Oct. 10, 2014 | By Euan Quigley*

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

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

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

Source: Red Eye On Demand

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

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.

Source: Easy Composites UK 

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 it 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%.

Source: Association of Rotation Moulders Australasia Inc

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


*Euan Quigley is Product Design Engineer & Director of ST3P 3D Print & Design, a Glasgow, Scotland based company that offers product design services and 3D printing services.

Posted in 3D Printing Technology

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Gregg Eshelman wrote at 8/21/2017 12:15:00 AM:

I printed several knobs for a classic car in PLA. I filled them with urethane resin. The interiors of the hollow knobs were made with ribs designed for the resin to flow around to ensure a solid mechanical bond. To make perfectly centered pilot holes for drilling out for mounting, I made snap together holders to suspend drill bits, shank end down in the resin fill, and a 3/8-16 threaded bolt in the shift knob. The resin was cured under pressure for 24 hours then the bits and bolt were removed, followed by post curing the knobs for 8 hours in 145F convection heat. A test was done first, without resin fill. No deforming or shrinking of the PLA was observed at that temperature and duration, so the knobs should be perfectly fine in a car that will never sit for hours in baking sunlight with the windows up. I also printed perfect fit vise clamping blocks so the knobs could be easily held for drilling out the mounting holes. The knobs were sprayed with high build primer, sanded smooth and painted. Turnaround time on the job was about 3 days total, from initial measurement of the crumbling and distorted originals through 3D design and printing of the knobs, supports and clamp blocks, to resin filling, curing and post curing. 3D printing and finishing one example of each knob then making silicone molds followed by serial casting of the knobs would have taken much longer for the one-off job. I'm working on some other knobs now where a good original is at hand so it will get reproduced by making a silicone mold and doing castings. I've 3D printed clamping blocks for drilling the cast knobs. Just making the mold and getting the first casting to post curing has taken longer. With the casting verified I'll be casting the lot of knobs then post curing all together.

Bob - South Africa wrote at 4/10/2016 2:59:36 PM:

I have (very) successfully impregnated a variety of 3D printed plastics using a high strength, low viscosity resin (about the viscosity of corn syrup). This is the stuff that usually used with vacuum bags and composite materials. So: print the part with 95 or 100% fill and do superficial cleanup. In a small jar, cover the part with liquid resin (i.e. entire part is just below surface - very important). Use a vacuum chamber and pull vacuum for 5 min, release the vacuum and the resin will get pushed into any remaining internal spaces (there are always internal air spaces even with a 100% fill print). Repeat 3 times. Just before the gel point, remove the part, thoroughly clean off any excess liquid resin from the surface (important!), leave to cure (I use a 65ºC print bed on sheet of aluminium foil for 3h). ABS parts are then good for 20 bar hydraulic fittings (engineering prototyping), no leaks. About 30% increase in strength. Enormous increase in stiffness. Part pre-tensioning effect (resin shrinks 3%). The plastic part will now also sand fantastically well and will have a slightly improved resistance to heat.

Michele B. wrote at 3/5/2016 10:09:56 AM:

Hi, recent studies has demonstrate that 300 micron layer is better than 100 micron layer, in terms of mechanical strength.

Mark wrote at 10/14/2014 10:49:40 AM:

Best tip I have is if you use ABS, dab the part with tissue soaked in ABS/acetone mix that's put on the bed.... This bonds the layers & fills. I've printed body parts for my car & done this to them, sanded and fitted (vents etc). Been driving around with them on for years now.

Rix wrote at 10/10/2014 3:33:12 PM:

Very good points. I can confirm many of many of these, as we learnt from experience by printing many gears for a small industrial machine. One thing I can add: it helps when printing in abs to use acetone to better fuse the layers. I would either rub it on by hand or use a vapour bath, but you lose detail or too much and your part melts.

Brian wrote at 10/10/2014 1:24:45 PM:

Nice article, I think you should put images after their heading next time, made it slightly confusing what the images were showing.

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