I bet there are many threads outlining the proper way to invest and process dentures on this planet. After doing it by hand for close to 40 years, I have learned a few hard and fast rules to get the best results, so I will share this without the aid of AI. Don't get me wrong I tried but it did not share the pearls I will share here, yes its a wall of text but MAYBE someone will find something helpful here.
Everything starts with your wax-up. The cleaner it is and the more naturally you carve the gingival roll and your transition to the tooth, the better. If you carve a sharp edge and leave that valley, it will be harder than if you carve and slightly flame this sharp edge. I also like to high-shine the wax with either an old-school cotton ball and water or a foam wheel on a mandrel.
When investing the pattern, there are many aids that allow for quick and clean separation. Many have stated putty, PVS, or denture insulating paste. These all work, but I am here to state you can do it with gypsum if you follow a specific protocol. I will share how I do it.
For compression packing, I would use plaster to invest the bottom half perfectly smooth and short of the model's landmarks that should be well-established in the master cast. If the model is of quality and density, the index marks need to have some separation, and a thin layer of sodium silicate applied to a saturated model will aid you in a clean separation from the bottom of the flask. When moving to the top half of the flask, I would mix a small amount of type 3 stone and plaster in a 1 to 1 mix and then paint only the gingival roll with my finger. While this starts to set up, I would paint the bottom half of the flask with some sort of separator. You can spend money on just about anything here, but we have used sodium silicate forever. I would then mix my top half of 100% type 3 stone, close the flask, and pour to the top of the teeth, use my fingers to expose the cusp tips. Let it set, and then we would mix our 3rd pour for our cap, paint more sodium silicate over the second pour, and then pour our cap and close the flask. This part is super important: use a lower expansion stone and mix to the proper ratio to aid you in not having the bite potentially change under compression.
Now, the next important part to getting clean results is how you de-wax the pattern. You must not liquefy the wax. This is where the most problems occur, and when you realize this, things will improve. If you are dumping the flask in boiling water, it's next to impossible to time every case to have the same results. We have found having a water bath at 125°F for 5 to 10 minutes will allow you to open the flask and peel out the bulk of the wax residue. If you have a heated pressure pot, this can aid you. Now, if you have a fancy boil-out tank that takes the clean water from the bottom of the tank to wash the remainder of the wax out of the flask, use it. If not, a tea kettle will do the same. Pro tip: do not go to the steamer first and use this instead of boiling water as you will liquefy the remainder of the wax and force it into the gingival roll. Now, don't get me wrong, I do use a steamer, but it's the last step that I do prior to prepping my teeth and applying the separator.
Ok, if you are still with me, the trick to getting the cleanest results is how and when you apply the separator and which separator you use will dictate this. We have to realize what is happening and why we are applying this tinfoil substitute. We want it to soak into the stone, not just lay on top. The little mix of the 1 to 1 ratio we put around the teeth will be more porous as well as softer, aiding the separator to penetrate and aiding you in a softer material to de-vest. The temperature of the flask is important, and how saturated your casts are will determine how the separator adheres. Too often, I have seen techs take boiling hot flasks, pour a bunch of separator in the flask, and then rinse it out only to pour or paint on more. This is not ideal and likely will lead to more stone sticking than not when you pack because the surface film will slough off and not allow good separation. Now, there will be some techs who say, why not use the plaster/stone mix for everything in the flask? The issue is compressive strength; this mix cannot stand up to the force you will apply while packing. This is why injection is so much easier to get your patterns to replicate your setup without slightly moving teeth.
Ok, so if you know the optimum temperature and you know the optimum saturation level of the investment, then you can apply the separator. I have used Ivoclar's separator for more than 20 years and feel it's the most consistent. Follow their instructions along with this preprep, and I can assure you, you will have exceptional results. So now that you have your separator on, prepping the basal surface of the teeth, if not done when arranging the teeth, is prudent, especially if you are using high-end teeth. This will allow the best chemical reaction to occur to help the acrylic etch the teeth and allow a chemical bond between them. There are bonders out there that will soften or etch, but if you clean the teeth as outlined, they are not needed. If you are a tech that is pouring monomer into the flask to clean teeth, just know you are damaging the tinfoil at this point as well. If you want to use a Q-tip and paint the teeth rather than dousing the entire flask, that's not a bad thing to do. BUT the absolute best way to get the teeth to etch and to bond is to bench set your packed cases before putting the compress into the curing tank. This 30 minutes to an hour time helps to have the resin soften this surface and will absolutely achieve a good chemical bond. When you start to cure the case fast, like many do trying to get the damn thing finished, the harder teeth struggle to soften and can lead to teeth debonding from the denture.
Ok, so now you know the tricks, but perhaps the most important part of processing is how and when you introduce the acrylic and at what stage of the exothermic reaction. Don't eyeball the ratio; measure and mix according to the IFU and only pack the case at the proper time. The lost art of cellophane sheets and incremental trial packing has been lost as the resins have improved with how they flow under compression. There is a very fine line at what stage you will achieve the best results; too stiff can slough off less than perfect applications of separator, too wet will etch and soften the separator as well. Follow the instructions for the specific acrylic you are using, and you will have a much better chance of having a clean separation.
Post curing cycle, if you did a final boil on your case, do not open a hot flask. Let it bench set slowly for optimum results and do not automatically shock the flask by putting it in cold water. I know everyone is in a rush, but time it appropriately with other duties and you will have better results.
Deflasking the case: If you have learned how to use nippers, you can de-vest quickly following my 3-stage pour investment. After knocking out the mass from the flask, nip off the top cap to expose the cusp tips of the teeth, and this will aid you in the jaw placement to protect and not damage the teeth. I start with the model side first and remove the plaster, then I nip the top half, starting at the heel of the denture and rotating my hand away from the pattern to shear off the facial in 2 to 3 nips. On lowers, it's the same, but I am a bit more careful not to break the model as I get to the lingual area. How you have boxed and poured your master model and your lingual wax up will influence how hard or easy this part is to remove.
Now, if you have followed all of this and you are at the point of cleaning the gingival roll, a stiff denture brush and some soap and water is all I have ever needed. If I do have any residual plaster/stone sticking, I would not use chemicals to clean, but I would use walnut shells and later in my career, we switched to glass beads at very low pressure.
With injection, many of the struggles have been overcome as there is much less flask pressure on the investment, but all of what I outlined for preparation and wax removal is still relevant.
Happy processing, brothers and sisters, and Happy New Year to you all.
