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An Idiots Guide to 3D Printing Strong Parts

So you want to print strong parts? This blog is going to look at how best to orientate your print, what materials to use and understand what continuous fibre 3D printing is. Firstly understanding the application that the part is …

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Tom Marlow-Gilks

April 2, 2020

So you want to print strong parts?

This blog is going to look at how best to orientate your print, what materials to use and understand what continuous fibre 3D printing is.

Firstly understanding the application that the part is being used in is vital. Knowing what type of stress the part is going to be subject to is essential whether that’s tension, compression or shearing forces.

Due to the nature of FDM/FFF printing (where parts are built layer upon layer to build up the overall shape) parts have what’s referred to as anisotropic properties. This is where parts are stronger in the XY directions and weaker in the Z direction due to the bonding between the layers. This is because the orientation the part is printed is crucial if you want to design a strong print. For example, if the part is in tension, then you want to ensure that the part is being pulled in the XY directions rather than the Z, to avoid delamination and fracture. Tensile strength in XY directions is up to 4-5 times stronger than in the Z direction.


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Anisotropic stength in 3D printed parts

What materials should I be using?

Again the application that your part is going to be used in is very important as it will affect the material selection. There are numerous different types of 3D printing materials available each with their own advantages and used for specific applications. Two materials that are commonly used when designing strong parts are Nylon and Polycarbonate. Nylon is a very durable material with excellent chemical resistance, however it isn’t stiff and has virtually no heat resistance. Polycarbonate is strong with good heat resistance but is only moderately durable and chemically resistant.

One way to improve material properties is to impregnate a standard polymer with particles of a secondary material and this is referred to a filled thermoplastic. A common type of filled thermoplastic is chopped carbon fiber reinforced nylon. This takes the excellent chemical resistance and durability of Nylon and considerably improves the strength, heat resistance and stiffness of the material by adding the carbon particles.

Markforged’s very own Onyx filament is a type of filled thermoplastic which is a mixture of Nylon and chopped carbon fibre which boasts not only high strength but excellent heat resistance, surface finish and chemical resistance. If even more strength, rigidity and durability is required then the Onyx filament can also be reinforced with a range of continuous fibres including carbon, fibreglass, HSHT fibreglass and Kevlar. If the Onyx filament is reinforced with layers of continuous fibre it can considerably improve the strength, withstanding stresses equivalent to Aluminium.


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Stress-Strain material comparison graphs

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Nylon property comparison

So how does continuous fibre 3D printing work?

Typically most 3D printed components aren’t solid. Instead various infill patterns are used to save material, cost and print time. When printing using continuous fibre reinforcement standard conventional infills are replaced with the fibres which significantly improves the mechanical properties of the part. The additional strength that these fibres add are dependent on the amount, orientation and type of fibre used. With the Markforged printers these parameters can all be controlled using their own slicing software called Eiger. This also gives you extra control regarding how to lay the fibres becuase we want to ensure that the reinforced strength is applied in the correct areas.


continuous-fiber-infill
Eiger continuous fibre infill

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