Transmission 

A FEW WAYS TO STRENGTHEN 3D PRINTED PARTS

A dead bit

A dead bit

There are some 3D print jobs that require a focus on part strength. We’ve had a few projects in our time that have led us to trying a few different strengthening techniques in order to guarantee that a part will survive when dropped repeatedly or that it will stand up to loading conditions you’re planning to put it under. This study seems to show that there is very little difference in strength between expensive commercial 3D printing machines and low cost desktop machines. All you need is a little bit of time spent thinking and tuning the settings.

This article is a sort of checklist I've put together that’s really more tailored for FDM (or FFF) desktop 3D printers. While sections 1 and 2 deal with steps you can take immediately, sections 3 and 4 require a little bit more preparation and thought.

1.0 MODEL GEOMETRY

Surfaces can be offset then thickened to give stronger thin sections

Surfaces can be offset then thickened to give stronger thin sections

1.1 THICKEN YOUR MODEL

    Starting with the most obvious technique, thin geometry usually produces weak parts. This isn’t helped by the fact that desktop FDM printers struggle to attain a decent quality with thin parts of a print (eg. layer separation, warping and nozzle clash). Think about whether or not it’s possible to thicken the model geometry there. Sometimes it won’t be possible to change the geometry on 1 or 2 planes, but the third plane allows for some added material.

    1.2 SCALE IT UP

    Maybe this is also really obvious, but scaling up the part has the same effect as thickening all the geometry at the same time. Be careful to think about whether or not this will have consequences for any mating parts, or any functional elements of the design.

    Fillets should be added to the bases of thin sections

    Fillets should be added to the bases of thin sections

     

    1.3 SMOOTH TRANSITIONS WITH FILLETS/ROUNDS & ADD RIBS TO WALLS

    During printing, it’s possible that the nozzles will knock thin parts off the print, causing displacement of the current layer. This makes thin parts even more wobbly. Use fillets, chamfers or blends to allow a sort of lead-in to a thin section, providing a stronger foundation for the thinner section. 

     

    2.0 LOOK AT YOUR PRINT SETTINGS

    2.1 PRINT ORIENTATION

    Do you have to print it standing up? Part are strongest in the X and Y axes, as the Z axis strength depends a lot on the properties of the layer. Sometimes the best orientation for printing is slanted diagonally, as the layers are usually not perpendicular to the direction of the load points or faces.

    2.2 LAYER HEIGHT

    When you print in smaller layer heights, the plastic is squashed down more, creating more surface area on the X/Y plane. Where the next layer isn't directly on top, a more squashed layer with higher surface area will have a higher contact area with the material. Higher contact area means higher layer adhesion, and the part is less prone to fail under tensile loading in the z axis. This means that a resolution of 100 microns will have stronger inter-layer bonds than the same print at 300 micron layer thickness.

    2.3 INFILL % AND TYPE

    This is another obvious one, but sometimes evades my thought pattern at first.  Changing infill percentage, infill type and occasionally angle can help to strengthen the 3D printed part. We’ve read a bit about how it’s pointless to print anything over 60-70% infill, however we have a client who needs parts done at 100% infill as 75% is not strong enough. One thing to note is that any infill setting above 75% will most likely impact on the outer surface of the part.

    2.4 PERIMETERS/SHELLS OR SHELL THICKNESS

    Following on from infill, another strengthening option is to increase the number of shells or perimeters in the slicing settings. We’ve found that 2 or 3 shells are usually enough, but some applications where loads are high or extremely localised, it may require 4.

    2.5 MATERIAL

    While we prefer to print in ABS 95% of the time, there are a few options for materials on desktop 3D printers - each with different strength properties. While ABS is a strong and flexible plastic, PLA is hard but stiff. Sometimes a flexible material will be stronger or more resistant to shocks, but when geometric stiffness is required, PLA will be better. Remember that though PLA is hard, it is relatively brittle. Where extra durability is required, it is possible to print in Nylon. Taulman 618 is a great nylon filament for FDM printers, although a bit of extra machine setup is usually required.

    3.0 POST-TREATMENT

    3.1 EPOXY OR POLYESTER RESIN

    Something that we’ve looked at recently is resin coating. For purposes when extremely accurate geometry is needed and sharp edges need to be preserved, this technique won’t be right for you. There are many different types of 2 part epoxy resin or polyester resin, each with different material properties and curing properties. Also, there will be a range of viscosities available. Don’t use 2 Part epoxy glue. It won’t work very well and produce a really lumpy finish.

    We use Polyester Clear Casting Resin as it’s thin enough to be spreadable all over intricate parts before it starts to cure. The resin starts to cure about 5 minutes after mixing, and takes about 24 hours to dry. It is also possible to use glass fibre shavings in the resin mix for extra strength, though this may impact on surface finish. The images below show the difference in the part (both parts were painted for a metal effect).

    Before polyester resin coating

    Before polyester resin coating

     After polyester resin coating

     After polyester resin coating

    After resin coating, we drop-tested two of the same model printed on the same printer with the same settings and material. The only reasonable difference was the resin coating. The resin coated part survived with no breakages at all, whereas the untreated part lost 5 or 6 different sections. We're going to continue to use this technique as our go-to strengthening method.

    3.2 CARBON/GLASS FIBRE LAMINATION

    Some parts may be suitable for carbon/glass fibre laminating. This isn't really suitable for intricate parts, as the part surface needs to be entirely wrapped in the fibre mesh; it's particularly suited to parts without holes or gaps. Once the part is wrapped in the fibre mesh, a layer of epoxy or polyester resin is applied over the mesh to solidify it in place. Bear in mind that this will add some extra thickness to the part.

    3.3 HEAT TREATMENT

    Although we've not tested this method, we've heard several reports that placing the part in an oven or using a heat gun/blowtorch to re-melt the outer surface of the plastic creates a stronger inter-layer bond. This sounds like a really dangerous method, as you risk melting the part completely, or distorting/warping certain features. If you're going to try this, start of at a lower temperature (and if using a heat gun, further away from the part then move gradually closer). 

    4.0 MOULD IT (OR MOLD IT, IF YOU'RE AMERICAN)

    Jeshua of 3DTOPO demonstrates his method of "lost PLA" casting

    4.1 PLASTER CASTING PARTS

    Printing off your model in ABS or PLA allows you to mould. Investment (lost wax) casting is possible. To do this, print off your part as is, then cast the part in plaster of paris. You can then remove the original plastic print by heating the plaster cast in a furnace above 230C. You can then pour molten metal or plastic to into the mould cavity and let it settle. To remove to final cast part, the mould is destroyed with a hammer and the excess plaster is washed off. Something to bear in mind when experimenting with this method is that there will be some shrinkage of the part, so you'll need to scale the mould pattern up by 2-3%.

    4.2 ROTO-MOULDING PARTS

      An alternative to casting parts, a plaster or silicone mould can be used for rotational moulding. The strength advantage of using roto-moulding to create hollow parts lies in the lack of build layers, and a single crystalline structure for the whole part as it cools as one.

      By pouring molten plastic/metal into the mould cavity, closing the mould and continuously rotating it on 2 axes, a hollow part can be achieved. There are numerous small/desktop roto-moulding machines available to buy, or you could build your own. The most common type of desktop roto-moulder consists of a central horizontal spinning X-axis to which is mounted a frame that spins on the Y or Z axis (rotates between them according to the X-axis). Usually they run off a single motor that control both axis via a gearing or pulley system. Each different mould shape will take some experimenting with rotation speed to ensure the molten plastic is spread over all the surfaces.

      Source: StudioMyFirst

      Source: StudioMyFirst