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I'm trying to make an evaluation of the available 3D printing options of polypropylene and I'm having difficulties comprehending material characteristics that make PP PP-like.

I realize this sounds a bit non-exact, but please bear with me. Polypropylene is very beneficial for this specific product because it handles repeated large plastic deformations very well and won't break. When you look at the material properties and compare them to other plastics for injection moulding, one characteristic stands out - elongation at break, which can be up to 150% or more for PP for injection moulding (this value is from a generic database, actual materials have similar values).

When I checked the HP's PP mateiral and Stratasys's Durus PP-like mateiral, their elongation at break was very low, around 20%, which is less than PA11 for SLS 3D printing.

Maybe elongation at break is not a good parameter to evaluate the response to large plastic deformation I mentioned. Maybe someone has more insight into this.

EDIT: I tested the PP printed part for bending fatigue. The part was printed on a HP fusion jet printer and the results are very similar to the injection molded part.

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  • $\begingroup$ look up properties on MatWeb. IMO, the mechanical properties of PP are okay at best for engineered parts. Whatever your application, there will almost certainly be a version of PBT, POM, PETx, LD/HDPE (all common for molded parts) that is mechanically superior. Where PP comes out ahead is chemical purity. Food grade, personal care products, medical, scientific research. // PP is on the soft and mushy side, but that also depends on polymer details - xlink, plasticizer. Look at the "water-bottle" PET-E blend for example... $\endgroup$
    – Pete W
    Jan 28 at 13:07
  • $\begingroup$ In my experience PP-like 3D printing materials aren't muck like PP at all. You're better off vac forming if you need a prototype to have useful material properties $\endgroup$ Jan 28 at 13:13
  • $\begingroup$ PS to my comment above - I can't speak for the 3D print process, not familiar with details of that. Tried to address what makes PP stand out from other commodity plastics in general. (its low chemical reactivity and purity) $\endgroup$
    – Pete W
    Jan 28 at 13:17
  • $\begingroup$ Could you update the question and specify where did you get the value for elongation at break? Is that value from one of the 3D printing materials also, or is it from a database for generic PP material? $\endgroup$
    – NMech
    Jan 28 at 13:25
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    $\begingroup$ @PeteW : Thanks for the info, I realize PP is not the best "engineering" material, but for this application it's actually perfect because the parts don't break after repeated plastic deformations. Also low reactivity is beneficial. $\endgroup$ Jan 28 at 13:41
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If you've done any 3d printing you must have noticed that the material gets deposited in very thin lines.

enter image description here

The end result if you look at it in detail is the following:

enter image description here

The two big differences with an injection molding machine are:

  • temperature at exit
  • adhesion between layers.

In the case of injection molding machine, the material get injected all at once in the mold. The material is pushed in to fill the mold.

In the case of 3d printing the material is deposited. So there is usually some time for the different layers to be deposited at the same x-y point (Assuming z is the direction of the layers).

The way the slicing (process to convert the stl to a 3d printed object) affects the mechanical properties. If you ever tried to build a 3d printed column you will see that it is very easy to snap if you build it along the height. The reason is that the molten material does not necessarily adhere optimally to the solidified material. The end result is a highly anisotropic material, which depends on the way the 3d printer head moved.

Adjusting the temperature might help with the problem, or changing the layer height, but the underlying problem largely remains.

Bottom line is that because the process is different you can never obtain the properties of a PP injection molding. You will always get a much lower value. And although this is just my guess, the reason that HP and other 3d Printing material manufacturers state those values is because they did some testing and came up with those "safe values". In a sense its a disclaimer, that although the material is PP, the process will result in reducing its actual strength properties.

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  • $\begingroup$ The image you posted is of an FDM process, the adhesion between layers with SLS is much much better. And HP claims that from their fancy multi jet fusion technology, the material is isotropic. This is of course bullshit, but from the cutout, the part actually didn't look porous. Perhaps you're right and the printed PP is actually far, far away from inj moulded parts in sense of mechanical properties. I will post an update, when we receive and test the parts. $\endgroup$ Jan 28 at 14:18
  • $\begingroup$ To be honest, I rushed through and did not see the SLS part. So yes you are right, I had FDM in mind, when I replied. I know that SLS is almost isotropic, but I don't have any experience with the process. I will look into it though and I will update my answer. However, I think the principles still apply. $\endgroup$
    – NMech
    Jan 28 at 15:57
  • $\begingroup$ yes, the principles still apply, the part has different properties in z-axis, but it is much better than FDM. $\endgroup$ Jan 28 at 16:51
  • $\begingroup$ I added the update to the original post $\endgroup$ Feb 22 at 11:09
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I tested the PP printed part for bending fatigue. The part was printed on a HP fusion jet printer and the results are very similar to the injection molded part.

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