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The need for a custom tray arises for a couple of reasons: The impression material is typically an elastomeric compound and these types of materials are most accurate when they have a uniform thickness. The custom tray allows the material to have a uniform thickness. Stock trays do not fit the mouth as well because of their universality. This is important for the pouring of models used later in the process and it is important when making a fixed partial denture. When using a custom tray a more accurate abutment distance is achieved. Before I say that it is detrimental for anyone to use a stock tray the differences in bulk of material were les than 1mm and the differences in abutment differences were 100 micrometers or less. Even so accuracy in dentistry is an important factor. I am not sure how often custom trays are used in the everyday practice of dentistry, but it is better to be exposed to everything than just a little. So this is how to make a custom tray:

Before any one gets their hopes up I did not have my camera when I made the custom tray so few pictures are available. The process is pretty simple though.  Here are the steps:

1. Take an alginate impression of your patient or typodont.
2. Pour up the impression using buff stone and add a base. A base is added to the bottom of an impression to make it taller and easier to work with. The base is just adding more stone to the bottom of the model. As easy as pie.
3. When the stone is set then lube it up with Vaseline (LOTS) and then take some dental wax (usually pink) and add a couple of layers around all the teeth and to the gingiva and only go down 2-3mm. When you are finished the model should have an even layer of wax on all the teeth and there should be an even layer on the gingiva that only goes down 2-3 mm. It helps to draw a line 2-3mm below the sulcus around the teeth before you add wax so you know how far down you should go. It also helps to put the wax into warm water so it is easier to mold it around the teeth. The wax acts as the spacer for the custom tray material.
4. In order for the spacer to work, occlusal stops are needed. These stops will stop the custom tray from squishing down all the way onto the teeth which would mess up the impression. They are usually located anteriorly and posteriorly. Cut out a rectangular hole in the wax anteriorly to expose the incisal edge of the anterior teeth. I made it long enough to get both central incisors and the mesial edges of the laterals. I then cut out a small square hole to expose the central fossa of the third molars. The size of the hole was a little smaller than the diameter of a pencil. Make the edges smooth.
5. The custom tray material is called Triad and it is light activated. Turn off your overhead lights when using this material. Add lots of Vaseline to the wax so the Triad doesn’t stick to it. Be liberal with the Vaseline. It comes in black plastic wrapping and is expensive, so don’t waste it. Add it to the wax in even amounts and push it down into the occlusal stop area. If the wax is smooth it will go in easier and the wax will be easier to remove when the tray has hardened. When you get to the edges of the wax then add the Triad to cover the wax and then go down beyond the wax by 2-3mm. This will make a lip in the custom tray when it has hardened which will fit the gingiva and prevent impression material from squishing out everywhere.
6. Add a small handle on the front using a little triad. This will help you seat and remove the custom tray. My handle sucked, but see the pictures below for an idea.
7. Make the triad look neat by smoothing it out with Vaseline on your finger. The soother and neater, the better. It is easier to smooth it out now then to smooth it out when it has cured.
8. To cure it place the whole thing into the curing box (I don’t know what it is called) and turn it on for 5 minutes! The wax will not melt in this time period. (Watch it closely though, don’t walk away). Then if the Triad feels pretty firm then you can pull it off the model. This is where the Vaseline comes in handy. Turn it over so the wax is face up and turn it on again for 2-3 minutes. Check it every once in a while to see if the wax can come out. Keep curing it with the wax face up until it can come out with some digging. WATCH IT CLOSELY, you don’t want the wax to melt and spill everywhere. When the wax has been taken out, cure the Triad for 5 more minutes while it is on the model and again off the model open side up for 5 minutes.
9. Take it back to your desk and smooth out any rough spots with your acrylic bur. Then take a number 8 round bur and punch holes into the custom tray. Punch them about 2cm apart all over EXCEPT NEAR THE TOOTH OF INTEREST. So if you are doing a crown on #30 then stay away from punching holes near tooth #30. Simple as that. You are now finished.

Look closely at the following picture and you can see the occlusal stops anteriorly and posteriorly.

This picture shows the holes everywhere except near tooth #3. It is also smooth. The handle suck on mine, but it is good enough for this project.