Strength of Printed vs Molded

Strength of Printed vs Molded Plastic
Is there any engineering data of the strength of printed vs molded or extruded.
It seems that the density of printed is less because of gaps between lines.
Also, melting between layers may be uneven, leading to connection weakness.

Printed parts are being used as structural members in printer construction and elsewhere.

Molded parts don't go we'll with community here. It's been tried before. Reprap site is about making parts that can make themselves. Molding defeats that model and is expensive in clean up and repair of parts.

I believe one number that has been thrown around in the past is printed parts with 100% infill are 60% the strength of injection moulded.

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Strength is only part of it flexing and elastic ability is important as well. Abs is more durable than pla

PLA extrudes much better than ABS, However ABS is better for higher heat applications. It depends really. Molded parts are definitely more economical for larger runs up until 100 widgets or so. I've been concerned about wear and tear, especially from UV exposure for outdoor applications. Also, the phenomenon known as "dry rot." But I've heard anecdotally that 3d printed parts are %30 as strong as molded parts.

WOOF, the 3D printing club at the University of Washington, tends to lean towards molded parts rather than printed. We print out printer parts, then mold them, and then cast them to make more printers. We can make about 20 printer kits (only the plastic parts, of course) in less than a day. It wouldn't make sense to try to print out all those parts, just because it would take forever.

Our group hasn't done any sort of definitive testing of the printed vs. poured parts, but it's clear that either one is more than strong enough for the applications we put it through. I believe the cast parts will break a bit more easily than printed parts, but that's hardly a fair comparison, considering the printed part is ABS, and the cast part is polyurethane, and different plastics have different properties.

the moulded vs printed debate has some interesting angles to it,

you would normally design parts bearing in mind the process involved in making the parts in the first place,
most of the strength issues with printed parts relate to the infill pattern, and how you express strength...

there are certain infill methods not found in slic3r which allow for crazy strength to weight ratios some of which i'm exploring now for printing very large pieces, eg entire printers

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Absolute strength in the same material them it's going to be cast, but strength to weight is pretty darn high from a 3D printed part.

I`m rather new to 3d printing, but it appealed to me in the first place because you can print parts that you will never be able to mold. if you talk about structural parts, you can optimize printed structures to only have "strength where you need it" and leave the rest out. if designed properly, you can make parts with a higher stiffness/weight ratio or with higher load bearing capability per weight (if this is what you define as strength) than with molded parts, even if the material itself is weaker.

From what i have seen there are very few parts where strength is an issue on the machine. If anything the problems arise from warping or parts that don't print quite right.

You can always insert bits of metal and the like to strengthen those bits that need it (coin bottle opener for example!)

@thejollygrimreaper: Can you elaborate on the stronger infill patterns? All I know is that the rectilinear grid found in Skeinforge is much stronger than the rectilinear setting in Slic3r.

use honeycomb in slic3r. it double walls and places lines across each layer.

Molded parts are limited generally to either thin uniform wall thickness parts (up to around 2.5mm thick), single side parts (used in sets to make all sides, like cases), with foaming methods ( up to around 12mm thick), or with gas assist that leave large voids in the internal cavities (like glorified blow molding with thicker parts). 3D Printing allows for internal structure, all faces being "finished", reverse tapers, flat sidewalls, voids designed internally, etc... all things not available to injection molding or any other current plastic processes. It is kind of like comparing blow molding, thermoforming, roto-molding, or injection molding to 3d printing. Each process has it's strong and weak suits.

About the ribbing caused by each individual printed layers, the Acetone vapor finishing's ability to smooth out the external surfaces really should greatly enhance the strength of ABS parts by eliminating external stress risers from individually bonded layers. It not only will make ABS parts look better, they will be significantly stronger.

