Resin wagons splitting

[ikcdab]

Having surmounted most of the 3d printing issues, I have successfully printed some 009 wagons.

This is using water washable elegoo resin.

The first batch were a failure as they warped and broke up horribly after curing. Maybe because I was printing flat sides 1mm thick.

Second attempt I added plenty of internal bracing along the top of the inside edge of the wagon.  This was far more successful and the wagons have been stable for two of three weeks.

Yesterday I decided to paint and add loads.  I undercoated with Wilco rattle can primer.  This says "contains acetone" so I guess it's cellulose paint.  Then topcoated with acrylics.  Last step was to add a load of sand fixed with sheet water down PVA.

All was fine, but today the wagons have started splitting. It's pretty much happening as I watch them. One wagon which was ok 5 mins ago, is not splitting. Out of 12 wagons, 8 have split.

So what is causing this? They were ok until I painted and added a load.

Is it the cellulose and acetone based paint reacting with the water based resin?

I really can't believe that the sand and watered-down PVA had caused it. I don't think it's expansion of the load.

Any ideas?

Ian 

 

 

 

[billbedford]

How deep are the "lines" between the planks?

[njee20]

Water washable resin is far more prone to failure. I’d switch to normal resin. 
 

That said that’s a pretty weird failure - it’s normally when they’ve not been cured internally properly. I’d echo Bill’s question - Ie how thick is the wall there? 1mm should be plenty thick enough, but if the plank gaps are 0.8mm you may have a problem…

 

[ikcdab]

11 hours ago, billbedford said:

How deep are the "lines" between the planks?

Hi Bill, the total tickness of the sides is 1mm. The plank gaps are indented 0.25mm on each side. So the remaining thickness (where it has cracked) is 0.5mm.

Sounds thin. But as i said, i printed these several weeks ago and they have sat happily in the shed with no movement over that time. Maybe its coincidence, but its only now that i have painted with the cellulose primer and added the sand/pva load that the cracking has happened.

I attach a screenshot of ther fusion360 model showing the reinforcing ring I added. The cracking occured below this.

The easy answer is to thicked up the sides below the reinforcing ring, but i am still mystified why the wagons split after being ok for several weeks.

Ian

[john new]

Just a thought. Card can warp if you glue a paper overlay to one side only. I know it can and sometimes does but unfortunately for this post can’t explain the physics as I don’t know why it does it. Is it the overspray creating a similar issue here? The primer or paint sticking but slightly shrinking as it dries perhaps.

[Jeff Smith]

I admit to knowing very little about 3D printing but from a structural point of view I would leave out the plank lines inside and add some vertical ribs below your reinforcing ring, perhaps opposite the outside strapping.  Also I would not use cellulose paint - stick to enamel or acrylic.

[Lazy]

 It may be worth printing two more wagons in order to identify the issue - one can be painted but not loaded and the other left unpainted but filled with the sand mixture.

It is noticeable that the splits are all happening in the same position. My guess is that the floor and stiffening ring prevent bowing at the top and bottom of the body so the middle planks give way instead.

Note that if a file repair program (eg netfabb) is used then it can make unwanted corrections including reduction of wall thickness.

 

[Player of trains]

In my experience water washable resin is somewhat absorbent and retains some flexibility compared to solvent washable resin, it may have absorbed moisture from the PVA glue and split along the weakest lamination of the material. Annoying they've split like that, warped prints with bowing sides genuinely look fantastic as old wagons when weathered appropriately. 

[stevel]

7 hours ago, ikcdab said:

Hi Bill, the total tickness of the sides is 1mm. The plank gaps are indented 0.25mm on each side. So the remaining thickness (where it has cracked) is 0.5mm.

Sounds thin. But as i said, i printed these several weeks ago and they have sat happily in the shed with no movement over that time. Maybe its coincidence, but its only now that i have painted with the cellulose primer and added the sand/pva load that the cracking has happened.

I attach a screenshot of ther fusion360 model showing the reinforcing ring I added. The cracking occured below this.

The easy answer is to thicked up the sides below the reinforcing ring, but i am still mystified why the wagons split after being ok for several weeks.

Ian

The ring inside looks way to thick in relation to the rest of the wagon,  and is probably setting up major stress points. If the wagon is going to loaded there is no need for this ring, just adding some support nubs, at intervals of 10mm, 1.5 x 1 x 3 mm in size will support a load, and if loaded, the inner detail is not required. If you wanted to have a removable load it could be made out foam, and if made in the wagon, use cling film to keep them separate, so it can be removed.

[billbedford]

12 hours ago, ikcdab said:

Hi Bill, the total tickness of the sides is 1mm. The plank gaps are indented 0.25mm on each side. So the remaining thickness (where it has cracked) is 0.5mm.

 

Wagons only had chamfers on the outside of the planks, so they are not needed on the insides, especially if you intend the wagons to be loaded. 

 

12 hours ago, ikcdab said:

Maybe its coincidence, but its only now that i have painted with the cellulose primer and added the sand/pva load that the cracking has happened.

I attach a screenshot of ther fusion360 model showing the reinforcing ring I added. The cracking occured below this.

The easy answer is to thicked up the sides below the reinforcing ring, but i am still mystified why the wagons split after being ok for several weeks.

 

Resins, in general, absorb solvents. If you clean with IPA, it's best to ensure the print has dried entirely before curing. Otherwise, there's a risk of it warping. The resin in the wagons likely absorbed enough acetone to swell up and split the sides. It's probably best to only use water-based paint on 3D prints. 

[Damo666]

On 23/05/2023 at 10:36, john new said:

Card can warp if you glue a paper overlay to one side only. I know it can and sometimes does but unfortunately for this post can’t explain the physics as I don’t know why it does it.

 

Different materials will expand/contract at different rates. You may have heard the term 'Coefficient of expansion'.

 

A good example is the auto-switch off mechanism in your electric kettle. This is a bimetalic thermostat, two different metals joined at one end.  A difference in temperature (from room temp to boiling point) causes one metal to expand more than the other, and breaks the circuit.

It's why sheets of ply will always have an odd number of layers, 5, 7, 9 etc plys in a board. It's also why, if you paint one face of a sheet of ply, you should paint the opposite side.

 

Getting back to card with a paper overlay. Whilst the drying glue may also have a factor in the warping, the thicker card and a thin paper will create stresses resulting in one moving more than the other. If you could glue a corresponding sheet of paper on the opposite side this would greatly reduce the risk of warping. (For the same reason, if you were getting a joinery shop to add a veneer to a sheet of ply they would put a counter or balancing veneer on the other side).

 

Apologies for being OT, but just hoped this might be of help.

 

Edited May 24, 2023 by Damo666
Slight clarity