Post-cure methods analysis


#1

Hey folks,

There’s been a lot of talk about post-cure methods. Everyone has their favorite, but it’s hard to be scientific about it. We now have the equipment to determine whether a part contains uncured resin, so we’re going to test the effectiveness of various methods.

Here’s the list of methods we’re already planning to test:

  1. Rinse in IPA, cure under UV lamp with in air
  2. Rinse in IPA, cure under UV in a nitrogen atmosphere
  3. Rinse in IPA, cure under UV while submerged in tap water
  4. Rinse in IPA, cure under UV while submerged in water with ammonium sulfite added (bonds with O2)
  5. Rinse in IPA, cure under UV while submerged in glycerin, then rinse with tap water
  6. Rinse in IPA, cure under UV, then submerge in water for 2 days
  7. Rinse in IPA, then alternate spraying with IPA and air (3x)

The resin is PR57K (Black Prototyping Resin).

The IPA rinse is a 5-minute soak in the first wash, then agitate for 30 seconds, then repeat for the second wash (10 minutes in IPA total).

UV post-curing time was 5 minutes for all samples. The UV curing box was an LC-3DPrint Box, which apparently is a NextDent product, although ours doesn’t have their logo on it. It outputs 300nm-550nm at 290W total. I don’t know how much 405nm is in there, so I emailed NextDent but haven’t heard back yet.

All the samples were cured in polypropylene cups that I was told would not block 405nm UV.

The idea behind #6 is that as the water is absorbed into the part, it frees trapped radicals, which continue the polymerization process.

#7 is technically not a post-cure method, but it serves as an additional control.

The model is a ASTM D638 Type V dog bone:

Update: Here are the results:

The results suggest that there’s not a huge difference in the effectiveness of any of the methods. Kind of a boring outcome, but it also frees you up from having to use overly complicated methods such as glycerin or nitrogen. Granted, these samples were dogbones, which don’t have any little crevices for resin to hide. If you’re regularly making prints that are difficult to clean, just stick with whatever method you’re using.


#2

I love to hear you are performing new tests !
Is the FTIR going to determine depth of cure?


#3

I’m actually not sure. My understanding is that it uses spectral analysis to determine the composition of the sample, so it would be able to tell the percentage of uncured vs cured resin. But I’m not sure if it can determine the depth of cure.


#4

Please also let us know:

  1. What the UV light source is and it’s properties (wattage, energy output, wavelength, etc…)
  2. What the containers are made from (what holds the water, nitrogen, glycerin…)
  3. Exposure time to UV source

#5

Good catch. Here are the answers, which I’ve also added to the original post.

UV post-curing time was 5 minutes for all samples.

The UV curing box was an LC-3DPrint Box, which apparently is a NextDent product, although ours doesn’t have their logo on it. It outputs 300nm-550nm at 290W total. I don’t know how much 405nm is in there, so I emailed NextDent but haven’t heard back yet.

All the samples were cured in polypropylene cups that I was told would not block 405nm UV.


#6

If you use the G&R Labs UV light meter are you going to get a false reading?
In other words, does non 405nm light pollute the measuring ?


#7

Good question. I just emailed G&R Labs, so I’ll let you know what they say.


#8

Updates:

  • NextDent says LC-3DPrint Box emits less than 5% of its light as 405nm. So out of 290W total, that would be somewhere around 14W of 405nm. But then you have to actually measure it where it’s hitting the part.
  • G&R says that the Model 222 will pick up some non-405nm light, so it’s not possible to use it to find only the 405nm light hitting the part.

Given that it won’t really be possible to exactly replicate the curing conditions without buying the same unit, let’s wait and see what the tests show.


#9

I finally have some results, which suggest that there’s not a huge difference between any of the methods.

Kind of a boring outcome, but it also frees you up from having to use overly complicated methods. Granted, these samples were dogbones, which don’t have any little crevices for resin to hide. If you’re regularly making prints that are difficult to clean, just stick with whatever method you’re using.


#10

Could you elaborate a bit, for those of us who are not very scientific?
Does Conversion refer to depth of cure?
Are we to understand that UV & Air yields the best post curing results?

I attribute my good casting results to the fact that I cook my models in glycerin at 325F for one hour after I cure under UV.

Would be good if you could add the presence of vacuum or heat to your results, there are those who claim it makes a huge difference.


#11

Absolutely, thanks for asking.

Conversion is the percentage of liquid resin cured into solid polymer. In theory, a perfectly cured part would show 100% conversion.

In practice, it’s a little fuzzier. According to this paper, “the limited conversion found in many network polymers is due to restricted mobility of radical chain ends, pendant methacrylate, and monomer imposed at high crosslink density.” This means that as monomers (small molecules) convert into polymers (long chain molecules), the cured polymers start to prevent additional monomers from moving around and adding themselves onto the ends of the chains. Once the polymer network fully solidifies, which is called the glass transition point, nothing can move around, and the conversion reaction stops. This happens well before reaching 100% conversion. In the dentistry world, UV-cured composite fillings are considered “cured” when they reach 43-75% conversion, and the same is true here.

