PLA material metrics on characteristics

[ebob]

There have been a couple of threads* touching on the behaviour of PLA, and seeing how central the material is to the system, I was thinking it might be useful to have some way of testing how good a particular one is.

Obviously we have the leg test stl, but that’s more of a binary go/no go. It would be nice to have something which lets you get a number out of it so you know just how good or bad one is. (And numbers are always nice).

*The particular threads that got me thinking are:

A 3D printed scale that also use the flexure mechanism

Sorry this may be a little bit off-topic. I remember I saw a fully 3D printed weighting scale/balance, which also smartly uses the elastic flexure of the 3D printed PLA structure, so that the weight scale/balance is spring-free, and all printed in one go. However, I can’t seem to find this design anywhere online. Just wonder if anyone in this community might have know this. Thanks very much! Andy

Actuator foot weakness - stage Y-axis actuator broke

I’m following up on this to see if anyone got anywhere with a PLA test foot. I’m currently trying to specify the PLA requirements for building quite a large number of units, and so am quite keen to get this right! My initial thought was to go for a Sunlu LightGold, which is a PLA Silk, but the comments about knowing just what is in the silk and the vagueness of manufacturers’ descriptions of their ingredients is making me nervous. So any models I can use as tests, maybe involving some gentle …

So something which encapsulates all that would be ideal. Maybe something where you manually rotate something until it breaks, and note how far you got when that happened?

Of course I’m hoping someone will point out my searching skills are terrible, and at a model which already does this, but if not, I’m happy to give it a go; design ideas welcome!

[WilliamW]

The final performance comes from the filament, the printer and the settings used to print. Your suggestion will not so much test the PLA as test the entire system - but luckily that is the more useful test to have as well as being easiest to design.

The leg test shows up problems with stringing or poor bridging. The simplest useful test for ‘strength’ would probably be something that measures layer to layer adhesion.

[ebob]

Yes, indeed, it’s not just the filament we care about, but the printer and the config settings too. Hence why having a test model that can be printed seems best suited to this.

For layer-to-layer adhesion testing, I’m considering a model along these lines: A frame consisting of a base plate, vertical walls, and a top plate. Together with a printed bolt, and of course some test pieces to destroy. All parts being 3D printed.

The bolt goes down through a threaded hole in the top plate, and is attached to a test piece.

The test piece contains a zone with a small area of layers, being the weak-point that we shall destroy.

The test piece is then connected to the bottom of the frame.

The test is performed by rotating the bolt, (unscrewing it upwards), and noting the degree of rotation at which the test piece fails, as it is either twisted or pulled apart.

I think with some cunning design of the way the bottom of the bolt is connected to the test piece, and the connection between the test piece and the bottom of the frame, we can devise two different test pieces that fit in the same frame, such that one can be used to measure resistance to twisting, and another that measures resistance to being pulled apart vertically with no twisting.

I’ll see how this goes, and report back.

[ebob]

I have been working on the design for a filament tester:

This is entirely 3D printed (requiring no other parts), and consists of a frame in which a sample is placed for testing.

Two tests are supported:

• Stretching the sample:

• Twisting the sample:

In both cases the strength of the sample can be recorded, either by measuring the amount of rotation of the test tool, or of the force exerted at the moment of failure.

Tester design

The tester consists of a frame. The frame is embedded in a base plate with holes at 25mm centres to allow for easy fixing to a bench (though basic testing can be done simply holding it in your hand)

The frame is printed in two parts, with the top part connected using dovetails to the walls of the frame. (This is a friction fit, there’s no need to glue it in)

The top of the frame contains a threaded hole, into which the tester tool is inserted. Rotating this changes the height of a connection nub. This nub mates with the sample, and depending on the sample shape, either twists the sample, or pulls it upwards.

To assist with measuring the rotation or forces involved, gradations are marked on the top of the frame at 10 degree intervals. The arm of the tool has a hole at 100mm from the centre of rotation in which a cord to a load cell etc could be attached.

Testing method

1. Insert the tester tool downwards into the frame
2. Insert sample into the frame ensuring it is fully under the centre, and mated with the nub of the tool.
3. Rotate tool to raise it, until the sample fails.
4. Note the angle of rotation of the tool, or the force applied to the tool at the moment of failure.

Repeat several times and average the results.

Notes on 3D printing the tool and samples

This is designed to be printed without the need for supports.

Attached are the individual STEP files.

The forum won’t let me upload a 3MF file here, so I have uploaded to MakerWorld:
3MF model on MakerWorld

The general settings I used are 2 walls, 25% gyroid infill (except for the threaded end of the tool where a modifier has been used to make it solid with 99 walls). A thin layer height of 0.08 is desirable for the threaded parts (the tool and the top of the frame) otherwise the threads will feel a bit lumpy as you rotate them, which interferes with measurements. For the rest, a standard layer height of 0.2 is fine.

I have successfully printed this in PLA on a Bambu P1S.

Samples appear to gain some strength as they cool after printing, so best not to test them immediately after printing them.

Files

Frame and tool:

FTesterFrameBody.step (62.9 KB)
FTesterFrameTop.step (2.2 MB)
FTesterBolt.step (219.2 KB)

Test pieces:

FTester-PiecePull2.step (47.5 KB)
FTester-PieceTwist2.step (42.3 KB)

[ebob]

As an update on the above model, (and there’s no way to phrase this without sounding prehistoric), I have invented the wheel…

This is a drop-in replacement for the lever, and allows a wire to be attached to it, wound round a couple of times, and then tied to a scale (eg a fishing scale or a luggage scale), so an increasing force can be applied whilst staying at the same angle to the the tester.

FTesterWheel.step (265.7 KB)

I have also updated the downloads at MakerWorld, as there’s some strength modifiers and settings that can’t be included in the step file.

3MF files on MakerWorld