Resin-printed COT track

[Martin Wynne]

@James Walters @Hayfield @Wayne Kinney

Hi James,

In a reply you made recently on your Bexhill West channel you wrote:

"As for resin printing the timbers, many have tried (including myself) and failed. They simply curl-up. No amount of strong adhesive or weighing-down seems to work. The FDM printed timbering bases are excellent, as is the laser-cut option. It simply isn't worthwhile resin printing the bases."

I have found the same. But there is a possible cure.

Soon after UV-curing (i.e. before the print has a chance to curl), fix it down to a flat surface. Immerse it in very hot or boiling water, and leave it there until cool. Dry it off with a hair dryer and leave it in a warmish place to dry out thoroughly.

Or heat-treat it in some other way.

There is also the option of hot UV-curing, which I have read gives better results, if some form of UV-curing oven can be found or improvised.

Wayne's excellent Finetrax kits are resin printed and supplied nicely flat, and I'm sure he mentioned heat-treatment in one of his early posts on RMweb.

It all adds to the faff of resin-printing compared with FDM, but having now got the COT exports working reasonably well in both 4mm and 7mm, it might be a workable option for those who want the crisper resin-printed chair detail in a one-piece base.

No doubt there is some experimenting to be done on different resins, how hot, how long for, how soon after curing, shrinkage adjustments needed in the export, etc. Anyone?

Badly curled chair rafts can also be straightened after dunking in a mug of very hot water. The chairs will be soft, so handle with care until cooled under the cold tap.

cheers,

Martin.

 

[Paul Boyd]

Wayne's excellent Finetrax kits are resin printed and supplied nicely flat, and I'm sure he mentioned heat-treatment in one of his early posts on RMweb.

Hi Martin,

The two bases on the Finetrax kits I bought were both curled along the timbers needing serious weighting and strong glue to hold them flat, as reported in my Llanastr post.

Cheers,
Paul

 

[Martin Wynne]

Hi Martin,

The two bases on the Finetrax kits I bought were both curled along the timbers needing serious weighting and strong glue to hold them flat, as reported in my Llanastr post.

Cheers,
Paul

@Paul Boyd

Hi Paul,

Thanks. The one I have here (unmade) was still nicely flat after a couple of years. But having just now looked at it again it has developed a slight curl recently. I will give it the boiling water treatment and see what happens.

cheers,

Martin.

 

[James Walters]

Hi Martin,

I agree that a little heat is helpful when re-shaping resin prints. I use a hair dryer to gently heat some of my wagon prints to reshape them if required, and if done well it seems to 'relax the casting' so-to-speak and the re-shaped part is noticeably stiffer and more stable than before. I've even built a little spit upon which the wagons are rotated whist they are slowly heated.
I've found that using very hot water speeds the process up, but the high temperature tends to leave the parts less stiff and stable afterwards when compared to slowly raising the temperature.
I'd love a materials scientist to explain what's going on in simple terms that I could understand.

I also noticed that dash of flexible resin in the mix also seems to help, as does using a quality resin over the consumer-grade types.

All the best,

James

 

[Martin Wynne]

Hi Martin,

I agree that a little heat is helpful when re-shaping resin prints. I use a hair dryer to gently heat some of my wagon prints to reshape them if required, and if done well it seems to 'relax the casting' so-to-speak and the re-shaped part is noticeably stiffer and more stable than before. I've even built a little spit upon which the wagons are rotated whist they are slowly heated.
I've found that using very hot water speeds the process up, but the high temperature tends to leave the parts less stiff and stable afterwards when compared to slowly raising the temperature.
I'd love a materials scientist to explain what's going on in simple terms that I could understand.

I also noticed that dash of flexible resin in the mix also seems to help, as does using a quality resin over the consumer-grade types.

All the best,

James

@James Walters

Thanks James. This is interesting:

cheers,

Martin.

 

[James Walters]

Thank you Martin, that's very interesting.

I've spent months developing the print strategy for my SE&CR hopper wagons. They now print very consistently with very little differential shrinkage upon curing. I've got them to the point whereby if they do distort, then it is only the sides of the hopper which tend to bow outwards. Gentle heating causes them to relax back into the correct form.
But what seems to work best, is if I heat them gently, and then hold them into a soft jaw fixture in a bench vice. This holds the sides parallel and has what I can only describe as a shock-cooling effect upon the part, as the heat is rapidly transferred to the cold vice jaws. Thereafter the hopper sides are perfectly parallel, and noticeably more rigid than before the heating process.
If I allow them to cool slowly, the part is more flexible.

I've also tested another interesting property, whereby I have placed weight on resin printed parts. Over time, parts subject to regular curing will deform or creep over time even with what seem to be insignificant loads, whereas those I have warmed and shock-cooled as above seem much more stable.

These tests haven't been at all scientific, but I'm sure the results I've observed are a 'thing', the video you shared above seems to confirm there is something here.

I shall do a test with my next batch of chairs. Cure some as usual, and variously heat-treat others.

 

[Terry Downes]

Martin, Thanks for sharing that very interesting video. I utilise a Formlabs resin printer, wash and cure system. The Formlabs cure can heat up to 80degrees but I generally heat whilst curing to 65degrees for 60 minutes and allow to cool naturally with perfect results but, the ends of long, thin items do tend to curl a little. E.g. the ends of this 4mm x100 S1 Chair raft is just beginning to curl up.

I will try out other annealing / cooling processes during my next prints to see if it affects / cures the minor warping.

Below are examples of my very early 4mm COT track experiments. The results were okay and the reasons l stopped pursuing it were, 1. time to resin print timbers, 2. difficulty in threading rails and 3. appearance of split wing rails. I think this was around 2 years ago so, as I have learnt more and COT track has evolved I think I'll have another go but, probably use plug chairs for the wing and check rails.