April 15, 2012

Dental Lab Model Fabrication

     In this demonstration I will be showing how to fabricate the working lab models from the die stone that was poured in the post from April 5th. There are many ways to fabricate a working model but I will just be illustrating how it is done at my lab.

Model Fabrication

The first step is to separate the die stone model that was poured from the dental impression. Slide a knife between the stone and the tray edge and twist the blade to release the model. It is best to hold the stone in your hand and have the tray pointing upwards. That way, if the impression were to suddenly release it would be the tray to fly across the room and not your model.

Inspect the poured model for any flaws such as bubbles. This looks ok.

Once separated, trim the base of the model flat and level, and the facial of the model using a wet model trimmer. Use eye protection.

After trimming, allow the model to dry a little. If you need to work fast simply spray the model with a compressed air hose to speed the drying process. Trim the lingual portion of the model with an arch trimmer if you have one. If not, you can use a handpiece. Do not create any undercuts on the model when trimming or it will be extremely difficult to separate from the base that will be poured later. 

Drill pin holes in the base of the die stone models so you can independently section the dies later on. One pin per abutment, pontic, and any other portions that will need to be removable to facilitate working on the case. Blow out the dust from the drilled hole with some compressed air.

Glue the brass pins into the holes. I use Krazy Glue Original because it bonds the stone and pin well and it flows easily. I apply a drop of glue to the pin threads then insert the pin into the drilled hole with the flat portion of the pin facing distally. Having the flat portion face the same direction each time makes placement of the dies back into the base easier where there are multiple units on one model. I also attach a rubber sleeve to the base of each pin which I will explain later.

Apply a stone separator to the die model using a brush. We use Super-Sep.

Allow the separator to dry a little, then you can pour a base for the die stone. Mix some Hydrocal 105 in a rubber bowl and pour into a base former until it is full. Vibrate the base former to remove any trapped air bubbles. Invert your die stone model so the pins are facing up and using an action like you were cleaning off the spatula apply yellow stone to the base of the model around the pins. Once the base is completely covered give the wet stone a little bit of a tap with your spatula to remove any air bubbles and ensure that it is smooth against the base of the model. place the die stone model into the base former that is filled with yellow stone and give the whole thing a couple of taps against the counter top to once again eliminate any air bubbles which may have been trapped. Allow the stone to harden for at least half an hour.

Once solid you can remove the model from the base former. Using the wet model trimmer, trim the base around the facial and flat at the posterior. Trim the base of the model to expose the brass pins and rubber ends. Let it all dry again.

After the model has dried somewhat, it is ready to be cut. Use a fine blade saw to cut the model. As I already stated, each die must be individually removable for working. If it makes it easier to cut out the dies, you can cut the model in half and remove the non working side before making the die cuts. Do NOT cut the margins or contacts off or you will have to start everything over again.

Remove the sectioned portions by pushing on the exposed pin bottoms at the base of the model. I use an old drill bit.

After all the segments have been removed clean up the base of the model with a sharp knife. Blow out any dust from the base with a air hose and be sure to use eye protection while doing that because tiny fragments of stone often fly towards your face at a high speed. Clean the die pieces with a brush. Do not brush the margins or contact areas because you may inadvertently chip or break the model.

Remove the black rubber prices from the bottom of the base and you can then replace all the die stone pieces. With the black rubber removed it creates an easy and clean looking access to the pin bottoms for easier removal of the dies.

Do a rough trim of the prepared dies around the margins to remove the bulk of the stone that will be removed during die preparation. I use the arch trimmer for this purpose.

Write the patient name or case number on the model so it can be identified and make sure you have poured the second pours of the models and you are done.

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