# The Importance of Curing in 3D Printing
## Introduction
[3] So I wanna talk about a really important topic that is curing your prints post processing. There's a few things that we need to discuss. One is how can we get better mechanical properties in a higher color stability from our prints utilizing the power of our curing units? [10] And then how are 3D printed resins differentiated from direct resins? I get this question all the time. Why not just do a direct resin for an inlay, onlay scenario? Why print it?
## Weaknesses of Direct Resins
[36] Well, the first thing I wanna try to mention about the kind of the weak link of direct resins, and this is something going back to some of the intrinsic limitations of methanrolate based composites, especially when polymerized in the mouth and site too, is that the degree of conversion of the methanrolate esters, the double cross linking or the cross linking between the long chain polymers for direct resins is the weakest link of a direct resin. [67] It only ever gets to 50 to 60% cured. There's many studies that look at this looking at things like FTIR. This is Fourier transmission electron microscopy, looking at peak spectrum of methanacrylate and looking at whether or not it was able to be polymerized and cross linked.
## Curing and Mechanical Properties
[93] And we also have some even more sophisticated technology than FTIR looking at degree of cure. And some of the old school ways, which is basically a vigorous hardness or the bottom of a composite, which is not very scientific. [109] Most of the studies, the propensity of the data indicates that direct resins when cured in the mouth, even with our best intentions, with our best curing units, typically lie about 60% maximum cure. That doesn't mean they're soft or hard, but it means that we're not maximizing methacrylate chemistry. It means we have things like higher wear.
## The Role of Heat in Curing
[131] We have the more likely to get plaque accumulation because we have unpolymerized methanrolid. We have higher leakage at the margin because we're not quite as hard as we could be. We have lower modulus of elasticity, etcetera etcetera. [158] We could go on like why the curing is important. Nevertheless, composites are a great direct resins are a great option. We just have never realized the maximum potential of this product category because of the limitation of the degree of cure.
## Studies on Curing
[173] Now, this is one of my favorite studies because it is a systematic review and meta analysis by Optam. And Optam showed that every time you add another surface to a direct resin based off of the data, you're gonna increase failure 30 to 40%. That's a huge increase. [193] And so we particularly have issues with large direct resins. Post large composites, tend to fail quite dramatically, and they oftentimes fail catastrophically through recurrent decay.
## Indirect Resins and Shrinkage
[206] Now one of the reasons why they have a propensity to fail to recurrent decay is not necessarily a degree of cure issue, but it's more of a polymerization, shrinkage, stress issue where these materials shrink about two to 3% volumetrically. [224] And so when you have a big restoration and you're curing it, even if you're well intentioned in doing increments and banking your cure, you're gonna get profound shrinkage stress which contracts towards the light source. So think of the floor of the box. Think of yourself carrying it at the marginal ridge.
## Comparing 3D Printed and Direct Resins
[241] The material down at the floor of the box shrinks up towards the light that opens up a micro gap and causes instant microleakage. So you set it up for failure the very same day you place it. Indirect resins don't suffer from that because we have instead of curing a huge two, three millimeter bolus of material, we are curing a 80 to 100 micron layer of resin with the resin cement. [263] And the volumetric shrinkage is so nominal that we don't get that same leakage.
## Maximizing Mechanical Properties
[275] So how do we compare these two like materials, 3D printed and direct resin? How do we prepare them differently, and how can we maximize the mechanical properties of our print? The first thing that I'm gonna tell you is that the propensity of the evidence indicates that heat is really important for the degree of conversion of the methanol ester, the photopolymer in the print resin. [307] So modern curing units, you'll start to see come on the market with high heat, 80 degrees Celsius, even a hundred degrees Celsius, which is awesome because that high heat increases the photocatalytic conversion of the methanacrylate bonds to make it deeply cured from within.
## The Impact of Glycerin
[333] So the light only kinda gets the outer surface, but that heat penetrates well within the material bulk. And as you could see from this study by Lee, he showed that 35, 60, and 80 degrees Celsius led to some profound changes in degree of cure. [348] And what was interesting is he looked at ambient air environment where you just cure it in the cure box versus a vacuum where it sucks all that air oxygen out of the chamber. So it's kinda like an inert oxygen free environment.
## Conclusion
[360] And then lastly, he looked at glycerin, which is vegetable glycerin. Like, we all have it at home. It's in grandma's house probably. It's at the grocery store. You could buy it on Amazon. Glycerin. Good old fashioned vegetable glycerin. [385] What Lee found was that vegetable glycerin when you cure your print in that solution, something kinda magical happens where the degree of cure is even better than when under vacuum. That's profound. And then if you combine that with heat, like some of the modern curing units like the NanoCure, they work synergistically to ramp up that degree of cure. So we're able to get beyond 90% cure. That is insane with a polymer, with a photopolymer.