@ugen:

essentially what you have inside the classic infill is a crossing over threads, however each layer is connected to the one below by a relatively small amount, so under a truss condition the internal infill leaves you with a parallelogram effect when it eventually fails,

by drawing in your own infill material not only can you save on plastic and make the strength to weight ratio higher but also stronger parts because you have solidly connected walls top to bottom, in saying that the current version of slic3r isn't capable of slicing some of the stuff i've been trying out, i've had to go to the thin-wall branch of it, not exactly an exercise i recommend many people do unless they know what they are doing with perl,

I'm also doing a "genericish" open scad script that will import an object hollow it out and then draw in a solidwall infil with both rectilinear pattens and star shaped patterns

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Ah, I see what you mean. I first thought you mean "legitimate" infill methods of other slicers, though.
Actually, I have been trying something similar with a 3D voronoi pattern in Blender. My goal is to create a variable density infill that is only strong in the places where you need it to be. With the new Nylon filament, I can imagine this getting even more interesting because you could make parts that are flexible in some places and dense and rigid in others.
Will provide pictures of a test object tomorrow.

thats pretty much the idea, although i'm thinking along the lines of for printing really large objects with bugger all plastic usage

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I wrote a pyramidal truss structure generation module a while ago, because the existing ones I could find weren't very good.

Pyramid Space Truss

I haven't done any specific tests, but a truss structure should beat infilled boxes at strength to weight ratio.

Well, yeah, I guess advanced infill options do have several uses. Also nice that you concentrate on OpenSCAD while I am using Blender. This way, both 3D software communities can profit.

Anyway, here are the photos. Actually, I did this experiment about half a year ago or so.



Actually, I just used Blender's voronoi shatter function on a cube and applied a boolean difference operation on another cube, so nothing all too fancy going on. The print was aborted about half way through because it took extremely long to finish.

The pyramid truss infill sounds interesting. This would be interesting for a (nearly) 100% printed printer frame, right?
Do you have photos of printed parts?

By the way, what do you think of the embeddable imgur gallery? Do you prefer the "traditional" long posts with a chain of images or should I continue posting images in this more compact way?

Edited 1 time(s). Last edit at 03/19/2013 07:50AM by uGen.

uGen Wrote:
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> The pyramid truss infill sounds interesting. This
> would be interesting for a (nearly) 100% printed
> printer frame, right?
> Do you have photos of printed parts?

A printed printer frame is exactly what I designed it for. This looks pretty rough because I didn't want to waste time while printing it and used the fastest and roughest settings, but here's an example of it in use:



Problem is that most cartesian (and delta, for that matter) printer designs require quite a lot of frame compared to arm designs, which can get away with L-shaped combination of bed support and Z tower. The printer in the picture is a parallel SCARA design I have been working on. The whole printer needs only five frame pieces, each of which fits into a 200x120x100 print area. The truss module also implements bolt patterns in different orientations so that pieces can be bolted together into a larger frame.

uGen Wrote:
-------------------------------------------------------
> The pyramid truss infill sounds interesting. This
> would be interesting for a (nearly) 100% printed
> printer frame, right?
> Do you have photos of printed parts?

A printed printer frame is exactly what I designed it for. This looks pretty rough because I didn't want to waste time while printing it and used the fastest and roughest settings, but here's an example of it in use:



The problem printer frames is that most cartesian (and delta, for that matter) printer designs require quite a lot of frame compared to arm designs, which can get away with L-shaped combination of bed support and Z tower. The printer in the picture is a parallel SCARA design I have been working on. The whole printer needs only five frame pieces, each of which fits into a 200x120x100 print area. The truss module also implements bolt patterns in different orientations so that pieces can be bolted together into a larger frame.

Whoa, that looks awesome! I expected a solid wall piece with a pyramidal truss inside, but looks so much more interesting. I can also imagine warp being a non-issue like this.
How would strength compare to a similar piece with solid walls by the way?

You could union the truss with a hollow box to make a smooth box with truss infill, but in this application I just unioned in the necessary pieces for external attachments. Using a box shell might be useful for printing thinner trusses. When printing an open truss the bars have to be at least 3-4 mm thick, or they will cool too much and the layer bonding will become a problem. With a solid outer layer they would probably stay hotter while printing.

And yes, warping problems seem to be much lesser than when trying to print a regular box with same mass. The truss bottom layer doesn't start to receive the warping forces until the truss starts to connect higher, and at this stage the lower parts have bonded well and the diagonals will in fact resist the warping.

I plan to print test pieces with both truss structure and regular honeycomb infill and destructively test them when I have time, but just handling one of the larger truss pieces gives you quite a wow feeling in how stiff it is against bending and twisting in any direction.