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My assumption is that if a part is injection molded, it will be stronger than a 3d printed part.

Is this assumption correct. For example, I think if I print a part in PET, it will not be as strong as an injection molded part of the same type.

Any science or tests I could look at out there?

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Hey Perry_1,

I believe your assumption is true as injection moulding is one solid part whereas 3D printing is done in layers meaning it may easily break along these layer lines. Also injection moulding is usually a solid mass, but printed parts usually have different infill and it is difficult to get 100% infill like injection as there may always be bubbles or groves forming at the layers. Unfortunately I don’t know of sources it tests to prove this.

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Hey @Perry_1,

This is a really difficult topic to accurately answer, partially due to the differences you encounter from one printer to the next, and also because “strength” is a very vague term when you’re talking about material properties.

From my own experience and others out there, it would appear a part printed at 100% infill on an appropriately calibrated printer can handle similar amounts of compression stress (squishing) as compared to a part made using conventional manufacturing methods and of the same material. The real issue happens when you have tensile stress (pulling apart vertically) and shear stress (pulling apart horizontally) on a 3D printed part. 3D printed parts fracture at much lower tensile stresses than injection molded parts, especially if the part is printed such that the z-direction will be bearing the brunt of the tensile force. Careful orientation of the part when printing can allow for the part to have very high tensile strength (comparable to injection molded), but there is very little that can be done to increase shear strength. Shear strength for a 3D printed part will always be lower than an injection molded part because you’re either pulling apart the from top to bottom or from side to side, and there’s no orientation that will correct that (with the exception of angled prints for SLA, but SLA resins tend to be very brittle and have little flexibility).

Sculpteo has some good information on material properties here.

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Hello @Perry_1

Questions:

- By 3D-printing do you mean FDM or SLA or SLS or SLM or LOM etc. etc.? They differ.

- By strength do you mean tensile strength or compressive strength or what type of strength? I’ll assume in this general context it’s the tensile strength you’re asking about.

I will share with you my knowledge on the tensile properties of Fused Deposition Modelling (FDM) parts.

The million dollar keyword you are looking for here is “anisotropy”. It means the tensile properties of the part are not the same in all directions. It’s like wood or a piece of steak. Cut along the grain and it’s easy, cut across the grain and it’s difficult.

FDM (and basically 98% of other Additive Manufacturing technologies) print 2D contours on top of another. This means within the 2D contour itself it is strong, that’s on the X-Y plane. Along the Z-axis (Vertical axis), the 2D-contours are stacked, but the bonding between the layers are not as great. So you very typically get delamination here. To give you a ballpark figure and sticking to tensile strength, the tensile strength on the X-Y plane is maybe around 70-85% of injection moulded parts of the same material, along the Z-axis you get around 30-50%. Again, these are not accurate figures but merely illustrate the weightage.

So now to teach you how to fish, go to scholar.google.com and use the terms “anisotropic properties”, “tensile properties”, “tensile strength” followed by your AM technology of choice and you will find various published scientific papers on this topic. Here’s an example just for you Home | Rensselaer at Work

Cheers,

James

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Thanks for the info!

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If I just answer this question I would say SLM but that’s for metal parts. For polymers I would say the SLA based CLIP technology has pretty good layer fusion as the curing is continuous. SLS is also not bad. FDM has the most adverse effects on layer adhesion.

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PS do you have a feel for which of these is strongest in shearing strength along the z axis?

You mention FDM or SLA or SLS or SLM or LOM.

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There are some great answers already posted that I can’t improve upon, including good tips on how to design and layout parts to place the greatest tensile load along the printed X-Y axes of the part. My only additional insight is that I use post-processing methods like coating with selected adhesives (frequently just multiple coats of clear spray paint) to improve the inter-layer bonding and therefore tensile strength along the Z axis when needed; this also helps the X and Y axes but to a lesser degree. It’s still not as strong as an injection molded part, but it might be good enough.

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Yes, injection molded parts are stronger than 3d printed parts.

Usually, if we ship 3d printed parts to our US or other country customers, we use a wooden/plywood case to protect them during transportation.

But if they are injection molded parts, usually no need to use wooden case but just hard paper carton.

sales01@rpimoulding.com Vicky www.rpimoulding.com