[424] I mean, think about the direct resin stuck at 50 to 60%. So we're maximizing that cure, but what's cool is you actually don't wanna get to a %. When you get to a %, which is nearly chemically impossible, but if you were to get to a % by also combining high pressure like composite blocks do in the milling world, then the issue is that you're very inert. You're hard to bond to. Your bond longevity is not good. If you paint a glaze on, it's not gonna stick. [452] Whereas with prints, here's the magical thing. When we pull it off of the printer, okay, right off of the printer, we're 60% cured on average. We're like a direct resin in the mouth. We could paint colors and stains and resin glazes, and it will stick tenaciously because it's only 60% cured. And then we bake it in the oven, high heat, glycerin, and we go ahead and get that cure level all the way up past 90% mechanical properties shoot through the roof, wear resistance increases, color stabilities increase.
[490] Let me just show you some data. The degree of conversion, this is a different study by Lim and Lim showed the same thing comparing glycerin to vacuum, 80 degrees Celsius. The best by far was glycerin. Comparing room temperature which is the r t here. So the white bars are room temperature. The black bars here in this graph are with 80 degrees Celsius heat. And then we have control which is just curing a normal air environment. [519] GL means glycerin, and then we have the VA for vacuum. It's very easy to understand this degree of conversion means how cured is it. And as the number goes up, the more polymerized you are.
[533] This was profound. And even though it's only at 80 in this particular study, it is still dramatically higher than when curing it in ambient air. Look at we're at, like, 55% there for the control group. That's profound, guys. Let's listen. This data changed the way that I thought about printing. [555] We've been curing in glycerin at mod for the past four years since we opened. That has been something that is really one of the reasons why some people will say, well, how are your how are your prints how are your veneers holding up so well? And my lower veneers guys are 3D printed. They're they're roughly about eighteen months old right now, and they they have not stained at all.
[583] Whereas some people out there will do 3D printed veneers and eighteen months later, they they literally look like you you can't even imagine how bad they look. So what is the big discrepancy here? Well, number one, it's post processing. Two is, like, what resin are you using because not all resins are created equal. And we'll get through that, in some other videos. But just right off the right off the bat, like if you're curing glycerin you're doing better. [608] So let's look further at some of this data. Here we have Vickers hardness. Okay. Vickers hardness is usually you have kind of a diamond shaped kind of probe that presses down into the material and then you're measuring the indentation that is formed from the diamond shaped probe volumetrically.
[628] And that basically tells you how hard the outer surface is. And for wear resistance, you want a higher hardness on the outer surface because the softer the material is, the faster it's gonna wear. Look at what happens here for Vickers hardness with glycerin and heat. It shot through the roof, almost double of the control. That is profound. [650] So that's the other thing I see. I don't see a lot of profound wear with my prints. I see high color stability, high wear resistance. Well, this could be why. We're curing and glycerin under high heat. The data doesn't lie.
[672] Now this this is really important, water solubility. Heat happens to be one of the most important things for water solubility to decrease. That is the ability of the material to dissolve in aqueous environment like the mouth. You want low water solubility. This is micrograms per cubic millimeter ultra low, okay, to begin with. [701] But look what happens when we apply heat which are the black bars compared to the white bars which are no heat. Glycerin certainly helps a little bit, but actually in this case, glycerin is not statistically different from vacuum and the control. So we don't have a large difference between glycerin in this stage. So glycerin is not quite important for decreasing water solubility, but heat is.
[726] So when you go on the market and you're trying to find a curing unit, like, the questions you should be asking are, like, how hot does it get? Have they been validated? The resin's been validated for the heat. And that's one reason why I happen to be kind of a SprintRay fan. The NanoCure unit gets up to a hundred degrees Celsius, and the resins have been validated with that heat. Right? [750] That's important. They understand the science behind behind heat cure. One of my favorite denture based materials is a Lucitone material. And what's cool about that is they were one of the first to mandate heat with their special curing unit. That's one of the reasons why they got the kind of godlike mechanical properties out of their printed Lucitone, which has been incredible on the market for the past four or five years.
[778] Heat. The secret was heat, and no other curing unit at the time had heat. I think lucitone gets up to 80 degrees Celsius in their dense supplies thrown a curing unit. Now one thing that glycerin does is dramatically impact color stability, which is super important for veneers. [795] And if you're in the part of the world where they have a lot of curry, curry is like the kryptonite to prints. Like, I don't know what it is, but there's something in the curry that dramatically stains the print resins. But look at glycerin here. When you look at the curry here for the control, which is just standard ambient curing like most curing units, vacuum could be very considered to be equal to nitrogen in a lot of ways.