As far as my results are concerned, it would appear that UV curing in air gives the best results. However, because all the results are within 15% of each other, and because samples can be considered cured with conversion percentages well below what I recorded, the results would also suggest that the method isn’t actually that important. In fact, with some resins, UV post-curing can actually cause them to have surface cracks.

With that being said, apparently it is theoretically possible that heating up samples after UV curing could allow the polymerization reaction to continue, which could be happening when you cook your models in glycerin. I’ll have to give that a try and see.

As for using a vacuum chamber, most of the resin components have such high molecular weight that it’s impossible to evaporate them, so it’s kind of a moot point.


#12

Top of my mind: with FTIR you might look for residual unsaturated bonds, which gives you the idea of how much unreacted oligomer is in your sample. But the FTIR specimen is usually a thin film, maybe I’m not up to date on this?
By the way what resin is the one used? PR48 has sartomer494 which has 3 unsaturated groups so maybe even in a “perfectly cured” specimen not all of them react beacause of steric hindrence.


#13

Hi, I’m really liking this post since postcuring is still quite a mistery to me.
Could you specify the resin you used, or did I miss it somewhere? I’m expecting your samples are too old now but did you notice difference in tackiness for each differnt post-curing methodology?
As far as UV-box I wouldn’t worry about 405nm since 1) the photoinitiator (TPO-L?) in the resin most likely absorbes on a wide range 2) I’d say only the amount of energy absorbed is important so you would just need to have the UV-box on for a longer time.


#14

RE: UV cure
Been noodling with an idea and wanted some feedback -

  1. take an old/cheap small microwave oven with a rotating carousel.
  2. Disconnect the microwave portion.
  3. install 3-4 LED UV lamps that are connected to the existing lighting wiring.
  4. Power up!

This would use the timer and carousel feature of the oven to even cure in open atmosphere. For oxygen inhibition, place the parts in a beaker of water. It would also look professional too :slight_smile:

amazon link


#15

I m sure you can build something with a more professional look at a low price, while making sure light hits from every angle.

For turntable, you could use one of those solar ones: http://www.ebay.com/itm/New-Solar-Powered-Jewelry-Watch-Phone-Rotating-Display-Stand-Turn-Table-Plate-/252432541983?var=&hash=item3ac626e91f:m:m4bH2LOFUMMgqZ7zUQQWyhg

For timer, I used one of those: http://www.ebay.com/itm/Automation-DC-12V-LED-Display-Digital-Delay-Timer-Control-Switch-Relay-Module-AD-/112095257278?hash=item1a1965e2be:g:dewAAOSwEzxYWv9U

For LED, I used the led strips, but there are much better options out there, provided that you pay attention to cooling.


#16

Pretty cool idea! You could use stronger, more expensive LEDs because you’d only need a couple of them.


#17

We looked at the ratio of the bonds for reacted vs. unreacted. To sample the specimen, I used sandpaper to scrape off the surface and put that over the FTIR sensor. And I used PR57K – thanks for catching that. I just updated the original post to mention it.


#18

@Yannis_Vauthier
I’ve tried the solar turntables - stopped working after a few weeks. Seems the solar panels aren’t meant to work so hard. Also, the LED strip I bought has since had over 50% of the bulbs burn out. Now, some of this could be that people would leave the thing on over the weekend…


#19

My concern with UV post curing (and I don’t know if this has been brought up in this forum) is that the penetration depth into your sample is not that large typically on the order of a millimeter. So the interior of larger models will remain at the cure level of the 3D printer. Is your method to measure the conversion a surface method? Would it be possible to make a fairly large part and cut through after post cure and analyze conversion at different depths see if the conversion is indeed being limited within the part?

Anyways why I am commenting here, some groups have used a heat post cure to cure resins in order to pour PDMS (which generally won’t cure in the presence of uncured resins).

As an aside, I have put some PR48 on a hot plate at ~80C for a while and it did cure. So I think heat should be looked at as a post cure method.


#20

Hi,

I notice that nobody has mentioned a microwave being used for curing…

Perhaps it’s because I use B9 resins a lot, and that’s what they recommend… but microwaving the parts in water for 5 minutes makes a HUGE difference to my castable models. (And seems to affect the casting results positively). Resins go from bendy, to very firm, to borderline brittle if I over cook them.

I tried it with the PR-48 too, but it tends to crack the models at 5 minutes (I assume it’s expanding due to the heat and as it’s firmer and cured better to begin with it’s getting brittle), so I usually only do it for a minute or 2.

Lots of folks on the B9 community used to put their castable resins in a conventional oven for a few hours to improve the curing and resulting burnout, but it seems like most (even the founders), now mainly just microwave in water or (rather more carefully), glycerin (it gets mega hot). A friend of mine also does it for his Form resins, so it certainly seems common enough practice for other printers…

Just thought I’d mention it!
-Barry