[812] And then glycerin. Glycerin's like the magical bullet here which minimizes now curry staining and what's cool is it also minimize coffee staining, grape staining. The other thing that the glycerin cure is really important for is also increasing that Vickers hardness, which decreases the plaque accumulation on the on the material. It's it's just less porous, I guess, if you will. [853] So we have all this evidence surmounting that says glycerin is really good for printing. Here's another study. This one was really cool. It's by a colleague, Nate Lawson, at a UAB looking at glycerin versus no glycerin curing. And look at the orange bars. This is color stain where the higher the staining, the higher the color change, the higher the bar.
[875] And all the blue bars represent no curing and glycerin where the orange bars represent curing and glycerin. And immediately we could see literally sometimes half to a third of the color change if you cured in glycerin. This is just another study confirming what Lim found, which also does what Lee found. So here we have Lawson finding the same thing. [900] Here's another Lawson study looking at, in this particular case, occlusal guard materials which are supposed to be clear and looking at intense aging, with various different materials. And what they found is again, glycerin protects the color of the material.
[918] This is just something that you just need to know. Now does it mean that you always need a cure in glycerin? No. In fact, SprintRay actually doesn't recommend it. It's not in their instructions for use. They frown upon it. However, operating outside of the instructions for use into the gray area, will you hurt things by curing in glycerin? Probably not. [941] My philosophy has always been this. This is important. I cure per the manufacturer's recommended cure time without glycerin. So in the NanoCure, let's say it's a crown and let's say it's the material called ceramic crown that's a like, a three minute cure time. So I'll cure it for the three minutes. Then I'll plop it in a little bowl of glycerin.
[964] So let me show you what that looks like. This is a glass petri dish. K. It's a boro aluminum silicate glass which lets light through UV, all the wavelengths. It also is easy to heat up. A little petri dish right here. Now what's cool about this is this will actually fit in the NanoCure. [987] This is a 80 millimeter, round disc. You can't go higher than 40 millimeters tall in the NanoCure, I believe. So you could look for petri dishes, agar plates, figure it out for your team. But there's a lot of these little glass petri dishes. Just remember, you can't go higher than about 40 millimeters. You'll get stuck in the door.
[1009] So you cure it and then you plop it in here. It's already been cured for the full time and then you cure it for an additional amount of time. We've been experimenting with this at mod and sending samples off to Texas A and M University looking at things like biocompatibility strength, Vickers hardness, degree of cure. [1030] And what we found is that anywhere from a additional half cure to full cure is really good depending on the material. And so what that means is, like, for example, if the ceramic crown material that you cured was, three minutes for the instructions for use, then you could do six minutes.
[1046] So you'll do three and then plop it in here and do an additional three and it does not seem to impact negatively the mechanical properties of the material. Some materials you could get away with just doing an additional half cure. And the difference for me is whether or not I candy coated it or not. [1060] So, if I didn't candy coat it, which you'll learn from some of our other videos, we we candy coat before we cure. If I didn't candy coat coat it, I do one and a half cure. So I'll cure it per instructions. I'll plop it in the bulk glycerin and just do a half cycle.
[1080] If I did candy coat it, which means I have an outer layer of kind of un polymerized resin, It is tacked before it's put in the curing unit, but still, we do a double cure. That's that's basically it. That's the guidelines that we use and it's been working so well for us. The materials have been holding up incredibly well. [1104] And I think it has to do with this glycerin that we've been implementing from the beginning here based off of actually, the study that was first conducted on this was the the least study and and it it really kinda changed the way I thought about curing.
[1114] Both heat and glycerin combined are a magic bullet. Now you don't ever really wanna get past two cure cycles. You'll you will start to get to a point of no return and even to a point of slightly weakening the material because you you start to actually, you could cause a brillness to occur, after well after two cure cycles. [1136] You could actually start to degrade the outer surface a little bit where it could get even, weaker. So so be careful there. And again, you're operating in a gray zone outside of the manufacturer's recommendations for most of these because most manufacturers are clueless about glycerin.
[1154] They wanna sell you like a nitrogen unit or tell you you don't need anything. You're fine as is and that's that's fine. If you're ever in doubt, just follow the manufacturer's guidelines. But if you wanna try to do things the mod way, this is how we've been doing it. And again, please don't ever violate the the two cycle cure time. You'll start to get into the zone of a point of, degradation. [1175] And to play it safe, do one and a half cures. You can't really go wrong there because, you're not actually truly curing it one and a half times because the the glycerin is gonna dissipate some of that light and some of the heat. And so it's it's not a full real full blown one and a half or two cycles. It's actually a little bit weaker due to the nature of it penetrating the glass jar and penetrating the glycerin to get to the item that's in there.
[1204] With that said guys, when you do this, your prints should be easier to polish because they're harder, they're more cured on the outer surface, which means also easier to polish. If you did coat it with a glaze, the glaze is more color stable. And so we have all these advantages of the glycerin cure. [1221] Check out some of our other videos where we actually, sit down and teach you guys how to finish prints, where we paint things, and we cure things together. Those are really fun to watch and for your team to kinda get an idea of, like, what we're talking about when we talk about the glycerin cure.
